'OSLO-
Location and Wetland Values: Some Pitfalls of Off site Wetland
Mitigation in the Chesapeake Watershed
Curtis C. Bohlen and Dennis M. King
University of Maryland System
Center for Environmental and Estuarine Studies
Chesapeake Biological Lab
P.O. Box 38
Solomons, MD 20688
To. appear in:
S. Nelson and P. Hill, eds., Towards a Sustainable Coastal Watershed: The
Chesapeake Experiment. Proceedings of a Conference: CRC Publication No.
149. Chesapeake Research Consortium, Edgewater, Maryland.
This work was supported under Cooperative Agreement CR-818227-CI from the U.S.
Environmental Protection Agency with the University of Maryland Center for Environmental
and Estuarine Studies. This support is gratefully acknowledged. '' .
EPA-230-R-94-020
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Location and Wetland Values: Some Pitfalls of Offsite Wetland
Mitigation in the Chesapeake Watershed
Curtis C. Bohlen and Dennis M. King
University of Maryland System
Center for Environmental and Estuarine Studies
Chesapeake Biological Lab
P.O. Box 38
Solomons, MD 20688
Introduction
Historical Mitigation
Wetland mitigation—compensating for the destruction or degradation
of an existing wetland by creating, restoring, or enhancing other wetland
resources—has long been, and continues to be, controversial. Compensatory
wetland mitigation emerged in the 1970s as a way for .sponsors of projects
with adverse impacts to wetlands to obtain wetlands permits they could not
otherwise get, by agreeing to undertake activities to offset a portion of those
impacts (Want-1993). A deep mistrust of mitigation emerged within the
environmental community at that time, because mitigation was not only
frequently ineffective, but also often resulted in greater impacts to existing
wetlands than would have been allowed in the absence of mitigation.
Permit seekers today must generally demonstrate that a proposed project
is in the public interest without mitigation, and also show that impacts to
wetlands have been, avoided and minimized to the maximum extent
practicable before a permit will be issued. Thus the decision to issue or deny a
permit has been separated from establishment of mitigation requirements,
and it is harder to argue that mitigation increases conversion of existing
wetlands. Nevertheless, mistrust of mitigation and concern about potential
abuses of mitigation remains strong within the environmental community.
Mitigation remains at the center of controversy over wetland policy in
part because of a poor record of compliance with mitigation requirements, ,
high reported failure rates for mitigation projects that are undertaken, and
low environmental value even of superficially successful projects.
Limitations of restoration science may account for some historical mitigation
failures, however, considerable evidence suggests that wetland mitigation's
poor record is more a result of institutional and regulatory failures (King
1991, King 1992, King and Bohlen 1994a, King arid Bohlen 1994b).
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A New Focus
In 1988, the National Wetland' Policy Forum developed a widely accepted
national goal of no net loss and an eventual net gain of wetland resources by
acreage and function (Conservation Foundation 1988). This goal has been .
formally adopted by the Chesapeake Bay Program, included as the explicit goal
of Maryland's Nontidal Wetlands Protection Act, and adopted by federal
regulators (Chesapeake Bay Program 1988, Maryland Code 1989,
Environmental Protection Agency and Department of the Army 1992). With
a clear goal' for wetland policy, regulators have been developing rules and
policies to improve mitigation practice. At the same time scientists have
been taking a •closer look at why mitigation projects succeed or fail and are
beginning to tease apart the technical, economic/and regulatory barriers to
environmentally sound mitigation policies (Bernstein and Zepp 1990, Crewz
and Lewis'1991, Erwin 1991, Florida Department of Environmental
Regulation 1991, Harvey and Josselyn 1986, Kusler and Kentula 1991, Race
1985, King and Bohlen 1993, Kentula et al 1992).
Little attention, however, has been given to the environmental and
economic consequences of "off-site" wetland mitigation—allowing wetland
impacts at one site to be offset by mitigation efforts somewhere else. In off-
site mitigation wetlands at one location are, in essence, traded for wetlands
somewhere else. When these trades are examined from a watershed or
landscape perspective, however, it becomes clear that wetlands at different
locations, even if functionally similar, do not generate the same ecological
and economic values and do not benefit the same people. These differences in
the level and distribution of ecological and economic values associated with
wetlands at different locations are the focus of the research summarized here.
