Wednesday, 19 October 2011

Muskrats are tolerant of multiple forms of human disturbance

Among the many studies that have investigated the effects of urbanization on wildlife, few, if any, have focused on a species primarily valued as an economic commodity. This knowledge gap was recently addressed by researchers from the University of Illinois who studied the effects of anthropogenic landscape alterations on muskrats (Ondatra zibethicus). These animals, native to North America and invasive in Europe, Asia, and South America, are a valuable source of fur (at one recent auction, their pelts sold for approximately $6 apiece); in some areas, they are even eaten. Muskrats are particularly interesting in an anthropogenic disturbance context because, as semiaquatic animals, they are exposed to human habitat modifications while both on land and in the water.

(A muskrat, Ondatra zibethicus)

The muskrat study was designed to measure the animals' tolerance of 2 major anthropogenic habitat changes: introduction of an invasive species (reed canary grass, Phalaris arundinacea) and degradation of territory suitability via modification or removal of preferred habitat. The reed canary grass is thought to have lower nutritional value than native vegetation, and its introduction has led to dramatic reconfigurations of local ecosystems. Thus, the researchers anticipated that they would find fewer muskrats in areas with high levels of this invasive species. However, the broader landscape-level characteristics of anthropogenic disturbance--such as the presence of urban habitat and associated increases in stream flow--were predicted to have a positive impact on muskrats by, among other things, releasing them from the risks of predation and trapping. 

The research was carried out in the Grand Prairie Region of central Illinois,  an area containing 5 watersheds with a variety of streams ranging from small headwaters to large rivers. The study sites were selected such that 50% were near incorporated towns, and the other 50% were in more rural (i.e., agricultural) areas; thus, the researchers were able to survey muskrat habitat along an anthropogenic disturbance gradient. Over a 2-year study period, 518 surveys were conducted along 200-m stretches of stream at 90 sites. 

(Reed canary grass, Phalaris arundinacea. Native to temperate Europe, Asia, and North America, this invasive species was introduced in the 1880's to provide forage for animals and help reduce erosion. According to a recent report by the Eastern Forest Environmental Threat Assessment Center, the species is now found in 43 of 50 U.S. states.)

The first goal during each survey was to locate evidence of muskrat occupancy--scat, tracks, signs of feeding, the presence of burrows, or, of course, muskrats themselves. The second goal was to record local and landscape-level habitat characteristics that might be associated with muskrat occupancy. These included variables associated with the stream and its banks--width and depth of the channel, percent sand in the soil, width of the wooded habitat on either side of the stream, and various aspects of the bank morphology--as well as type, amount, and dominant vegetation in the land cover within a 500-m buffer around each site. The scientists then constructed "occupancy models," which pinpoint the habitat characteristics most closely associated with the presence of muskrats.

During both years of the study, muskrats were detected in over half the sites surveyed. The probability of occupancy could be predicted by 4 major habitat characteristics: size of the stream, height of the stream bank, percentage of sand in the soil, and amount of urban land cover. Specifically, muskrats were more likely to be found in territories with larger streams that had higher banks (in which the muskrats dig their burrows), more clay-ey soil (which is easier to shape a burrow from), and higher levels of impervious surface cover and human structures. Surprisingly, and despite the fact that reed canary grass was the dominant species at approximately half of the sampling stations at each site, the presence of this nonnative plant did not seem to have an impact on muskrat occupancy. In fact, muskrats were frequently observed carrying reed canary grass clippings, suggesting that they were eating it, using it to build nests, or both.

(Champaign-Urbana (aka "Chambana"), Illinois--the center of the study area.)

The ability of muskrats to happily inhabit human-modified areas suggests that these animals are urban adaptors. This is not entirely surprising given their small body size and behavioral plasticity--two traits common in species that are tolerant of anthropogenic disturbances. Overall, anthropogenic habitats like the ones studied here provide many potential benefits for muskrats. Streams in and near urban areas are often deep and quick-running as a result of high levels of runoff from impervious surfaces. This means that they probably provide good cover from potential muskrat predators, such as American mink (Neovison vison), coyotes (Canis latrans), and red foxes (Vulpes vulpes). Human environments are characterized by the presence of culverts, the use of which helps muskrats avoid potential collisions with cars. Additionally, anthropogenic areas typically contain many small bodies of water (such as decorative ponds) that can act as "stepping stones" for dispersing individuals.

(An American mink, Neovison vison--the most common muskrat predator. Studies of invasive mink in Europe show that these animals avoid human-disturbed habitats, which means that urban muskrats are probably safe from this particular source of harm.)

It is possible that muskrats in other locations at the extreme ends of the "disturbance spectrum"--such as those in more built-up cities or even "wilder" rural areas--might show different responses to the human disturbance factors measured here. It is also not clear whether the animals suffer nutritional deficits from the potential inclusion of reed canary grass in their diets. Finally, there is little information on the longer-term dynamics of territory selection and abandonment. Thus, additional data need to be collected before any definitive conclusions can be drawn about the health of muskrat populations in disturbed areas.  

Cotner, L.A., Schooley, R.L. 2011. Habitat occupancy by riparian muskrats reveals tolerance to urbanization and invasive vegetation. Journal of Wildlife Management 75(7):1637-1645.

Thanks to the following websites for providing the images used in this post:

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