Summer Precipitation Predicts Spatial Distributions of Semiaquatic Mammals.

Climate change is predicted to increase the frequency of droughts and intensity of seasonal precipitation in many regions. Semiaquatic mammals should be vulnerable to this increased variability in precipitation, especially in human-modified landscapes where dispersal to suitable habitat or temporary refugia may be limited. Using six years of presence-absence data (2007-2012) spanning years of record-breaking drought and flood conditions, we evaluated regional occupancy dynamics of American mink (Neovison vison) and muskrats (Ondatra zibethicus) in a highly altered agroecosystem in Illinois, USA. We used noninvasive sign surveys and a multiseason occupancy modeling approach to estimate annual occupancy rates for both species and related these rates to summer precipitation. We also tracked radiomarked individuals to assess mortality risk for both species when moving in terrestrial areas. Annual model-averaged estimates of occupancy for mink and muskrat were correlated positively to summer precipitation. Mink and muskrats were widespread during a year (2008) with above-average precipitation. However, estimates of site occupancy declined substantially for mink (0.56) and especially muskrats (0.09) during the severe drought of 2012. Mink are generalist predators that probably use terrestrial habitat during droughts. However, mink had substantially greater risk of mortality away from streams. In comparison, muskrats are more restricted to aquatic habitats and likely suffered high mortality during the drought. Our patterns are striking, but a more mechanistic understanding is needed of how semiaquatic species in human-modified ecosystems will respond ecologically in situ to extreme weather events predicted by climate-change models.

Polyaromatic hydrocarbons and elements in sediments associated with a suburban railway.

Railroad operations are a potential source for contamination of aquatic ecosystems. We examined concentrations of polyaromatic hydrocarbons (PAHs) and selected elements in sediments collected during 2009-2011 from streams, ditches, or ponds bisected or bordered by the former Elgin, Joliet, and Eastern rail line in the western Chicago, Illinois, metropolitan area. Summed PAH concentrations were greater in sediments collected downstream than in those collected upstream of the railroad and were negatively associated with distance within 500 m of the tracks. Phenanthrene and dibenzo (a,h)anthracene concentrations at some locations exceeded probable effect thresholds for risks to aquatic life. Although maximum levels of chromium (Cr) were below levels of concern, we did not determine the valence state of Cr; thus, risks to aquatic life could not be fully evaluated. Nickel and mercury concentrations exceeded lower effect levels, and vanadium concentrations exceeded chronic toxicity thresholds at some locations, although we did not detect an association between these elements and the presence of the railroad. Lead and arsenic concentrations were greater in proximity to the railroad; however, concentrations were below thresholds of concern for aquatic life. Our results suggest that the railroad and associated activities are contributing some environmental contaminants to waterways in close proximity to it, particularly in a downstream direction. Risks to aquatic life may be greater than implied by observed concentrations of individual contaminants, as synergistic adverse effects are likely to occur with exposure to complex mixtures.

Risk factors for Toxoplasma gondii exposure in semiaquatic mammals in a freshwater ecosystem.

We assessed risk factors for Toxoplasma gondii exposure in semiaquatic mammals in east-central Illinois, US. This agricultural region has extensive drainage systems that could potentially transport T. gondii oocysts into the watershed. We used muskrats (Ondatra zibethicus) and American mink (Neovison vison) as sentinels of watershed contamination. We predicted individuals from larger subwatersheds would more likely be antibody-positive for T. gondii, as they were exposed to drainage from larger areas. We also evaluated amount of urban land cover within the subwatershed, proximity to farmsteads, and age of individuals in competing models of T. gondii infection. Antibodies to T. gondii were assayed in animal sera by modified agglutination tests (titer 25 or higher) and detected in 18 (60%) of 30 muskrats and 20 (77%) of 26 mink. Infection rates were ≥1.7 times higher than those typical for mammals in upland habitats in this region. Subwatershed size and age class were important predictors of T. gondii infection in muskrats (R(2) = 0.35). Models incorporating urban land cover and proximity to farmsteads had little support. None of our models of antibody prevalence in mink were well supported, possibly because mink are less strictly associated with riparian habitats. Because ~91% of our study area is devoted to agricultural production and urbanization, transport of T. gondii into freshwater ecosystems is likely facilitated by modified drainage practices common in these areas.

Hematologic and blood chemistry reference values for free-ranging muskrats (Ondatra zibethicus).

Baseline hematologic and serum chemistry values are used by veterinarians and wildlife biologists to identify abnormally high or low levels of particular blood parameters in a target species. This vital information can assist animal care providers in making informed decisions on the care of wildlife and help to determine diagnoses for certain illnesses. Published blood parameter values are not available for wild-caught muskrats (Ondatra zibethicus). We measured 27 blood parameter values from 29 free-ranging, riparian muskrats caught from June-November 2008 in east-central Illinois, USA, and compared mean values between adults and juveniles. Adult muskrats had higher levels of globulins (F(1,27)=6.394, P ≤ 0.018) and eosinophils (F(1,25)=6.883, P ≤ 0.015) than did juvenile muskrats, possibly because of increased exposure to parasites and allergens over time.

Heavy metal exposure, reproductive activity, and demographic patterns in white-footed mice (Peromyscus leucopus) inhabiting a contaminated floodplain wetland.