On-site vs. Off-site Mitigation
Existing regulations and other policies favor mitigation at the site of the
wetland impact over off-site mitigation. Cogent criticisms of this, approach
have been raised, both on environmental and economic grounds (Willard
and Klarquist 1992, Shabman et al. 1994, White House Office of .
Environmental Policy 1993). Environmentally, critics of a preference for on-
site mitigation point out that it sometimes results in construction of wetland
mitigation projects in questionable locations, such as within highway
cloverleaves, or in the middle of large commercial developments. Such
siting decisions may limit some wetland functions, by all but ensuring the
mitigation wetland will suffer from poor water quality, provide inferior •
wildlife habitat, and offer little in the way of aesthetic, recreational, or'.'
educational value. Economically, the preference, for pn-site wetland
mitigation often increases mitigation costs by forcing mitigation in areas
where providing appropriate wetland hydrology is difficult, and land is
expensive.
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The controversy over on-site versus off-site wetland mitigation has
taken on more importance as interest in wetland mitigation banking has
increased. Mitigation banking refers to the practice of using one or more
(usually large) wetland creation or restoration projects to provide mitigation
for impacts from many smaller wetland impacts. By its very nature^
mitigation banking involves off-site mitigation. Thus issues and trade offs
related to on-site versus off-site wetland mitigation are critical for
understanding the long term environmental, economic, and policy
implications of the widespread adoption of mitigation banking.
Tlie Urban-Rural Landscape Gradient
In the modern world, human activity alters; even the most remote
landscapes (McKibben 1989), but some landscapes are affected to a much
greater degree than others. Urban areas are more intensively manipulated by
humans to achieve economic and social ends than are areas far from urban
centers. Rural landscapes, although sometimes shaped in less direct ways, are
no less profoundly influenced by human activity. The notion that urban
areas as structured by humans, and rural areas are structured by nature
ignores the often profound human transformation of landscapes that has
occurred even in the most remote rural hinterlands that surround urban
centers (Cronon 1991). The rural landscape is not natural, nor is the urban
landscape artificial. Both landscapes are the result of an ongoing interaction
between human economic and social systems and physical, chemical, and •
ecological processes.
Landscapes and the ecological and human systems of which they are
composed change in systematic, if complex, ways as one moves from an
urban center, through suburbs, to high intensity agriculture, through low-
intensity agriculture, to non-agricultural forest, and wildlands.. This urban-
rural landscape gradient (McDonnell and Pickett 1990), although often
unrecognized by policy makers, in fact underlies and determines the success
of much water quality and wetland policy. .
Anthropogenic wetland impacts, in particular, are strongly structured by
the urban-rural landscape gradient. Wetland impacts in rural areas are
driven, in large part, by forestry and agricultural practices; losses in urban,
suburban, and resort areas, by private and public development activities.
Moreover, private development activities in urban-suburban and resort areas
are less likely than their rural counterparts to avoid wetland impacts
altogether, or reduce impacts to the point that mitigation is not required.
Sponsors of more profitable suburban projects are generally able to afford the
transaction costs associated with seeking an individual wetland permit, and
in a more developed landscape, avoiding wetland impacts is typically more
difficult because of constraints imposed by existing land uses. Thus
individual wetland permits, and associated demand for wetland mitigation,
are not randomly scattered across the landscape/but instead cluster in
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suburban and resort areas, and in-areas where, human activity is-generally
more intensive. . ,
Wetland mitigation costs also-vary along the urban-rural landscape
gradient, with higher costs in urban settings and lower costs further from
urban centers. The price of land is typically highest in urban areas, and the
costs of some inputs to wetland mitigation projects (e.g. labor, costs for *
disposal of fill material) also decline as the mitigation site becomes more
distant from urban areas. Qualitative differences in project characteristics
between urban and rural projects also have a profound effect on total project
costs. Urban and suburban watersheds are often degraded by hydrologic
modification, as well as by chemical and thermal effects of stormwater runoff.
