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Water: Monitoring & Assessment

6. Birds as Indicators of Prairie Wetland Integrity

Bioindicators for Assessing Ecological Integrity of Prairie Wetlands
Report # EPA/ 600/ R-96/ 082
September 1995

Contents:

6.1 Ecological Significance
6.2 Potential Indicator Metrics
6.3 Previous and Ongoing Monitoring
6.4 Response to Stressors

6.4.1 Birds as Indicators of Hydrologic Factors
6.4.2 Birds as Indicators of Changes in Vegetative Cover
6.4.3 Birds as Indicators of Wetland Salinity
6.4.4 Birds as Indicators of Sedimentation and Turbidity
6.4.5 Birds as Indicators of Excessive Nutrient Loads and Anoxia
6.4.6 Birds as Indicators of Pesticide and Heavy Metal Contamination

6.5 Monitoring Techniques

6.5.1 General Surveys
6.5.2 Reproductive Success
6.5.3 Time Budget Analysis
6.5.4 Bioassay Methods
6.5.5 Bioaccumulation

6.6 Variability and Reference Points

6.6.1 Spatial Variability
6.6.2 Temporal Variability

6.7 Collection of Ancillary Data
6.8 Sampling Design and Required Level of Sampling Effort

6.8.1 General Considerations
6.8.2 Asymptotic Richness: Results of Analysis
6.8.3 Power of Detection: Results of Analysis

6.9 Summary


6.1 Ecological Significance

Birds are an obvious feature of prairie wetlands during the growing season. Birds inhabiting prairie potholes include waterfowl, shorebirds, large wading birds, and songbirds. The ecological significance of birds in prairie wetlands stems from at least two characteristics:

  • They are highly mobile, moving frequently among as many as 20 prairie potholes during a growing season, and between prairie potholes and wetlands in other regions during migrations. In the course of these movements, they often passively carry with them various invertebrates and seeds, which subsequently become established in new areas.
  • Their movements and feeding within a wetland can alter vegetation structure (especially submersed plants), invertebrate densities, and the mixing of sediments (which in turn can affect wetland fertility).

The usefulness of birds as indicators of ecosystem integrity has been widely discussed (e.g., Morrison 1986, Reichholf 1976, Temple and Wiens 1989). Specific factors that make birds attractive as indicators of wetland integrity include:

  • Ease of monitoring (usually no samples to process); identification is simple and capable non-scientists are often willing to assist with surveys.
  • Availablity of established survey protocols
  • Tendency of some species (e.g., many raptors and wading birds) to accumulate toxic substances because of their position at the end of food chains.
  • Longer life span than other bioindicators; this may make them more sensitive to some cumulative impacts and more able than other groups to integrate the effects of episodic events.
  • Usefulness for in situ assessments (confined or behaviorally imprinted individuals).
  • Availability of the only relatively extensive nationwide data bases on trends, habitat needs, distribution.
  • Availability of moderately extensive bioassay data bases.

Characteristics that are disadvantageous to using birds as indicators of wetland integrity include:

  • They are essentially absent from most prairie wetlands in winter.
  • Their mobility makes it difficult to locate site-specific causes of mortality (could be factors that operate thousands of miles away).
  • Their mobility makes it difficult to assume that wetlands used by birds also support other organisms, because birds may only be resting, rather than feeding on these wetlands.
  • No opportunities exist for routine analysis of decay-resistant remains (as with diatoms and some invertebrates), which otherwise could provide a means for establishing historical reference conditions in a wetland.
  • Some individuals present in the breeding season do not breed, and their presence in a wetland then denotes little about the condition of the species' population.
  • Although survey protocols are well-established, they contain many nonsystematic biases, e.g., some species are much easier to detect than others.
  • In general, bird community structure is highly controlled by physical habitat, predation, and perhaps mortality as a result of being hunted by humans, rather than by contaminants.

In summary, whereas birds are likely to be poor indicators of the integrity of a specific wetland, their trends in species composition and relative abundance when measured throughout a region can integrate changes occurring in wetlands across the region. Given the current availability of data and tested protocols, they are the only taxonomic group capable of serving this purpose.

6.2 Potential Indicator Metrics

The following measurements and metrics deserve consideration, as applied to bird communities, for use in characterizing conditions in reference wetlands, identifying the relative degree of past disturbance to a prairie wetland complex, or assessing the current inhibition of key processes:

  • Richness of species and functional groups (per unit area, or per number of randomly-chosen individuals).
  • Number of individuals per unit area, by season.
  • Relative dominance and richness of species that are characteristically associated with a particular habitat condition (e.g., grazing-sensitive species).
  • Reproductive success, including the following (see Sheehan et al. 1987 for definitions):
    • numbers of breeding pairs
    • nest density
    • clutch size
    • nest success
    • hatch success
    • number of broods produced
    • brood size at fledging
    • broods per pair
    • recruitment
  • Daily duration of specific activities (e.g., feeding, roosting) in the wetland complex, i.e., time budget analysis.
  • Interannual variability in richness, density, and reproductive success.
  • Bioaccumulation.

The specific ways some of these metrics have been or could be interpreted as an indication of stressed conditions are described in Section 6.4.

6.3 Previous and Ongoing Monitoring

Of the more than 80 publications describing field studies of birds in prairie wetlands, only 14 (18%) involved species other than waterfowl. The parameters most commonly measured in waterfowl studies are the frequency of nests and broods. An impressive 16 surveys covered more than 100 wetlands, but 15 studies were based on only a single year's data.

Few studies have systematically surveyed non-waterfowl species outside the breeding period. Collection of data on habitat use by migrant shorebirds in particular has been limited (Eldridge 1992, Eldridge and Krapu 1993), despite the fact that available evidence suggests that prairie wetlands are used extensively. In some instances, counts of shorebirds in the northern prairies exceed those known from any other location on the Central Flyway of North America (G. Krapu, personal communication, NPSC, Jamestown, ND). Compared to other mid-continent populations, populations of migratory shorebirds that occur in Dakota wetlands contain a higher proportion (55%) of long-distance migrants which depend most heavily on wetlands to replenish their energy supplies during migration (Skagen and Knopf 1993).

No state agencies are currently monitoring birds for the specific purpose of using the data to estimate the ecological integrity of prairie wetlands. At a regional level, USEPA's EMAP investigated the use of estimates of four breeding waterfowl species as indicators of landscape quality. Other ongoing regional efforts (Appendix K) include (a) the USFWS's annual breeding waterfowl surveys (18-mile long aerial and ground transect surveys), (b) the USFWS's annual Breeding Bird Survey (25-mile long roadside transects; an average of 69 routes have been run in U.S. and Canadian parts of the prairie region, and contain an average of 14 years of data); and (c) Breeding Bird Censuses (plot-based intensive surveys). At more localized levels, birds are being tested for possible use as indicators of the success of (a) wetland restoration efforts in Iowa (Dinsmore et al. 1993, Zenner and LaGrange 1993), and (b) cover management practices of the Conservation Reserve Program. Research on ecological relationships affecting waterfowl in particular continues to be conducted by NPSC and universities.

