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Water: Monitoring & Assessment

12.0 Mammal Communities

Impacts on Quality of Inland Wetlands of the United States:
A Survey of Indicators, Techniques, and Applications of Community Level Biomonitoring Data
Excerpts from Report #EPA/600/3-90/073
(now out of print)


In general, wetlands are permanently inhabited by fewer mammal species than are upland ecosystems. However, the association of some mammals with wetlands is very strong. These include river otter, muskrat, nutria, beaver, mink, raccoon, swamp rabbit, marsh rice rat, and others. In contrast to most wetland birds, many wetland mammals are herbivores or omnivores, i.e., they consume wetland plants directly or have a mixed animal-plant diet. Muskrat in particular can have major impacts on wetland herbaceous plants (e.g., McCabe 1982). Advantages and disadvantages of using mammals as indicators are summarized in Appendix A.

As with birds, because a majority of mammals use wetlands at least briefly at some time during the year, defining what truly constitutes "wetland-dependent" is difficult. For example, individual bobcats and black and grizzly bears use wetlands extensively in some regions (e.g., Helgren and Vaughn 1989), but it is sometimes unclear whether this is the general preference of the species, and if so, whether alternative habitats infrequently visited by humans are suitable substitutes. Some species of mammals have been categorized according to wetland dependency by Brooks and Croonquist 1990, Durham et al. 1985, and Fritzell 1988.

In one comparison of existing data, prairie pothole wetlands were reported to support fewer species of mammals than either northern bogs/fens, or southern bottomland hardwoods (Fritzell 1988). Response to particular stressors is described below.

Enrichment/Eutrophication. The effects of enrichment on overall community structure of wetland mammals has not been documented, and indicator assemblages of species "most sensitive" to eutrophication remain speculative.

Organic Loading/Reduced DO. Attempts have been made in a few instances to measure the effects of severe organic loading, e.g., from wastewater outfalls, on overall community structure of wetland mammals. However, results generally have been equivocal and indicator assemblages of species "most sensitive" to organic loading remain speculative.

It can be hypothesized that, where introduction of organic wastes results in anoxic conditions lethal to mammal foods (e.g., fish and some amphibians), community composition may shift from fish-eating species (e.g., otter, mink) to vegetarian or invertebrate-eating species and opportunists (e.g., muskrat, opossum).

Contaminant Toxicity. The effects of bioaccumulation of contaminants in wetland mammal tissues have sometimes been measured. Species assemblages for indicating the physical effects of oil spills can be easily identified based on characteristic behaviors of some wetland mammals. However, the effects of pesticides, heavy metals, and other contaminants on overall structure of wetland mammal communities are poorly documented in wetlands, and indicator assemblages of "most sensitive species" remain mostly speculative for these stressors.

Acidification. Effects of acidification on the overall community structure of wetland mammals apparently have not been documented and indicator assemblages of "most sensitive" species remain speculative. It can be hypothesized that, where acidification becomes severe, community composition may shift from fish-eating species (e.g., otter, mink) to vegetarian or invertebrate-eating species and opportunists (e.g., muskrat, opossum).

Salinization. The effects of salinization, e.g., from irrigation return water and oil drilling wastes, on overall community structure of mammals has not been documented in wetlands, and indicator assemblages of "most sensitive species" remain speculative.

Sedimentation/Burial. Excessive sedimentation can alter food sources of wetland mammal communities. However, the effects of sedimentation/ burial on overall community structure of wetland mammals has not been documented, and indicator assemblages of "most sensitive" species remain speculative.

Vegetation Removal. Many mammals are sensitive to the presence and type of vegetation and its juxtapositioning with open water. Species richness of small mammals in wetlands has been correlated with complexity of vegetation structure (Arner et al. 1976, Landin 1985, Maki et al. 1980, Nordquist and Birney 1980, Stockwell 1985, Searls 1974, Simons 1985). Vegetation removal and associated long-term destruction of den sites in both wooded and emergent wetlands has resulted in changes in furbearer populations and small mammal communities (Krapu et al. 1970, Malecki and Sullivan 1987, Possardt and Dodge 1978), while restoration of riparian vegetation has led to increases in use by mink (Burgess and Bider 1980). However, many small mammals are more abundant in the denser herbaceous ground cover that results from overstory removal, as shown in a Texas riparian system by Dickson and Williamson (988). Grazing at levels recommended by the Soil Conservation Service had no significant effect on abundance or distribution pattern of small mammals in a Colorado cottonwood floodplain (Samson et al. 1988).

Species in Iowa considered by Geier and Best (1980) to be least tolerant of vegetation change include Microtus pennsylvanicus, Spermophilus tridecemlineatus, Reithrodontomys megalotis, Peromyscus maniculatus, and Mus musculus. Species considered "moderately tolerant" included Sorex cinereus and Blarina brevicauda . The Eastern chipmunk (Tamias striatus ) and white-footed mouse (Peromyscus leucopus ) were considered the most tolerant in Iowa, and this was also found to be true in the Vermont study of Dodge et al. (1976) and Possardt and Dodge (1978). Species considered most sensitive to riparian vegetation removal in Vermont were jumping mice (Zapus hudsonicus and Napeozapus insignis ) and shrews (Blarina brevicauda, Sorex cinereus ).

