Ann. Rev. Ecol. Syst. 1984. 15:353-91
Copyright C 1984 by Annual Reviews Inc. All rights reserved
THE ROLE OF DISTURBANCE
IN NATURAL COMMUNITIES
Wayne P. Sousa
Department of Zoology, University of California, Berkeley, California 94720
INTRODUCTION
Two features characterize all natural communities. First, they are dynamic
systems. The densities and age-structures of populations change with time, as
do the relative abundances of species; local extinctions are commonplace (37).
For many communities, a self-reproducing climax state may only exist as an
average condition on a relatively large spatial scale, and even that has yet to be
rigorously demonstrated (36). The idea that equilibrium is rarely achieved on
the local scale was expressed decades ago by a number of forest ecologists (e.g.
101, 168). One might even argue that continued application of the concept of
climax to natural systems is simply an exercise in metaphysics (41). While this
view may seem extreme, major climatic shifts often recur at time intervals
shorter than that required for a community to reach competitive equilibrium or
alter the geographical distributions of species (6, 21, 43, 76, 92). Climatic
variation of this kind influences ecological patterns over large areas, sometimes
encompassing entire continents. Other agents of temporal change in natural
communities operate over a wide range of smaller spatial scales (47, 242).
Second, natural communities are spatially heterogeneous. This statement is
true at any scale of resolution (242), but it is especially apparent on what is
commonly referred to as the regional scale. (By region I mean an area that
potentially encompasses more than one colonizable patch.) Across any land or
seascape, one observes a mosaic of patches identified by spatial discontinuities
in the distributions of populations (153, 159, 161, 231, 239, 240). Closer
examination often reveals a smaller-scale patchwork of same-aged individuals
(e.g. 85-87, 101, 146, 199, 204, 217-220, 235, 246).
Discrete patch boundaries sometimes reflect species-specific responses to
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354 SOUSA
steep gradients in the physical environment. Such responses ac
small part of the spatial heterogeneity found in natural comm
Even where background physical conditions are relatively uniform across a
site, opportunities for recruitment, growth, reproduction, and survival vary
spatially, reflecting variation in the intensity of biological interactions, resource
availability, and microclimatological conditions. By itself, this spatial
and temporal variation in the density of "safe sites" for establishment (sensu 83)
may only partially explain local differences in the demography of populations.
The availability of propagules sometimes limits rates of establishment (84, 94,
215). Since environmental characteristics and population parameters change
with time, the mosaic patterns are themselves dynamic.
To interpret and predict the patterns observed in nature accurately, our
methods of study must embrace temporal and spatial variability as essential
features of population and community dynamics. There is now abundant
evidence that in the absence of such variability many species would cease to
exist. Inherent in this view is the recognition that traces of history are etched in
the structures of many, if not most, natural communities. Often, present-day
patterns can only be interpreted if the organisms themselves yield clues (e.g.
fire scars) as to the identity and timing of historical events or if the assemblage
has been monitored continuously for a long time.
Disturbance is both a major source of temporal and spatial heterogeneity in
the structure and dynamics of natural communities and an agent of natural
selection in the evolution of life histories. These roles are clearly interdependent.
The differential expression of life history attributes under different
regimes of disturbance produces much of the spatial and temporal heterogeneity
one observes in natural assemblages. On the other hand, the heterogeneity in
environmental conditions (both biological and physical) induced by disturbance
is probably a key part of the "habitat templet" (194) that selects among
life history variants.
This review emphasizes the impact of disturbance on the numerical abundance
of populations and on the relative abundance of species in guilds and
communities. Disturbance also has an important influence, however, on ecosystem-level
processes such as primary and secondary production, biomass
accumulation, energetics, and nutrient cycling (e.g. 17, 20, 136, 197, 222,
233). Indeed, too often studies focused at the population or community level
overlook potentially significant effects of ecosystem-level processes on
population dynamics. In forests, for instance, disturbance sometimes causes a
net increase in the amount of soil nitrogen available to early colonizers. It is
probably not coincidental that the seeds of a number of pioneer plant species
germinate in response to high levels of soil nitrates (9, 157).
Assemblages of both sessile and mobile organisms are subject to disturbance.
The effects of disturbance have been much more thoroughly studied in
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ROLE OF DISTURBANCE 355
the former, however, simply because sessile organisms are easier to observe
and quantify. Because of this disparity in our understanding of the role of
disturbance in the two sorts of assemblages and because of apparent fundamental
differences in their responses to disturbance, I have treated mobile
organisms in a separate section at the end.
WHAT IS A DISTURBANCE?
Traditionally, disturbances have been viewed as uncommon, irregular events
that cause abrupt structural changes in natural communities and move them
away from static, near equilibrium conditions (104, 235). This definition has
little utility in light of the following observations:
1. Evidence from long-term censuses suggests that few natural populations
or communities persist at or near an equilibrium condition on a local scale (37).
There is no clear demarcation between assemblages in an equilibrium state and
those that are not.
2. The change caused by any force can vary from negligible to extreme,
depending on the intensity of the force and the vulnerability of the target
organisms. How does one objectively decide what degree of change along this
continuum constitutes a disturbance? The response of perennial species to
regular seasonal change in the physical environment is a case in point. When
temperatures or rainfall oscillate close to their long-term seasonal averages,
organisms respond physiologically and/or behaviorally to ameliorate possible
negative effects of the change. With more extreme seasonal fluctuations in the
physical environment, the limits of effective physiological or behavioral response
are exceeded. At first, this may cause reductions in growth and reproduction,
but if the stress becomes severe enough, organisms will die. The
number killed can vary from one or just a few individuals to entire populations.
Thus, the same basic phenomenon can elicit responses ranging from physiological
acclimatization to population *extinction, depending on the magnitude of
the variation. Since lethal and sublethal responses to such changes (e.g.
seasonal migrations of birds; see 103) can markedly alter the community
structure, the "objective definition of a threshold at which a periodicity becomes
a disturbance [is] difficult at best" (104).
Moreover, the levels of environmental fluctuation to which a present-day
species responds with effective homeostatic mechanisms probably represented
a far greater hazard early in the species' evolutionary history. Differential
mortality and/or reproductive success in the past among individuals differing in
genotype probably contributed to the evolution of the homeostatic mechanisms.
Therefore, an environmental fluctuation that once caused disturbance
does so no longer. This dynamic evolutionary relationship between an organism
and the environmental stresses it encounters is ongoing and subject to
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356 SOUSA
numerous constraints; no organism can perfectly track fluctuations in its
environment. The evolutionary moderation of stress is exhibited in its most
extreme form by species that depend on environmental disruption for the
completion of their life cycles and the persistence of their populations (227).
Allen & Starr (5) argue that at this point the disruptive event ceases to be a
disturbance at all.
Given the complexities discussed above, it seems wisest to adopt the view
(104) that disturbance lies near one extreme of the continuum of natural
perturbations that affect organisms. In the context of this review, a disturbance
is a discrete, punctuated killing, displacement, or damaging of one or more
individuals (or colonies) that directly or indirectly creates an opportunity for
new individuals (or colonies) to become established.
AGENTS OF DISTURBANCE
Both physical and biological processes act as agents of disturbance. The former
are the kind most often associated with the term disturbance, and their role in
natural communities is the primary focus of this review. Examples include
fires, ice storms, floods, drought, high winds, landslides, large waves, and
desiccation stress. Agents of biological disturbance (45, 235) encompass
everything from predation or grazing to nonpredatory behaviors that inadvertently
kill or displace other organisms [e.g. digging by mammals and ants in
grasslands (114, 163, 179) or by elasmobranchs in marine soft sediments (207,
216)].
The impact of biological agents of disturbance seems generally similar to
that of physical agents. Organisms are killed, thereby creating opportunities for
recruitment. The timing of biological disturbance, however, is probably subject
to a somewhat more complex set of controls. Rates of predation depend on
the functional, numerical, and developmental responses of the predator (139).
These responses, in turn, are influenced by the physical environment, habitat
complexity, presence of alternate prey, availability of prey refuges, and the
impact of higher level predators and parasites. For example, the timing of
insect outbreaks in forests may be simultaneously influenced by parasitic
infection (8), spatial heterogeneity in forest structure (138), and the recent
history of physical disturbance (181, 205, 235).
DISTURBANCE IN ASSEMBLAGES OF SESSILE
ORGANISMS
A full understanding of the dynamics of populations within habitats subject to
disturbance requires knowledge of the regime of disturbance and of the subsequent
patterns of recolonization and succession in the disturbed patches. These
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ROLE OF DISTURBANCE 357
patterns are a product of certain characteristics of the original disturbance and
the life histories of the species available to reoccupy the disturbed site.