6.4 Response to Stressors

6.4.1 Birds as Indicators of Hydrologic Stressors

Species Composition

Birds are affected both directly and indirectly by hydrologic changes. The assemblage of breeding birds that have established territories in a particular prairie wetland can generally indicate the present water depths of the wetland. For example, the regular presence of western grebes and certain diving ducks can indicate relatively deep water (> 2 m) and consequently, the likely seasonal persistence of water in an individual wetland. Much of the information on depth requirements is summarized by Fredrickson and Taylor (1982), Fredrickson and Reid (1986), and Short (1989). Species that are likely to be the most sensitive indicators of water levels might be those that (a) nest along water edges, (b) feed on mudflats (e.g., shorebirds), (c) require a particular combination of wetland hydroperiod types in a region (e.g., Kantrud and Stewart 1984). In contrast, species (e.g., marsh wren, some diving ducks) that characteristically nest well above the water level might be less directly vulnerable, and thus are probably weaker indicators.

Only when data are combined at a regional level is it likely that trends in bird community composition will reflect trends in the hydrologic integrity of wetlands overall. Species composition of the bird community in a single prairie wetland is a poor indicator of past hydrologic stresses to that particular wetland, because most birds can move freely among wetlands and among regions (although this can reduce reproductive success). As documented by radiotelemetric and modeling studies, many species appear to require wetland complexes (a particular combination of wetland hydrologic types at a particular density on the landscape or in close proximity to each other; Cowardin 1969, Kantrud and Stewart 1984, Weller 1975, Flake 1979, Patterson 1976, Talent et al. 1982, Rotella and Ratti 1992a,b). Years of regional drought temporarily reduce the number and perhaps the variety of wetland types, and consequently cause drastic changes in species composition for an indefinite number of years thereafter (Hammond and Johnson 1984). Interannual fluctuations in bird numbers are likely to be smaller in landscapes containing intact wetland complexes, because the complexes support a "shifting mosaic" of water depths that provides at least minimally suitable habitat regardless of regional drought or flood conditions (Skagen and Knopf 1994).

Single-species Indicators

Mallards and other waterfowl species are widely monitored throughout the prairie region. It is not apparent, however, that simple presence of a single species, its nest, and/or broods in a particular wetland is evidence of good hydrologic integrity. Mallards, for example, seem to inhabit a wide range of wetland types (as defined by hydrologic permanence). There are likely to be many situations where hydrologic conditions in wetland complexes are sufficient to support one or a few species such as mallard, but are too degraded (e.g., through drainage) to support many other species, including some plants, invertebrates, and other vertebrates that are crucial contributors to regional biodiversity because of their narrower habitat preferences.

Species Richness

At an individual wetland level, avian species richness is often greater in semipermanent and permanent wetlands than in temporary and perhaps seasonal wetlands (Weber et al. 1982, Faanes 1982). By reducing the number and perhaps the variety of wetland types, sustained regional drought diminishes species richness in many individual wetlands and wetland complexes. Among six restored prairie wetlands that were sampled in Iowa for two years, avian richness was greater during the wetter year in all but one wetland, where it did not change (Hemesath 1991). Richness in wetlands restored after being drained for > 30 years did not differ significantly from richness in wetlands drained more recently. Birds colonized formerly drained wetlands within one year of restoration; other prairie wetland studies report similar results (Sewell 1989, LaGrange and Dinsmore 1989, Zenner and LaGrange 1993).

Density and Biomass

Among waterfowl, pair density during the early summer is usually greater in temporary and seasonal wetlands that have ponded water, than in semipermanent and permanent wetlands (Krapu and Duebbert 1974, Kantrud and Stewart 1977, Ruwaldt et al. 1979). When presence/absence of ponded water is not considered, seasonal wetlands have the highest pair densities. However, later during the summer and perhaps during dry years, the more permanent wetland types support the greatest number of individuals per wetland area (Stewart and Kantrud 1971, 1973, Duebbert and Frank 1984, Talent et al. 1982). In contrast, for birds as a whole (all species combined), breeding densities are greatest in semipermanent wetlands (Faanes 1982). By reducing the number and perhaps the variety of wetland types, sustained regional drought diminishes density of birds in many individual wetlands and wetland complexes (Greenwood et al. 1995).

Reproductive Success

Reproductive success of waterfowl and undoubtedly other prairie birds is diminished during drought years (Higgins et al. 1992, Greenwood et al. 1995).

6.4.2 Birds as Indicators of Changes in Changes in vegetative cover

Species Composition

Birds in prairie wetlands respond strongly to changes in vegetation density and type, both within wetlands (Weller and Spatcher 1965, Lokemoen 1973) and in the surrounding landscape (Duebbert and Kantrud 1974, Huber and Steuter 1984). Many species, primarily waterfowl and shorebirds, benefit from (or tolerate) reduced ground cover and increased openings in dense stands of vegetation (Keith 1961; Weller and Spatcher 1965; Weller and Fredrickson 1974; Krapu et al. 1979; Kaminski and Prince 1981b, 1984; Blixt 1993; McMurl et al. 1993). For example, breeding waterfowl in four semipermanent wetlands responded positively to thinning of dense cat-tail stands for at least four years after the stands had been thinned by herbicides (Solberg and Higgins 1993a). The waterfowl also used the treated wetlands to a greater degree than they used untreated wetlands that had natural openings in the vegetation. However, effects of increased open water on waterfowl as a result of another experimental application of herbicides (Blixt 1993) were equivocal.

Several species, including sora (Fannucchi et al. 1986), other rails (Weller et al. 1991), northern harrier, short-eared owl, and ring-necked pheasant (USDA Soil Conservation Service 1985, Homan et al. 1993) do not necessarily benefit from reduced cover density. One North Dakota study that used herbicides to reduce vegetation cover found a reduction in densities of marsh wren, red-winged blackbird, and common yellowthroat, up to two years after application (Linz et al. 1993, Blixt 1993). A Minnesota study found no positive correlation between cover ratio and numbers of yellow-headed blackbird, song sparrow, or sora (Olson 1992). Limited surveys of restored wetlands in Iowa seldom found certain species that occurred only in natural (vs. restored) wetlands -- least bittern, American bittern, sora, Virginia rail -- or more abundantly -- common yellowthroat, red-winged blackbird, swamp sparrow (Delphey and Dinsmore 1993, Dinsmore et al. 1993). Among waterfowl species, the northern pintail and northern shoveler appear to tolerate or benefit from partial removal of cover in surrounding landscapes (e.g., from grazing) to a greater degree than do teal, gadwall, and American wigeon (Stewart and Kantrud 1973).

Impacts of cover removal might be most evident among (a) species that nest in uplands, (b) species with relatively large territories, and/or (c) species that nest early in the growing season, before there is appreciable new growth by crops or pasture grasses (Batt et al. 1989). Species in prairie wetlands that appear to benefit from light-intensity grazing (or mowing during the prior autumn) include Wilson's phalarope, common yellowthroat, and red-winged blackbird; species most sensitive to heavy grazing include LeConte's sparrow and sedge wren (Kantrud 1981). At a regional level, changes in the frequency or range sizes of the species cited above (and others) might indicate changes in the overall condition of vegetative cover. Literature on bird response to vegetation removal in wetlands is summarized by Skovlin (1984) and Kantrud (1986a). Since Kantrud's 1986 synopsis was published, an additional 15 research studies on the topic have been published (Appendix J). Based on the literature, Short (1989) categorized 88 species that breed in the prairie region according to the vegetative cover types they prefer for nesting and foraging.