Geier and Best (1980) predicted that a reduction in shrub cover would reduce populations of T. striatus and S. cinereus. T. striatus would be especially affected by the selective removal of eastern red cedar. Populations of T. striatus, Peromyscus leucopus , and the two shrew species would suffer from the loss of woody plant debris (logs, brushpiles, and stumps).

Despite these initial efforts, indicator assemblages of mammals "most sensitive" to vegetation removal remain speculative in most of the U.S., and the effects of vegetation removal on overall community structure of mammals have not been well-documented in wetlands.

Thermal alteration. The effects of thermal alteration on overall community structure of mammals apparently have not been documented in wetlands, and "most-sensitive" indicator assemblages remain speculative.

Dehydration/Inundation. Changes in wetland water level and soil moisture alter the quantity and quality of mammal habitat, and may trigger immigration and emmigration of particular species. The effects of dehydration may be particularly severe if they occur during hibernation, due to the effects of exposure. In northern wetlands, muskrats, for example, require deep water in winter for successful hibernation (Bellrose and Low 1943). Although muskrats and minks appeared to tolerate temporary flooding in an Illinois forested floodplain, opossums, red foxes, gray foxes, striped skunks, and woodchucks were evicted by flood conditions (Yaeger 1949).

In northern Florida cypress ponds, Harris and Vickers (1984) found an increase in relative abundance of rice rats and a decrease in cotton rats with any addition of water. In a series of Maine bogs, species richness of small mammals was highest in the driest part of the bog, near the upland edge (Stockwell 1985). In prairie pothole wetlands, small mammals select habitats based on soil moisture levels (Pendleton 1984). In Colorado (Olson and Knopf 1988), mammal species richness, relative diversity, and faunal similarity were greater in upland communities than in riparian wetlands. Richness was also less in Washington riparian areas than in adjoining uplands, although presence of water-placed woody material within the wetter areas mediated this effect to some degree (Mason 1989).

Fossorial mammals (e.g., moles and shrews) that inhabitat subsurface areas may be particularly sensitive to moisture level changes. However, local changes in moisture regimes and other aspects of wetland habitat quality are frequently not reflected by indicator species of mammals because of the ability of mammals to move freely, in and out of impacted areas.

Despite these initial efforts, indicator assemblages of mammals "most sensitive" to habitat dehydration or inundation remain speculative in most of the U.S., and the effects of these stressors on overall community structure of mammals have not been well-documented in wetlands.

Fragmentation/Isolation of Habitat. Although habitat fragmentation has been widely implicated in the decline of some large mammals, we found little explicit documentation of overall mammal community response to fragmentation of regional wetland resources. One can surmise that as the distance between wetlands containing wetland-dependent mammals becomes greater, and/or hydrologic connections and vegetated corridors become severed by dehydrated channels, bank-clearing, or (particularly) roads, the more sensitive mammals or those which do not disperse easily might be most affected. Although individual mammals, being highly mobile, can disperse to new areas having the proper combination of wetland types at a sufficient density, they probably do so at risk of greater predation and energetic loss.

Sensitive species can be grouped into "guilds" that exhibit similar responses to fragmentation. For example, Brooks et al. (1989, 1990) found significant differences in mammal communities in disturbed vs. undisturbed watersheds, and recommended that stream corridors be at least 100 m in width. Home range sizes of wetland mammals have also been used for defining wildlife guilds and required buffer strip sizes (Brown et al. 1989). However, home range sizes can vary greatly by season and habitat type. They can be determined from observations of presence/absence in wetland patches of various sizes and degrees of isolation, or by using radiotelemetry (Hegdal and Colvin 1986 describe techniques).


Some factors that could be important to measure and (if possible) standardize among wetlands when monitoring anthropogenic effects on community structure of mammals include:

distribution of water depth classes, vegetation and woody debris (type, and vertical and horizontal diversity and arrangement), current velocity, distance and connectedness to other wetlands of similar or different type, surrounding land cover (particularly within 500 feet of wetland perimeter), wetland size, ratio of open water to vegetated wetland and its spatial interspersion, and the duration, frequency, and seasonal timing of regular inundation, as well as time elapsed since the last severe inundation or drought.

Methods for surveying mammal communities are described in Cooperrider et al. (1986), Halvorson (1984), and others.