The Regime of Disturbance
How an investigator characterizes a regime of disturbance depends on the
particular disruptive force and responses being studied. The most commonly
used descriptors (e.g. 35, 62, 86, 192, 223) are listed below:
1. Areal extent-the size of the disturbed area
2. Magnitude-consists of the following two components:
a. Intensity a measure of the strength of the disturbing force (e.g. fire
temperature, wind speed, wave velocity)
b. Severity-a measure of the damage caused by the disturbing force
[Both of these terms have often been used interchangeably (e.g. 33,
192). Severity seems to denote better the amount of damage caused by
a disturbance. ]
3. Frequency-the number of disturbances per unit time. Separate terms are
used for the average frequency of disturbance at the local and the regional
spatial scales:
a. Random point frequency the mean number of disturbances per unit
time at a random point within a region; this is often expressed as the
recurrence or return interval (i.e. the average time between distur-
bances)
b. Regionalfrequency-the total number of disturbances that occur in a
geographical area per unit time
4. Predictability measured by the variance in the mean time between distur-
bances
5. Turnover rate or rotation period-the mean time required to disturb the
entire area in question
Regimes of disturbance vary considerably along a number of spatial and
temporal scales. For example, the well-studied forests of North America
exhibit a wide range of variation in both present-day and presettlement disturbance
regimes (19). Near one extreme is the mixed mesophytic cove forest of
the southern Appalachian mountains (176). Scattered deaths of single trees or
at most a few neighboring trees by windthrow, lightning, or glaze storms form
the predominant pattern of disturbance. Fire is uncommon. According to one
study, treefall gaps range up to 1490 m2, but the average gap is only 31 M2. For
a number of sites, the average percentage of the forest canopy converted to gaps
per year ranges from 0. 5 % to 2. 0% (grand average = 1. 2%), so that the rotation
interval for the canopy layer is 50-200 years. The annual rate at which new gap
area is generated by treefall did not vary markedly over time.
The presettlement regimes of disturbance in the conifer and hardwood forests
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358 SOUSA
of the Boundary Waters Canoe Area (BWCA) of northern Minnesota lie near
the opposite extreme on the geographical scale of variation. Fire is the primary
source of disturbance in all forest types there (85-87). The presettlement fire
regime of "near boreal" conifer forests consisted of crown fires and/or severe
surface fires with a return interval of 50-100 years. A typical fire in this forest
type usually burned a large area, probably 400-4000 ha. Pine forests experienced
a regime of moderate surface fires with a return interval of approximately
36 years, as well as an overlying regime of severe surface or crown fires that
occurred about once every 180 years. The latter fires burned 40-400 ha of old
growth stands. Adjacent enclaves of mixed hardwood forest were burned in
400-4000 ha patches by severe surface and crown fires (where conifers were
prevalent) about every 80 years. High intensity surface and crown fires kill
most of the trees in a burned stand, and regeneration initiates from dispersed or
stored seeds. Extensive fire scar analyses indicate that all present-day stands in
BWCA originated after fire. Smaller-scale disturbances such as windthrow
were insignificant by comparison. Before the adoption of effective fire suppression
procedures, an average of 0.8% of the entire BWCA study area (405,000
ha) burned per year, and the average fire rotation period was about 100 years
long (85, 87).
Surprisingly, turnover rates estimated for forests in these two areas are quite
comparable, despite the striking dissimilarity in the predominant agent of
disturbance. The critical difference is that the Minnesota forests experience
infrequent large-scale disturbances whereas the disturbances in the Appalachian
cove forests are much more regular in time and smaller in area. In
addition, the severity of disturbance caused by intense fires is much greater
than that caused by windthrow.
I intentionally chose two rather extreme temperate forest examples to demonstrate
the wide range of disturbance regimes that occur within this habitat
in different geographical locations in North America. Forests in other regions
of the continent-for example, the Harvard Forest Tract in New Hampshire
(90)-experience a more balanced mix of disturbance by fire and windthrow.
A knowledge of long-term average climatological conditions is useful in
explaining coarse differences in the geographic patterns of disturbance. For
example, the average annual pattern of precipitation determines the relative
importance of fire versus treefall as agents of disturbance in forest communities
(29, 87, 235). Similarly, differences in winter temperatures explain why
floating ice commonly scours the rocky intertidal shores of New England and
eastern Canada but not those along the Pacific coast of the United States (192,
234).
Local patterns of disturbance cannot be predicted from a knowledge of
large-scale climatic variation, however. The regime of disturbance at any one
site depends on a multitude of local physical and biological factors. In the
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ROLE OF DISTURBANCE 359
following sections, I briefly describe some of the more common, and betterstudied,
local physical agents in natural communities.
WIND Wind is an important agent of disturbance in many temperate and
tropical forests, where it creates gaps of various sizes in the forest canopy by
blowing down large branches or trees (19, 22, 84, 176, 237, 238). Rates of
treefall vary substantially over time and across a landscape. In most areas,
treefall is seasonal. It occurs most frequently during seasons with strong
gusting winds and high rainfall (e.g. 22, 232).
Spatial variation in treefall rates is attributable to a number of factors
including differences in topography and soil type. Treefall is more likely to
occur where there are high prevailing winds and at sites that lie in the path of
hurricanes (22, 232, 236, 238). In some forests, surviving trees at the edges of
existing gaps may be more likely to blow down in subsequent storms (22, 84,
237, 238); but in other forests, they are not (176). The risk of windthrow is
often greater for trees that grow on steep slopes or in soils where a stable root
hold cannot be established-for example, in wet, sandy, and some very fertile
soils (22, 84, 176).
As trees grow older and taller, they are more likely to be blown down (22,
176). In part, this is because of the increasing forces a stationary, and relatively
inflexible, object like a tree experiences as it grows larger in a moving fluid
environment. This is particularly true if the flow sometimes accelerates, as
when the wind gusts (224: 94). Other factors also contribute to the increasing
risk of blowdown as a tree ages, including the weakening effects of insect
attack, lightning strikes, disease, and physiological stress. A heavy load of
epiphytes, which often develops on older rain forest trees, may contribute
further to the chance that a tree will be blown down in storms (201).
Gap size is related to the manner in which a tree falls (22; G. B. Williamson,
unpublished manuscript). As one would expect, gap area is positively correlated
with tree size in the most common case where the tree falls laterally owing
to wind stress. In contrast, some species of trees die in a standing position and
gradually collapse downward. This phenomenon produces smaller gaps, and
their size is usually uncorrelated with the size of the tree at the time of death.
Limb fall usually creates even smaller gaps.
The very largest gaps are generated by multiple tree falls. Such gaps are
created when several trees fall synchronously in "domino" fashion or when a
new gap is contiguous or overlaps with an older one. In tropical rain forest,
extensive liana connections may increase the rate of multiple synchronous
treefall (F. E. Putz, cited in 22).
FIRE The local intensity, frequency, and areal extent of fire in terrestrial plant
communities is controlled by complex interrelations among the following six
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360 SOUSA
factors (29, 30, 72, 87, 227): (a) frequency and seasonality of ignition sources,
(b) moisture content of the fuel (i.e. the potentially flammable living and dead
plant material), (c) the rate of fuel accumulation, (d) structural and chemical
characteristics of the fuel, (e) mosaic nature of the landscape, and (J) local
weather conditions at the time of the fire. Since all of these factors vary over
space and time, there is considerable heterogeneity in local fire regimes and
consequently in the effects of fire on vegetation.
Lightning is the most significant natural source of fire (205, 226, 227). Far
less common natural sources include spontaneous combustion (221), sparks
from falling rocks (89, 135), and volcanic eruptions (219, 220, 227). The
frequency of lightning strikes alone, however, rarely explains local patterns of
fire occurrence, even where there is little anthropogenic influence on the fire
regime. Only about 0.03% of lightning discharges that strike vegetated areas of
the world result in wildfires (205). Thus, while it is true that fires must have a
source of ignition and that seasonal patterns of thunderstorm activity may
influence their timing (87), the other five factors listed above determine, in a
proximate sense, the fire regime in a particular area. They determine whether or
not ignition will occur, how intense the fire will be, and what area it will cover.
The moisture content of the fuel determines the likelihood of ignition and the
ability of the fuel to carry a fire. The amount of moisture is influenced, in turn,
by a number of local factors including the aspect of the site, the water retention
properties of the underlying soil, and wind conditions (226). In certain geographical
regions, there are only brief periods during a "normal" year when
fuels are dry enough to ignite and carry a fire. Consequently, the most
significant fires (i.e. the most intense, severe, and extensive) burn during
periodic droughts that recur at intervals of 10-20 years or longer (66, 85-87,
204, 249). Substantial amounts of fuel accumulate between droughts, so that
when fires occur they are exceedingly intense. Similarly, fire only invades
semipermanent wetlands such as shrub bogs (29) during droughts. Fires occur
much more frequently and regularly in areas that have predictable annual dry
seasons as well as occasional droughts [e.g. grasslands and savanna (29, 93,
120, 144, 226), chaparral and forests of the Sierra Nevada Mountains in
California (109, 111), and the dry sclerophyll forest of Australia (31)].
The rate of fuel accumulation can influence the frequency and intensity of
fires because it determines how much fuel will be available for burning at any
given time. Assuming that the likelihood of fire is positively related to the
standing mass of fuels, fires should occur most often at sites where fuels
accumulate the most quickly, all else being equal. (This assumption is critically
evaluated below.) Also, the more fuel that has accumulated since the last burn,
the more intense a fire will be.