Single-species Indicators

It is likely that conditions of vegetative cover that are suitable for nesting mallards and other waterfowl are suitable for sustaining relative high levels of avian richness generally. Nonetheless, cover needs vary among waterfowl species, and there are some bird species (e.g., piping plover) that do not generally occur in wetlands that are optimal for waterfowl. Definitions and measurements of wetland integrity must be broad enough to account for needs of such species.

Species Richness

High species richness within prairie wetlands typically occurs where there is a mix of vegetation types, and/or a mixture of about 30-50% open water with 50-70% vegetation (Weller and Spatcher 1965, Weller and Fredrickson 1973, Hemesath 1991, Kaminski and Prince 1981b, 1984, Olson 1992). However, avian richness in prairie wetlands cannot always be predicted by vegetation structural diversity (Olson 1992).

As dense stands of vegetation are thinned, the diversity of bird species using a wetland typically increases or remains stable (Blixt et al. 1993), especially if open water begins to occupy spaces cleared in the vegetation (Harris et al. 1983, Kaminski and Prince 1981b, 1984). Thus, "moderate" levels of grazing, herbicide application, mowing, and/or tillage, if occurring at a time of year that does not disturb nests, either have no effect (Kaminski and Prince 1981a,b, 1984) or increase wetland bird species richness (Kantrud 1981). However, severe grazing, mowing, fire, or herbicide application at inappropriate times is detrimental to waterfowl (Kantrud and Stewart 1984, Higgins 1977, Higgins et al. 1992). Also, wetlands that are tilled during the breeding season tend to support fewer non-waterfowl wetland bird species than do untilled wetlands (Weber et al. 1982). Ongoing studies of CRP lands by the NPSC are intended to determine relationships of avian richness to patch sizes of unfarmed land. The sizes, types, and distribution of wetlands on the study plots are being recorded incidentally.

Density and Biomass

Field data show that waterbird density tends to be greatest in prairie wetlands with the most even balance between open water and vegetation (Stewart and Kantrud 1971, Weller and Spatcher 1965, Weller and Fredrickson 1973, Kaminski and Prince 1981a,b, 1984, McMurl et al. 1993). In an Iowa lakeside marsh, open water patches that were 0.01 ha in size (about 10 x 10 m) were little-used by waterbirds, but patches > 0.02 ha were used by several species, especially if they exceeded 100 m in their longest dimension (Weller 1975). Between years, changes in cover density may also mediate the response of breeding waterfowl to limnological factors (Lillie and Evrard 1994).

Waterfowl pair densities in tilled wetlands (especially wetlands with little crop debris) are only 20% of densities in untilled wetlands (Stewart and Kantrud 1977), and are lower than in grazed wetlands (Barker et al. 1990). However, Kantrud (1981) found the total number of individual birds (of all species) to increase with grazing intensity in North Dakota. In South Dakota, Bue et al. (1952) found virtually no duck nests in areas grazed more than 15 cattle-days per acre per year.

Reproductive Success

Many studies have shown that reduced reproductive success in waterfowl can be a strong indicator of loss of cover in a wetland or surrounding landscape due to grazing, herbicides, cultivation, or other factors (Duebbert and Frank 1984, Higgins 1977, Dwernychuk and Boag 1973). Analysis of data from Canadian prairie wetlands indicates that waterfowl populations might decrease once cropland occupies more than 56% of a landscape (a rectangular 25.6-km2 area), and average nest success might decrease four percentage points for every 10 percentage points increase in cropland (Greenwood et al. 1995).

6.4.3 Birds as Indicators of Wetland Salinity

Species Composition

Many waterfowl avoid hypersaline or alkali prairie wetlands unless freshwater wetlands are located nearby (Kantrud and Stewart 1977, Lokemoen and Woodward 1992). However, a few other waterbird species occur regularly in alkali wetlands during the breeding season (e.g., American avocet, phalaropes, killdeer) or migration (e.g., tundra swan; white-rumped, semipalmated, and Baird's sandpipers; see Faanes (1982), Kantrud (1986b), Eldridge and Krapu (1993), and Earnst (1994). These relatively salt-tolerant species also occur in less saline wetlands, but their abundance often is greatest in hypersaline wetlands, and is related to sharp seasonal peaks in the abundance of brine shrimp and other salt-tolerant invertebrates. Although changes in the frequency or range sizes of these species at a regional level might indicate changes in the occurrence of hypersaline wetlands, birds generally do not appear to be sensitive indicators of less extreme variations in salinity, especially at a site level.

Single-species Indicators

It is likely that saline conditions that are suitable for nesting mallards and other waterfowl are suitable for sustaining relatively high levels of biodiversity in general. Nonetheless, there are some bird species (e.g., American avocet) and many plants that do not generally occur in wetlands whose salinity is optimal for waterfowl. Definitions and measurements of wetland integrity must be broad enough to account for needs of such species.

Species Richness

Avian richness is generally low in hypersaline or alkali wetlands of the prairie region (Faanes 1982). Avian richness is probably predicted by salinity only among wetlands that are the most saline.

Density and Biomass

The density of birds nesting in saline wetlands is generally low (Faanes 1982); this is particularly true of waterfowl (Savard et al. 1994). Pair densities of breeding waterfowl in alkali (highly saline) prairie wetlands are only one-tenth the densities in fresher wetlands (Kantrud and Stewart 1977). However, densities of some migrating waterbirds can be high in saline wetlands, sometimes exceeding densities in many fresher wetlands (Kingsford and Porter 1994).

Reproductive Success

Although moderately saline wetlands can be highly productive, reproductive success of some waterfowl species is limited in highly saline wetlands if they cannot gain access to areas having fresh water. For example, mallard ducklings are generally not present (or experience reduced growth) in wetlands with salt concentrations greater than 10-20 µS/cm unless freshwater springs are present (Swanson et al. 1983, Swanson et al. 1988). In a survey of part of the Canadian prairie, 2% of the wetlands were found to be potentially too saline to support waterfowl reproduction (Leighton and Wobeser 1994).

6.4.4 Birds as Indicators of Sedimentation and Turbidity

Species Composition

Bird species (e.g., redhead) that feed on submersed plants and their associated invertebrates can be defined, and they are likely to be affected the most by turbid conditions in prairie wetlands. At a regional level, changes in the occurrence, frequency, or range sizes of such species might indicate overall trends in turbidity and sedimentation. A regression analysis In a statistical regression, Flake et al. (1977) reported that turbidity negatively influenced numbers of mallard pairs in stock ponds in western North Dakota, whereas a regression analysis in British Columbia (Savard et al. 1994) found positive correlation between wetland turbidity and dabbling duck densities.

Species Richness

Density and Biomass

Reproductive Success

Changes in avian richness, density, biomass, or condition in response to increased turbidity and sedimentation are being investigated by ongoing work sponsored by the NPSC and US USEPA.