Thus restoration and creation projects must be carefully engineered and
constructed to handle the stresses imposed by the surrounding urban setting.
Moreover, many desirable sites in urban and suburban landscapes where
wetland creation or restoration would be relatively inexpensive are '
unavailable for mitigation purpose's because of existing or alternative high-
value land uses. Thus urban and suburban mitigation must often be located
in second or third best sites, further driving up project costs and risks.
On-site wetland mitigation (by definition) occurs in the same landscape
context as the wetland impacts which it is meant to offset. Yet off-site
mitigation is unlikely to be distributed across the urban-rural gradient in the
same way that wetland impacts are. mdividuals, firms, and government
institutions have a strong cost-based incentive to construct mitigation in
rural settings, even for impacts in urban and suburban settings. Without
clear guidance about ecological and economic trade offs, regulators may
perceive that they have little reason to resist replacing urban or suburban
wetlands with wetlands in rural areas because both the probability of
successful wetland creation or restoration and the overall quality of the
resulting wetland, are generally thought to increase as one moves from urban
to more natural settings (NRC 1992). Thus expanded use of off-site wetland
mitigation and widespread implementation of wetland mitigation banking is
likely to lead to significant, unplanned changes in the distribution of
wetlands across the urban-rural landscape gradient. The long term
consequences of such a redistribution, even if it does not involve any loss of
wetland acreage or function, are not well explored.
' • i "
Basic Approach
In the following sections, we provide a preliminary examination of the
potential consequences of .replacing wetlands in urban and suburban areas
with wetlands in areas further from urban centers. The basic approach we
will take will be to consider how the location and landscape context of
wetlands affect the values they provide to human society. -First, we introduce
some conceptual tools for understanding wetland functions and values in a'
landscape context. We then present three examples in which location plays
an important role in determining,wetland functions, values, or both. Finally,
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we present analytic tools that may help wetland managers and others assess
how location and landscape context will affect wetland functions and values
on a case specific basis.
Conceptual Tools
Wetlands are natural systems that exist independent of their significance
to humans. Unlike an automobile that has a specific purpose for which it was
designed, a wetland has no specific purpose, yet it generates a variety of
benefits to humans. Conventional treatment of wetland benefits recognizes
this fact and distinguishes between wetland functions, and wetland values.
Objectively measurable properties of a wetland (e.g. size of a breeding duck
population, denitrification rates) are considered functions. Subjective
experience of the wetland or the flow of products and services it produces (e.g.
good bird watching, improved downstream water quality) are considered
values. To a first approximation, functions are the domain of natural
scientists, values, the domain' of social scientists, philosophers, and
politicians. Functions are a, property of the wetland alone; values are a
property of the interaction of the wetland and its functions with social,
political, economic and ethical systems.
Values of wetlands and other natural systems come in many forms.
While the market values produced by wetlands (timber, fish, hunting rights, .
etc.) are often the easiest to understand and measure, wetland values are
much broader, and include: non-consumptive use values (e.g., education),
non-use values (e.g., flood protection), option and bequest values (value of
potential future wetland benefits, to oneself or later generations), and
existence values (values of knowing something exists). Moreover, some
wetland values accrue far from the wetland thai: provides them because of
movement of water and migratory fish, mammals, and birds. These off-site
values are no less important than on-site values, but they are much harder to
understand and measure, and much harder to manage.
The practical difficulties of tracing chains of causation from wetland
functions to discrete wetland values are formidable. Figure 1 suggests the
complexity of the task for a single wetland function, sediment trapping.
Many wetlands trap and retain sediments because water flow velocities in
wetlands are typically too low for sediments to remain suspended in the
water column. Thus, due to purely physical processes, many sediments that
enter wetlands in association with flooding events or through overland flow
settle out in the wetland, reducing the sediment loads of adjacent water
bodies. Reduced sediment loads alter a series of ecological and physical
properties of downstream environments, each of which in turn has linked
ecological, physical, and economic impacts. Many distinct values are
ultimately affected by wetland sediment trapping functions. Moreover,
several causative pathways may lead from this function to changes in a single
wetland value (e.g. support of biodiversity). This complexity is further
compounded by the geographic distance that often exists between the site of
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wetland functions (the wetland) and the sites at which values accrue (Bohlen
1992). Thus wetland functions and wetland values are related, but not in the
simple and direct way that is sometimes assumed in a wetland management
context, in which one must evaluate the extent to which one wetland is
equivalent to another in environmental and economic terms.