The rate at which fuels accumulate equals the difference between their rates
of production and decomposition (29, 30). Site conditions or characteristics of
the vegetation itself that enhance the production of fuels and/or slow their
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ROLE OF DISTURBANCE 361
decomposition result in higher rates of fuel accumulation. Insect and parasite
attacks, disease, competition, windthrow, lightning strikes, and senescence
convert living vegetation into dead fuels. Since dead plant material is often
drier and therefore more flammable than living plant tissue, the likelihood of
intense fires is greater in older stands. There are exceptions to this pattern when
past fires have killed the vegetation but not consumed it completely. In this
case, dead "carry over" fuels may be abundant in the early stages of stand
regeneration, creating a high risk of fire even in a young stand (e.g. 66, 86).
The rate at which dead fuels decompose is controlled by climatological conditions
(i.e. temperature and humidity) and by the characteristics of the vegetation.
Sclerophyllous foliage, for example, is decay resistant and decomposes
relatively slowly.
Structural characteristics of the fuel influence fire intensity (29, 174). When
fuels are distributed in widely separated strata-e.g. in savannas or frequently
burned pine woods-relatively low-intensity surface or ground fires are the
rule. In shrublands and long unburned coniferous.forest, the vertical distribution
of fuels is more continuous, and the surface to volume ratio of the
vegetation is large. As a consequence, high intensity crown fires are common
in these vegetation types.
Fuel flammability is also a function of the chemical composition of the
foliage (29, 174, 175). Fuels rich in secondary organic compounds are ignited
more easily and bum more intensely. In some plant species, the concentrations
of extractable organics vary seasonally and increase with age [e.g. the chaparral
shrub Adenostoma fasciculatum (158, 175)].
The vegetational mosaic and local topographic features strongly influence
the point fire frequency and the areal extent of fires (29, 86, 227). Only a
fraction of the fires that burn through the vegetation at a particular point start
within the stand that includes that point. Therefore, the point fire frequency
depends to a large degree on the rate at which fire encroaches from surrounding
areas. This rate is influenced by the ability of neighboring phases of the
vegetational mosaic to carry a fire and by the extent and orientation of natural
and artificial firebreaks.
Finally, weather conditions at the time of a fire affect its intensity and size
(87). Such conditions include the level of precipitation, air temperature and
humidity, wind speed, and wind direction, especially with respect to the spatial
distribution of fuels and the position and orientation of fire barriers. A fire's
impact is also influenced by its rate of spread (30, 93), which depends on both
weather conditions and the winds, convection currents, etc., that the fire itself
produces. In general, rapidly moving fires consume less fuel and burn at a
lower temperature.
In summary, a reciprocal relationship appears to exist between vegetation
and fire. The state of the vegetation affects the fire; and the interfire interval
regulates the composition, structure, and quantity of living vegetation and dead
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362 SOUSA
fuels. All else being equal, lightning fires are more likely to start in older stands
of vegetation and to be more intense than fires that start in younger stands. In
many instances, however, this description of fire behavior may be overly
simplistic (86). Once a fire starts, it can spread into stands of many different
ages. "All else" is rarely equal; the intensity and areal extent of any particular
fire may be influenced to a considerable degree by factors other than the
characteristics of the accumulated fuel.
WATER MOTION In marine and freshwater environments, moving water
exerts forces on sessile organisms, just as wind does in terrestrial environments.
In addition, suspended particulate matter (e.g. sediment) or larger
objects transported by the moving water (e.g. logs or cobbles) may strike and
abrade the substratum over which the water flows. Aquatic organisms may also
be killed either by burial under sediments that have been displaced and redeposited
by moving water (e.g. 126, 192) or by exposure to air when water motion
changes drainage regimes (e.g. coral reef crests; J. H. Connell, personal
communication).
Detailed study of the natural regimes of disturbance caused by water motion
in freshwater habitats-in streams in particular-is in its fledgling stages (e.g.
88, 113, 131, 131a); little is known about the frequency, areal extent, and
intensity of such disturbances. The disruptive influence of moving water has
been much more thoroughly studied in marine habitats, particularly along
temperate, rocky intertidal shores (e.g. 45, 153, 189, 192) and on tropical coral
reefs (e. g. 33-35, 54, 155, 247). Wave action is a major agent of disturbance in
these habitats. As with wind and fire, the regime of wave-induced disturbance
varies in space and time and is influenced by physical and biological components
of the environment. Wave energy is maximal during seasons of high
storm activity. Therefore, the disturbance it causes is highly seasonal (35, 45,
54, 153, 189, 192). The frequency of disturbance is also strongly influenced by
the physiological and morphological characteristics of the organisms in question
and the properties of the substratum to which the organisms are attached.
As a sessile marine organism grows larger in an environment subject to
periodically accelerating water motion, i.e. in wave-swept habitats, its risk of
being detached or broken by wave stress often increases (50, 117, 192). Older
individuals are more likely to suffer injury or death from wave forces if
weakening wounds caused by boring organisms, predators, and grazers
accumulate with age (35, 192). Epiphytic or epizoic overgrowth can also
increase the chance that an organism will be dislodged by wave action (192).
The risk of damage or death by a given wave force varies among species that
differ in shape, flexibility, or internal structure, among other factors. For
example, tree or bush-like corals (e.g. Acropora spp.) are more susceptible to
wave damage than mound or sheet-like corals (33-35). Similarly, some
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ROLE OF DISTURBANCE 363
aggregations of sessile organisms become more vulnerable to disruption as
their individual members increase in size and number. Dense, multilayered
beds of the mussel Mytilus californianus are less stable and more likely to be
torn from the rock surface by wave forces than less dense, single-layered beds
(82, 151, 153).
In some aquatic habitats, the stability of the substratum directly determines
the rates of disturbance. In streams and on the seashore, strong water motion
overturns loose rocks, damaging the attached organisms. The frequency of this
kind of disturbance declines with increasing rock size (125, 131, 131a, 150,
189).
The sizes of the areas disturbed by wave action vary considerably (35, 192).
Patches cleared in mussel beds during stormy winter months and at more
exposed sites are much larger on average than those created during calm
summer months or at protected sites (153). Similarly, larger boulders are
overturned more often during winter storms and at sites exposed to heavy wave
action than at other seasons or sites (189). In both systems, the rate of
disturbance differs from one year to the next; reflecting annual variation in the
intensity and frequency of storms. Similarly, there was substantial spatial and
temporal heterogeneity in the regime of disturbance caused by four hurricanes
that struck the reef at Heron Island, Australia (35; J. H. Connell, personal
communication). Though such storms are seasonal, their effects are unpredictable
from year to year. The underlying causes of the variation in this case are
not as clear as in the mussel bed and boulder examples.
PATTERNS OF DISTURBANCE IN ASSEMBLAGES OF SESSILE ORGANISMS
Within any particular habitat, a variety of agents operate independently or in
concert to generate the overall regime of physical disturbance to which organisms
respond. The heterogeneity of natural disturbance regimes is due, in part,
to local variation in the intensity, timing, and spatial distribution of potentially
disturbing forces. Often, however, heterogeneity in local patterns of disturbance
is better explained by temporal and spatial variation in the "intrinsic
vulnerability" (192) of the organisms affected by these forces. Conspecific
individuals differing in age and size usually differ in their vulnerability to a
particular force. Increasing size and/or age decreases the risk of damage or
death from some forces (e.g. of woody vegetation from ground fires; intertidal
organisms from desiccation stress; coral colonies from sediment burial) but
increases vulnerability to others (e.g. of large, old trees to windthrow; older
stands of trees and bushes to crown fires; larger intertidal organisms and
branched corals to wave forces).
Vulnerability to a given force is also species- and assemblage-specific. Not
uncommonly, the vulnerability of a community to disturbance changes over the
course of succession (e.g. 191), owing to differences in the physiology,
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364 SOUSA
morphology, or growth habit of the species characteristic of each seral stage.
The risk of disturbance by fire, for example, often changes with the successional
age of the vegetation because seral stages differ in fuel characteristics. Such
differences can influence the temporal pattern of disturbance. Some terrestrial
plant communities appear to exhibit cycles of inflammability that are controlled
by the rate of accumulation of combustible plant material (95, 134, 235). There
may be similar cycles of disturbance on mussel-dominated shores of the Pacific
Northwest coast of the United States. The rate of succession to a community
dominated by mussels appears to set a lower limit of 7-8 years on the interval
between successive major disturbances of assemblages occupying a particular
area of substratum (153).
In many forests, gaps in the canopy are generated primarily by windthrow of
isolated individual trees or small groups of neighboring trees. In these cases,
gap regeneration is a small-scale, spatially and temporally asynchronous process.
There are remarkable, though relatively rare, exceptions to this pattern
where large tracts of forest are leveled by wind (e.g. 19, 24a, 56, 168, 170,
232, 237). To my knowledge, however, a large-scale, cyclic pattern of windinduced
tree mortality has only been rigorously demonstrated in some highaltitude
balsam fir forests (170, 196, 198).
Such cyclic patterns of disturbance are certainly intriguing and have partially
inspired the hypothesis that species have evolved physiological mechanisms
and morphologies that promote disturbance and/or determine its characteristics
(see below). The phenomenon of regular disturbance controlled by the biotic
component of the environment is by no means universal, however. A significant
amount of the variability in the impact of a disturbing force may be
unrelated to biotic properties. In fact, it has been argued that patterns of local
fires in some presettlement forests were largely random with respect to vegetation
type and best explained by patterns of lightning ignition and the vagaries of
the winds and weather during a fire (86, p. 399). Large-scale disturbances such
as mass movements caused by landslides (60, 170, 199, 217-220), earthquakes
(68, 219), and volcanic eruptions (219, 220) usually occur at random with
respect to the age, successional state, or species composition of the populations
and communities they disrupt.
