Water: Monitoring & Assessment
5.1 Ecological Significance
Bioindicators for Assessing Ecological Integrity of Prairie Wetlands
Report # EPA/ 600/ R-96/ 082
5.5.1 General Considerations
5.1 Ecological Significance
There appears to be considerable concern among scientists about possible worldwide declines in amphibians (Barinaga 1990, Wyman 1990), yet relatively little is known about the relative sensitivities or ecological significance of amphibians (frogs, salamanders) in prairie wetlands. Larval stages of most species feed largely on algae, whereas adults are mostly insectivorous and are consumed by birds (e.g., bitterns, egrets, pelicans). Populations and biomass of amphibians can reach relatively high levels under some conditions, but species richness is lower than for other groups discussed in this report.
5.2 Potential Indicator Metrics
The following measurements and metrics deserve consideration, as applied to amphibian 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:
- Number of individuals per unit area, by season.
- Reproductive success.
- Interannual variability in density.
5.3 Previous and Ongoing Monitoring
Apparently only two broadscale attempts have been made to survey amphibians in prairie wetlands. One is a county-wide survey conducted in Iowa (Lannoo et al. 1993). The other was sponsored by EMAP, and it involved surveys in 11 North Dakota wetlands in summer 1993. Productivity of salamanders in some prairie wetlands is reported by Deutschmann and Peterka (1988).
5.4 Response to Stressors
Few studies exist of amphibians in prairie wetlands. Species are too few to allow amphibian richness to be used meaningfully at the individual site level as an indicator of impaired wetland integrity. Nonetheless, at a regional level the distribution, abundance, and perhaps richness of amphibians might be a sensitive indicator of overall changes in wetland water regimes, sedimentation, eutrophication, and other stressors. Amphibians appear to be particularly sensitive to pesticides because they absorb many chemical substances directly through their skin (Harfenist et al. 1989). At a site level, various biomarkers might be used as indicators (e.g., Leboulenger et al. 1982, Licht et al. 1983, Moore and Miller 1984), as well as the fecundity, incidence of deformities, and bioaccumulation.
5.5.1 General Considerations
Amphibians can be sampled using methods and equipment described by Bury and Raphael 1988, Halvorson 1984, Jones 1986, Scott 1982, Vogt and Hine 1982, Moser et al. 1993, Heyer et al. 1994, and others.
Sampling amphibians effectively normally requires several repeated visits to a wetland, or to a series of wetlands along a survey route. Amphibians are best sampled during the mid- to late growing season when maximum numbers of developing juveniles (e.g., tadpoles) are present. However, many species are easily found only after the first few days of rain following a drought, during late-summer thunderstorms, during the first spring thaw in northern areas, during mid-day basking hours, or at night (Kaplan 1981). Occasionally, traditional winter hibernation areas can be located and used to count individuals representing a larger (but undefinable) area.
5.5.2 Equipment and Methods
Amphibians are sampled using pitfall traps and funnels (often with drift fences and bait), visual belt transects, direct capture methods, and vocalization recording. EMAP-sponsored efforts to collect amphibians in prairie wetlands have used a setup involving drift fences funnels.
Pitfall traps and funnels are perhaps the method most widely used (Jones 1986). Pitfall traps involve implanting a container in the soil, either on the periphery of the wetland or within it (if surface water is absent), with the lip of the container placed flush with the ground surface. Amphibians stumble in and cannot climb the steep sides to escape. Because some species can drown if the container fills with rainwater, Jones (1986) recommends placing floatable material (e.g., styrofoam) in the container to reduce mortality. Pitfall traps are impractical in all but the most temporary parts of wetlands, because otherwise the water table is so close to the land surface that pits fill rapidly with water.
Pitfall and funnel traps often produce more species per sampling effort than direct capture methods (Jones 1986). With funnel traps, animals enter a screened area and cannot find the opening to escape. They are subsequently identified, counted, measured, and released. To reduce loss of trapped animals to predation, traps and funnels are checked regularly (at least every other day) and can be shaded, and/or filled with sufficient moist plant litter to minimize physiologic stress to animals. Funnel openings are usually oriented toward land for greatest effectiveness. The size of the trap, baits used, and trap placement can affect the species that are caught. Trap and funnel methods can provide relatively quantitative data, when arranged systematically and level-of-effort (e.g., "trap-hours") is standardized. The efficiency of traps and funnels can be increased by channeling movements of amphibians in the direction of the trap or funnel. This is commonly done with "drift fences" (Gibbons and Bennett 1974). These are fences constructed of wire screen or polyethylene plastic, with lengths upwards of 15 m. Traps are placed at both ends of the drift fence, along the fence at various points, or at the junction of several intersecting fences. The bottom edge of the fence is implanted in the ground, or at least no space is provided for amphibians to crawl under the fence.