Examples
Function Depends on Location—Biodiversity
i " ,
Metapopulations are collections o.f interdependent, semi-autonomous
populations of an animal or plant, species of interest (Gilpin and Hanski 1991).
Although each component population may persist for many years,
occasionally, site-specific processes may lead to the extirpation of one or more
of the component populations. Recolonization from adjacent, extant
populations can reestablish the population. A component population that
goes extinct under one set of conditions (e.g., a dry year) may be highly
successful under other conditions (a wet year), which may nevertheless
threaten other populations with extinction. Thus populations may be
dependent on one-another for recolonization in the event of a local
extinction, as well as for infrequent outcrossing to maintain genetic diversity.
the metapopulation as a whole is thus more resilient than the populations of
which it is composed.
Metapopulations in fluvial (stream and river) and flood plain
ecosystems are likely to have unique features because of stream geometry..
Unlike habitat islands in an upland matrix, fluvial habitat patches are linked
by a branched, and directed stream network. Small, low order streams joiri to
become larger order streams, which eventually make large river systems.
Water flow moves organisms and propagules preferentially downstream,
from low-order streams to higher order streams, which often provide
qualitatively different habitat conditions (Vannote 1972). This branched
topology has profound implications for dispersal patterns within fluvial
ecosystems. Locations, especially in small, low order streams, that appear
close together on a topographic map may be separated by large stream
distances, and thus may be effectively isolated from one another for dispersal
.'. of aquatic species. Moreover, dispersal probabilities for many species along ',
stream reaches may differ depending on whether dispersal is occurring
upstream or down. These properties suggest that fluvial metapopulations
may be strongly dependent on key populations located at or near branch
points in fiver and stream systems that can link otherwise separated
upstream populations. . '
Humans have long relied on surface waters for irrigation, drinking
water, transportation and waste removal services. Thus many urban areas
worldwide are located at the confluence of rivers and streams, at the mouths
of rivers, and on estuaries. These locations, selected by humans as locations '
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for large settlements, may be (if this description of metapopulation dynamics
in fluvial systems is correct) likely to harbor key metapopulation links for a
variety of aquatic, flood plain, and river bank species (see figure 2). Thus
human patterns of land use may sever dynamically important links among
the component populations of fluvial metapopulations. Stream reaches in
and immediately downstream from sites of human habitation are the most
likely stream reaches to show signs of degradation under anthropogenic
impacts. Yet protection of biodiversity in the larger stream network may
require management of those same stream reaches to provide at least some
base level of habitat for fluvial plant and animal species.
Thus wetland mitigation practice that overlooks urban and suburban
mitigation efforts in favor of rural projects may jeopardize habitat functions
in urban and suburban areas that play an important ecological role in the
larger landscape pattern, and thus may be 'especially valuable.
Value Depends on Location—Stormwater Management
The value of certain wetland functions depend completely or nearly
completely on where they occur within the context of human-altered
landscapes. Where wetland functions are significant because they ameliorate
negative consequences of human activity, or because they support productive
human activities, they may require proximity of human populations or the
presence nearby of specific human uses, impacts, and needs.
Stormwater engineers have been experimenting with the use of artificial
wetlands for Stormwater management (e.g., Schueler 1992). However, in
addition to control of Stormwater quantity, Stormwater wetlands provide
improved water quality through trapping and biological processing of
sediments, nutrients and toxic organic compounds. These values of
Stormwater management wetlands depend upon their location within an
urban or suburban context; most wetlands embedded in more natural
landscapes can not provide similar functions. Without excess Stormwater
generated by impervious surfaces like parking lots, roads and rooftops/there
can be no benefits from Stormwater attenuation. Without elevated
concentrations of pollutants in Stormwater, a wetland's ability to trap or
eliminate pollutants has little value.
