Density-dependent habitat selection in muskrats: a test of the ideal free distribution model

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.

Active density-dependent habitat selection in a controlled population of small mammals

Density-dependent habitat selection has numerous and far-reaching implications to population dynamics and evolutionary processes. Although several studies suggest that organisms choose and occupy high-quality habitats over poorer ones, definitive experiments demonstrating active selection, by the same individuals at the appropriate population scale, are lacking. We conducted a reciprocal food supplementation experiment to assess whether voles would first occupy a habitat receiving extra food, then change their preference to track food supplements moved to another habitat. Meadow voles, as predicted, were more abundant in food-supplemented habitat than in others. Density declined when food supplements ceased because the voles moved to the new habitat receiving extra food. Although males and females appeared to follow different strategies, meadow-vole densities reflected habitat quality because voles actively selected the best habitat available. It is thus clear that behavioral decisions on habitat use can motivate patterns of abundance, frequency, and gene flow that have widespread effects on subsequent evolution.

Local- and landscape-level den selection of striped skunks on the Canadian prairies

We examined the seasonal landscape and habitat use patterns of striped skunks ( Mephitis mephitis Schreber, 1776). We tracked 52 male and 72 female skunks from September 1999 to June 2003 in Saskatchewan, Canada. At the local level, den structures differed by sex and season. In autumn/winter, all skunks preferred buildings, whereas in spring/summer females selectively used underground burrows and rock piles for parturition and rearing of young, and males used aboveground retreats. Den sites were closer to crop fields, roads, water sources, and macrohabitat edges than random sites. At the landscape level, den sites were associated positively with weighted mean shape index of crop fields, mean patch size of water bodies, total edge of water bodies, and weighted mean fractal dimensions of grassland, woodland, and farmsteads, suggesting that wetland edges and habitat complexity are important in den selection by skunks. Compositional analysis revealed sex- and season-specific differences at the population level. Both sexes preferred grassland/pastures and farmstead habitats for establishing den sites. In autumn/winter, skunks preferred grassland/pastures for winter dens. However, in spring/summer skunks preferred farmsteads for resting sites. Our results suggest that skunks respond to landscape and habitat features surrounding den sites, and not just specific den or local habitat characteristics.

Density-dependent habitat selection by brown-headed cowbirds ( Molothrus ater) in tallgrass prairie

Local distributions of avian brood parasites among their host habitats may depend upon conspecific parasite density. We used isodar analysis to test for density-dependent habitat selection in brown-headed cowbirds ( Molothrus ater) among tallgrass prairie adjacent to wooded edges, and prairie interior habitat (>100 m from wooded edges) with and without experimental perches. Eight study sites containing these three habitat treatments were established along a geographical gradient in cowbird abundance within the Flint Hills region of Eastern Kansas and Oklahoma, USA. The focal host species of our study, the dickcissel ( Spiza americana), is the most abundant and preferred cowbird host in the prairie of this region. Cowbird relative abundance and cowbird:host abundance ratios were used as estimates of female cowbird density, whereas cowbird egg density was measured as parasitism frequency (percent of dickcissel nests parasitized), and parasitism intensity (number of cowbird eggs per parasitized nest). Geographical variation in cowbird abundance was independent of host abundance. Within study sites, host abundance was highest in wooded edge plots, intermediate in the experimental perch plots, and lowest in prairie interior. Cowbirds exhibited a pattern of density-dependent selection of prairie edge versus experimental perch and interior habitats. On sites where measures of cowbird density were lowest, all cowbird density estimates (female cowbirds and their eggs) were highest near (< or =100 m) wooded edges, where host and perch availability are highest. However, as overall cowbird density increased geographically, these density estimates increased more rapidly in experimental perch plots and prairie interiors. Variation in cowbird abundance and cowbird:host ratios suggested density-dependent cowbird selection of experimental perch over prairie interior habitat, but parasitism levels on dickcissel nests were similar among these two habitats at all levels of local cowbird parasitism. The density-dependent pattern of cowbird distribution among prairie edge and interior suggested that density effects on perceived cowbird fitness are greatest at wooded edges. A positive relationship between daily nest mortality rates of parasitized nests during the nestling period with parasitism intensity levels per nest suggested a density-dependent effect on cowbird reproductive success. However, this relationship was similar among habitats, such that all habitats should have been perceived as being equally suitable to cowbirds at all densities. Other unmeasured effects on cowbird habitat suitability (e.g., reduced cowbird success in edge-dwelling host nests, cowbird despotism at edges) might have affected cowbird habitat selection. Managers attempting to minimize cowbird parasitism on sensitive cowbird hosts should consider that hosts in otherwise less-preferred cowbird habitats (e.g., habitat interiors) are at greater risk of being parasitized where cowbirds become particularly abundant.

