2000 Blackwell Science Ltd. http://www.blackwell-science.com/ddi 93
BIODIVERSITY RESEARCH
Diversity and Distributions (2000) 6, 93­107
Blackwell Science, Ltd
Naturalization and invasion of alien plants:
concepts and definitions
DAVID M. RICHARDSON1
, PETR PYSEK2
, MARCEL REJMÁNEK3
,
MICHAEL G. BARBOUR4
, F. DANE PANETTA5
and CAROL J. WEST6 1
Institute for
Plant Conservation, Botany Department, University of Cape Town, Rondebosch 7701, South Africa.
E-mail: rich@botzoo.uct.ac.za, 2
Institute of Botany, Academy of Sciences of the Czech Republic,
Cz-252 43 Pruhonice, Czech Republic. E-mail: pysek@ibot.cas.cz, 3
Section of Evolution and Ecology,
University of California, Davis, CA 95616, USA. E-mail: mrejmanek@ucdavis.edu, 4
Department of
Environmental Horticulture, University of California, Davis, CA 95616, USA, E-mail: mgbarbour@ucdavis.edu,
5
Alan Fletcher Research Station, Department of Natural Resources, PO Box 36, Sherwood, Queensland
4075, Australia, E-mail: dane.panetta@dnr.qld.gov.au, 6
Southland Conservancy, Department of
Conservation, PO Box 743, Invercargill, New Zealand, E-mail: cwest@doc.govt.nz
Abstract. Much confusion exists in the Englishlanguage
literature on plant invasions concerning
the terms `naturalized' and `invasive' and their associated
concepts. Several authors have used these
terms in proposing schemes for conceptualizing the
sequence of events from introduction to invasion,
but often imprecisely, erroneously or in contradictory
ways. This greatly complicates the formulation
of robust generalizations in invasion ecology.
Based on an extensive and critical survey of
the literature we defined a minimum set of key
terms related to a graphic scheme which conceptualizes
the naturalization/invasion process.
Introduction means that the plant (or its propagule)
has been transported by humans across a
major geographical barrier. Naturalization starts
when abiotic and biotic barriers to survival are
surmounted and when various barriers to regular
reproduction are overcome. Invasion further requires
that introduced plants produce reproductive offspring
in areas distant from sites of introduction
(approximate scales: > 100 m over < 50 years for
taxa spreading by seeds and other propagules;
> 6 m/3 years for taxa spreading by roots, rhizomes,
stolons or creeping stems). Taxa that can cope with
the abiotic environment and biota in the general
area may invade disturbed, seminatural communities.
Invasion of successionally mature, undisturbed
communities usually requires that the alien
taxon overcomes a different category of barriers.
We propose that the term `invasive' should be
used without any inference to environmental or
economic impact. Terms like `pests' and `weeds'
are suitable labels for the 50­80% of invaders
that have harmful effects. About 10% of invasive
plants that change the character, condition,
form, or nature of ecosystems over substantial
areas may be termed `transformers'.
Key words. aliens, barriers, biological invasions,
casual alien plants, invaders, naturalized species,
pests, plant introduction, transformers, weeds.
INTRODUCTION
The expanding field of invasion ecology has
seen a proliferation of terms to describe various
concepts. There has also been considerable
confusion and misuse of existing terminology.
Invasion ecology has perhaps suffered more
than most scientific disciplines since the notion
of `invasion' frequently evokes anthropocentric
concepts (aggression, assault, attack, encroachment,
incursion, infringement, intrusion, onslaught,
raid, etc.). Several recent contributions
94 D. M. Richardson et al.
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have debated the relative merits of various
terms in invasion ecology with special reference
to plants (e.g. Jarvis, 1979; Mack, 1985; Di
Castri, 1990; Carr, 1993; Binggeli, 1994; Pysek,
1995; Weber, 1997). Much of the debate on
terminology is essentially semantic, and poses
little threat to the development of an increasingly
robust understanding of invasions (other
than bedevilling computerized searches for references).
An area of confusion with more profound
ramifications for invasion ecology exists around
the lack of clarity and consistency in the use
of the term `naturalized'. One problem involves
the widespread use of this term as a synonym
for `invasive', thus lumping two overlapping, but
not identical, phases in the naturalization/invasion
process.
This confusion has clear practical implications.
The shift from `naturalized, noninvasive'
to `invasive' (as we understand these concepts;
see later) represents the point at which the
presence of an alien species may start to have
detectable ecological or economic consequences.
The absence of clearly defined, and widely
accepted, terminology to describe the status of
alien plants confounds the objective formulation
of priorities for management (e.g. Westman,
1990).