Drift fences and pit traps can be more effective and less biased in capturing amphibians than walking transects, electroshocking, or searching and digging through litter. However, drift fences are expensive; time and cost estimates for drift fence trapping are provided by Gibbons and Semlitsch (1982). Drift fence/pitfall trap methods are less effective for quantifying populations of frogs, toads, large snakes, terrestrial turtles, and salamanders than for quantifying populations of small snakes (Jones 1986). Sizes and shapes of containers and associated drift fences and their configurations vary greatly, depending partly on target species and wetland type. Various designs are described by Vickers et al. 1985, Stockwell 1985, and others. For sampling seasonal and semipermanent prairie wetlands, Euliss (personal communication, NPSC, Jamestown, ND) has designed and used a particularly effective drift fence and trap array.
The above methods require multiple visits to a wetland, first to set up and later to check traps. Amphibians can also be monitored directly, that is, during a single visit, or without having to wait for traps to catch individuals. However, direct methods usually do not provide accurate quantitative data on abundance. Unless frequent visits are made and the correct microhabitats are searched at the proper times of year, direct methods are also unlikely to yield good estimates of species dominance or richness. However, they can provide a useful complement to trap methods, locating species that are not easily trapped.
The simplest type of direct search involves scanning a wetland with binoculars to observe the more obviously visible forms such as basking frogs. In some cases, floating egg masses of amphibians can also be detected visually and identified to species. Observational methods can be done formally along defined transects. Searches on foot, perhaps employing many people shoulder-to-shoulder (e.g., Marshall and Buell 1955) have been used, but could be impractical and destructive of habitat in many wetlands. To enhance opportunities for encountering amphibians during direct searches, electrofishing can be used, at least for retrieving larger salamanders and frogs. Frogs can sometimes be located more easily at night, as their eyes reflect light in the beam of a flashlight. Vocalizations of many frogs and toads are easily identified (commercially-available recordings are available to learn these) and can be used to augment observations along survey routes. Frogs and toads can sometimes be induced to vocalize by introducing sharp, loud sounds or played-back tape recordings of vocalizations.
5.6 Variability and Reference Points
Although 40 species of amphibians and reptiles have been found in South Dakota and 25 in North Dakota, perhaps only three -- the tiger salamander, leopard frog, and chorus frog -- appear to be widespread and intimately associated specifically with prairie wetlands (Hubbard et al. 1988). Biweekly sampling of 17 seasonal and semipermanent wetlands in North Dakota captured six species of reptiles and amphibians (N. Euliss, personal communication, NPSC, Jamestown, ND). A survey of prairie wetlands in Iowa found seven amphibian species; two species present in the 1920's (mudpuppy, Necturus maculosus; Blanchard's cricket frog, Acris crepitans blanchardi) apparently were no longer present (Lannoo et al. 1993). Minnesota prairie wetlands support at least four toad species (Oldfield and Moriarty 1994).
Density, Biomass, Production
A 2-year study of prairie lakes in North Dakota revealed a maximum density of 5000 larval salamanders per ha. Maximum biomass was 180 kg/ha and maximum annual production was 565 kg/ha (Deutschman and Peterka 1988).
5.7 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 amphibians:
water depth, temperature (site elevation, aspect), conductivity and baseline chemistry of waters and sediments (especially pH, DO, and suspended sediment), shade, amount and distribution of cover (logs, muskrat houses, etc.), cover ratio, extent of plant litter, vegetation type, and the duration, frequency, seasonal timing of regular inundation, and 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.
5.8 Sampling Design and Required Level of Sampling Effort
No quantified estimates of interwetland or interannual variability were found in the published literature from the region, and no data sets were obtained for analysis, so requisite sample sizes cannot be estimated.
Although apparently constituting a large portion of the annual animal production of some semipermanent wetlands, amphibian communities have seldom been investigated in prairie wetlands. This is due in part to their relatively low species richness and spotty spatial and temporal distribution. Limited information suggests amphibians in prairie potholes may be highly sensitive to some chemical contaminants and to landscape-level fragmentation of wetland resources. Considerably more research is required before amphibian species composition, richness, and biomass can be used as unambiguous indicators of prairie wetland condition.