Assessment of source-sink theory for predicting demographic rates among habitats that exhibit temporal changes in quality

Within the framework of Pulliam's source-sink model we tested predictions of habitat-specific demography on a "closed" population of muskrats (Ondatra zibethicus L.) occupying three contiguous habitats that exhibited temporal changes in quality and quantity. We were able to distinguish between dispersal among habitats and mortality within each habitat, and induce temporal variation in operational-habitat availability and suitability by manipulating water level. Temporal variation in population size and density among habitats supported the source-sink model and was primarily associated with habitat-specific survival rates. For example, the mean annual over-winter mortality rate of individuals in the principal source habitat (0.87) was less than in the sink habitat (0.94), and subsequently the mean annual finite rate of increase (λ) was positive in the source habitat (λ = 1.41) and negative in the sink habitat (λ = 0.90). The high recruitment rate of juveniles in the prime habitat during autumn was also associated with significant emigration of juveniles from this habitat. Emigration of adults among habitats provided support for the role of spacing behaviour in regulating the breeding density of muskrats in prime habitat. However, in contrast to the assumption of source-sink theory, year-to-year variation in survival rate in the more marginal habitats appeared to be explained more by temporal changes in habitat suitability than by density. Significant emigration of juveniles from the sink habitat was not predicted and was largely dependent on current habitat conditions. In addition, the mean annual emigration rate was lowest in the principal source habitat (0.30) and highest in the more marginal habitats (0.62). Failure to detect directional emigration from prime to marginal habitats in the spring, as predicted by the source-sink model, was likely due to declining local population size. In environments where spatial differences in habitat quality are not static, and annual change in local population size is largely independent of density, current source-sink models must be modified to better predict individual dispersal strategies.

Influence of the density of a sika deer population on activity, habitat use, and group size

The influence of population density of sika deer (Cervus nippon) on their activity, habitat use, and habitat-related group size was studied in the area of Japan's Tanzawa Mountains in early winter. The percentage of active sika deer was higher in January 1994 (a low-density period) than in January 1995 (a high-density period). The pattern of habitat use differed between periods. During the high-density period, deer increased their relative use of lower quality habitats. A possible mechanism and implications of this are discussed. Changes in population density had a positive effect on group size in sika deer, though the variation among habitat types supported the hypothesis that food biomass is an important determinant of group size.

Density-dependence in habitat utilisation by wood mice in a Sitka spruce successional mosaic: the roles of immigration, emigration, and variation among local demographic parameters

Density dependence in habitat utilisation by a population of wood mice (Apodemus sylvaticus L.) was investigated within a habitat mosaic produced by the felling of mature Sitka spruce (Picea sitchensis Bong. Carrière). A capture-mark-recapture study was carried out from May 1990 to June 1992, with trapping in five 0.81-ha grids marked in mature forest and clear-fellings of different ages. Intergrid movements were frequent (15.3% of all recaptures). A significant linear negative relationship was found between overall population sizes and degree of heterogeneity in habitat utilisation, although the former explained only about one-quarter of the variation in the latter (r2= 0.233). Wood mice were more evenly distributed among grids when numbers in the area were high, indicating density-dependence in their habitat utilisation. Demography was studied at subpopulation (grid) level to determine whether the observed pattern was due to movements among grids or to intergrid variation in the balance between recruitment and losses. Partial regression coefficients in a multiple regression showed that emigration/immigration had a lesser role in explaining the pattern, although the former was significant in explaining part of subpopulation decreases and the latter was significant in explaining part of their increases. Differences in the balance between recruitment and loss rates among local subpopulations accounted for most of the observed density-dependent changes in habitat utilisation. These results do not support the view that such changes are mostly due to active choices made by individuals moving among habitats.

Effect of size-dependent muskrat (Ondatra zibethica) predation on the spatial distribution of a freshwater clam, Anodonta piscinalis Nilsson (Unionidae, Bivalvia)

We studied the effect of central-place foraging by muskrats on the spatial distribution of freshwater clam Anodonta piscinalis. We also analysed the prey-size preference of muskrats. We collected A. piscinalis shells from four muskrat middens representing different prey populations and sampled the clam populations quantitatively. Muskrats had clear effects on the spatial distribution of the clams. At all study sites the area close to shore had no clams. The width of the empty area was correlated with the number of shells found in the muskrat midden. The density of clams decreased and their mean size increased with the distance from muskrat midden at two of the sites. Muskrats did not prey on clams smaller than 50 mm. Muskrats preferred 60- to 70-mm clams at three of the sites and 85- to 90-mm clams at the fourth. In an analysis conducted using ages, a selection gradient on the growth rate of clams was found for three of the study populations. However, spatial refuge from predation and inconsistency of selection may slow down or counterbalance the evolutionary response to predation.