Many recent studies have described various
processes and/or phases that may be identified
in the invasion of a new region by an introduced
taxon. In conceptualizing these phases
it is useful to consider the limiting factors that
restrict the spread of introduced taxa in a
region as a series of `barriers' [the concept of
barriers was probably first used in connection
with biological migrations by De Candolle (1820;
p. 45) and later by Cain (1944; p. 149)]. This
approach has proved useful in deriving generalizations
on various aspects of invasion ecology
(e.g. Kruger et al., 1986; Carr, 1993; Reader &
Bricker, 1994; Mack, 1996; Richardson & Higgins,
1998). However, the approach loses much of its
value if the different stages in invasion are imprecisely
defined. The lack of clarity in this regard
has unfortunately been exacerbated by the uncritical
use of terminology and concepts in several
prominent reviews. To cite but one example,
Cousens & Mortimer (1995; p. 21), in a chapter
on `The dynamics of geographical range expansion',
follow Groves (1986) in advocating that
the process of invasion of an unoccupied region
by new taxa may be divided into the following
three phases:
1. INTRODUCTION. As a result of dispersal,
propagules ... arrive at a site beyond their previous
geographical range and establish populations
of adult plants.
2. COLONIZATION. The plants in the founding
population reproduce and increase in number
to form a colony that is self-perpetuating.
3. NATURALIZATION. The species establishes
new self-perpetuating populations, undergoes
widespread dispersal and becomes incorporated
within the resident flora.
In our view, this scheme confuses stages in
the naturalization/invasion process. Introduction
and establishment (understood as survival,
not as reproduction) are clearly fundamental
requirements for invasion, and we have no serious
problem with lumping these under the
broad heading INTRODUCTION. However, we
believe that the phases COLONIZATION and
NATURALIZATION in the above scheme are
incorrectly defined. What is described as COLONIZATION
above corresponds with what we
believe is an integral part of `naturalization'
(see later) whereas NATURALIZATION in the
above scheme conforms to our understanding
of `invasive'.
Does it matter that different authors perceive
the naturalization/invasion process differently?
We suggest that it is fundamentally
important to resolve this issue. Only a small
fraction of all introduced taxa reproduce and
spread over large areas; most taxa fail at some
stage before reaching such levels of success.
One of the main tasks of invasion ecology is
to explain why some taxa are better invaders
than others. This is greatly complicated if the
criteria for judging success are poorly demarcated.
Similarly, attempts to define correlates of
invasibility of different regions are misleading
in the absence of objective criteria for
the inclusion of taxa on lists of `invasive'
organisms.
One recent example illustrates the type of
problem that confronts invasion ecologists. Haber
(2000) presents a very valuable list of vascular
plants of the Monteverde cloud forest in
Costa Rica. However, only about 60% of alien
Naturalization and invasion of alien plants 95
 2000 Blackwell Science Ltd, Diversity and Distributions, 6, 93­107
species are recognized as such (labelled with
an asterisk). An explanation that these are `species
introduced or escaped from cultivation'
(p. 518) helps us very little, since asterisked
taxa span the continuum from taxa persisting
temporarily after cultivation (Brugmansia candida)
to fully naturalized and invasive species (Impatiens
walleriana). Many recent reviews on various
aspects of biological invasions have used just
such lists uncritically, with the result that many
generalizations that have been proposed are
spurious. Obviously, more attention must be
paid to clear categorization and terminology in
manuals, floras, and checklists which include
alien taxa.
In this paper we: (a) examine how the term
`naturalized' is being used in relation to what we
consider to be its precise meaning; (b) explore
reasons for the lack of consistency; (c) discuss
some problems with the current imprecise usage
of terms; and (d) suggest ways to confine the use
of `naturalized' and other terms in plant invasion
ecology.
METHODS
We first examined definitions of `naturalized'
in various dictionaries and encyclopedias of
ecology and other widely cited books dealing
with both animal and plant invasions. Next,
we searched the TREECD database (1939­98),
a CAB Abstracts Forestry Database, published
by CAB International, Wallingford, Oxford,
UK, for publications with the words `naturaliz/
sed' or `naturaliz/sation' in the title, abstract or
`descriptors' (keywords). As the name implies,
this database contains references pertaining to
woody plants; this is appropriate as invasions
of woody taxa serve very well for illustrating
the general problem of imprecise terminology in
plant invasion ecology. After editing our list to
remove various nonbiological or otherwise inappropriate
papers, our database comprised 157
papers. We also examined all recent books dealing
with plant invasions, and our own large
personal collections of invasion-related publications.
Various regional and national floras and
catalogues of invasive taxa were examined to
determine the criteria that are used to distinguish
between the various categories of alien
plants.
RESULTS
The definitions of `naturalized' in dictionaries,
encyclopedias and widely cited books span a
wide range (Appendix 1). The main areas of
divergence in this sample of `authoritative' definitions
relate to: (a) the role of human assistance
considered necessary to establish the alien (see
definitions 1 and 2 in Appendix 1); (b) the
assumption that, to be naturalized, a taxon
must invade natural vegetation (see 3, 4, 5 and
7 in Appendix 1); and (c) imprecise statements
on the degree to which the alien is established
and/or reproducing and self-sustaining (see 6, 8
and 10 in Appendix 1).
