Biodiversity
Copyright National Academy of Sciences. All rights reserved.
CHAPTER 36
RESTORING DIVERSITY IN SALT
MARSHES
JOY B.ZEDLER
Professor of Biology, San Diego State University, San Diego, California
Along the U.S. coastline, development has reduced the area of coastal wetlands and
endangered certain wetland-dependent species. Despite the threats to biodiversity,
development of wetland habitat is still permitted by regulatory agencies if project damages
can be mitigated by improving degraded wetlands or creating new wetlands from uplands.
For example, the California Coastal Act allows one-fourth of a degraded wetland to be
destroyed if the remaining three-fourths is enhanced. The expectation is that increased
habitat quality will compensate for decreased quantity.
The concept sounds reasonable, but biodiversity is continuing to decline. Why? First,
the process allows a loss of habitat acreage. Second, there is no assurance that wetland
ecosystems can be manipulated to fulfill restoration promises. The magnitude of the
problem is well illustrated by examples from southern California, where more than 75% of
the coastal wetland acreage has already been destroyed, where wetland-dependent species
have become endangered with extinction, and where coastal development pressures rank
highest in the nation. This chapter reviews several restoration plans and implementation
projects and suggests measures needed to reverse the trend of declining diversity.
RESTORATION PLANS
Several large development projects in southern California wetlands have recently
been approved by the California Coastal Commission (see Figure 36–1). Three federally
endangered species are affected by such projects: the California least tern (Sterna
albifrons browni), light-footed clapper rail (Rallus longirostris levipes), and salt marsh
bird's beak (Cordylanthus maritimus spp. maritimus; see Figure 36–2).
RESTORING DIVERSITY IN SALT MARSHES 317
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FIGURE 36–1 Sites of some coastal development projects in southern
California. In all, there are 26 coastal wetlands between Point Conception and
the Mexico-U.S. border.
FIGURE 36–2 The salt marsh bird's beak grows near the upper wetland edge. As
an annual plant, its seeds germinate after winter rainfall to maintain the
population; as a hemiparasite, its seedlings grow roots that can attach to those of
other plants, thereby increasing its supplies of water and nutrients. Photo by
J.Zedler.
RESTORING DIVERSITY IN SALT MARSHES 318
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There would also be an impact on the Belding's Savannah sparrow (Passerculus
sandwichensis beldingi), which is listed as endangered by the state, and on several plant
species of regional concern (Ferren, 1985).
Projects That Show Losses in Wetland Area
At Bolsa Chica Wetland, more than 1,200 acres (480 hectares) of lagoonal wetland
will be reduced to 951 acres (366 hectares) of restored wetland (California State Coastal
Conservancy, 1984). Mitigation plans are not final, but the draft concept includes cutting
an ocean inlet to serve a new marina. Inland from the marina are sites for restored
wetlands with controlled tidal flushing. Uplands designated as “environmentally sensitive
habitat areas” that lie within the lowland area and that will be destroyed during
development are to be relocated adjacent to the restored wetland in a bluff-edge (linear)
park. The draft concept plan accommodates development, but does not ensure maintenance
of biodiversity. The restoration activities are based on the assumption that habitat values
can be created and moved about at will.
In Los Angeles Harbor, about 400 acres (160 hectares) of shallow water fisheries
habitat will be filled to construct new port facilities. At this project site, there is no habitat
available to be restored—all the wetlands have been filled or dredged. Thus, off-site
mitigation has been approved. Batiquitos Lagoon, more than 80 miles (130 kilometers)
south of Los Angeles, will be dredged to create deep-water habitat and increase tidal
flushing. According to plans (California State Coastal Conservancy, 1986), the net loss of
aquatic habitat in Los Angeles will be mitigated by altering (not increasing) habitat
elsewhere. The dredging of Batiquitos Lagoon will remove sediments and, at least
temporarily, solve the occasional problems of algal blooms (odors and fish kills after
sewage spills). However, maximizing tidal flushing at Batiquitos Lagoon (to replace
fisheries habitat in Los Angeles Harbor) will destroy existing salt marsh habitat
(Figure 36–3) and reduce the area of shallow water and mudflat habitat. The mitigation
plan contains two strikes against biodiversity—the loss of area and the loss of existing
functional wetland types.
At Aqua Hedionda Lagoon, about 14 acres (5.6 hectares) of wetland were filled to
build a four-lane road. The mitigation plan (U.S. Army Corps of Engineers, 1985)
promised to enhance diversity and increase the functional capacity of the lagoon.
Brackish-water ponds were planned for a wetland transitional area (itself a rare habitat
type); a 2-acre (0.8-hectare) dredge spoil island was to be built for bird nesting; and a 7acre
(2.8-hectare) debris basin was proposed within a riparian area to reduce sedimentation
into the lagoon. Flaws in the plan became clear when construction of the brackish ponds
began. Pits were dug to a depth of 6 feet (1.8 meters) without encountering groundwater.