Mammals occur in wetlands throughout the year. Mammal density and richness may be reduced during and immediately after floods in riverine wetlands. Surveys covering several wetlands, if not conducted simultaneously, should occur within consecutive days, unless severe weather conditions intervene. For efficient censusing, advantage can be taken of species that congregate seasonally in wetlands (e.g., white-tailed deer in northern cedar swamps). Diurnally, detection of most species is greatest at night. Visual surveys of larger, day-active species can be conducted from ground level, from elevated observation posts, or aerially. Low-altitude overflights or aerial photography can be used to identify some beaver dams and beaver and muskrat lodges, and to census moose and large mammals in open country. Ground-level, direct-observation techniques cannot be used effectively in wetlands with tall vegetation (mid-season emergent marshes, forested wetlands).

Many methods have been developed for monitoring wetland mammal communities, and generally rely on various types of traps. Tracks, scat, den trees, burrows, vocalizations, eyeshine, and other sign may also be counted using point counts, line transects, or similar methods. Some species can be attracted to scent stations or salt blocks. Most non-capture methods can be used in virtually all types of wetlands. Methods differ mainly in the degree of quantification they provide, the level-of-effort required, and the taxa they are most effective in censusing. Thus, whenever possible a variety of methods should be used.

Spring-loaded snap traps, live (cage) traps, pitfall traps, and funnel traps are widely used for capturing mammals. Animals are attracted by bait or, in the case of pitfall traps, stumble into a confining pit and usually cannot 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.

The efficiency of traps and funnels can be increased by channeling small animal movements 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 of at least 5-15 m. Lengths less than 2.5 m are not very effective (Bury and Corn 1987). 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 emplanted in the ground, or at least no space is provided for non-burrowing animals to crawl under the fence. Sizes and shapes of containers and associated drift fences and their configurations vary greatly, depending partly on target species and wetland type. Trap and funnel methods can provide relatively quantitative data, when arranged systematically and level-of-effort (e.g., "trap-hours") is standardized.

The size of the trap, baits used, and trap placement can affect the species that are caught. Thus, a variety of methods should be used if possible (Szaro et al. 1988). Snap traps are effective for cricetids and many other small rodents (e.g., meadow vole, short-tail shrew, house mouse, western harvest mouse, masked shrew)(Geier and Best 1980), whereas pitfall traps are more effective for rodents that are primarily insectivorous and/or fossorial (moles and shrews)(Szaro 1988). Funnel traps are ineffective in capturing many forest mammals (Bury and Corn 1987). If only a single type of capture method can be used and the aim is to capture the widest variety of small mammals, then in Pacific Northwest forests, Bury and Corn (1987) recommend use of pit traps over a continuous 60-day period; a list of the most common species could be compiled by using pitfall traps only for a typical 10-day trapping period. However, the high water table in many wetlands can render pitfall traps impractical due to flooding. In these situations, spring-loaded traps mounted on floating platforms are effective for detecting some species (pers. comm., T. Roberts, Waterways Experiment Station, Vicksburg, MS).

Examples of community-level mammal studies in wetlands include, for example: Cross 1985 (Oregon), Geier and Best 1980 (Iowa), Landin 1985 (Mississippi), McConnell and Samuel 1985 (West Virginia), Olson and Knopf 1988 (Colorado), Scelsi (n.d.)(New Jersey), and Urbanek and Klimstra 1986 (Illinois).


In general, quantitative data on structure of the entire mammalian community of wetlands has not been uniformly collected from a series of statistically representative wetlands in any region of the country. Thus, it is currently impossible to state what are "normal" levels for parameters such as mammal density, species richness, or biomass, and their temporal and spatial variability, in any type of wetland.

We found only a few published studies that quantified the entire mammalian community (or a large proportion of it) among a set of wetlands: Brooks et al. 1985, 1987, 1989, Geier and Best 1980, Landin 1985, Nordquist and Birney 1980, Pardue et al. 1975, Stockwell 1985, and Urbanek and Klimstra 1986 (Illinois).

We found no journal articles that quantified year-to-year or long-term variation in mammalian community structure in wetlands, but conceivably such unpublished data may be available from sites of the U.S. Department of Energy's National Environmental Research Park system, sites of the National Science Foundation's Long Term Ecological Research (LTER) program, and regional studies of the ELF military communications facility (Blake et al. 1987).

Quantitative data on composition of wetland mammalian communities is virtually lacking from all regions except parts of the Northeast and some riparian systems. Information on impacts is limited mostly to studies of hydrologic effects and vegetation removal; especially little is known of impacts to community structure from contaminants, salinization, sedimentation, and habitat fragmentation.

Qualitative lists of "expected" mammals in wetlands can be easily developed in most regions from Niering (1985), Fritzell (1988), and the "Vertebrate Characterization Abstracts" database managed by The Nature Conservancy and various state Natural Heritage Programs. Limited qualitative information may be available by wetland type from some of the "community profile" publications of the USFWS (Appendix C).

However, fine gradations in degree of dependency of individual species upon wetlands have not been defined. Quantitative data are most available for harvested species, while the majority of wetland mammals, which are not harvested, are seldom studied quantitatively in wetlands.

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