In the 157 original papers we studied we
were able to distinguish four main categories of
use of the words and concepts `naturalized' and
`naturalization'. In 15% of papers insufficient
details were given to categorize the intended
meaning.
1 `Naturalized' [the conventional meaning]
(23% of articles analysed)
This usage refers to alien plants that reproduce
and sustain populations without direct intervention
by humans, often producing plentiful
offspring, mainly close to parent plants. Such
plants do not necessarily invade natural and
seminatural vegetation to any extent (although
many do). This understanding corresponds to
usage by De Candolle (1855; p. 643); Thellung
(1911­1912); Sukopp & Trepl (1987); Stace (1991;
p. 1190); Huston (1994); Scott (1996); Sell &
Murrell (1996; p. 376); Williamson (1996; e.g.
p. 37); Crawley (1997; p. 617); Lincoln et al.
(1998); Provost (1998), among others. This
concept conforms with that of `established'
in Williamson (1996; p. 37) and Vitousek et al.
(1997).
2 `Naturalized' meaning self-sustaining
populations in natural or seminatural
vegetation (8%)
This usage is similar to `1' except that it
considers taxa to be naturalized only if they
reproduce (regularly) outside human-dominated
systems. For example, `... an alien plant is
considered to be naturalized when it grows
96 D. M. Richardson et al.
 2000 Blackwell Science Ltd, Diversity and Distributions, 6, 93­107
and reproduces in the wild state unaided by
humans' (Esler, 1987). This concept is expanded
upon by Heenan et al. (1998) who define `fully
naturalized' species as those that `form a wild
population self-maintained by seed or vegetative
reproduction or occur repeatedly in the wild'.
Clement & Foster (1994) consider naturalized
species to be `established extensively amongst
native vegetation so as to appear native' (see
also Ghate & Vartak, 1990). Other examples
of this usage include Walker (1989); Allaby
(1994) and Cronk & Fuller (1995). This usage
coincides with the term `established' as defined
by Kloot (1987).
3 `Naturalized' as a synonym for `alien'
[non-native] (25%)
These papers usually refer to plants that are
reproducing in the new environment, but give no
indication of how well the taxon is established
or whether it is spreading. Typical examples of
this usage are given in Holub & Jirásek (1967)
and Sachse (1995).
4 `Naturalized' as a synonym for
`invasive' (29%)
More publications were placed in this category
than in any other. A typical example of this
usage is: `this paper studies the distribution
of ... which has spread and naturalized in ...'.
We conclude that a very large proportion of
recent publications describing aspects of plant
invasions have used the terminology loosely and
their lack of rigour has led to confusion.
How are `naturalized' and `invasive' species
distinguished in regional lists of alien species?
Most such lists include some information
on the frequency of occurrence of the given
taxon. Such information is usually qualitative,
using terms such as `adventive', `acquired', `established',
`casual', `persistent' (Ahti & Hämet-Ahti,
1971; Kloot, 1987; Clement & Foster, 1994);
decreasing/increasing (Esler, 1987); established
vs. not-established (Knops et al., 1995); expanding
vs. nonexpanding (Dafni & Heller, 1990);
rare/local/common (Corlett, 1988, 1992); having
low/moderate/high impact (Brown & Gubb, 1986),
etc. Such definitions do not always enable one to
judge whether the given taxa are `naturalized' or
`invasive' (see later for recommended definitions).
DISCUSSION
A century of `naturalization'
One reason for the lack of clarity that has
developed in the terminology of plant invasions
in English is that different meanings are
attached to the term `naturalized' in different
languages. With the emergence of English as
the primary language of scientific discourse, the
transfer of terminology between other (mainly
European) languages and English has introduced
some confusion (this problem has also arisen
in other fields of ecology; see, e.g. Mooney,
1998; p. 3). The SCOPE programme on the
ecology of biological invasions in the 1980s
(Drake et al., 1989; see also Mooney, 1998;
pp. 93­111) brought together perspectives on
invasions from many parts of the world. The
publication, in English, of many contributions
from non-English speaking regions, in
the absence of a clear lexicon, helped to compound
the confusion regarding terminology. This
is especially evident in the volume on `Biological
invasions in Europe and the Mediterranean
Basin' (di Castri et al., 1990), which contains
a bewildering array of terms and concepts to
categorize alien taxa. Starfinger et al. (1998) also
collated many useful insights from European
authors, but again introduced many new (at
least to the international literature in English)
terms to describe phases of invasion or categories
of invasive plants, notably in the chapter by
Falinski (1998). One reason for the complicated
classification schemes used in Central Europe
may be the strong `classificatory' tradition in
this part of the world (see, e.g. Mayr, 1982;
Jahn, 1998).