This caution aside, the regime of disturbance in many communities reflects
the interplay between the properties of organisms and the characteristics of the
physical forces that cause disturbance. Thus, the categories of endogenous and
exogenous disturbance (sensu 19, 20) are difficult, if not impossible to apply
(176, 235). It is therefore hard to unambiguously classify a particular successional
sequence as autogenic or allogenic (145).
Present-day disturbance regimes may be very different from the regimes
characteristic of communities unaffected by humans. Fire regimes in vegetation
surrounding inhabited areas were surely altered when indigenous peoples
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ROLE OF DISTURBANCE 365
began to use fire thousands of years ago as a tool for managing vegetational
cover and game animal populations (29, 31, 112, 166, 226). Modern man's
influence has been much more pervasive.
While the activities of indigenous peoples and early settlers may have
initially increased the frequency of fire, the adoption of effective fire suppression
techniques during the last century has sharply reduced its frequency in
many vegetation types (e.g. 62, 112, 119, 134, 137, 173, 204, 206). In North
America only the unexploited boreal forests of northern Canada and Alaska
(87) and forests with extremely long natural fire cycles (66, 172) have remained
relatively unaffected by this policy of fire suppression. Where fire cycles have
been unnaturally lengthened, large quantities of fuel accumulate and infrequent,
unusually intense fires are the result (53, 134). In other habitats, such as
swamps where draining has lowered the water table (29), fires are much more
frequent now than in presettlement times. These changes in the fire regime have
had a marked effect on vegetation patterns (see above citations).
The effects exerted by humans are not limited to disturbance regimes caused
by fire. For example, forest logging practices produce much larger clearings
than those usually created by windthrow, and the temporal and spatial patterns
of harvesting are often quite distinct from natural treefall. Flood control
procedures combined with human modification and degradation of watersheds
can lead to extreme flooding and sedimentation of abnormally long duration in
coastal wetlands. This altered regime of disturbance has much more severe
effects on the biota of the floodplains and estuaries than the normal, lower
volume discharge of freshwater runoff during the rainy season (147, 250).
Human-caused deterioration of riparian watersheds can have similar negative
effects on stream communities (214).
The ubiquity of human-caused alterations in natural disturbance regimes
significantly complicates evolutionary interpretation of present-day patterns of
morphology, physiology, and life history in relation to physical disturbance
(e.g. 109). Only in some communities, such as forests, is accurate reconstruction
of presettlement regimes of disturbance possible. The same concern
applies to human alterations of biotic components of the environment (47. D.
Lindberg, J. Estes, K. Warheit, in preparation). Cautious consideration of such
effects should precede speculation about the evolutionary mechanisms underlying
present-day patterns.
The Repopulation of Disturbed Sites by Sessile Organisms
Propagules of sessile organisms, be they sexually or asexually produced (sensu
143), are rarely able to invade and become established in areas densely
occupied by other organisms (22, 35, 36, 46, 74, 77-80, 83, 176, 192, 231,
252). Resident organisms inhibit recruitment from propagules by a variety of
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366 SOUSA
mechanisms. Occupants may consume the dispersed propagules, as in some
assemblages of sessile, suspension-feeding invertebrates (35, 190, 192, 246),
or the residents may simply have preempted the available space. In other cases,
residents modify site conditions in ways that inhibit the germination or metamorphosis
of those propagules that do reach the substratum. They may release
toxic chemicals (allelopathy) or reduce the supply of essential resources such as
food, light, nutrients, and water.
Physical disturbance is one of the major mechanisms that break this inhibition
and generate conditions favorable for recruitment, growth, and reproduction.
Disturbances not only reduce or eliminate the cover of resident organisms,
thereby lessening competition for resources that are present on the site, but in
some cases they indirectly replenish some of the depleted nutrients [e.g.
nutrient-rich ash produced by fire (17), the accumulation of detritus in pits dug
by foraging rays (216), and nutrients leached from rotting treefall debris in
forest light gaps (e.g. 22, 222)]. In addition, the disturbance may eliminate
toxic chemicals that have accumulated in or on the substratum [e.g. volatilization
of allelopathic chemicals by fire (70); see 30 for a recent discussion of the
controversial role of allelopathy, and its interaction with fire, in shrublands].
The disturbance may also temporarily reduce the density of predators or
parasites of the propagules (70).
The rate and pattern of reestablishment following a disturbance depend on
the following elements:
1. The morphological and reproductive traits of species that are present on the
site when the disturbance occurs. Such traits determine, in part, the likelihood
that these species will survive the event and rapidly reoccupy the site.
2. The reproductive biology of species that were not present on the site when it
is disturbed but have occupied it previously or live within dispersal distance
of it.
3. Characteristics of the disturbed patch including:
a. the intensity and severity of the disturbance that created it,
b. its size and shape,
c. its location and degree of isolation from sources of colonists,
d. the heterogeneity of its internal environment, and
e. the time it was created.
Below, under subheadings 3a-e, I briefly discuss the influence of the characteristics
of these elements on the recolonization of a disturbed patch. This
information is certainly not sufficient to predict the abundance and demography
of a species within a particular habitat or even the likelihood that the species
will persist there. Such properties of a population depend on the complex interaction
of an organism's complete life cycle (including events that occur during
disturbance-free periods) with the overall regime of disturbance in the habitat.
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ROLE OF DISTURBANCE 367
INTENSITY AND SEVERITY OF DISTURBANCE The influence of the intensity
of disturbance on recolonization has been most thoroughly studied in communities
exposed to fire (e.g. 2, 70, 71, 108). Trees and shrubs whose aboveground
buds are covered by a thick layer of heat-resistant bark are more likely to
survive fires of moderate intensity, even if there is some scorching and
defoliation. Hotter fires may kill all aboveground tissues, and only those
species that are able to resprout from underground buds associated with
rhizomes, roots, root crowns, or lignotubers will survive and regrow vegeta-
tively.
The probability that a plant will survive a fire (even with some bud protection)
may decline with increasing age and number of exposures to fire (70, 71).
The degree of vegetative regeneration depends on the season in which burning
occurs if such regrowth draws on a seasonally fluctuating pool of stored
carbohydrates (e.g. 175). There is also the interesting case (100) in which
mutualistic ant associates mediate the impact of fire on resprouting swollenthorn
acacia plants. The obligate acacia-ants clear foreign vegetation from
around the base and branches of the acacia, thus reducing the chance that the
plant will be killed by a fire. If the fire is hot enough to kill aboveground shoots
but not the ants, the colony will move into the regenerating sucker and defend it
against encroaching vegetation and herbivorous insects.
The mortality of even those species that possess subterranean buds increases
with fire intensity (7 1). This relationship holds especially for underground fires
in layers of peat or humus that directly damage tissues below the soil surface
and often kill all of the vegetation on a site. Thus, the probability that a site will
be repopulated by the vegetative regeneration of surviving residents decreases
with the intensity of the disturbance. This same pattern seems to apply to many
species of intertidal algae (191, 192) and colonial marine invertebrates (35). On
coral reefs, for example, fragments of colonies -that are broken loose in storms
can establish themselves in clearings and fill the space by asexual reproduction
(91). The survival of these asexual propagules is probably inversely related to
storm intensity, since heavy wave action breaks fragments into smaller pieces
and scours off much of the living tissue; this greatly reduces the number that can
successfully colonize disturbed sites (116).
Repopulation of disturbed sites by other kinds of propagules, including those
produced by sexual reproduction, can also be influenced by disturbance intensity.
Once again, the best examples come from plant communities exposed to fire
(e.g. 2, 30, 70, 71, 107-109, 252). The flowering of some plant species
characteristic of fire-prone habitats is stimulated by low intensity surface fires.
Other species produce seed that is stored in fruits on the plant until fire of a
particular intensity triggers their dehiscence [e.g. serotinous pine cones (225-
227, 251)]. Still other species release hard-coated seeds that remain dormant on
or in the soil until fire, in concert with other environmental-factors, stimulates
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368 SOUSA
germination. The cues that stimulate germination and the mechanisms involved
vary among species (30). In northern conifer forests, the intensity of the fire is
particularly important to patterns of seedling establishment. Hot fires consume
the organic layer, including its seed bank, and expose mineral soil that is
favorable to the establishment of conifers. Less intense fires leave the organic
layer and its seed bank intact; thus, species whose seeds have accumulated in
this layer will dominate the regenerating vegetation, and conifer seedlings will
be scarce (86).
Plants vary substantially, both within and among species, in the degree to
which they respond to fire with vegetative resprouting versus germination from
seed (70, 108). Similarly, patterns of serotiny in pines differ markedly among
populations and species (132, 156).