We examined the concentrations of selected metals and selenium (Se) in the tissues of white-footed mice (Peromyscus leucopus) collected at a constructed wetland originally created as a retention basin for sediments dredged from Lake DePue, Illinois. These sediments were contaminated with high concentrations of cadmium (Cd), zinc (Zn), and other elements as a result of nearby smelting operations. White-footed mice inhabiting the former retention basin experienced greater exposure to Cd, Pb, and Se than those from nearby reference sites. Concentrations of Cu and Zn in livers of mice from the contaminated wetland and adjacent floodplain reference site were greater than in mice from the more-distant reference sites. Judging by concentrations in their kidneys, white-footed mice inhabiting the floodplain adjacent to the contaminated wetland had greater exposure to Cd than those from the two more-distant reference sites. Concentrations of Hg in tissues of mice did not vary appreciably among sites. Concentrations of Cd and Se in the tissues of some white-footed mice from the contaminated wetland exceeded critical concentrations observed in experimental studies of laboratory mice and rats; with few exceptions tissue Pb concentrations were below published effects levels. However, we did not detect changes in abundance, demographics, or reproductive activity that might suggest population-level effects of contaminant exposure. Mean weight of embryos expressed as a function of crown-rump length did not differ among locations sampled, and no gross lesions indicative of exposure to heavy metals were observed. Kidney and liver weight, corrected for body weight, were nominally, though not significantly, lowest in both male and female mice from areas of increased Cd and Pb exposure. Metals dredged from Lake DePue were still bioavailable 25 years after deposition. However, small mammal populations are resilient to environmental stressors and we did not detect differences in population parameters suggesting that the population of white-footed mice inhabiting the contaminated wetland was at risk from increased exposure to these contaminants.

Comparative phylogeography of eastern chipmunks and white-footed mice in relation to the individualistic nature of species.

Palaeoecological studies have demonstrated that ecological communities as a whole did not remain stable throughout the climatic fluctuations of the Quaternary. The result is that long-term associations of species cannot be inferred by contemporary associations in ecological communities. Therefore, the evolutionary significance of any contemporary ecological interactions among species and of the biotic community within which species have evolved also cannot be assumed from contemporary conditions. Comparative phylogeographic data provide a method to identify species within ecological communities that have shared biogeographic histories. We present an example of a long-term association between populations of two mammalian species, eastern chipmunks (Tamias striatus) and white-footed mice (Peromyscus leucopus), which are commonly associated with deciduous forest habitats. The distribution of mitochondrial DNA variation in T. striatus and P. leucopus from previously glaciated regions of the eastern United States support the hypothesis that, in at least part of their range, genetic lineages of the two species have expanded from similar population sources since the Last Glacial Maximum. In addition, the spatial concordance of genetic lineages of T. striatus and P. leucopus with the oak-savannah forest formations of Wisconsin and Illinois, suggest that populations associated with this community colonized the area in association with a set of arboreal species that comprise their deciduous forest habitat.

A survey for West Nile virus in bats from Illinois.

A blocking enzyme-linked immunosorbent assay was used to test 97 serum samples from big brown bats (Eptesicus fuscus) captured in six counties in Illinois between May 2002 and February 2004 for West Nile virus (WNV) antibodies. One female big brown bat tested positive for WNV antibodies. Samples of kidney, liver, and heart tissue were collected from 312 bats of seven species that were submitted to the Illinois (USA) Department of Public Health or the Illinois Department of Agriculture diagnostic laboratories between January 2001 and December 2003. Tissue samples were tested for WNV using TaqMan reverse transcriptase polymerase chain reaction and all were negative. Prevalence of WNV antibodies in the bats (1%) was lower than previously reported for other flaviviruses, but similar to the prevalence (2%) of WNV antibodies reported in bats from New Jersey and New York, USA. Additional research is needed to determine potential impact of WNV infections on bats and to determine whether they play a role in the WNV transmission cycle.

Surviving the ice: Northern refugia and postglacial colonization.

The contemporary distribution of biological diversity cannot be understood without knowledge of how organisms responded to the geological and climatic history of Earth. In particular, Quaternary expansions and contractions of glacial ice sheets are thought to have played an important role in shaping the distribution of biodiversity among current populations in the north-temperate region. In the central U.S., fossil and palynological data provide support for the maintenance of a large southeastern refuge during the last glacial maximum, and many temperate organisms are believed to have responded to glacial expansion by shifting their ranges to southern refugia and recolonizing northward to track the receding ice sheets. Thus, organisms are assumed to track favorable climates, and species ranges are expected to have shifted significantly. Here we present data from a deciduous forest vertebrate, the eastern chipmunk (Tamias striatus) in the central U.S., indicating the maintenance of multiple refugial sources as well as a southward expansion from a northern refugium. These results challenge the view that, during glacial maxima, organisms must have migrated south out of their ranges to track favorable climates.

Effects of kangaroo rat exclusion on vegetation structure and plant species diversity in the Chihuahuan Desert.

Long-term (1977-90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.

Why spacing behavior does not stabilize density in cyclic populations of microtine rodents.

There are several published hypotheses that consider spacing behavior to be a significant factor causing the multiannual density fluctuations characteristic of some microtine rodent populations. Recent modeling efforts have concluded, however, that spacing behavior should have a stabilizing rather than a destabilizing effect on population dynamics. Why doesn't spacing behavior stabilize these cyclic populations? We argue that while spacing behavior does have a stabilizing influence on population dynamics by limiting the number of breeding individuals, reproduction continues and population size is not limited in an asymptotic manner. Rather, microtine social organization produces demographic changes within a population that allow density cycles to occur under certain conditions. Using a simulation model, we demonstrate that in a strongly seasonal environment populations with low density dependence in reproduction will cycle whereas populations with high density dependence in reproduction will have relatively stable densities. Given such complicating factors as the "annual species" nature of microtine rodents, occasionally intense predation, and the tendency for territoriality to break down during the non-breeding season, individuals with low density dependence in reproduction will always be able to invade and eventually dominate populations with high density dependence in reproduction, regardless of the resulting destabilization of population dynamics.