While natural wetlands are sometimes geomprphically unstable in the
flashy stream systems of urban landscapes, where they do persist (including in
many suburban landscapes), natural wetlands can provide similar Stormwater
treatment and water quality benefits to those provided by engineered
wetlands. Thus off-site wetland mitigation, if improperly handled, runs the
risk of trading a wetland that provides valuable services ameliorating the
consequences of suburbanization for one that simply because of its location,
can not. . .
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Value Depends on,Proximity to Humans—Forest-Interior Birds
Functional Comparisons. From a purely functional perspective/the quality of
habitat for forest-interior birds, and especially neotropical migrants, provided
by a forested wetland will be greatest if that forested wetland is located far •; .
from urban areas in a largely rural, non-agricultural landscape. On an acre-
. by-acre basis, therefore, a forested wetland in an urban.setting is likely.to
support fewer birds and harbor fewer birds species than a similar-wetland in a
less altered landscape; forested wetlands in suburban and agricultural /
landscapes are likely to be intermediate in habitat quality.
The differences in habitat quality are a consequence of three properties of
the changing landscapes across the urban-rural gradient. First, contiguous
forest patches (of which a forested wetland would be part) are likely to be
small in an urban landscape, intermediate in agricultural and suburban
landscapes, and largest in non-agricultural rural areas. Second, the lands
surrounding forest patches in an urban landscape provide few resources for -•
forest interior birds. In a suburban or rural landscapes, in contrast,
surrounding land includes a mix of land uses, including forest patches, lightly
•forested suburban areas and agricultural lands, some of which may provide
resources valuable to forest interior species. Finally, extant forested areas in
urban and suburban landscapes tend to be fragmented by roads, power lines,
and other linear structures, while forest patches in agricultural and non-
agricultural rural areas tend to be less fragmented. Each of these changes tends
to make urban wetland forest less likely to support a robust population of
forest interior birds than wetland forests in other landscapes.
Value Comparison. The value of a forested wetland as habitat for forest
interior birds, unlike purely biophysical measures of wetland performance, is
sensitive not only to habitat quality, but also to the way humans interact with
and perceive that quality. Undoubtedly, individuals vary widely in whether
and how they appreciate (value) forest interior birds. Some may be content
merely to know that rare warblers continue to .exist. Others may value the
experience of hearing the songs of warblers and other birds, while some
serious birders may enjoy the opportunity to-add an unusual species to their
life-list. To still others, the wetland, and its birds may provide, an outdoor
classroom or laboratory in which to learn and through which to enrich the
enjoyment of life for themselves and others.
Many of the diverse ways that people appreciate wetland forest birds
depend on physical proximity of the person to the bird. One can not enjoy the
sound of bird song if one is too far away to'hear it. Moreover, the benefits to
one person hearing a bird singing are little affected by the fact that others have
seen or heard the bird that same day (bird song is a non-consumptive good).
Thus the total, society-wide benefits of wetland forest bird habitat generally
increases as more people are able to share in them—at least.until the presence
of many humans deteriorates environmental systems that support the birds,
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or destroys the broader aesthetic experiences of which hearing bird song is a
part.
In addition, as the abundance of suitable habitat for forest interior birds
in the nearby landscape increases, the value of adding another hectare of
suitable habitat (the marginal value of habitat) is likely to decline. In general,
habitat for warblers and other forest interior birds is relatively scarce in most
urban landscapes and relatively abundant in rural areas. Thus construction
of a forested wetland in a rural landscape whose forested wetlands are already
abundant is likely to provide fewer values than construction of a forested
wetland that provides similar habitat quality in an urban or suburban
landscape.
Thus there is an important tradeoff between habitat quality which often
increases from urban to rural landscapes, and the societal value of habitat,
which may decrease along the same landscape gradient. Whether a full
assessment of this tradeoff would favor preservation or construction of
wetlands close to or far from urban areas will differ from case to case, but is
clearly important in evaluating off-site mitigation. Critical tradeoffs can only
be evaluated by carefully considering specific wetland management goals, the
ecology of wetland systems and the way humans perceive and value
wetlands.