A buried seed strategy (20, p. 108) is also employed by some plant species of
mesic or wet terrestrial environments in response to disturbances other than fire
(11, 13, 22, 25, 39, 40, 78, 79, 83, 84, 130). Large quantities of dormant seed
may be stored in the soil. Some of it will have been produced by resident
plants-usually early successional species mature individuals of which no
longer occupy the site. The remainder will have been dispersed to the site, in
many cases transported by animals (e.g. 20, 25, 40, 84, 130, 211). The
creation of a large gap in the vegetation, e.g. by treefall, alters the soil
environment and/or the light regime in such a way that seed germination is
stimulated (9, 11, 20, 130, 157). Cues for germination vary among species, as
does the viability and length of dormancy of their buried seeds. Though an
equivalent recolonization mechanism may exist in marine and freshwater
communities, its existence has yet to be rigorously demonstrated.
The seeds of other mesic or wet environment plant species do not accumulate
in a long-lived seed pool. Instead, they germinate immediately upon dispersal
or soon thereafter; germination may be briefly delayed until favorable seasonal
climatic conditions develop (25, 67). This pattern is exhibited by some shadeintolerant,
early successional species whose light, often wind-dispersed seeds
colonize transient or frequently disturbed habitats (e.g. floodplains and river
banks). It is particularly characteristic, however, of middle and late successional
tree species in temperate and tropical closed-canopy forests (22, 25, 67, 84,
176). Such species produce relatively large seeds that often are dispersed close
to the parent tree. The large seed reserve permits germination and establishment
in the low light conditions prevalent under a closed canopy and may
facilitate root penetration of dense litter on the forest floor. The seedlings are
shade tolerant and persist in the understory for varying lengths of time,
depending on the species. They grow slowly, if at all, until a gap opens in the
canopy over them, permitting a pulse of growth. A suppressed individual rarely
can recruit to the canopy layer after a single disturbance. Before it can do so,
either the hole in the canopy is closed by lateral expansion of the crowns of
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ROLE OF DISTURBANCE 369
surviving trees on the gap border or the gap becomes so choked with regenerating
vegetation that the resources needed to support further growth are used up.
Thus, several episodes of low intensity canopy disturbance in the immediate
vicinity of the suppressed individual are usually required before it grows into
the canopy (176). The disturbance must be severe enough to open a gap, but not
so intense that it kills the understory plants. Suppressed individuals are often
damaged by falling trees and other debris, but most survive and are able to
resprout.
Following an intense disturbance such as that produced by a landslide (60,
68, 170, 199, 217-220), a volcanic mudflow (49), a long-overturned boulder
on the seashore (191, 192), erosion and redeposition of alluvial sediments on
floodplains (63, 141, 200), or a receding glacier (42) neither resident organisms
nor stored dormant propagules survive. All recolonization must come
from either propagules dispersed into the open patch from surrounding areas or
vegetative ingrowth of neighboring individuals or clones.
PATCH SIZE AND SHAPE The size and shape of a patch indirectly influence its
repopulation in several ways. The internal physical and biological environments
may vary with patch size. For example, there is an increase in light
intensity and its daily duration, in mean soil and air temperature (and their
ranges), and in subsurface soil moisture as the size of forest treefall gaps
increases (assuming a constant severity of disturbance). The levels of these
microclimatological variables are substantially higher in gaps than under a
forest canopy (20, 22, 25, 51, 176, 237, 238). Conversely, humidity decreases
with gap size; the initial competition for nutrients with surviving trees at the
edge of the gap may also. [However, there have been no detailed studies of
nutrient dynamics within a single treefall gap (222)1. A number of these
patterns have also been observed in disturbed sites in shrublands (30). These
size-related differences in the characteristics of the physical environment can
influence the germination of seeds, resprouting from buds, and the subsequent
survival and growth of seedlings and saplings within a gap.
In aquatic environments, organisms surrounding a clearing can influence the
patterns of water flow in and around the open patch and thereby affect the
availability of food and the density of settling propagules. These organisms
may also consume incoming propagules or otherwise directly interfere with
their settlement or recruitment. In intertidal habitats, however, the presence of
organisms on the edge of a clearing may moderate the internal microclimate of
the patch at low tide when it is exposed to the air. These influences will be more
strongly felt in small clearings than in large ones (35, 192).
Small patches have a greater perimeter-to-area ratio than large patches,
which has a number of consequences for patterns and rates of recolonization
(133). The vegetative ingrowth of clonal organisms or the lateral encroachment
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370 SOUSA
of attached but semimobile solitary organisms will make a proportionately
greater contribution than dispersed propagules to the recolonization of small as
compared to large clearings if these organisms occur along patch edges. This
phenomenon has been demonstrated in beds of intertidal mussels (153, 193,
202) and in subtidal assemblages of colonial invertebrates (35, 106, 154).
Numerous factors influence the rate at which a patch of a given size is filled by
vegetative ingrowth, including the morphology and rate of expansion of the
invading colony, clone, or shifting assemblage of individuals.
Similarly, in some instances the rate of colonization by dispersed propagules
varies with patch size. The number of nearby adults per unit patch area is
usually greater for small patches than for large ones. This differential may
result in a greater density of recruited propagules and therefore more rapid
recolonization of small patches. This pattern will only develop when propagules
are not dispersed far from their parents and when there is no overriding
negative influence on recruitment by organisms that surround the patch. When
the size of a clearing is large relative to the dispersal ability of potential
colonists, invasion from the edge of the patch may be slow; its closure by
recruitment from dispersed propagules may take several generations (94, 195,
230).
In some systems, the abundance of mobile consumer species within disturbed
patches and their influence on recolonization varies predictably with the
size of the patch. In intertidal mussel beds, for example, small clearings contain
higher densities of grazers, particularly limpets, than do large patches (153,
193, 202). This relationship is analogous to the association between rates of
vegetative ingrowth and patch size. The surrounding bed of mussels appears to
serve as a refuge for small grazers from wave shock, desiccation stress, and
possibly predation. These grazers migrate into the open patch from the edge to
feed on colonizing algae. Since the length of edge is proportionately greater in
small patches, a higher density of grazers accumulates in small clearings than in
large ones. As a consequence of this variation in grazing pressure, different
assemblages of macroalgae develop in patches of different size (193, 202).
This pattern is likely to occur in any system where natural enemies prefer
or are forced to live largely within the phase(s) of the community mosaic that
surrounds a patch, while their prey occupy the interior of the patch. Bartholomew's
(10) demonstration that vertebrates concentrate their grazing and
seed predation within a narrow zone along the edge of chaparral stands suggests
that their influence on the revegetation of clearings may vary with clearing size
in the same way.
Given that the physical and biological environments of patches often vary
with patch size, it is not surprising that species are differentially distributed
among clearings of various sizes in a number of communities. These include
intertidal assemblages (192, 193), subtidal epifaunal assemblages (35, 97,
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ROLE OF DISTURBANCE 371
110), tropical forests (22, 51, 84, 237), temperate forests (20, 57, 176, 243),
and old fields (12, 44, 74, 78).
Patch shape also influences the degree to which surrounding organisms
affect within-patch dynamics. The more irregularly shaped a clearing of a given
area is, the higher the ratio of perimeter length to area and the greater the
influence (both positive and negative) organisms in the immediate neighborhood
of the patch will have on its recolonization (192). For example, long,
narrow gaps in a forest canopy allow less light to penetrate than more circular
gaps (22; 149; J. Tomanek, 1960, cited in 176).
THE LOCATION OF THE PATCH AND ITS DEGREE OF ISOLATION FROM
SOURCES OF PROPAGULES The proximity of a patch to sources of colonists
can greatly influence the mode and rate of colonization (84, 164, 192, 193). For
example, colonization by expanding clones is only possible if they occur on or
near the borders of the patch. Discrete patches of substratum, such as cobbles
on the seashore, are physically isolated from.other substrata. When they are
denuded by a disturbance, invasion by neighboring clones is impossible, and
dispersed propagules must initiate recolonization (35, 192). Patch location is
particularly important for species that do not disperse their propagules very far,
e.g. many terrestrial and some marine plants (20, 46, 84, 94, 152, 164, 192,
193). In contrast, the larvae of many, but not all (e.g. 69), marine invertebrates
spend days, weeks, or months in the plankton before they are competent to
settle; consequently they can be dispersed long distances from the parent
organism. Recruitment of such species to disturbed patches rarely correlates
with the abundance of propagule-releasing adults in the immediate vicinity of
the clearing (35, 192, 215).
WITHIN-PATCH ENVIRONMENTAL HETEROGENEITY The environment
within a patch cleared by disturbance is seldom, if ever, homogeneous (22, 84,
176). In many cases, this internal heterogeneity influences the process of
recolonization. In forest lightgaps, soil conditions and light intensity vary with
position in the gap. Fallen dead plant material may significantly alter the local
microclimate and distribution of soil nutrients. Conditions along the bole of a
fallen tree may be very different from those under and around its crown, and
both may differ from conditions near the upturned root mass where mineral soil
may be exposed (see references in 176). The association of certain species with
particular areas of the forest floor adjacent to a fallen tree suggests a degree of
specialization to these microenvironments (149, 176, 210). The decomposing
log itself may be a critical germination site for some species (e.g. 66, 218,
220). This local heterogeneity in the conditions for germination and growth can
enhance the diversity of vegetation within the gap (e.g. 149, 208).