Areas that were modified included transition habitat, pickleweed marsh (Figure 36–4),
brackish marsh, and riparian habitat. The wetland lost both acreage and habitat quality.
All these projects show a net loss in wetland habitat area. Proponents argue that the
lost areas are already degraded. However, they could be enhanced to maintain
biodiversity. The fact that four wetland-dependent species have become endangered in
Southern California while coastal wetlands have shrunk by 75% indicates a cause-effect
relationship. There is some minimum area required to
RESTORING DIVERSITY IN SALT MARSHES 319
Biodiversity
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support regional biodiversity and a limit to the number of species that can be packed into
individual wetlands. Populations are dynamic; some migrate and use several wetlands,
whereas others experience local declines and must reinvade from another refuge. The need
to maximize area available for wetland species is indicated by population declines that
have followed human disturbance and environmental catastrophes.
Several species may be lost simultaneously if a wetland experiences multiple
catastrophes. At Tijuana Estuary, the combination of destabilized dune sands (following
long-term trampling), the winter storms of 1983, dune washovers, and channel
sedimentation led to closure of the ocean inlet in April 1984. The drought of 1984
coincided with an 8-month nontidal period. The population of endangered light-footed
clapper rails dropped from about 40 pairs to 0 and did not fully recover after tidal flushing
was restored (16 pairs were present in 1987). In addition, there were major declines of
three salt-marsh plant species—cordgrass (Spartina foliosa), annual pickleweed
(Salicornia bigelovii), and sea-blite (Suaeda esteroa)—and none has recovered to pre-1983
levels. This salt marsh has shifted from the region's most-diverse to a species-poor wetland
(Zedler and Nordby, 1986).
FIGURE 36–3 Salt flats may appear to have low habitat value, but many
unusual insects, some of them threatened with extinction, are found only in these
open areas. In the winter, runoff and high tides inundate the areas, and they
become highly productive ecosystems. What appears to be barren in summer is
heavily used by shorebirds and dabbling ducks in winter, as migrants visit the
flats and feed on the abundant insects and algae. Photo by J.Zedler.
RESTORING DIVERSITY IN SALT MARSHES 320
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FIGURE 36–4 Pickleweed marsh may seem monotonous, but close inspection
will reveal a variety of insects, invertebrates, and dozens of species of
microscopic algae. Individuals and trails of the California horn snail (Cerithidea
californica) are visible in the tidal pool. Photo by J.Zedler.
Maintenance of the region's resources through years of wet and dry periods, with and
without closure to tidal flushing, requires that each habitat type be maintained at several
different wetlands so there will be refuges during periods of environmental extremes.
Further losses in habitat area cannot be justified.
Projects That Replace Functional Wetland Habitat with
Modified Wetland Habitat
Some restoration projects retain acreage but exchange one type of habitat for another.
In these cases, functional wetland habitats may be destroyed in order to create some other
habitat type. Following are some examples.
The City of Chula Vista's Bayfront Development Plan calls for several developments
near and in the last major salt marsh within San Diego Bay (90% of the Bay's wetland has
already been developed). The plan includes a multistory hotel and a nature center to be
built on an island that is surrounded by Sweetwater Marsh and San Diego Bay. Residential
and commercial buildings would surround the marsh. To provide access, three roads are to
be built over the wetland. The plan will also require modification of the wetland for the
construction of debris basins. Wetland restoration is planned to mitigate impacts. The U.S.
Fish and Wildlife Service has concluded that portions of the project jeopardize the
following
RESTORING DIVERSITY IN SALT MARSHES 321
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endangered species: California least tern, light-footed clapper rail, California brown
pelican (Pelicanus occidentalis), and salt marsh bird's beak.
At Los Cerritos Wetland near Los Angeles, a complicated plan (California State
Coastal Conservancy, 1982) proposes development of some wetland in exchange for an
equal area of wetland creation. In all, 129 acres (51.6 hectares) of wetland will be
retained. Some dikes that now prevent tidal flushing will be breached; some new areas
will be graded to allow tidal flow. The restored wetlands will be divided into four
segments and surrounded by high-density urban uses. Buffers between the wetland and
development are as narrow as 25 feet (7.5 meters) for much of the project. A main concern
is that existing wetland habitat will be lost and that the artificially created replacements
cannot guarantee maintenance of biodiversity.
At Upper Newport Bay, a sediment control plan within a California State Ecological
Reserve has received wide political support, in part because dredging in the upper bay
reduces sedimentation in the lower bay's marina. Sedimentation in the upper bay is a
long-term threat to the marsh habitat, but sudden changes in hydrology may have a
negative impact on the habitat of endangered species. Upper Newport Bay has the highest
density of light-footed clapper rails in the state of California and some of the region's most
robust cordgrass vegetation. Shallow-water and transitional habitats are being traded for
deeper channels, and the value of the new habitats to biodiversity is uncertain.