Analytic Tools
The evaluation of off-site wetland mitigation alternatives is a complex
process. As the preceding examples show, the evaluation requires regulators
to weigh a wide variety of ecological, social, and economic factors in order to
assess whether or not a particular mitigation proposal should be accepted.
We offer here two simple analytic tools that may be useful to wetland
managers and others when considering on-site and off-site mitigation
alternatives.
j
Functions and Values Across a Landscape Gradient
The functions and values provided by a created or restored wetland are
both likely to change in. predictable ways as a proposed mitigation site changes
from an urban setting to a suburban or rural landscape. Values, however, do
not necessarily change by the same magnitude or even in the same direction
as functions. Moreover, each wetland function and each wetland value may
exhibit different patterns of change across the landscape gradient. A simple
graphical framework for understanding these patterns on a qualitative basis is
presented in figure 3. ,
Functions. As one moves from urban centers through suburban areas to
rural landscapes, the functions of an acre of wetland change. In general the
overall quality of a wetland constructed for mitigation purposes is likely to
increase. Specific wetland functions, however, may increase, decrease, or
change in some more complex way. Some wetland functions (e.g. flood
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storage capacity) are determined primarily by properties of the wetland itself,
and show little change with landscape context* Other functions are controlled
by dynamic relationships between 'the wetland and its surroundings and
show substantial changes.in response to changing landscape context.
Habitat functions and sediment trapping functions demonstrate
contrasting ways that wetland functions may vary according to the condition
of the surrounding landscape./Most habitat functions of a created or restored
•wetland can be expected to increase as one moves further from1 urban centers,
and the proximity of the wetland to and average quality of adjacent habitat
patches improves. Thus habitat functions typically will be inversely related to
the level of urbanization of the surrounding landscape.
The degree to which a wetland traps sediment, on the other hand,
depends on both wetland hydrology and the influx of sediments from the
surrounding landscape. Without sediment influx induced by nearby land use
practices, the wetland may have the capacity, but notrthe opportunity, to trap
sediment. The clearing of land for" road construction and development of ,
suburban subdivisions is a major source of sediments to aquatic
environments in the Chesapeake watershed. Conventional agricultural
practices, in which soil bare of crops or crop residues may be exposed for
'extended periods provides another important source. Thus the opportunity
to trap sediments is likely to be greatest in agricultural and suburban
landscapes, intermediate in urban landscapes, and lowest in extra-agricultural
rural landscapes. ,-.--.'
Values. Even in the absence of functional differences, wetlands \,
embedded in different landscape contexts can generate substantially different
values and, perhaps more importantly, provide those values to different
segments of society. First, the marginal value of a particular wetland function
(the value of a small change in the level of the function) may be expected to
decline as one moves from landscapes where wetlands and healthy .-
environmental assets are relatively scarce to landscapes where they are
relatively abundant. Construction of a forested flood plain wetland for ;
mitigation along a stream that already has substantial forested flood plain, for
example, may be of little value to society; if the stream in question is already
healthy, water quality and ecosystem support services provided by the extra
wetland area may be of little consequence. Construction of a similar wetland
in a suburban area, in contrast, may offer new recreational opportunities, and
make a significant difference, in the health of adjacent stream systems.
The value of certain wetland functions are also affected by the wetland's
proximity.and accessibility to human populations, and thus by the
distribution of populations, roads, and other transportation' facilities
surrounding the wetland. Values associated with consumptive and non-
consumptive uses of wetlands such as,hunting or bird watching drop off
quickly as the distance from the wetland increases. Non-use, values associated
with the continued existence of a rare animal or plant species, on the other
hand, are less dependent on access or proximity and within certain limits
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r
drop off only slightly with distance. Off-site values and non-use values such
as flood protection, water quality benefits and general ecosystem support fall
somewhere in between, with values depending on proximity to ecological
systems linked to the wetland, but not necessarily to the wetland itself.
These kinds of relationships can be depicted as shown in Figure 3b where
the overall value of a hypothetical wetland function is shown to depend on
the per capita marginal value of the function and also on the size of the
population that receives the resulting value. By considering, at several
locations along an urban-rural landscape gradient, (1) the scarcity value of
specific wetland functions of management interest and (2) the number and
kinds of people who will receive value at various locations, it should be
possible to get a clearer idea of how wetland values change across the
landscape gradient, and thus make .better wetland mitigation decisions.