6.4.5 Birds as Indicators of Excessive Nutrient Loads and Anoxia

Species Composition

Birds have not been documented to be influenced directly or measurably by the nutrient status of prairie wetlands, and accordingly they are probably unsuitable indicators of this stressor. Effects of nutrient enrichment are likely to be expressed as increases in density of vegetation cover or turbidity (from algal blooms), to which birds respond mostly negatively (see Sections 6.4.2 and 6.4.4). In wetlands in other regions, the abundance and/or on-site diversity of songbirds (Brightman 1976, Hanowski and Niemi 1989) and sometimes waterfowl (Belanger and Couture 1988, Piest and Sowls 1985) have tended to increase with increased abundance of aquatic invertebrates, as resulted from wetland enrichment. However, some anecdotal observations in prairie wetlands suggest that wetlands experiencing prolonged anaerobic conditions following major runoff-induced algal blooms tend to support lower densities of birds (G. Krapu, NPSC, Jamestown, ND).

Single-species Indicators

Simple presence of a single species, its nest, and/or broods in a particular wetland is insufficient evidence that the wetland is relatively enriched.

Species Richness

Migrant shorebirds and gulls often appear to concentrate at nutrient-enriched sites, e.g., wastewater lagoons, both in other regions (Campbell 1984, Fuller and Glue 1980) and in the prairies (Swanson 1977a, Maxson 1981). Thus, overall avian diversity might be greater in moderately enriched prairie wetlands than in unenriched ones. However, definitive data are lacking.

Density and Biomass

Large numbers of waterbirds congregate at all seasons in wastewater wetlands where the cover of emergent vegetation is at least partly controlled (e.g., Maxson 1981, Swanson 1977a, Brady and Giron-Pendleton 1983). This high level of use is attributable largely to the ready availability of high densities of invertebrate foods in these areas. Nonetheless, little is known of waterbird responses in prairie wetlands subject to eutrophication from fertilizer runoff.

Reproductive Success

Data are lacking to describe the effects of enrichment of prairie wetlands on waterbird reproductive success.

6.4.6 Birds as Indicators of Pesticide and Heavy Metal Contamination

Species Composition

Whereas few if any pesticides appear to be acutely toxic to waterbirds in prairie wetlands when applied as prescribed, many indirect effects, i.e., mortality of foods upon which waterbirds depend and loss of nesting cover, can be significant (Grue et al. 1986, 1988, 1989; Mineau 1987; Sheehan et al. 1987; Tome et al. 1990, 1991). Some studies from other regions (Hunter et al. 1986) and more recently in prairie wetlands (McCarthy and Henry 1993, Martin and Solomon 1990) have demonstrated indirect impacts to individual birds as a result of pesticide-induced loss of foods. Whether loss of plant and/or invertebrate foods reduces bird populations depends partly on the degree to which birds in a particular situation can avoid contaminated foods or shift (without physiologic damage) their preferences from impacted foods to non-impacted foods.

Foods chosen by birds that inhabit prairie wetlands are relatively species-specific. Thus, the exposure of wetlands to a pesticide or other contaminant that kills only a particular insect or plant species or group might be reflected by the absence or widespread decline of just those bird species that are associated with the target organism/group. For example, a local decline in populations or breeding success of American wigeon and gadwall -- ducks that rely most heavily on plant foods -- could signify impacts from herbicides. A local decline in northern shoveler could indicate impacts to nektonic invertebrates that are its primary food. Invertebrate food choices of various other prairie waterbirds are summarized from the literature in Appendix B. As noted above, however, the effects on any species of loss of a particular food will depend on the likelihood of unimpacted foods being selected and meeting the physiologic needs of birds. Seasonal timing is also important.

Direct toxicity levels and descriptions of the effects of several heavy metals, selenium, and synthetic organics are given in Hudson et al. (1984), USEPA's "TERRETOX" data base, and in the USFWS's "Contaminant Hazard Reviews" series that summarizes data on arsenic, cadmium, chromium, lead, mercury, selenium, mirex, carbofuran, toxaphene, PCBs, and chlorpyrifos.

Single-species Indicators

Contaminant levels or population declines of a single species is seldom sufficient to indicate that a particular wetland has been exposed to the contaminant.

Species Richness

Density and Biomass

Declines in avian richness, and perhaps density and biomass, would be expected at wetland complexes or regions heavily contaminated by pesticides or heavy metals. However, data from prairie wetlands are lacking.

Reproductive Success, Fledgling Growth, Population Demographics

Many studies have documented birds failing to reproduce or grow successfully in wetlands severely contaminated with heavy metals (e.g., Scheuhammer 1987, Kraus 1989) and particular pesticides, e.g., phorate (Dieter et al. 1995).

Bioaccumulation

Selenium levels of > 0.050 mg/L, or > 0.030 mg/g of body weight, pose a potential risk to many waterbird species because selenium is rapidly accumulated in food chains and body tissues (Welsh et al. 1993). Analysis of waterbirds collected from > 24 wetlands in the prairie region, mostly on National Wildlife Refuges, 1986-1992, revealed problems with selenium accumulation in only a few localities (Welsh and Olson 1991, Ludden 1990). Selenium was not detected in water samples from any of 238 wetlands in a part of the Canadian prairie with selenium-rich soils (Leighton and Wobeser 1994). Incidences of organochlorines, PCB's, and mercury accumulating in prairie birds, especially raptorial and fish-eating species, have been reported (Jackson 1986, USFWS 1989, DeSmet and Shoesmith 1990, Larson 1990, Welsh and Olson 1991).

Physical Condition, Deformities, Behavior

Eggshell thinning, physical deformities of embryos and hatching birds, and feather loss in adult birds, are symptoms of severe contamination of wetland food chains with certain chemicals, such as selenium (Scheuhammer 1987, Ohlendorf et al. 1990). Drooping wings and abnormal neck posture can indicate poisoning by carbamate or organophosphate insecticides (Facemire 1991).

Biomarkers

The USFWS's Biomonitoring of Environmental Status and Trends (BEST) program has proposed use of several biomarkers, including the following relatively well-established ones:

1. Delta-aminolevulinic Acid Dehydratase (ALAD). Elevated concentrations of this enzyme in birds and perhaps amphibians can indicate sublethal exposure, within the previous month, to lead from highway runoff or birdshot.

2. Acetylcholinesterase (AChE).

Depressed concentrations of this enzyme in birds, amphibians, and invertebrates can indicate exposure, generally within a few hours or days, to organophosphorus and carbamate insecticides (Ludke et al. 1975), and perhaps to some heavy metals.

3. Cytochrome P450 Monooxygenase System (MO).

Elevated concentrations of this enzyme in birds can indicate exposure, within the previous few days or weeks, to various organic hydrocarbons.

4. Hexacarboxylic Acid Porphyrin (HCP).

Elevated concentrations of this enzyme in birds can indicate ongoing exposure to various organic hydrocarbons.

5. Retinol (Vitamin A).

Depressed concentrations of this enzyme can indicate reduced viability of individual birds (Wobeser and Kost 1992).

