396
eutrophication have been more pervasive and persistent than previously assumed (e.g., Smol et aI.,
1983; Reavie et aI., 1995), but that even small shifts
in trophic status might have substantial but complex effects on fish assemblage organization (e.g.,
Plante and Downing, 1993; Evans et aI., 1996;
Kelso et aI., 1996; Steedman et aI., 1996). Over periods of decades, such changes in fish assemblages
could subsequently feed back to either hasten or
slow eutrophication processes initially driven by
changes in nutrient or carbon loading.
Because these influences have not been widely
recognized, the long-term vulnerability of lakes to
human activities (and other causes of landscape
change or species dispersal) has likely been seriously underestimated in the past. The potential regional, as well as local, influences of human alteration of landscapes should be carefully considered
in any assessment of lake ecosystems. Across many
regions where recent human influences are pervasive, most or all lake communities may be altered
from their historical condition due to both externally driven eutrophication and changes in fish assemblages and trophic structure. In such circumstances, location of unaltered reference systems
may be difficult or impossible; historical analysis
from lake bed cores may be the only remaining evidence of such pervasive changes. In core records,
temporal changes in fish and many invertebate taxa
usually must be inferred from evidence of general
trophic status provided by the few diatom and invertebrate taxa that are consistently preserved as
fossils. From a conservation point of view, except
in regions that have not seen extensive human activity (such as much of arctic North America), any
lakes that have escaped the ecological changes
commonly wrought by introduced taxa should be
recognized as potentially having exceedingly high
value as reservoirs of biological diversity and as
examples of naturally functioning ecosystems. Unfortunately, we know of no published, comprehensive regional conservation assessment for lake
ecosystems that can be recommended as an example or prototype.
26.4 Assessing Conservation
Status of Freshwater Biota
Recently, scientists have begun to document the advanced global extent of biotic deterioration and impoverishment of freshwater ecosystems. Allan and
Flecker (1993) review evidence of the declining
status of aquatic biota worldwide and discuss the
Assessment of Biotic Patterns in Freshwater Ecosystems
varied causes of such declines, which include the
loss, deterioration, and fragmentation of habitat; introduction and invasion of nonnative species; overexploitation; and effects of artificial propagation.
Regional assessments of fish (Williams et aI., 1989;
Nehlsen et aI., 1991), river mussels (Williams et
aI., 1993), and other taxa (Master, 1990) have
drawn similar conclusions about the multifarious
causes of decline and the apparent overriding importance of habitat alteration. These regional status assessments have consisted largely of qualitative assessments of species status based on
synthesis of expert judgment of the available, typically fragmentary data on distribution, population
trends, and threats. Typical data sources include
distribution and occurrence data from state Natural
Heritage Program databases, other state and federal
surveys, population trend data gained from agency
monitoring efforts or inferred indirectly from
sources such as catch in fisheries, site-specific information compiled from research studies conducted at scattered sites, and qualitative information about the distribution of threats from many
sources. These assessments are at best conservative
in documenting biotic declines, because taxa, regions, and sites for which no information is available are usually not listed. Some assessments have
attempted to correct for missing biases through
mapping protocols based on explicit rules for interpolation from known sites (e.g., Frissell, 1993).
However, although it is useful to map regional patterns of biotic decline, in regional assessments the
absence of information for a given site should not
automatically be assumed to mean that its conservation values are low.
To display conservation status data in a spatially
explicit way, Frissell (1993) translated information
from several relevant reviews and recent studies of
fish conservation status along the Pacific Coast into
a regional map of fish endangerment. When the
numbers of taxa known to be declining, depressed,
or already extinct in each watershed were mapped,
zones of high endangerment were associated with
urban areas and concentrations of dams and diversions. However, a background pattern was also apparent across most of the region, in which two to
three or more fish taxa were imperiled or extinct,
indicating that fish endangerment is a problem even
in basins lacking dams, diversions, or concentrated
human populations. Fish populations in these areas
may be affected by the habitat-altering effects of
pervasive land-use practices (e.g., logging, grazing,
mining, and cropland agriculture). Frissell (1993)
also presented examples of range-wide status maps
for selected species, showing that several species
eutrophication have been more pervasive and persistent than previously assumed (e.g., Smol et aI.,
1983; Reavie et aI., 1995), but that even small shifts
in trophic status might have substantial but complex effects on fish assemblage organization (e.g.,
Plante and Downing, 1993; Evans et aI., 1996;
Kelso et aI., 1996; Steedman et aI., 1996). Over periods of decades, such changes in fish assemblages
could subsequently feed back to either hasten or
slow eutrophication processes initially driven by
changes in nutrient or carbon loading.