If one is concerned only with the number of song birds in the region, it
may make sense to favor mitigation sites in rural areas where land costs are
low, the surrounding landscape is relatively healthy, and mitigation success is
likely. On the other hand if one is concerned about how many people have
the opportunity to hear song birds or the consequences for society at large of
the existence of urban populations who have no regard for song birds there
may be reason to favor more urban mitigation sites despite the higher costs
and more exacting construction requirements. ,
Wetland Values and Functions at Multiple Scales
A variety of ecological phenomena, especially at the ecosystem and
landscape levels can be understood as consequences of dynamic systems
operating at a multiple interdependent spatial and temporal scales (O'Neill et
al. 1986). Considerable insight into wetland ecology, management, and
politics can be 'gained by consideration of the scales over which wetland
functions and values occur and are managed. Understanding the multiple
scales of wetland management may be especially important in the context of
off-site wetland mitigation and mitigation banking.
Figure 4 presents a framework for analyzing wetland management
efforts in terms of the scales of strong functional dependence of natural
processes, and the scales over which the values of wetlands are perceived.
The diagram is based on three observations: (1) Some, but not all wetland
functions are dependent on phenomena that occur far from the wetland. (2)
Certain wetland values accrue primarily within the wetland, to wetland
owners and others with direct physical access to the wetland, while other
wetland values accrue at least in part to people far from the wetland itself.
And (3) the scales of functional dependence (which determines effective
scales of management), and the scales over which benefits accrue (which
provides incentive for management) need not be the same.
Functional Dependence. Wetland functions, (and thus the values that
derive from them), depend on the physical and biological environment of the
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wetland. However, the scale over which that dependence is significant for
management purposes differs among'wetland functions. Thus production of
timber in forested wetlands depends loosely, if at all on conditions outside of
the wetland itself, and timber production can be managed on a wetland by
• wetland or even hectare by hectare basis with little need to consider
conditions elsewhere. Waterfowl populations in the, Chesapeake region, in
contrast, can not be effectively managed without consideration of conditions
hundreds and even thousands of miles away along migration routes, and in
breeding areas. -,
- For any wetland function, one can identify a range of spatial scales over
which actions may be necessary for effective management. For most wetland
functions, the smallest scale of management is on the order of a few meters or
tens of meters, but the largest scale that needs to be considered varies widely.
Wetland functions near -the top of figure 4 will be difficult to manage without
considering larger scale (regional, .continental) phenomena. Wetland
functions near the bottom of the figure can be managed primarily through
local action.
Scales Over Which Values Accrue. Values derived from specific wetland
functions also accrue at certain relatively well defined spatial scales. The
value of a tidal wetland for producing salt marsh hay, for example, accrues
directly to the landowner or farmer. The benefits of waterfowl hunting
accrue to those with physical access to the wetland. In contrast, benefits of
flood attenuation by flood plain wetlands typically do not accrue to the
landowner, but to those living some distance downstream. And benefits of
the "water quality functions of wetlands are often realized in lakes,
impoundments and estuaries like Chesapeake Bay that may be hundreds of
miles away. The benefits of certain wetland functions (e.g., carbon '
sequestration, biodiversity) may even accrue over global scales. Figure 4
displays some of these differences in the spatial scale over which wetland
values accrue. Wetland'values shown toward the'.right hand side of the
figure are those that may be expected to accrue a substantial distance from the
wetland itself; wetland values toward the left accrue primarily in or near the
wetland itself. ' •
Implications. The scales over which the benefits of specific wetland
functions accrue, and the scales over which those same wetland functions can
be managed do not necessarily match. Where the two scales do not match,
wetland managers with the strongest political or financial incentives to
protect wetland functions may be unable to do so. At one extreme, one may
consider a globally significant wetland function—carbon metabolism. Carbon
sequestration and production of methane are wetland.functions that depend
on anaerobic conditions within wetland soils. These conditions are hardly '
influenced by properties of the landscape outside of the wetland, except
insofar as those conditions alter properties of the wetland, yet the effects of
wetland carbon metabolism on atmospheric 'chemistry have global
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January 24,1996 'DRAFT ! . Page 13
consequences. Landowners, who are best placed to manage wetlands as
carbon sinks, have little incentive to do so, while the incentive to manage for
carbon uptake and storage is felt most strongly ait national and international
levels, where appropriate management tools short of coercive measures are
few.