6. Thyroid hormones.

Depressed concentrations of various thyroid hormones in birds can indicate ongoing exposure to various organic hydrocarbons.

Laboratory costs for analysis of any of the above biomarkers generally range from $15 to $75 per sample, processed at a rate of about 20 to 30 samples per day. Other potential biomarkers for use with terrestrial vertebrates are described in Harder and Kirkpatrick (1994).

6.5 Monitoring Techniques

6.5.1 General Surveys

Methods for censusing waterfowl in prairie wetlands are summarized by Hammond (1969, Klett and Johnson (1981, Klett et al. (1986, Ball et al. (1988, and Higgins et al. (1992. Although not specific to prairie wetlands, Eng (1986) and Kirby (1980) also discuss censusing of waterfowl. Methods for censusing marsh and shorebirds are discussed by Connors (1986), Clark and Murkin (1989, and Weller (1986). Methods for surveying entire bird communities within individual habitats are described by Burnham et al. (1980), Halvorson (1984), Ralph and Scott (1981), Verner (1985), Verner and Ritter (1985), and others.

Observations that are part of a survey covering several wetlands should occur simultaneously or be made within consecutive days unless severe weather conditions intervene. If the objective is to compare between-year trends in a species, total species, or species richness, then simple count methods (e.g., transects) are probably appropriate. However, if the objective is to rank wetland types or relative abundance of species, more time-consuming censusing are required to develop estimates of density (Steele et al. 1984). Determination of indices of relative annual abundance, rather than exhaustive population censusing, is suitable for most purposes (Emlen 1981).

Although most common songbirds will not be disturbed by frequent visits by monitoring personnel, raptors, waterfowl, other large or colonial species, and ground-nesting species can be susceptible. Wetland songbird surveys are commonly conducted during May through July, when breeding birds are most detectable by song. Species detection (especially of most songbirds) is greatest during early morning hours. However, in winter some species are active at mid-day. Night-time coverage is sometimes warranted, not only for typically nocturnal species such as owls, but also for waterfowl and wading birds which sometimes use different prairie wetland types for roosting and for feeding (e.g., Swanson and Sargeant 1972). Secretive species (e.g., rails, some passerines) have sometimes been surveyed more effectively by playing back tape recorded calls, use of predator decoys, use of dogs, and by dragging ropes or chains through wetlands (e.g., Glahn 1974, Ralph and Scott 1981, Gibbs and Melvin 1993).

Surveys can be conducted from ground level, from elevated observation posts, or aerially. Ground-level, visual techniques cannot be used effectively in wetlands with tall vegetation. Boats are typically used for surveys of wetlands wider than about 100 meters, as visibility from shore, even using a spotting scope, becomes restricted.

6.5.2 Reproductive Success

Where birds that colonize bird boxes are present, boxes provide a convenient means of monitoring reproductive success, with minimal disturbance and without the labor of having to find nests. In other regions, they have been used successfully to monitor impacts from heavy metals (Kraus 1989, Peterson and McEwan 1990) and acid precipitation (St. Louis and Barlow 1993).

6.5.3 Time Budget Analysis

Studies on prairie wetlands (Murkin and Kadlec 1986a, Eldridge and Krapu 1993) have demonstrated that estimates of bird density are not necessarily sufficient to indicate a degraded wetland condition (i.e., a wetland with diminished invertebrate densities), yet documenting the hours of use of the wetland by various species can successfully indicate such a condition. Such a time budget approach usually requires purchase and installation of video equipment that automatically photographs portions of the wetland at specified intervals. From viewing the tapes, the duration of each activity (e.g. feeding) of visible birds in each photographed zone can be determined. It would be costly to implement for studies intended to survey more than a few wetlands.

6.5.4 Bioassay Methods

A review of laboratory, outdoor mesocosm, or in situ bioassay methods involving birds or other wildlife is beyond the scope of this report. Use of bioassays to explore direct contaminant toxicity to birds has been relatively limited in prairie wetlands.

6.5.5 Bioaccumulation

Methods for assessing bioaccumulation of contaminants in bird tissues are described in Moser and Rope (1993a).

6.6 Variability and Reference Points

6.6.1 Spatial Variability

Species Richness

As a point of reference, of the 302 bird species that occur regularly at some season in the prairie region (Faanes and Stewart 1982), about 104 occur regularly in wetlands, 35 solely as migrants, and 69 additionally as nesters (interpretion based on published literature and personal experience, see Appendix C). During the nesting season, this wetland component of the avifauna represents 78% of all 88 nesting species that Short (1989) notes are found regularly in any habitat in the prairies.

In Iowa, a breeding-season survey of 17 restored prairie wetlands found 2-18 species per wetland (Hemesath 1991). No species occurred in all wetlands and four species occurred only in one. A survey of another 11 Iowa wetlands, both natural and restored, reported a cumulative total of 22 species (Delphey and Dinsmore 1993). In yet another survey in Iowa, covering 30 natural wetlands ranging in size from 0.2 to 182 ha, Brown and Dinsmore (1988, 1991) found a range of 2-17 species per wetland, similar to the 7.2 species per wetland found by Dinsmore et al. (1993). , in which each wetland was visited 1-2 times annually to include the breeding period, found an average of three species per wetland (range 0-45 species) (Igl and Johnson, unpub. data, NPSC, Jamestown, ND). Richness varied among six subregions of the prairie region, from 1.38 species per wetland in one subregion to 3.74 species per wetland in another. Cumulatively (among all the wetlands), 101 species were found the first year and 113 the second. In a single-visit survey of 95 randomly selected plots in North Dakota's wetland subregions, Kantrud (1981) found an average of about seven breeding species per 31.5-ha plot. Avian richness was less variable among landforms than was avian density. About 750 acres of one North Dakota wetland (Kraft Slough) supported 29 breeding species (Krapu and Duebbert 1974). In a one-year survey of breeding species in the Sheyenne Lake area of North Dakota, Faanes (1982) found 36 species in a cumulative area of 76 ha of permanent wetland, 25 species in 24 ha of seasonal wetland, 22 species in 20 ha of semipermanent wetland, and 17 species in 44 ha of alkali (saline) wetlands.

A major source of information on the species composition, distribution, and relative abundance of prairie birds is the USFWS's Breeding Bird Survey (BBS). This is a data base containing data collected, in some instances, as far back as 1966. Birds seen or heard at each of 50, 3-minute stops along 25-mile roadside routes are recorded. Both waterfowl and nonwaterfowl are surveyed, as well as both wetland and nonwetland habitat (which are not identified specifically by the survey). At a regional level, the USFWS has calculated trends in abundance of each species; trends of wetland species are shown in Appendix C ("Priority" column).

For Appendix C of this report, data from 160 routes in BBS strata 37, 38, and 40 were tabulated for the years 1966-1993. Each of these strata is located predominantly within the region commonly considered to be the prairie pothole wetland region. Along these 160 BBS routes, a cumulative total of 74 wetland species (71% of all wetland species that occur regularly at any season in the region) have been recorded at least once during 27 years. During any year, the median number of wetland species per route is 15 (14% of the region's wetland avifauna) and ranges from 46 species (44%) on the richest route during its highest-count year to three (3%) on the poorest route during its lowest year. Among years, the richest route averages 37 wetland species, and the poorest route averages five wetland species.