Because these influences have not been widely
recognized, the long-term vulnerability of lakes to
human activities (and other causes of landscape
change or species dispersal) has likely been seriously underestimated in the past. The potential regional, as well as local, influences of human alteration of landscapes should be carefully considered
in any assessment of lake ecosystems. Across many
regions where recent human influences are pervasive, most or all lake communities may be altered
from their historical condition due to both externally driven eutrophication and changes in fish assemblages and trophic structure. In such circumstances, location of unaltered reference systems
may be difficult or impossible; historical analysis
from lake bed cores may be the only remaining evidence of such pervasive changes. In core records,
temporal changes in fish and many invertebate taxa
usually must be inferred from evidence of general
trophic status provided by the few diatom and invertebrate taxa that are consistently preserved as
fossils. From a conservation point of view, except
in regions that have not seen extensive human activity (such as much of arctic North America), any
lakes that have escaped the ecological changes
commonly wrought by introduced taxa should be
recognized as potentially having exceedingly high
value as reservoirs of biological diversity and as
examples of naturally functioning ecosystems. Unfortunately, we know of no published, comprehensive regional conservation assessment for lake
ecosystems that can be recommended as an example or prototype.
26.4 Assessing Conservation
Status of Freshwater Biota
Recently, scientists have begun to document the advanced global extent of biotic deterioration and impoverishment of freshwater ecosystems. Allan and
Flecker (1993) review evidence of the declining
status of aquatic biota worldwide and discuss the
Assessment of Biotic Patterns in Freshwater Ecosystems
varied causes of such declines, which include the
loss, deterioration, and fragmentation of habitat; introduction and invasion of nonnative species; overexploitation; and effects of artificial propagation.
Regional assessments of fish (Williams et aI., 1989;
Nehlsen et aI., 1991), river mussels (Williams et
aI., 1993), and other taxa (Master, 1990) have
drawn similar conclusions about the multifarious
causes of decline and the apparent overriding importance of habitat alteration. These regional status assessments have consisted largely of qualitative assessments of species status based on
synthesis of expert judgment of the available, typically fragmentary data on distribution, population
trends, and threats. Typical data sources include
distribution and occurrence data from state Natural
Heritage Program databases, other state and federal
surveys, population trend data gained from agency
monitoring efforts or inferred indirectly from
sources such as catch in fisheries, site-specific information compiled from research studies conducted at scattered sites, and qualitative information about the distribution of threats from many
sources. These assessments are at best conservative
in documenting biotic declines, because taxa, regions, and sites for which no information is available are usually not listed. Some assessments have
attempted to correct for missing biases through
mapping protocols based on explicit rules for interpolation from known sites (e.g., Frissell, 1993).
However, although it is useful to map regional patterns of biotic decline, in regional assessments the
absence of information for a given site should not
automatically be assumed to mean that its conservation values are low.
To display conservation status data in a spatially
explicit way, Frissell (1993) translated information
from several relevant reviews and recent studies of
fish conservation status along the Pacific Coast into
a regional map of fish endangerment. When the
numbers of taxa known to be declining, depressed,
or already extinct in each watershed were mapped,
zones of high endangerment were associated with
urban areas and concentrations of dams and diversions. However, a background pattern was also apparent across most of the region, in which two to
three or more fish taxa were imperiled or extinct,
indicating that fish endangerment is a problem even
in basins lacking dams, diversions, or concentrated
human populations. Fish populations in these areas
may be affected by the habitat-altering effects of
pervasive land-use practices (e.g., logging, grazing,
mining, and cropland agriculture). Frissell (1993)
also presented examples of range-wide status maps
for selected species, showing that several species