We suspect that another reason is that the
magnitude and dimensions of invasions have
changed considerably since the term `naturalized'
was first used. The term was already fairly
widely used in the English biological literature
in the middle 19th century. For example,
Darwin (1859) used the terms `naturalized',
`naturalization' and `naturalizing' 35 times (27,
7 and 1 times, respectively) in the first edition
of `On the origin of species'. This was the time
Naturalization and invasion of alien plants 97
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when naturalists were starting to record the
widespread phenomenon of introduced taxa
behaving `like a native' (definition 4 in Appendix
1) in new areas. The term `naturalized' has
been applied uncritically since the middle of the
19th century and now its meaning has become
imprecise. We are left with a term whose denotation
means `the process whereby a species is
made to act like a native', but it has now taken
on a slightly different connotation.
It is instructive to consider some meanings
attached to `naturalized' in European languages
other than English. Most Central European
phytogeographical studies were published in
German. In this literature the term `naturalization'
refers to a process of `existence' of an
alien taxon in its adventive region (including its
incorporation into native vegetation) which can
have various degrees or levels (Schroeder, 1969;
Müller, 1995; Trepl, 1995). This usage appears
to originate from the work of A. Thellung
who, in the early 1900s, attempted to define
and discuss terms like `native', `introduced'
and `casual' in French, German and English
(see Trepl, 1995). The `naturalization process'
as understood by these European authors, is
more or less synonymous with what is now
more often called the `invasion process' (see, e.g.
Holub & Jirásek, 1967; Sachse, 1995) or what
we call the naturalization/invasion process. This
view is discussed in detail by Holub & Jirásek
(1967) who wrote about `Grad ihrer Naturalization'
(degree of naturalization) and defined the
naturalization of human-accompanying plants
(`Naturalization der Anthropophyten') as `eine
bis zur Einbürgerung gehende Anpassung der im
Gebiete ursprünglich fremden Pflanzen' (i.e. a
process of adaptation of alien plants that can
lead to their incorporation into native vegetation).
They explicitly mention particular degrees
of naturalization, from the stage of `ephemerophytes'
(casual alien plants; see Table 1),
to temporarily established aliens, to those
that are permanently present in either natural
or human-modified habitats. Holub & Jirásek
(1967) proposed special terms for plants introduced
intentionally (`Akklimatization') or unintentionally
(`Xenonaturalization').
It may be argued that the bewildering array
of concepts and terms used to describe nonnative
plants in Europe is due to the very long
history of human habitation of the region,
which has resulted in many taxa being introduced
at different times and in differing numbers.
The ways that these plants have organized
themselves in the environment may seem to
demand a complicated hierarchy of concepts and
terms to describe the extent to which they are
part of the flora. However, standardization of
terminology in this area is highly desirable.
What follows is an attempt to define a minimum
number of useful terms.
Towards a standardized terminology for
plant invasion ecology
The current search for robust generalizations
and theories in invasion ecology (e.g. Jarvis,
1979; Williamson, 1996; Shigesada & Kawasaki,
1997; Daehler, 1998; Goodin et al., 1998; Pysek,
1998; Lonsdale, 1999; Rejmánek, 1999a) demands
further elucidation of the naturalization/invasion
process. To this end, we propose the simple
conceptualization of the process as shown in
Fig. 1 (terminology given in Table 1). Following
this scheme, invasion is a process requiring a
taxon to overcome various abiotic and biotic
barriers. Phases of the process can be defined
on the basis of the relevant barrier(s) that
are (or are not) overcome. Introduction means
that the plant (or its propagule) has overcome,
through human agency, a major geographical
barrier (A in Fig. 1). Many introduced taxa
survive as casuals (also `waifs', `persisting after
cultivation'); such taxa can reproduce sexually
or vegetatively, but fail to maintain their
populations over longer periods. Casuals therefore
must rely on repeated introduction for
their persistence. Naturalization only starts when
environmental barriers (B) do not prevent
individuals from surviving and when various
barriers to regular reproduction (C) are overcome.
Therefore a taxon can be considered
successfully naturalized after overcoming barriers
A, B and C. At this stage populations
are sufficiently large that the probability of
extinction due to environmental stochasticity
is low (MacArthur, 1972; Menges, 2000; see
also Panetta & Randall, 1994 with regard to
Emex australis). Several authors have attempted
to define the degree to which an alien taxon is
naturalized. For example, in Flora Europaea
98D.M.Richardsonetal.
2000BlackwellScienceLtd,DiversityandDistributions,6,93­107
Table 1 Recommended terminology in plant invasion ecology
Alien plants1
Plant taxa in a given area whose presence there is due to intentional or accidental introduction as a result of human activity (synonyms:
exotic plants, non-native plants; nonindigenous plants).