In marine communities, within-patch heterogeneity in the characteristics of
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372 SOUSA
the substratum may also affect patterns and rates of patch colonization (35,
192). Settlement and/or survival of propagules on hard substrata is sometimes
influenced by relatively small-scale differences in surface texture or composition.
Larger-scale differences in the rock surface such as cracks and crevices
can provide refuges from predators and grazers.
Within-patch heterogeneity in the form of a spatial refuge afforded by a
particular substratum can also be important in terrestrial communities. The
perennial herb, Lomatium farinosum, suffers far lower mortality caused by
small mammals in shallow rocky soils than in deeper, less rocky soils (209)
because mammals are better able to create runways, dig burrows, and feed on
plant roots in deeper soils.
TIME OF PATCH CREATION When an open patch is created by disturbance
will indirectly affect colonization if, as is commonly the case, the availability
of propagules varies over time. Production of propagules is seasonal for at least
some species in most habitats [e.g. tropical rain forest (63, 67, 84, 237),
temperate forest (25), temperate intertidal zone (192), temperate subtidal
epifauna (150, 203), coral reefs (35), and chaparral (107)]. Yearly variation in
the production of propagules is often great (see also 80). Since large patches
often remain open to colonization longer than small ones, temporal variation in
the availability of propagules may have less of an impact on the long-term
development of their assemblages (35). In some systems, the largest patches of
open space are only generated in certain seasons, whereas small patches are
produced year-round (35, 192). In this case, there should be less variation in the
composition of colonists among large patches than among small ones (189),
assuming that yearly variation in propagule availability is smaller than seasonal
variation.
Temporal variation in the production of propagules may also be less important
where propagules are stored for long periods in the soil or on the parent
organism. Such storage effectively dampens variation in the number of propagules
that are immediately available to recolonize a disturbed site. If the
interval between successive disturbances is short, however, there may not be
sufficient time to replenish the pool of stored propagules, particularly if this
depends on the matiuration of individuals that became established since the last
disturbance. Under these conditions, few if any stored propagules will be
available to repopulate the site, and local extinction may result (70, 251, 253).
Therefore, in fire-prone habitats the dynamics of a population that relies on
stored propagules for regeneration depends on the interrelation between fire
frequency and fire intensity, the rate of propagule production as a function of
plant age/size, and the viability of stored propagules. The relationship between
inputs to and losses from a pool of dormant propagules is equally important in
closed-canopy mesic forests where windthrow is the predominant form of
disturbance (25).
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ROLE OF DISTURBANCE 373
The timing of disturbance can have a significant effect on patterns of
recolonization even when reproduction is stimulated by the disturbance event
itself. For example, patterns of fire-induced flowering and seed production can
vary with the time of year the plant is exposed to fire (e.g. 2, 70).
CORRELATIONS AMONG THE CHARACTERISTICS OF A DISTURBANCE REGIME
AND COMMUNITY RESPONSE The characteristics of natural disturbances
are often correlated. Disturbances that affect large areas are generally
the least frequent and the most severe (i.e. leave the fewest survivors). The
above correlation has been observed on coral reefs (35), rocky seashores (192),
and in some terrestrial plant communities (235). The simplest explanation for
this pattern is that the events that trigger massive earth movements, severe
droughts, and large storms occur less frequently than those that produce
smaller-scale, less intense disruptive forces. In other cases, this correlation is
due to the interacting influences of the biotic and abiotic components of the
system on patterns of disturbance. This sort of interaction best describes
correlations among characteristics of a fire regime. If ignition is restricted
to infrequent periods of severe drought, considerable fuel will have accumulated,
and the resulting fire will be intense, killing much of the vegetation over
a large area. Alternatively, if dry periods favorable to ignition occur annually,
fires will be frequent but usually of low intensity and small areal extent
because the short interval between bums will not allow much fuel to accum-
ulate.
One general consequence of this correlation among disturbance characteristics
is that there is a fairly continuous relationship between, the predominant
mode by which disturbed sites are recolonized and patch size. At one extreme
are large clearings with no survivors and no dormant propagules. These areas
will initially be recolonized by species producing widely dispersed propagules
at the time the patch is opened. Germination or metamorphosis of the propagules
soon after dispersal and rapid early growth are advantageous for successful
establishment (73). If they are not redisturbed subsequently, such
patches often undergo a long period of succession as other species slowly
invade and gradually replace earlier colonists. At the other extreme, the
smallest disturbances are filled almost exclusively, and relatively quickly, by
the vegetative growth of survivors living either within the clearing or on its
edge. Little or no successional replacement takes place in this case.
Repopulation of patches whose size and severity of destruction falls between
these two extremes occurs by one or more of the following mechanisms: (a)
germination or metamorphosis from recently dispersed or stored propagules,
(b) growth from a suppressed juvenile state, or (c) regrowth from damaged
tissues. The first mechanism predominates in larger patches where conditions
are more open and where the disturbance has been severe enough to kill most
adults and juveniles. Quite often, mass seeding of such patches in forests
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374 SOUSA
produces extensive even-aged stands (85-87, 101, 141, 146, 199, 204, 217-
220).
The latter two mechanisms are more important in smaller patches where
some survivors remain and where competition for resources such as light and
nutrients is likely to be intense. Here, larger individual size at the outset should
be advantageous for procuring the limited resources available. In small lightgaps,
for example, suppressed seedlings or saplings usually outcompete individuals
that germinate from seed after the gap has opened (25). Similarly, in
coral reef communities, attached fragments of corals probably have an advantage
over metamorphosing planula larvae in small clearings oln coral reefs (35).
Because fragments are larger at initial colonization, they can grow to a large
size more quickly. At this larger size, they are better able to hold off colonies
invading from the edge of the clearing.
It is often impossible to predict a priori the relative contribution of each of
these mechanisms to the process of recolonization in any particular clearing.
This will depend on the environmental setting and the life histories of the
particular species involved. Chance events also play a significant role in many
cases.
Comments on Evolutionary Responses to Disturbance
Disturbances are clearly an important cause of local heterogeneity in the
environmental conditions relevant to the recruitment, growth, survival, and
reproduction of organisms. Circumstantial evidence strongly suggests that
under the constraints of adaptive compromise, the differential reproductive
success of individuals has resulted in specialization to particular phases of the
environmental mosaic generated by disturbance (e.g. 11-13, 25, 35, 51, 52,
77-80, 164, 171).
Disturbance causes environmental heterogeneity along two dimensions-the
temporal and the spatial. The temporal component derives from asynchrony in
patch creation, causing the assemblages of neighboring areas to differ in age.
Species may partition this temporal component of heterogeneity by colonizing
and/or growing to maturity only during a particular stage of succession (159) or
only in patches created at a particular time of year.
The spatial component results from variation in patch characteristics other
than age, e.g. size, microclimate, and location. Species may differentially
exploit patches that differ in one or more of these characteristics. Levin (124)
refers to these temporal and spatial components of disturbance-induced heterogeneity
as phase difference and local uniqueness, respectively. Undoubtedly,
most species exploit a combination of these two forms of environmental
variation. Despite the specialization described above, many species overlap
considerably in the environmental conditions suitable for completion of their
life cycles.
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ROLE OF DISTURBANCE 375
Several authors have suggested that organisms are incapable of evolving
adaptive responses to disturbances that occur at long intervals relative to their
generation time (e.g. 15, 83, 152, 207). As Harper states:
I distinguish (as ends of a continuum) disasters and catastrophes. A disaster recurs frequently
enough for there to be reasonable expectation of occurrence within the life cycles of
successive generations . . . the selective consequence may be expected to leave relevant
genetic and evolutionary memories in succeeding generations. A "catastrophe" occurs
sufficiently rarely that few of its selective consequences are relevant to the fitness of
succeeding generations. The selective consequence of disasters is therefore likely to be to
increase short-term fitness and the consequence of catastrophes is to decrease it (83, p. 627).
I see at least two problems with this idea. The first is largely a matter of
semantics. It is unfortunate that Harper chose to assign the specific terms
disaster and catastrophe to opposite ends of the continuum of disturbances. In
principle, it seems a poor practice to designate specific categories without
providing objective criteria for distinguishing among them. How much "relevant
genetic memory" must be passed to future generations for an event to be
classified as a disaster and not a catastrophe? While I doubt that Harper
intended these terms to be adopted in empirical research, several workers have
recently applied these terms to real situations (e.g. 152, 207). Such usage is
best discouraged both for the reason just discussed and because these words are
so commonly, but ambiguously, used in everyday speech.
The second problem with this view is the tacit assumption that the observed
"fit" of an organism to its environment is attributable largely to the optimizing
process of natural selection (75). It overlooks the real and lasting influence that
a "catastrophe" may exert on the genotypes and phenotypes of succeeding
generations, even if its immediate effects are density independent and largely
random with respect to genotype. Until we know much more about the relative
influences of different evolutionary processes, it is difficult to say whether
natural selection by frequent disturbance has a greater evolutionary impact than
the population bottlenecks, local extinctions, and founder effects associated
with rare catastrophes (228).