Scale-explicit analysis may also highlight some of the most significant
and most often neglected consequences of off-site wetland mitigation and
mitigation banking. A decision to accept off-site wetland mitigation is a
decision to accept a trade of wetland functions and values between locations.
Successful off-site wetland mitigation may have little effect on values that
accrue primarily on the scale of the wetland. Production of timber, for
example, will simply have moved from one location to another, but need not
be diminished overall. Similarly, off-site mitigation is unlikely to have
much effect on wetland values that accrue over spatial scales larger than the
distance between the original and mitigation wetlands. If microbial ecology at
the two sites is similar, the global carbon balance will not be altered
appreciably if a wetland is lost near Annapolis, and replaced twenty miles
away in southern Anne Arundel County, Maryland. Off-site mitigation will,
however, affect wetland values that accrue outside of the wetland itself, but
on a spatial scale similar to or smaller than the distance between the original
and mitigation wetland: Such values are most likely to include benefits to
local and regional populations of humans, fish, and wildlife, and benefits that
accrue to downstream ecosystems and human communities, including value
of wetlands as components of a regional habitat mosaics, energetic and
functional linkages between wetlands and adjacent stream and estuarine
ecosystems, water quality benefits, protection from flooding and storm .
damage, and other primarily regional wetland benefits.
Conclusions
Wetland mitigation efforts and wetland mitigation-policies have been
based, to a large extent on site-by-site comparisons of wetland functions and
values. While such a local approach is sufficient when off-site wetland
mitigation is rare, it does not provide the necessary perspective to address the
complex issues raised by widespread adoption of off-site mitigation, or by the
proliferation of mitigation banking programs. Both trends require the
development of conceptual frameworks and management tools that explicitly
address the spatial structure of wetlands, watersheds, and landscapes from
ecological and economic perspectives.
Even relatively simple analytic and conceptual tools can help clarify the
trade offs wetland managers should consider when evaluating off-site
wetland mitigation alternatives. Consideration of how wetland functions
and values are likely to vary in different landscape contexts, and
consideration of the scales'over .which wetland functions develop, and over
which wetland values accrue is crucial for evaluation of wetland policy
within a landscape or watershed perspective. Analytic tools developed for
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January 24, 1996
DRAFT
Page 14
this purpose
and values atfe
and in determinin
rules to be us
:an help wetland managers recognize which wetland functions
^ most at risk whenjconsidering off-site mitigation alternatives,
ling of the size and location of trading territories and trading
id by regional; mitigation banks. "
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January 24,1996 . DRAFT- .; Page 15
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~r
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i
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Figure
Tracing Economic Significance of Wetland
Sediment Trapping Function
F
Species Abundance
& Diversity
"*"
Decreased Biological
Diversity
Loss of Wetland .
Function
Increase in Sediment
Entering Water Body
-J
— 1
^-
Lower Commercial
& Recreational
Fish Catches.
Higher Fishing Costs
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Metapopulations
In Fluvial Ecosystems
Separated
Metapopulatlons\
Dagradod River-
Reach
Isolated Downstream
Population
Figure 2
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Wet/and Functions and Values
Accross a Landscape Gradient
High
Low
High B
Low
Ecological Function
Societal Values
Urban Suburban Agricultural Other Rural
Location of Wetland
Figure 3
-------
Geographic Scales of Wetland
Benefits and Management
Figure 4
1 y
| 100Km,
c
<§
^ 1Km,
0
"ra
<5? 10m,
Habitat for Forest Biodiversity
^ Interior Birds /
Production of
Wild Trout Anadromous Fishes
Production /
/
Timber 'Education
Production N . . .
/ "4-- SSL
10m 1 Km 100Km 10,OOOKm
Scale of Derived Benefits
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