Casual alien plants Alien plants that may flourish and even reproduce occasionally in an area, but which do not form self-replacing populations, and which rely
on repeated introductions for their persistence (includes taxa labelled in the literature as `waifs', `transients', `occasional escapes' and
`persisting after cultivation', and corresponds to De Candolle's (1855, p. 643) usage of the term `adventive'2
).
Naturalized plants Alien plants that reproduce consistently (cf. casual alien plants) and sustain populations over many life cycles without direct intervention by
humans (or in spite of human intervention); they often recruit offspring freely, usually close to adult plants, and do not necessarily invade
natural, seminatural or human-made ecosystems.
Invasive plants3
Naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from parent plants
(approximate scales: > 100 m; < 50 years for taxa spreading by seeds and other propagules4
; > 6 m/3 years for taxa spreading by roots,
rhizomes, stolons, or creeping stems), and thus have the potential to spread over a considerable area.
Weeds Plants (not necessarily alien) that grow in sites where they are not wanted and which usually have detectable economic or environmental
effects (synonyms: plant pests, harmful species; problem plants). `Environmental weeds' are alien plant taxa that invade natural vegetation,
usually adversely affecting native biodiversity and/or ecosystem functioning (Humphries et al., 1991; Randall, 1997).
Transformers5
A subset of invasive plants which change the character, condition, form or nature of ecosystems over a substantial area relative to the extent
of that ecosystem.
Notes:
1 Some authors suggest that distinction should be made between non-native plants introduced by humans in prehistorical times and those that arrived recently.
For example, in Central Europe, `archeophytes' and `neophytes' refer to taxa introduced before or after 1492, respectively (Mandák & Pysek, 1998). In other
parts of the world (e.g. Australia; Kloot, 1987), a distinction is sometimes made between taxa that arrived before or after European colonization. On the time
scale of decades (necessarily the focus of our treatment), cases of introduction of alien taxa via means not associated with human activity (e.g. by seeds attached
to the plumage of migrating birds, or via passive dispersal in strong winds, or by sea currents) are certainly rare (e.g. spontaneous (re)colonization of the
Krakatau Islands; Whittaker et al., 1989). Such events have to be treated as natural phenomena. How far must a taxon have been moved before it may be
reasonably termed `alien'? Clearly, the `> 100 km' suggested in Fig. 1 is a rough approximation. Some plant taxa are noted as `alien' at localities where native
populations occur within 100 km (e.g. Phragmites australis in West Virginia; Hutton & Clarkson, 1961). Nonetheless, 100 km can, in our view, be used as a
practical cut-off in most cases. Under some circumstances, shorter critical distances may be considered (e.g. 30 km, as 32 km separates Great Britain from the
mainland Europe). Taxa can be alien to continents, islands, bio- or ecoregions, states, or counties.
2 The term `adventive' has been later used in a much broader sense (casual + naturalized) by many authors (e.g. Muhlenbach, 1979; Burda, 1991; Provost, 1998).
3 A potential complication with this definition concerns taxa which spread previously, but do not spread currently because the total range of suitable habitats and
landscapes has been occupied. Such taxa should still be termed invasive because local eradication will undoubtedly lead to re-invasion.
4 This also applies for dioecious taxa reproducing by vegetative propagules (e.g. Salix spp. via broken branches; Henderson, 1991; Hussey et al., 1997). For
dioecious taxa that rely exclusively on seeds for reproduction, `< 50 years' applies only after the introduction of both sexes.
5 Transformers are those taxa that have clear ecosystem impacts. Several categories of transformers may be distinguished: (a) excessive users of resources (water --
Tamarix spp.; Acacia mearnsii; light -- Pueraria lobata, Rubus armeniacus; water and light -- Arundo donax; light and oxygen -- Salvinia molesta, Eichhornia crassipes);
(b) donors of limiting resources (nitrogen -- Acacia spp., Lupinus arboreus, Myrica faya, Salvinia molesta); (c) fire promoters/suppressors (promoters -- Bromus tectorum,
Melaleuca quinquenervia, Melinis minutiflora; suppressors -- Mimosa pigra); (d) Sand stabilizers (Ammophila arenaria); (e) Erosion promoters (Andropogon virginicus
in Hawaii, Impatiens glandulifera in Europe); (f ) Colonizers of intertidal mudflats/sediment stabilizers (Rhizophora mangle, Spartina spp.); (g) Litter accumulators
(Eucalyptus spp., Lepidium latifolium, Pinus strobus, Taeniatherum caput-medusae); (h) Salt accumulators/redistributors (Mesembryanthemum crystallinum, Tamarix spp.).
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(Tutin et al., 1964) a taxon is considered to be
`effectively naturalized' once it has `established
in a single station for at least 25 years, or is
reported as naturalized in a number of widely
separated localities'. Invasion, i.e. spread into
areas away from sites of introduction, requires
that introduced plants also overcome barriers
to dispersal within the new region (D) and can
cope with the abiotic environment and biota
in the general area (E). Many then invade disturbed,
seminatural communities. Invasion of
successionally mature, undisturbed communities
usually requires that the alien taxon overcomes
resistance posed by a different category of
factors (Richardson et al., 2000); (barrier F in
Fig. 1).