A final caveat seems warranted. Hypotheses concerning the adaptive nature
of observed patterns of life history, morphology, and behavior are easy to
propose. It has often been suggested, or even taken as fact, that certain
characteristics have evolved by natural selection in response to recurring
physical disturbance. Furthermore, some species that depend on particular
kinds of disturbance for completion of their life cycles or whose fitness is
otherwise enhanced by such disturbances exhibit morphological or physiological
features that seem to increase the likelihood that these disturbances will
occur. This correlation has elicited the hypothesis that the latter features,
sometimes referred to as "disturbance facilitating," have evolved by natural
selection (e.g. 140, 244).
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376 SOUSA
While there is little question that physical disturbance can act as a potent
agent of natural selection and may play a part in other evolutionary processes as
well one must be careful not to invoke natural selection as an explanation
without considering alternatives. For example, many features of plants classically
considered to be "fire-adapted," including flammability and the ability to
resprout after fire, may have evolved in response to alternative selective forces
such as herbivory or drought (29, 30, 70, 71) or by a different evolutionary
mechanism altogether (75). "The immediate utility of an organic structure often
says nothing at all aboutithe reason for its being" (75:593). Whether "disturbance-dependent"
organisms have evolved specific mechanisms that increase
the probability of particular kinds and/or patterns of disturbance favorable to
their persistence is an interesting, though probably an unanswerable, question.
The Role of Disturbance in Population and Community
Dynamics
WITHIN-PATCH DYNAMICS
Demography I have mentioned a variety of factors that influence the reestablishment
of populations within a disturbed patch of habitat. Subpopulations
inhabiting different regions of a patch may differ in age structure, genetic
composition, and life history characteristics (210). The selective effects of
intra- and interspecific interaction may vary over small distances within a
single patch-e.g. at its edge versus at its center. The evolutionary consequences
of such variation have yet to be fully explored.
Interspecific interactions and the patterns of subsequent disturbance strongly
influence the absolute and relative abundances of species within a patch,
including the probability of continued local persistence.
Species diversity The resources made available by a disturbance are soon
exploited by colonists and regenerating survivors, and a successional sequence
of species replacements usually ensues (36, 143, 190). In the course of most
successions, one or a few competitively dominant and/or long-lived species
come to monopolize the resources of the disturbed patch (e.g. 36, 45, 113, 127,
129, 151, 153, 160, 190). In such cases, the time to local extinction of early
successional species depends on the characteristics of the patch for example,
its size and degree of isolation (35, 192) and of the species participating in the
succession. When late successional species are able to invade the open patch,
dominance will be quickly attained. So, small patches surrounded by adults of
these species should be dominated more quickly than larger and/or more
isolated patches.
In situations where hierarchical competitive interactions or differential
longevities [see the inhibition model of succession in (36)] will probably lead to
the monopolization of patch resources, disturbance can maintain withinThis
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ROLE OF DISTURBANCE 377
patch diversity by one of two mechanisms-compensatory mortality or intermediate
disturbance (33, 34). Compensatory mortality refers to the situation
in which the potential late successional dominant suffers a disproportionately
high rate of disturbance-related mortality as compared to other species that it
might otherwise exclude from the patch. Selective predation by the starfish
Pisaster on the competitively dominant mussel Mytilus californianus ( 151) is a
classical example of compensatory biological disturbance. Likewise, selective
herbivory can maintain high local diversity in plant communities (128). Physical
disturbance associated with heavy wave action can act in a similar manner
on mussel-dominated intertidal shores (153) and coral reefs (33, 34); spates can
have the same influence in stream communities (113).
Physical disturbance that does not cause compensatory mortality may
nonetheless maintain within-patch diversity. To do so, a physical disturbance
must renew resources, such as space, at a rate sufficient to allow continued
recruitment and persistence of species that would otherwise be driven locally
extinct. It must not occur so often or with such intensity, however, that many
species are eliminated. Therefore, the disturbance must occur with some
intermediate frequency and intensity/severity (33, 34), hence the term intermediate
disturbance. These intermediate scales of disturbance allow species
to accumulate within the patch but prevent it from becoming dominated by one
or a few of them. The assemblage is maintained in a nonequilibrium state, and
assuming that the system is open [i.e. dispersal can freely occur among patches
(26)], local coexistence of species is ensured (33, 34, 88, 160, 189). The scales
of disturbance that will maintain the highest within-patch diversity in any
particular assemblage depend on factors such as the rate of competitive exclusion
(96) and the relative rates of recruitment.
The hypotheses of disturbance-mediated coexistence "assume a transitive
hierarchical ranking of competitive abilities among the species, with competitive
outcomes being consistent and asymmetrical, i.e. one of a pair of competitors
always winning over the other" (35). For some assemblages of sessile
organisms, this assumption does not seem to hold, however. In some cases, the
competitive abilities of species are about equal and the outcome of their
interaction is largely stochastic (e.g. 1, 35, 73, 98, 177). The winner in any
particular interaction is determined by its order of invasion, its relative size,
and the angle of encounter with the competitor, among other factors. In other
cases, species are arranged in a relatively intransitive competitive network
defined by "the occurrence of a loop in an otherwise hierarchical sequence of
interference competitive abilities" (23:223-24). The degree of intransitivity in
such a network can vary, depending on the competitive symmetry of its
members (35).
Even if there is an overall hierarchy, a high degree of symmetry in competitive
interactions among high-ranking species or a competitive network involving
such species should reduce the likelihood that within-patch diversity will
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378 SOUSA
decline in the later stages of succession (167). Reciprocal replacement of
species, as has been documented in some forest stands (e.g. 248), might also
maintain high diversity in the absence of large-scale disturbance. Where such
relationships among competitors exist, disturbance might seem unnecessary to
maintain within-patch diversity. In the absence of disturbance, however, within-patch
diversity in these systems may eventually decline owing to differences
in the growth rates and competitive abilities of the species (23) or simply
because the populations of some species fluctuate to low densities and go
extinct by chance. Even rare disturbances can then be crucial in the long-term
maintenance of local species richness (35). If nothing else, such nonhierarchical
relationships may slow the rate of competitive exclusion (23, 102, 167) and
thus increase the opportunity for mechanisms such as physical disturbance to
maintain local diversity.
THE REGIONAL DYNAMICS OF POPULATIONS AND COMMUNITIES
The persistence of populations Most sessile organisms are, to varying degrees,
"fugitive species." On a small enough spatial scale, probably no population
persists indefinitely (37). The replacement of adults requires dispersal of
propagules to sites favorable for recruitment. A population will persist in a
given area only if that area provides a sufficient number of safe sites per unit
time to guarantee successful recruitment during the adults' lifetimes. The
minimum area needed for persistence will vary among species. A species
whose offspring can survive and grow under environmental conditions similar
to those experienced by the adults usually does not disperse its propagules far,
and it potentially can persist in a relatively small area. In contrast, if the
offspring require conditions significantly different from those found in the area
occupied by the adults, propagules are often dispersed a long distance, and
persistence will only occur on a relatively large spatial scale.
As noted earlier, many species depend on disturbance to create conditions
favorable for the recruitment, growth, and reproduction of their offspring. The
regional dynamics and abundance of such a species will reflect the interplay of
its life history and the regime of disturbance. For such a species to persist,
disturbances must generate colonizable space within the dispersal range of
extant populations and within the period of time it takes for these populations to
go extinct (e.g. 152).
Species diversity The species diversity of a region is a function of both the
number and relative abundances of species that persist there. The regional
abundance of a species can depend on the rate of disturbance (i.e. the area or
number of patches cleared per unit time) and its predictability (as just discussed).
In addition, regional population dynamics can be influenced by the size
distribution of open patches and how synchronously they are produced.
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ROLE OF DISTURBANCE 379
Miller (133) assumes, and Abugov (3) demonstrates theoretically (as have
others), that regional diversity peaks at intermediate rates of disturbance. At
high mean disturbance rates, early successional species dominate most patches,
while at low mean disturbance rates, late successional species do. These studies
showed, however, that the distribution of patch sizes and the degree to which
disturbances were phased (i.e. how synchronously they occurred) could strongly
influence the level of diversity maintained by any given level of disturbance.
Assuming that small patches are usually invaded and dominated more
quickly than large ones, Miller (133) concludes that large patches should favor
colonizing species (i.e. early successional species) while small patches should
favor competitive species (i.e. late successional species). Given these assumptions,
he shows that at a low overall disturbance rate, regional diversity will be
higher if the mean size of cleared patches is large, thus favoring the continued
persistence of colonizing species. When the rate of disturbance is high, regional
diversity will be greater when the areas disturbed are small, since this will
ensure the continued persistence of competitive species. Alternative ways in
which patch size may influence regional dynamics have also been discussed
(35, 192).
Abugov (3:289) has examined how the phasing of disturbance might affect
regional diversity. He describes phasing as follows:
... the disturbance of a patch is unphased if its probability of being cleared during each time
interval is independent of whether any other patches are being cleared during the same time
interval. Conversely, other patches may be disturbed in phase. Each time a disturbance clears
one of these patches it clears them all.
In the simplest case, as phasing increases, so does the proportion of colonizing
species in the assemblage. Thus, an increase in the phasing of disturbance
will decrease diversity when colonizing species predominate but will increase
diversity when competitively dominant species occupy most of the space. More
complicated patterns are also possible if one allows for changes in the relative
competitive and migratory abilities of the colonizing species. To my knowledge,
these theoretical predictions have yet to be tested empirically.