Fewer data are available to describe bird richness during migration periods. In a 28-visit springtime survey of 13 varied wetlands in Brookings County, South Dakota, the cumulative species total ranged from 0 to 21 species per wetland (Brady and Giron-Pendleton 1983). A cumulative species total of 137 species (60 of them breeding) was found during a year-round, 2-year study of a 176-ha restored prairie wetland in Minnesota (Svedarsky et al. 1993).

Density

Many investigators have documented waterfowl pair densities, both in wetlands and in adjoining habitat, during the breeding season. Density estimates of waterfowl are difficult to compare, and habitat relationships are difficult to define, because unit of area can be measured in numerous ways (include the entire wetland? or just the open water part?) (Savard et al. 1994). Densities or regional population estimates based on probabilistic sampling of hundreds of wetlands of all types are reported by Stewart and Kantrud (1974), Brewster et al. (1976), Kantrud and Stewart (1977), Higgins (1977), Ruwaldt et al. (1979), Krapu et al. (1983), and Duebbert and Frank (1984. Densities from smaller numbers of wetlands are reported by Mundinger (1976), Krapu and Green (1974), Higgins et al. (1992), and many others.

Pair densities and/or regional abundance estimates of non-waterfowl species (as well) are reported from multiple wetlands by Stewart and Kantrud (1972b), Kantrud (1981), Weber et al. (1982), Faanes (1982), Kantrud and Stewart (1984), Brown and Dinsmore (1986), and Igl and Johnson (unpubl. data, NPSC, Jamestown, ND). The Igl and Johnson study (described on page 117 and in Appendix L) found an average of 5.43 pairs of all species per wetland in the prairie region of North Dakota (median = 0, range = 0-744 pairs). The number of pairs believed to actually be breeding averaged 3.12 per wetland (median = 0, range = 0-397 breeding pairs). Mean density of breeding pairs varied spatially (among six subregions of the prairie region) from 2.04 to 13.92 pairs per wetland. The maximum pairs per wetland also varied among subregions, from 27 pairs in a Agassiz Lake Plain wetland to 397 pairs in a Northwestern Drift Plain wetland. About 750 acres of one North Dakota wetland (Kraft Slough) supported 2934 breeding pairs (Krapu and Duebbert 1974). Similar data from other areas of the prairie region (including some non-wetland habitat) are reported as part of the Breeding Bird Censuses published in the journals American Birds and Audubon Field Notes.

Considering just the 1967-87 period, there were only 76 instances in which any of the 7255 ten-mile route segments (that constitute BBS routes in the prairie region) failed to contain a single wetland species, and no segments have been devoid of wetland species during every year that they have been visited. During any year, wetland species are typically found at greater than nine (18%) of the 50 stops along each prairie BBS route (range = 1-50), and the median number of individuals (all wetland species combined) per route is 159. The number of individuals of wetland species varies spatially from 1133 on the richest route to 30 on the poorest during years when numbers vary the least among routes. During years when numbers vary the greatest among routes, the number of individuals varies from 1247 on the richest route to six on the poorest.

Species Composition

Of the 67 wetland species ever recorded from prairie BBS routes, a majority have been recorded on at least 62% of the routes. However, of the 128 species found in prairie wetlands by Igl and Johnson (unpubl. data, NPSC, Jamestown, ND), none were found in more than 41% of the individual wetlands, and a majority were present only in less than 1% of the wetlands surveyed.

Bioaccumulation

No published data pertaining to spatial variability of bioaccumulation in prairie wetlands were found.

Reproductive Success

Dozens of studies in prairie wetlands and adjoining grasslands have documented reproductive success rates of waterfowl. Nest success rates of about 50% are typical for many species (Solberg and Higgins 1993b), but spatial and annual variability is great.

6.6.2 Temporal Variability

Species Richness

Between years, the variety of breeding birds on a single prairie wetland can range from near 0 species to over 20, depending largely on water conditions. Among six restored prairie wetlands that were sampled in Iowa for two years, richness changed dramatically between years in two of the wetlands (from 1 to 5-6 species, as the wetland matured during its first post-restoration year), moderately in 3, and not at all in 1 (Hemesath 1991). In another survey in Iowa, the species richness in each of 30 natural wetlands did not change between two consecutive years in 14 (47%) of the wetlands, and there was no statistically significant difference in species richness between the years (Brown and Dinsmore 1988).

Igl and Johnson's two years of data from North Dakota prairie wetlands show that the number of species per wetland changed from 2.71 in 1992 (a dry year) to 3.22 in 1993 (a wet year). The greatest interannual variation was in the Agassiz Lake Plain subregion, where richness per wetland changed from 0.88 in 1992 to 1.77 in 1993.

Along most BBS routes in the prairie region, the number of wetland species has varied by a factor of less than 2.6 between years(2), but in one extreme instance changed from 1 to 11 species between years. Bird richness in prairie wetlands is lowest in winter, but the few birds present at that season are highly dependent on the vegetative cover of the wetlands.

Density

In a wetland complex near Woodworth, North Dakota that was surveyed annually for 17 years, the density of waterfowl pairs varied annually from 19 to 56/km2 and averaged 40/km2 (Higgins et al. 1992). Brood densities ranged from 10 to 63/km2 and averaged 12/km2. Mallard densities over a 20-year period in another part of eastern North Dakota varied at least fourfold over a multiyear period (Krapu et al. 1983). Among waterfowl species, the northern pintail, green-winged teal, northern shoveler, and American wigeon appear to have the greatest interannual variability (Stewart and Kantrud 1974).

Igl and Johnson's two years of data from North Dakota prairie wetlands show that the mean number of pairs changed from 5.62 in 1992 (a dry year) to 5.64 in 1993 (a wet year). As was true of species richness, the greatest interannual variation was in the Agassiz Lake Plain subregion, where mean number of pairs per wetland changed from 1.48 in 1992 to 2.48 in 1993, indicating a rapid response to improved water conditions. Among wetlands that contained birds both years, the largest changes in numbers of pairs occurred in a wetland that experienced an interannual increase of 22 pairs (30% increase) and the largest decrease occurred in a wetland that experienced an interannual decrease of 49 pairs (12% decrease). Among wetlands that contained birds both years, the species that showed the greatest interannual change (averaged among all wetlands) were ruddy duck, eared grebe, bank swallow, black tern, American wigeon, gadwall, redhead (declined between years), and Forster's tern, green-winged teal, American coot, double-crested cormorant, and Franklin's gull (increased between years). Such species might be good candidates as indicators of environmental change. However, in some situations the annual fluctuations in waterfowl densities are caused by different environmental factors in different wetlands (Lillie and Evrard 1994).