Fig. 1 A schematic representation of major barriers limiting the spread of introduced plants (see text for
further details) The barriers are: (A) Major geographical barrier(s) (intercontinental and/or infra-continental;
approximate scale: > 100 km; see Table 1, Footnote 1); (B) Environmental barriers (abiotic and biotic) at
the site of introduction; (C) Reproduction barriers (prevention of consistent and long-term vegetative
and/or generative production of offspring); (D) Local/regional dispersal barriers; (E) Environmental
barrier(s) in human-modified or alien-dominated vegetation; and (F) Environmental barriers in natural or
seminatural vegetation. Arrows a through f indicate the paths followed by taxa to reach different states
from introduced to invasive in natural vegetation. Crossing of the barriers is not irreversible. For example,
climatic fluctuations can either pose new barriers (which could drive alien taxa to extinction at local and/or
regional scales), or enable the taxon to survive or spread.
100 D. M. Richardson et al.
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Deciding whether a taxon should be labelled
`naturalized, noninvasive' (`c' or `d' in Fig. 1)
or `invasive' (`e' or `f' in Fig. 1) can be problematic.
The two phases are not discrete and
there may be a continuum of situations for
different taxa, or a single taxon may move
through the continuum over time or in space. Is
it feasible to make the distinction more objective?
Heenan et al. (1998), when considering the
distinction between `casual' and `naturalized'
species (essentially `naturalized' and `invasive'
according to the definitions in Table 1) conclude
that `the main issues ... are the amount
of sexual or asexual spread, and the distance
of propagules from the parent plant'. We
agree. Richardson et al. (1994), when considering
the distinction between naturalized and invasive
Pinus spp. in the southern hemisphere wrote:
`For a species to be "invasive" [as in Fig. 1 and
Table 1] we required clear evidence that it regenerated
naturally and recruited seedlings more
than 100 m from parent plants ...'. These authors
thus excluded many other pine species, including
naturalized species, which may have been able
to overcome barriers A through C in Fig. 1,
but had not been able to disperse far from
sources. This distinction works well for pines
and for other tree and shrub taxa that rely
on seeds for dispersal. However, two additional
qualifications are needed. First, because seedling
mortality is usually much higher than
adult mortality (Silvertown, 1987; Fig. 5.18),
finding a few seedlings > 100 m from parents
still does not mean that invasion has started.
Consequently, `reproductive offspring' should be
used instead of `seedlings'. Second, 100 m can
be a long or short distance, depending on the
time since introduction. We therefore suggest
that `> 100 m from parent plants' should be
associated with the time interval < 50 years.
This should not be interpreted as > 2 m/yr. More
than 100 m is a necessary component of this
definition for plant taxa that depend on propagules
dispersed by wind, water, animals, and
unintentionally by humans (e.g. with equipment
used in agriculture and forestry). This definition
is inevitably somewhat arbitrary but, in
our experience in many parts of the world, useful
in practice. According to this definition, many
(but not all) Eucalyptus species introduced to
Africa and the Americas are naturalized but
not invasive (M. Rejmánek & D.M. Richardson,
unpublished data). Similarly, many woody species
reported as `invasive' from Berlin (Kowarik, 1995)
are in fact just naturalized.
Finally, for plant taxa spreading exclusively
vegetatively by roots, rhizomes, stolons or creeping
stems, a somewhat different definition is
needed. Waiting 50 years to see whether a plant
clone expands over 100 m is clearly impractical.
However, there will probably be general agreement
that an alien taxon that spreads consistently
by means of vegetative growth > 2 m per
year is invasive. To incorporate this condition
into an operational definition, `6 m/3 years'
could be used as a criterion for taxa spreading
by the vegetative means listed above. In
California, Ammophila arenaria and Delairea
odorata (= Senecio mikanioides) certainly belong
in this category.
Another issue requires some clarification.
Plant ecologists sometimes use `invasion' in a
slightly different context to that discussed above.
For example; `tree invasion and establishment
in old fields' (Myster, 1993); `limited invasion
and reproduction of loblolly pines in a
large South Carolina old field' (Golley et al.,
1994); `invasion and persistence of bird-dispersed,
subtropical thicket and forest species in fireprone
coastal fynbos' (Cowling et al., 1997) or
`shrub invasion of grassland' (Brown & Archer,
1999). We believe that in the context of such
studies (local vegetation succession), it is more
appropriate to use the term `colonization' (sensu
Brown & Gibson, 1983; p. 559: `the immigration
of a [taxon] into a new habitat and the
founding of a new population'). Another term
that is used is `encroachment' (e.g. Hodgkin,
1984). There are certainly many opportunities
for integrating insights from invasion ecology
with those from research on local vegetation
dynamics (Rejmánek, 1989; Vitousek, 1990;
Richardson & Bond, 1991; Davis et al., 2000).