At Buena Vista Lagoon, sediment control measures were also taken. Shallow-water
areas were deepened, and dredge spoils were placed alongside them in the wetland. The
dredge spoil islands became hypersaline, bricklike substrates that have not developed the
desired vegetation or significantly improved the status of the least tern population.
CONCLUSIONS CONCERNING RESTORATION PLANNING
Several observations on the status of restoration plans can be made:
• Many large projects result in a loss of wetland acreage.
• Mitigation measures for lost habitat often involve changing one type of wetland
habitat into another, rather than creating wetland from upland habitat.
• Proposed projects are planned and reviewed individually rather than with a
regional perspective. While cumulative impacts may be considered, there is no
regional coordination to set priorities and guide decision making. There is no
way to ensure that the wetlands with the greatest potential for maintaining
clapper rails will be managed for clapper rails.
• There is no single source of information on restoration projects, no center that
keeps records on changes in biodiversity to ensure that resource agencies are
aware of changes in individual wetlands, no comprehensive monitoring
programs to assess changes in biodiversity (although some endangered species
are censused annually), and no mechanism to require suitable and comparable
methods for the few monitoring programs that have been planned.
RESTORING DIVERSITY IN SALT MARSHES 322
Biodiversity
Copyright National Academy of Sciences. All rights reserved.
IMPLEMENTATION OF RESTORATION PROJECTS
To enhance, restore, or create wetland habitat requires manipulation of the physical
environment (especially the topography and the degree and timing of fresh- and seawater
influence) as well as the biota (e.g., by introducing target species and eliminating
undesirable ones). Research in this area is just beginning; most of the work has been done
on a trial-and-error basis, and the evaluation criteria are not yet standard.
Assessing Success
Restoration success must be measured in time scales that relate to the species being
managed and to the periodicity of extreme environmental conditions characteristic of the
region. Successful creation of clapper rail habitat cannot be measured by censusing
mortality of cordgrass a few weeks after transplantation. Rather, such projects need to be
followed at least until clapper rails establish breeding populations. Measures of restoration
success must be done within spatial scales that relate to whole ecosystems. The degree to
which breaching a dike and restoring tidal flushing can enhance a lagoon must be
measured beyond channel biota and water quality, because there will also be substantial
impacts on intertidal marshes. Likewise, dredging to improve fish diversity cannot be
considered successful if endangered birds become extinct in the process. In short,
restoration success must be measured at the ecosystem level and with long-term
evaluation. To date, this has not been done.
Summary of Trials
All the projects described above incorporate some element of habitat creation or
restoration, for which there are no guaranteed benefits to threatened species. Many
projects have been designed to restore wetlands and mitigate losses, but in no case have
ecosystem functions been duplicated, nor have endangered species been rescued from the
threat of extinction. Projects to reduce sedimentation have as one goal the creation of fish
and benthic invertebrate habitat (sometimes to provide food for the California least tern).
Marsh restoration projects have focused on vegetation used by target bird species
(cordgrass for clapper rails, pickleweed for Belding's Savannah sparrows).
While some wetland plant species can be transplanted successfully and others will
invade voluntarily given suitable conditions (Zedler, 1984), there are only a few cases
where the marsh ecosystem has been monitored for several years (Broome et al., 1986;
Homziak et al., 1982) and no example of a threatened species that has been increased as
desired. Attempts to restore wetlands in southern California have generally failed to attract
target species. In a few cases, the California least tern has nested on dredge spoil islands
—but not always where its use was planned. One briefly successful site was an 80-acre
(132-hectare) island in south San Diego Bay, which was planned for salt marsh and fish
habitat.
RESTORING DIVERSITY IN SALT MARSHES 323
Biodiversity
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Conclusions Concerning Implementation
Four observations on the status of wetland habitat restoration in southern California
can be made.
• Selected plant species (e.g., cordgrass) can be transplanted successfully.
• No plant or animal populations have been taken off the endangered list as a
result of restoration projects.
• Wetland restoration assessments have not been made for entire ecosystems but
have been limited to one or a few target species.
• No studies have been conducted to determine the minimum wetland area or
configuration of multiple wetlands required to maintain regional biodiversity.
Therefore, it is premature to conclude that an artificial tidal wetland can develop and
replace the functions of a natural one. Furthermore, there is no evidence that restoration of
degraded wetland habitat can compensate for lost habitat area.