Data for prairie BBS routes during 1967-1987 indicates that the number of individuals of wetland species can vary interannually by a factor of 49 (the most temporally dynamic route), but on most routes varies by a factor of less than 3.58(3) . Along the routes, interannual trends in nonwaterfowl wetland birds weakly mimic trends in waterfowl. Specifically, the number of nonwaterfowl individuals (summed across routes(4) ) is correlated (r = 0.38, p< 0.09, n = 21) with waterfowl individuals summed across routes, and the frequency of BBS stops at which waterfowl were present is correlated (r = 0.51, p< 0.02, n = 21) with frequency of stops at which nonwaterfowl were present. The mean richness per route of all wetland species hit lows in 1971 and 1981, despite coverage of a normal number of routes during those years.

Among 60 wetland species for which there are sufficient BBS data to calculate long-term trends by subregion within the prairie region (see Appendix C), 49 species (82%) have declined in one or more of the three subregions (the number is 44 species if only the trends that are statistically significant are included). By subregion, the eastern and central subregions appear to have a larger percent (67%) decreasing species than the western subregion (44% decliners).

Reproductive Success

Based on analysis of data from over 3000 nests during an 18-year period in the prairie region, Klett et al. (1988) concluded that average nest success had changed little for most waterfowl species in most subregions. Duck production in wetlands of the Woodworth complex ranged from 15 to 61 broods per 100 pairs over a 17-year period, and averaged 30 broods per 100 pairs (Higgins et al. 1992).

Bioaccumulation

Based on seven years of data from North Dakota wetlands, Welsh et al. (1993) speculated that bioaccumulation of selenium might be greater during drought years.

6.7 Collection of Ancillary Data

It is easier to separate the anthropogenic from the natural causes of impairment of community structure if data are collected or inferred simultaneously on the following variables of particular importance to wetland birds:

distribution of water depth classes, vegetation (type, and vertical and horizontal diversity and arrangement), conductivity and baseline chemistry of waters and sediments (especially conductivity), distance and connectedness to other wetlands of similar or different type, surrounding land cover (particularly within 500 feet of wetland perimeter), shoreline slope, wetland size, cover ratio, spatial interspersion among vegetation classes, and the duration, frequency, and seasonal timing of regular inundation, as well as time elapsed since the last severe inundation or drought.

All of these features vary to a large degree naturally, as well as in response to human activities such as soil tillage, compaction, and erosion; fertilizer and pesticide application; and water regime modification. In addition, disturbance from the presence of humans visiting wetlands can directly alter the bird community composition of the wetlands.

6.8 Sampling Design and Required Level of Sampling Effort

6.8.1 General Considerations

"Scale" is an important issue in monitoring the birds of prairie wetlands. Methods used to determine bird density and richness within a wetland become impractical and even inaccurate when the objective is to make comparisons among many wetlands or wetland complexes, especially at regional scales. Likewise, regional-scale methods are often too coarse for application to individual wetlands.

When monitoring birds within prairie wetlands, point count methods have been used most often. In Iowa, Delphey and Dinsmore (1993) and Brown and Dinsmore (1986) used fixed-radius (18 m) circular plots; the first one was placed randomly in a wetland and the rest were placed equidistantly around the wetland until the investigators could not locate a plot at least 60 m from another (or when a total of five were established). Alternatively, when surveying wetlands with particularly tall, dense vegetation and limited access, the survey points might be placed such that the largest portion of the wetland is visible from the fewest points. A third option would be to allocate points in proportion to the sizes of various vegetation zones and water depths, if these strata can be delimited beforehand. Where quantitative estimates of populations are not needed, less formal survey methods can be used. For example, in bird surveys of quarter-sections (e.g., Kantrud and Stewart 1984), observers have simply walked in as straight a line as possible through all habitats they recognize within a fixed area.

Costs of surveying birds depend on the number of visits that need to be made per wetland, the number of points to be visited, and the duration of observations at each point. As with other taxa, if numbers of individuals are to be estimated, the intensity of effort should reflect the expected variability (coefficient of variation) and the desired precision. If the objective is to assess species composition and biodiversity, species accumulation curves should also be plotted, as described below (Section 6.8.2) and in Section 1.5.

Some biologists in other regions suggest that, for reasonably accurate estimates of breeding bird richness in a wetland, three visits spread over the breeding season is usually desirable (Brooks et al. 1989, Weller 1986). This is advisable because some waterfowl species breed in May, most songbirds breed in June, and the remaining songbirds breed in July and August. In studies of breeding birds in Iowa wetlands, the number of visits per wetland ranged from two (LaGrange and Dinsmore 1989b) to five (Delphey and Dinsmore 1993). The time spent per observation point ranged from six to eight minutes. Only a single visit was made to most of the 128 areas covered by a survey of nongame birds conducted by the NPSC.

The foregoing discussion has described sampling of individual wetlands or quarter-sections. If the objective is to estimate species composition, richness, and numbers at only a regional level, then a different design can be used. For example, the Breeding Bird Survey bases estimates of avian distribution, relative abundance, and trends on just a single 3-minute visit annually to hundreds of points in a region. To conduct a regional survey of prairie avifauna, Stewart and Kantrud (1972a, 1973) and Kantrud and Stewart (1984) selected quarter-sections (64.7 ha) as plots. The plots were randomly selected by a cluster sampling (without replacement) process, in which 120-130 quarter sections were grouped as 30 clusters, with clusters reflecting the major landforms of the prairie region. A stratified random and cluster sampling design was also implemented, in two stages, in regional avian surveys by Brewster et al. (1976) and Ruwaldt et al. (1979). They found that cluster sampling reduced the number of zero observations and travel time, and thus increased the number of wetlands that could be visited. In just one field season, they were able to do an avian survey involving two visits to 500 quarter-sections (64.7 ha each). Four of these quarter-sections were selected, one in each of the four compass directions, from a corner of each of 125 townships which had been selected randomly, for a total of 500. Two hours were spent surveying birds in each quarter-section (about two minutes per ha). A two-person survey of 128 quarter-sections was been able to cover about 160 acres in 1-2 hours (about three minutes per observer per ha) (L. Igl, personal communication, NPSC, Jamestown, ND). Because he was surveying a much smaller region and had somewhat different objectives, Faanes (1982) used smaller (16.2 ha) plots which he was able to visit for longer duration.

If not only richness, but density, must be determined, then at least eight visits are probably needed (Ralph and Scott 1981).

6.8.2 Asymptotic Richness: Results of Analysis

For this report, we used two data sets to estimate species accumulation and asymptotic richness(5) . One was the BBS data, 1967-1993, from all routes in the prairie region, and the other was the Igl and Johnson study that covered 452 individual wetlands during 1992-1993. For the BBS data set, we examined only the assemblage of species that are most characteristic of wetlands (see introduction to Appendix C), whereas for the Igl and Johnson data, we examined all species.

Analysis of the Breeding Bird Survey (BBS) Data

Several issues were considered in the analysis of the BBS data:

1. Number of Years. We analyzed data from the two routes having the greatest species richness in each of the three subregions of the prairie (strata 37, 38, and 40). On these routes, half the wetland species found during the entire interannual period of the route could generally be found during any two years. To find 90% of the wetland species collectively present during the entire period of coverage required a number of years equivalent to 37-83% of the route's total years of coverage. Of the six species-rich routes, the one with the longest coverage required 15 years (range, 9-22 years) to detect 90% of the 49 species that were found collectively during the entire 27 years of that count.