Patch dynamics and landscape pattern Recent research on the role of disturbance
in natural communities has increasingly focused on patterns at the
regional scale, where the diversifying influence of disturbance is easy to
observe. Patchy and locally asynchronous disturbance transforms the land or
seascape into a continuously changing mosaic of patches of different sizes and
ages (153, 159, 172, 173, 189, 239). The spatial orientation of each of the
phases of this mosaic may strongly influence regional dynamics, particularly if
the dispersal range of propagules is limited (159, 192). The dynamics of this
patchwork of successional stages is beginning to receive theoretical attention
(e.g. 153, 184, 185).
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380 SOUSA
Do within-patch, nonequilibrium conditions average to an equilibrium pattern
when on considers the mean dynamics of an area containing many such
patches? Bormann & Likens (19, 20) refer to this large-scale equilibrium as a
"shifting-mosaic steady-state." Many workers would answer the above question
affirmatively (e.g. 19, 20, 196, 198, 249); but Romme (172, 173) found
strong cyclic, rather than steady-state, dynamics in the vegetation of a 73 km2
watershed in Yellowstone National Park. Recent simulation studies (185)
indicate that the likelihood of a large-scale steady state is a function of the total
landscape area and the size of the individual disturbances. The larger the area
affected by a single disturbance, the more extensive the landscape must be to
average out its effects. Landscapes that are small in absolute area (including
those that have been fragmented by human activity) or that experience typical
disturbances covering many thousands of hectares (e.g. 66) are unlikely to be in
equilibrium.
DISTURBANCE AND MOBILE ANIMALS
With some notable exceptions, ecologists have largely overlooked the significant
role of physical disturbance in the biology of mobile animals. Appreciation
of the direct and indirect influences of disturbance on mobile animals has
been slow to develop for at least two reasons. First, the direct effects of
disturbance on mobile organisms are not as easy to observe and measure as
those on sessile organisms. Second, during the last two decades many of the
most influential investigators of the ecology of mobile animals vertebrates in
particular have been proponents of a competition-based equilibrium theory of
community organization (104, 241). Once equilibrium is assumed, there is
little cause to examine the influence of supposedly rare and inconsequential
disturbances.
In recent years, this view has become quite controversial (178). Physical
disturbance can be a major cause of local disequilibrium when it kills appreciable
numbers of animals, as demonstrated by the following incomplete list of
examples: cold temperatures butterflies (24, 58, 182), coral reef fish (18).
stream fish (104, 214); oxygen depletion lake fishes (213); drought pond
fish (121), stream fish (122), salamanders (99); flooding-desert fish (32);
hurricanes-coral reef fish (123); fire vertebrates (14, 28, 31, 169), soil and
litter invertebrates (4, 31, 169); miscellaneous agents intertidal invertebrates
(192).
Severe climatic conditions can indirectly exert a strong negative impact on
populations of mobile organisms by eliminating or producing shortages in vital
resources. For example, prolonged drought stresses or kills the food plants of
butterflies and causes large declines in, or even extinctions of, local populations
(59, 182). A similar risk of local extinction is faced by any parasite whose
host is subject to physical disturbance (165).
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ROLE OF DISTURBANCE 381
Even when a climatically induced shortage in resources does not cause much
mortality, organisms that remain in the area may experience increased intraand
interspecific competition. Highly mobile species may move in search of
more benign sites where resources are more plentiful. Increased competition
and emigration during harsh climatic periods can reduce local population
densities. These effects are well documented in recent long-term experimental
and observational studies of lizard (55, 186) and bird (187) assemblages.
As was true for sessile organisms, mobile animals vary considerably in their
vulnerability to disturbing forces and in their ability to avoid them. Animal
responses to fire are a case in point (4, 14, 31, 118, 169). Highly mobile species
can escape harm from all but the most quickly spreading fires by moving just
ahead of the advancing flames or by seeking refuge out of the fire's path in
patches of unburnt vegetation. If fleeing animals are overtaken, they can
survive (as long as the fire is not too intense) by taking refuge in burrows or
under rocks. Consequently, low to medium intensity fires often have relatively
little direct impact on vertebrate populations. The main effect of such fires is to
alter temporarily the spatial distribution and density of these populations. The
burned site is frequently reoccupied soon after the fire has passed, though
population densities may not return to prefire levels until the appropriate
vegetation has regenerated on the site. In contrast, relatively immobile soil and
litter invertebrates have no means of escaping an advancing fire; even a low
intensity burn can kill large numbers of these organisms. The rate at which the
burned area is recolonized by such organisms depends on the species' dispersal
abilities and the proximity of source populations in surrounding areas of
unburnt vegetation.
Physical disturbances also cause short- and long-term changes in the habitat
that can have major indirect effects on populations of mobile species. Some of
these changes can be detrimental to mobile animal populations, as the following
examples demonstrate. As a result of unusually heavy rains, large quantities
of sediment were deposited in a southern California coastal lagoon. These
deposits reduced the low tide volume of the lagoon and eliminated much of
the previously extensive cover of eel grass. Probably for lack of sufficient
habitat-as opposed to direct mortality caused by the sedimentation-the density
of water column fishes declined significantly (148). Similar declines
in fish populations have been observed on storm-damaged coral reefs (105,
229). Here too, the reduction in fish numbers was due more to storm-induced
alterations in the habitat than to any direct mortality caused by storm con-
ditions.
Disturbances can also have indirect positive effects on mobile animal
populations. Those that cause some immediate mortality may in the long run
produce a net increase in population size and vitality. Fires, for example, may
kill some birds and mammals, but they also reduce the populations of parasites
that afflict some species (14).
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382 SOUSA
Disturbances from such agents as fire, windthrow, and water motion indirectly
affect populations of mobile animals by influencing the composition
and structure of sessile assemblages. Sessile organisms provide cover and/or
food, thus constituting a major component of the habitat of mobile animals. As
this paper has emphasized, physical disturbance transforms sessile assemblages
into mosaics of different seral stages. Sessile organisms comprising each
of these stages differ in many characteristics critical to the welfare of mobile
animals, including structural complexity, species composition, microclimate,
and the quantity and quality of the food and shelter available. Although mobile
animal species vary in their habitat requirements, many prefer the productive
conditions associated with areas undergoing regeneration from recent disturbance.
The quantity and palatability of plant foods are often higher in earlier
stages of succession (27, 65, 81, 173). Some species of birds forage preferentially
for fruits or insects in forest light gaps (180, 211) .
Other species exploit the tissues of dead or dying plants as a resource and
preferentially colonize recently disturbed sites where this material is abundant.
For example, bark beetles and other boring insects are strongly attracted to
injured or dead trees (181).
The compositions of insect, bird, and mammal assemblages change with the
successional stage of the vegetation (e.g. 7, 16, 28,61,64, 115, 142, 173, 181,
183, 188, 206, 245). The population density and species richness of mobile
animals often decline in later successional stages, paralleling similar trends in
the vegetation. Some species, however, become more abundant as time elapses
since the last disturbance. Such patterns reflect species-specific responses to
changes in vegetation structure and composition, food availability, and interspecific
interactions that accompany successional shifts in the plant community.
Similar changes in mobile animal assemblages occur over the course of
succession in sessile communities on rocky seashores (e.g. 48). On the landscape
scale, mosaic patterns in sessile assemblages generated by disturbance
are likely to have a profound effect on the regional population dynamics of
mobile animals (65, 81, 173, 242).
CONCLUSIONS
I have attempted to summarize some of the key themes that run through t
and rapidly expanding literature on disturbance in natural communities. These
include: (a) the factors that determine natural regimes of disturbance, (b)
organismal-level responses to disturbance and their evolution, and (c) the
influence of disturbance on population and community structure and dynamics
at both the local and regional scales.
Although all natural communities probably experience disturbance at some
spatial and temporal scale, historically its role in community dynamics has
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ROLE OF DISTURBANCE 383
been largely overlooked, except by some temperate-forest ecologists. There are
many reasons for this neglect, but one of the prime causes may be that major
disturbances often recur at intervals longer than the duration of an average
research project or even than the lifespan of the investigator (146). Thus, the
effects of disturbance cannot always be directly observed, which may lead one
to conclude that disturbance is unimportant. Even a very long recurrence
interval does not necessarily indicate that the impact of disturbance on the
community is inconsequential, however. When the affected organisms are
long-lived, the "compositional effects of disturbance can persist for centuries or
even millennia" (66:216)).
There is a growing realization that disturbance may play as great a role in
community dynamics as do biological interactions such as competition and
predation, which have received far more empirical and theoretical attention
from ecologists. The interplay between disturbance and these biological processes
seems to account for a major portion of the organization and spatial
patterning of natural communities.
ACKNOWLEDGMENTS
I thank J. Connell, P. Frank, A. Meyer, B. Mitchell, B. Okainura, G.
Roderick, and S. Swarbrick for their very helpful comments on an earlier draft
of the manuscript. I also thank those authors who provided prepublication
drafts of chapters from a forthcoming book on disturbance edited by S. T. A.
Pickett and P. S. White (162).
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