Nevertheless, although successful alien species
often behave very much like native colonizers
(Thompson et al., 1995), we suggest that separate
terms should be maintained in these fields.
After Elton's (1958) book on `The ecology of
invasions by animals and plants' and recent
international programmes on biological invasions
(Mooney, 1998; pp. 93­111), most ecologists
probably equate `biological invasions' and
Naturalization and invasion of alien plants 101
 2000 Blackwell Science Ltd, Diversity and Distributions, 6, 93­107
even `invasions' with the dynamics of alien
species, rather than the colonization of coastal
dunes, abandoned pastures or other ecosystems
by native species.
In palaeoecological publications, the term
`invasion' (Pielou, 1991; Davis et al., 1998) is
used interchangeably with `spread' (Birks, 1989;
Tallantire, 1972) and `migration' (Davis & Sugita,
1997; Cain et al., 1998). As the last-mentioned
term seems to appear most often, its preferential
usage is recommended. However, as the
spatial scales of `palaeoinvasions' and `neoinvasions'
are often comparable and the context is
always clear, the term `invasion' is certainly
appropriate for palaeoecological studies. In fact,
its use clearly indicates that co-operation between
palaeoecologists and invasion ecologists is for
their mutual benefit (Pitelka and the Plant
Migration Workshop Group, 1997; Rejmánek,
1999b).
Implications for national policies and
international conventions
This paper has focused mainly on the definitions
of the terms `naturalized' and `invasive' and
on proposing a framework for defining these
and other phases in the naturalization/invasion
process. A related issue that should also be
mentioned here concerns the various definitions
applied to `alien' and `invasive' species in
national and international conventions. Article
8(h) of the Convention on Biological Diversity
states that: `Each Contracting Party shall, as
far as possible and as appropriate: Prevent the
introduction, control or eradicate those alien
species which threaten ecosystems, habitats or
species'. This implies (but does not define precisely)
that species that cause such damage
form a subset of alien species, and that methods
are needed to manage such species (and not
all alien species). The Executive Order on `invasive
species' issued by the President of the USA
on 3 February 1999 defines invasive species
as `alien species whose introduction does or
is likely to cause economic or environmental
harm or harm to human health'. This represents
an attempt to define the subset of alien
species for which various control measures need
to be implemented, and conforms with the
usage of some authors who consider plants to
be `invasive' only when they cause obvious
ecological and/or economic damage (e.g. Cronk
& Fuller, 1995; p. 1; Mack, 1997). Examples of
definitions following this notion are:
`Invasive species: Non-native species disrupting
and replacing native species' (Biotech Resources,
1995­98). `Invasive species means an alien species
which becomes established in natural or seminatural
ecosystems or habitat, is an agent of
change, and threatens native biological diversity'
(IUCN, 1999).
These definitions of `invasive' differ from that in
Table 1 in that they add connotations of impact.
Whether there will ever be a standardized procedure
for assessment of the impacts of invasive
species is difficult to predict (Williamson, 1998;
Parker et al., 1999). However, well-established
terms for harmful species (native or alien) already
exist: pests for all kinds of organisms and
weeds for plants (Holzner, 1982; Randall, 1997).
Our estimate is that between 50 and 80%
of invasive plant species can be classified as
pests or weeds, depending on actual impacts
and human perceptions. The remaining species
are `benign invaders' whose environmental
or economic impacts are beyond any practical
detection limits in most situations. Examples
of such `benign invaders' are Aira praecox,
Arabidopsis thaliana, Cakile edentula, Cypselea
humifusa, Elatine ambigua, Epipactis helleborine,
Lindernia procumbens, Lythrum tribracteatum,
Myosotis stricta, Petrorhagia dubia, Spergularia
platensis, Teesdalia coronopifolia, and Velezia
rigida.
Elton's (1958) classic book on invasions is
a clear starting point for invasion ecology as
a new discipline. Unfortunately, Elton never
defined the terms `invasion' or `invader'. Most
of his examples are introduced species that
had profound economic and/or environmental
impacts since the best data are available for
such taxa. However, Elton (1958) also discussed
the spread of some alien species which do not
have any obvious impacts (e.g. Lamium album
and several fresh-water shrimps). He also used
the term `invaders' with reference to the intermixing
of faunas during the Pliocene. Therefore,
it seems that he applied the terms `invasion' and
`invader' rather broadly, without obligatory connotations
of impact. This is also how the term
`invasion' was understood by other ecologists of
102 D. M. Richardson et al.
 2000 Blackwell Science Ltd, Diversity and Distributions, 6, 93­107
the time (`Invasion -- The movement of plants
from one area to another, and their colonization
in the latter; Clements, 1949; p. 284), and
how it was originally defined by Goeze (1882;
p. 109). This is how the term is used by most
ecologists today, and how it is commonly used
in textbooks with reference to alien species [e.g.