PROSPECTS FOR THE FUTURE
To restore biodiversity in the nation's coastal wetlands, we must understand the
factors controlling these ecosystems and develop the ability to modify them to meet
desired management goals. We must make substantial advances in ecotechnology—the
scientifically sound manipulation of ecosystems to maintain natural diversity and achieve
specific management objectives. The field is relatively new in ecology. Only one journal,
Restoration and Management Notes, and a few books focus on the topic. Although most
of the work in this area concerns disturbed ecosystems, all ecosystems need some
management to maintain their natural hydrology as well as air and water quality.
Ecosystems of greatest concern tend to be those whose areas have been reduced and
whose species are threatened with extinction. Rare species are difficult to study, because
the conditions that allowed them to thrive no longer exist. Manipulative experimentation,
required to establish cause-effect relationships, cannot always be done without threatening
the endangered populations even further. Bringing animals or plants (even seeds) into the
laboratory may reduce field populations to levels that jeopardize population recruitment.
Thus, maintenance of biodiversity must be based on an understanding of the factors that
control the ecosystems in which rare species persist—the type of long-term, ecosystemlevel
research now funded by the National Science Foundation, the National Oceanic and
Atmospheric Administration through its Sea Grant Program, and other agencies. A new
research emphasis could allow major advances to be made in wetland ecotechnology. I
recommend manipulative experimentation, first in replicate mesocosms (medium-size
artificial ecosystems), followed by experimental restoration at the ecosystem level. This
approach was adopted by the U.S. Environmental Protection Agency in their research plan
for the nation's wetlands (Zedler and Kentula, 1985).
Ecosystem-level experiments have not been incorporated into wetland restoration
projects. The contention that artificial or restored wetlands can maintain biodiv
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ersity must be tested. Every restoration project can include experimentation in its design,
e.g., to provide different tidal flows; to test different hydroperiods, salinities, and nutrient
inputs; to use different transplantation regimes; or to vary the width of buffer zones, with
treatments appropriately replicated. Detailed, long-term evaluation of the experiments will
document success or failure to maintain natural diversity. In either event, we will learn
whether it can be done and why it succeeds or fails. The present practice of poorly
planned, unreplicated, undocumented trials leads mainly to errors whose causes cannot be
identified. Only as our understanding of factors controlling wetland ecosystems improves
can we ensure the restoration and maintenance of biodiversity.
ACKNOWLEDGMENTS
Research on wetland restoration was funded in part by NOAA, National Sea Grant
College Program, Department of Commerce, under grant number NA80AA-D-00120,
project number R/CZ-51, through the California Sea Grant College Program, and in part
by the California State Resources Agency.
REFERENCES
Broome, S.W., E.D.Seneca, and W.W.Woodhouse, Jr. 1986. Long-term growth and development of
transplants of the salt-marsh grass Spartina alterniflora. Estuaries 9:63–74.
California State Coastal Conservancy. 1982. Los Cerritos Wetlands: Alternative Wetland Restoration
Plans Report. State of California—Resources Agency, State Coastal Conservancy, Oakland.
49 pp. +appendixes.
California State Coastal Conservancy. 1984. Staff Presentation for Public Hearing: Bolsa Chica
Habitat Conservation Plan. State of California—Resources Agency, State Coastal
Conservancy, Oakland. 31 pp. +exhibits.
California State Coastal Conservancy. 1986. Batiquitos Lagoon Enhancement Plan Draft. State of
California—Resources Agency, State Coastal Conservancy, Oakland. 183 pp. +appendixes.
Ferren, W., Jr. 1985. Carpinteria Salt Marsh. Publication #4, Herbarium, University of California,
Santa Barbara, Calif. 300 pp.
Homziak, J., M.S.Fonseca, and W.J.Kenworthy. 1982. Macrobenthic community structure in a
transplanted eelgrass (Zostera marina) meadow. Mar. Ecol. Prog. Ser. 9:211–221.
U.S. Army Corps of Engineers. 1985. Public Notice of Application for Permit, Application No. 85–
137-AA. U.S. Army Corps of Engineers, Los Angeles District. 13 pp.
Zedler, J.B. 1984. Salt Marsh Restoration: A Guidebook for Southern California. Report No. TCSGCP-009.
California Sea Grant, La Jolla, Calif. 46 pp.
Zedler, J.B. In press. Salt marsh restoration: Lessons from California. In J.Cairns, ed. Rehabilitating
Damaged Ecosystems. CRC Press, Boca Raton, Fla.
Zedler, J.B., and M.Kentula. 1985. Wetland Research Plan. Corvallis Environmental Laboratory,
U.S. Environmental Protection Agency, Corvallis, Oreg. 118 pp.
Zedler, J.B., and C.S.Nordby. 1986. The Ecology of Tijuana Estuary: An Estuarine Profile. U.S. Fish
Wildl. Serv. Biol. Rep. 85(7.5). 104 pp.
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