2. Number of Routes. In each of the three subregions, we analyzed data from the two years in which the most species were detected. Half the wetland species found collectively among all 29-40 routes run per subregion during these years could generally be found on any 2-3 routes. To find 90% of the wetland species collectively present on all these routes during a given year required between 15 and 19 routes (or 44-59% of the total routes run during those years in the subregion). During the year (1993) in which the most BBS routes were run, 16 routes (range, 5-32) were needed to detect 90% of the species in each of two of the subregions.

Another subset of the data were also examined to estimate the requisite number of routes. This subset included just 11 routes that had been run during the same 26 or 27 years. This analysis indicated that half the wetland species present collectively among all these routes during the entire 26-27 years could be detected on just two routes at least sometime during that period. Finding 90% of the species would require four routes (range, 2-7).

3. Number of Route Segments. The objective of the analyses described above was to estimate the requisite number of routes. Each route contains 50 point counts whose totals are aggregated into five subtotals, each representing 10 point counts conducted over a 5-mile segment. To analyze these segment subtotals, we considered just three routes, selected on the basis of their being the richest routes run during any year in their subregion. This analysis indicated that two segments were sufficient to detect half the wetland species found collectively with all five segments, and four segments (range, 2-5) were needed to detect 90% of the species total.

Analysis of the Igl and Johnson data

Species accumulation among wetlands was determined separately for 1992 and 1993, and for breeding individuals vs. total individuals (presumed nonbreeding as well as breeding). Species accumulation among wetlands rose and began to level off sooner for breeding individuals than for total individuals, and was slightly faster in 1993 than in 1992 (Appendix O). If only half the number of wetlands had been sampled, the cumulative species list would have been about 75-80% as large.

6.8.3 Power of Detection: Results of Analysis

Analysis of the BBS data

The BBS monitoring program was better able to detect inter-route differences in the total sampled number of wetland species than in the total number of stops containing wetland species or in the total number of individuals of wetland species. The data suggested that, for the prairie pothole region as a whole, a sample size of 10 BBS routes would allow detection of inter-route differences of six wetland species, 29 stops containing wetland species, or 140 individuals of wetland species. When variability is reduced by analyzing data from just the routes that have had the most consistent coverage over the years, the respective detection limits are two species, 25 stops, and 120 individuals. The data suggest that conducting additional BBS routes in the prairie region, beyond about six routes, has diminishing effects on increasing the precision of the richness-per-route estimates. Increasing beyond 10 routes has diminishing effects on the increase in precision of the other two variables (stops with wetland species, total individuals of wetland species).

Analysis of the Igl and Johnson data

If wetlands similar to those examined by Igl and Johnson are visited twice annually for two years, approximately 10 wetlands would need to be visited to detect a difference of two species between any two wetlands. To detect a difference of 10 breeding pairs, about 35 wetlands would need to be visited.

6.9 Summary

The species composition of bird communities, and to a lesser degree their species richness, demonstrates diagnostic responses to changes in water levels and duration, and to vegetative cover conditions, within a prairie wetland complex (Table 4). Birds also may respond over the long term to changing wetland nutrient levels, sedimentation, and contaminant levels, but existing information is too limited and confounding effects are too prevalent to currently allow widespread use of birds to diagnose impairment of prairie wetlands from these stressors. Even for the responses to water regime and vegetation change, the ability to use birds to distinguish natural from anthropogenic levels of wetland disturbance is currently limited.

Bird communities are practical to monitor because sampling is nondestructive, and identification is relatively simple. Although their high mobility confounds attempts to use birds as indicators of the condition of individual wetlands, changes in species composition within a wetland complex or subregion can demonstrate impacts to wetlands that are occurring at such broad scales. Birds are the only group suitable and practical for indicating such impacts.

Individual prairie wetlands that are semipermanently flooded generally contain about three pairs of breeding birds, representing three species. Some wetland complexes and larger individual wetlands can support at least 40 breeding species and 400 pairs. Between years, the variety of breeding birds on a single prairie wetland can range from near 0 species to over 20 species, depending mainly on water conditions.

Additional data collected and applied primarily at a landscape or regional scale are needed to support hydrologic and water quality criteria for nesting waterfowl and migratory shorebirds. Related information is needed on the degree to which surrounding condition of upland habitats influences the hydrologic and water quality requirements of wetland birds. Data are clearly needed on factors that influence use of prairie wetlands by migratory shorebirds, and on the impacts of sedimentation and nutrient enrichment on the sustainability of wetlands as habitat for waterbirds.

Table 4. Summary Evaluations of Possible Invertebrate Indicators of Stressors in Prairie Wetlands.

Evaluations are based on technical considerations, not cost or practicality. A rating of FAIR or POOR is assigned when too few data (FD) suggest potential as an indicator, or when confounding effects (CE) of other variables often overshadow those of the listed stressor, with regard to effects on the indicator.

Stressors Possible Indicators (when measured at a regional or wetland-complex scale) Evaluation
Hydrologic stressors Species Composition
Single-species Indicators
Richness
Density, Biomass
Reproductive Success
FAIR (CE)
POOR
FAIR (CE)
GOOD
GOOD
Changes in vegetative cover Species Composition
Single-species Indicators
Richness
Density, Biomass
Reproductive Success
GOOD
GOOD
FAIR (CE)
FAIR (CE)
GOOD
Salinity Species Composition
Single-species Indicators
Richness
Density, Biomass
Reproductive Success
FAIR (CE)
FAIR (CE)
POOR
POOR
FAIR (CE)
Sedimentation & turbidity Species Composition
Richness
Density, Biomass
Reproductive Success
FAIR (FD)
FAIR (FD)
POOR (FD)
POOR (FD)
Excessive nutrients & anoxia Species Composition
Single-species Indicators
Richness
Density, Biomass
Reproductive Success
POOR
POOR (CE)
FAIR (CE)
FAIR (CE)
POOR (FD)
Herbicides Species Composition
Single-species Indicators
Richness
Density, Biomass
Reproductive Success
FAIR (FD)
POOR (FD)
POOR (FD)
POOR (FD)
POOR (CE)
Insecticides Species Composition
Single-species Indicators
Richness
Density, Biomass
Reproductive Success
Bioaccumulation
Physical Condition, Behavior
Biomarkers
FAIR (FD)
POOR (FD)
POOR (FD)
POOR (FD)
GOOD
FAIR (CE)
FAIR (CE)
GOOD
Heavy Metals Species Composition
Single-species Indicators
Richness
Density, Biomass
Reproductive Success
Bioaccumulation
Physical Condition, Behavior
Biomarkers
POOR
POOR (FD)
POOR (FD)
POOR (FD)
GOOD
GOOD
FAIR (CE)
GOOD

1. 1

2. 2     Calculated as the number of species on the route during the year having the most species, minus the year having the fewest.

3. 3     Calculated as the number of individuals on the route during the year having the most species, minus the year having the fewest.

4. 4     based on 15-21 years of data from the 12 BBS routes having the most years of coverage

5. 5     The monitoring design and data structure of the BBS data set that was used are detailed in Appendix 12.


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