`Invasions of exotic species into new geographical
areas sometimes occur naturally and without
human agency. However, human actions
have increased this trickle to a flood ... Many
introduced species are assimilated into communities
without much obvious effect' (Townsend
et al., 2000; p. 500)].
There may well be a need for a universally
acceptable, and objectively applicable, term for
the most damaging invasive plant taxa within
given regions, or globally. In our view, a potentially
useful term to use in this regard is `transformer
species', proposed by Wells et al. (1986)
and referring to a subset of invasive plants
which `change the character, condition, form or
nature of a natural ecosystem over a substantial
area' (see examples in Table 1). It is these
species, comprising perhaps only about 10% of
invasive species, that have profound effects on
biodiversity and that clearly demand a major
allocation of resources for containment/control/
eradication.
Based on the above, we suggest that `invasive'
should be used with reference to the `biogeographic/demographic'
status of a species
without any connotation of impact.
We believe that the simple scheme proposed
in Fig. 1 and Table 1 captures the most important
concepts and defines them in ways that will
permit widespread application and acceptance.
We hope that authors will consider using the
terms `alien', `naturalized' and `invasive' as
defined here. If they prefer different terminology
then, at least, precise definitions should be
provided.
ACKNOWLEDGMENTS
We thank Mark Williamson and other members
of the Scientific Steering Committee of the Global
Invasive Species Programme (GISP) for useful
comments and discussion on many aspects
dealt with in this paper. The views expressed
here do not necessarily reflect those of GISP.
Debra Ayres, Mick Crawley, Joseph DiTomaso,
John Hoffmann, Fred Hrusa, Lloyd Loope,
Shirley Pierce, John Randall, Rod Randall and
Rob Whittaker provided extremely useful comments
on a near-final version of the manuscript.
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Naturalization and invasion of alien plants 107
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Appendix 1 Definitions of `naturalized' in dictionaries and encyclopedias of ecology, floras and other volumes
1 Scott (1966) CONCISE ENCYCLOPEDIA: BIOLOGY
`The successful establishment of animals and plants ... in an area in which they had not previously
existed. In the restricted sense ... used only when the process is fully intentional and planned by
human agency. In the broader sense ... also includes ... those whose introduction is entirely accidental
and whose presence at the time of introduction may be unknown ...'
2 Gray (1967) THE DICTIONARY OF BIOLOGICAL SCIENCES
`The enforced adaptation, usually with human aid, of an organism to a foreign environment'
3 Walker (1989) CHAMBERS BIOLOGY DICTIONARY
`Introduced from another region but growing, reproducing and maintaining itself in competition with
the native vegetation'
4 Allaby (1994) THE CONCISE OXFORD DICTIONARY OF ECOLOGY
`Applied to a species that was originally imported from another country but now behaves like a native
in that it maintains itself without further human intervention and has invaded native communities'
5 Huston (1994; p. 322) BIOLOGICAL DIVERSITY
`... are able to establish in existing plant communities and become naturalized and of these 10 only
five are actually able to spread beyond the site of introduction'
6 Lever (1994; p. 279) NATURALIZED ANIMALS
`Naturalization: The introduction of animals and plants to places where they are not indigenous, but
in which they may flourish under the same conditions of those that are native. More particularly, the
establishment of self-perpetuating populations unsupported by and independent of man of an
introduced species in a free-living state in the wild'
7 Cronk & Fuller (1995; p. 15) PLANT INVADERS; THE THREAT TO NATURAL ECOSYSTEMS
`Once introduced, an invasive plant must become established beyond the site of initial introduction to
form large self-sustaining populations in natural or seminatural vegetation [ ... ] This stage is known
as naturalization ...'
8 Sell & Murrell (1996; p. 376) FLORA OF GREAT BRITAIN AND IRELAND (VOLUME 5)
`An alien plant which has become self-perpetuating in the British Isles'
9 Williamson (1996; p. 33) BIOLOGICAL INVASIONS
`Established -- with a self-perpetuating population, naturalized, feral and breeding successfully,
released and breeding successfully'
10 Crawley (1997; p. 617) PLANT ECOLOGY (2nd ed)
`alien plant species that have formed self-replacing populations based on recruitment from seed or
spread of vegetative fragments'
11 Lincoln, Boxshall & Clark (1998) A DICTIONARY OF ECOLOGY, EVOLUTION AND
SYSTEMATICS
`used of an alien or introduced species that has become successfully established'
12 Olejniczak (1998) THE ENCYCLOPEDIA OF ECOLOGY AND ENVIRONMENTAL
MANAGEMENT
`Describing species that have established themselves and are flourishing following their introduction to
a non-native region'
Responsible editor: Robert J. Whittaker