Chronic Wasting Disease in Deer and Elk: a Critique of Current Models and Their Application

Chronic wasting disease (CWD), a fatal transmissible spongiform encephalopathy of deer (Odocoileus spp.) and elk (Cervus elaphus), presents a challenge to wildlife managers because little is known about its transmission, yet it could severely threaten wildlife populations if action is not taken rapidly. Published mathematical models predict that CWD could devastate populations of free-living deer and elk, prompting wildlife managers to attempt large-scale eradication of deer in hopes of containing CWD outbreaks. Our objective is to critically examine the theoretical and empirical support for current models of CWD epizootiology, in light of herd health-management actions. We identify a critical, untested premise (i.e., strictly frequency-dependent transmission) that underlies the dire model predictions. We re-evaluate published comparisons of model output with field data and find little support for published model structures. Given the uncertainty surrounding the future effects of chronic wasting disease on deer and elk populations, and the potential costs of unnecessarily culling large numbers of charismatic and valuable animals, we propose that consideration of alternative models and management actions in a decision–theoretic framework is necessary for wildlife management actions to retain their scientific basis

Distribution of the Long-tailed Weasel (Mustela frenata) in Illinois

Knowing the geographic distribution of any species is an important first step toward understanding its ecology. Long-tailed weasels (Mustela frenata) are secretive and difficult to detect, so little is known about their distribution in Illinois. To fill this gap in the knowledge, we solicited information from museum curators about the sites where long-tailed weasel specimens were collected in Illinois and surveyed biologists and recreational trappers to document locations of where they had seen or captured long-tailed weasels. We present an updated map of long-tailed weasel distribution, documenting presence in at least 60 of the 102 counties in Illinois. Long-tailed weasels were seen or captured in a variety of habitats, including fields, forests, and forest edges. Most survey respondents considered long-tailed weasels to be relatively scarce. The information we have collected more than doubles the number of counties where long-tailed weasels are known to exist, and indicates that this small carnivore is widely distributed throughout the state

Behavioral Indicators of Predator Space Use: Studying Species Interactions Through the Behavior of Predators

Predation has major impacts on survival and reproductive success for many species. To quantify these effects, ecologists often choose to intensively study prey populations to measure predation rates and/or estimate predator abundance. But in some cases, predation rates are less strongly related to predator abundance per se than to spatial and temporal patterns of predator space use; thus, quantifying the latter may provide meaningful surrogates of predation rates that scale up to larger areas. This is particularly true when safety for prey, especially sessile and vulnerable prey, is strongly linked to predator-free space. Our own research programs have used two general types of behavioral indicators to quantify space use by predators: giving-up densities, as a surrogate for patch quitting harvest rates, and activity density. We discus.s two general mechanisms by which predator-free (or predator-poor) space is created and link these mechanisms to behavioral indicators that can be easily collected in the field. We then summarize our past work on prédation on passerine nests and moth pupae to demonstrate how using behavioral indicators of space use can reveal much about the impact of a predator on its prey. We demonstrate that behavioral indicators can be used for the following: (1) leading indicators for predation rates, (2) surrogates for information otherwise difficult to obtain, (3) integrative measures of the strength of species interactions, and (4) to reveal the outcomes of ecological interactions, such as prey persistence

Estimating Abundance of Adult Trachemys scripta with Camera Traps: Accuracy, Precision and Probabilities of Capture for a Closed Population

Low and variable rates of capture are common problems when estimating abundance of freshwater turtles with capture-mark-recapture (CMR). We speculated camera traps would allow us to obtain reliable estimates of abundance by re-sighting marked Trachemys scripta elegans (Sliders) as they basked on man-made rafts during a 20-day surveillance period. We evaluated the method by releasing Sliders in a fenced enclosure to compare estimates from CMR to true abundance. We also evaluated probabilities of detection and retention of marks. Permanence of marks applied with marine epoxy satisfied assumptions for CMR. Camera traps detected 23 of 25 Sliders. Our ability to discern marks from photos was good (110 of 114 re-sightings). The proportion of marked Sliders detected per day was 0.22; detection varied with day of surveillance (1–20) and maximum air temperature the preceding day. All CMR models providing valid estimates of abundance included the true number of marked Sliders in their confidence intervals and yielded point estimates within 27% of the true value. An estimate of abundance from the top CMR model exceeded the true value by 22%, with a wide confidence interval. Model averaging improved the point estimate (17% over true) and produced a narrower confidence interval. A favorable comparison of estimated and true abundance validated camera traps as a tool for estimating abundance of adult Sliders. We believe camera traps could prove useful for detecting biases caused by primary methods of capture, refining estimates of abundance from other methods and collecting data at multiple locations consistently, simultaneously and frugally compared to manual methods alone

Ranging Behavior of Marsh Rice Rats in a Southern Illinois Wetland Complex

The marsh rice rat (Oryzomys palustris) inhabits wetlands that are often fragmented and isolated by upland cover types. Persistence of marsh rice rat populations and metapopulations likely depends on their ability to enter and traverse the upland matrix, yet basic information, such as home-range size and landcover use patterns, is lacking. Our goal was to quantify home-range size and habitat selection by marsh rice rats in southern Illinois. Between March and November 2011, we radiocollared 21 male rice rats (8 subadults and 13 adults) that were each located 7 to 24 times each via triangulation and homing. We estimated home-range size, compared landcover composition within kernel home ranges to what was available in the surrounding landscape, and quantified daily movement distances. Mean (±SE) home ranges were 3.53 ± 0.66 ha based on 95% kernel isopleths and 1.85 ± 0.49 ha based on minimum convex polygons. Home ranges were largest for individuals followed in early summer, but home-range sizes were similar for adults and subadults. Rice rats’ use of emergent wetland vegetation was greater than availability, indicating they preferred emergent wetlands habitat at the home-range level. However, upland cover types made up \u3e40% of each home range, on average. Daily movements averaged 46.6 ± 3.4 m (maximum: 396 m), and rice rats were located up to 464 m from the nearest wetland. Based on by far the largest sample size (in individuals and locations per individual) available for space use of the marsh rice rat, our findings support the characterization of male rice rats as highly vagile and suggest that rice rats move through upland cover more frequently than previously described

Modeling the Effects of Reservoir Competence Decay and Demographic Turnover in Lyme Disease Ecology

Lyme disease risk is related to the abundance of infected nymphal ticks, which in turn depends on the abundance and reservoir competence of wild hosts. Reservoir competence of a host (i.e., probability that an infected host will infect a feeding vector) often declines over time after inoculation, and small mammalian reservoirs typically undergo rapid population growth during the period when vector ticks feed. These processes can affect disease risk in the context of site-specific tick abundance and host community composition. We modeled the effects of reservoir decay and host demographic turnover on Lyme disease risk using a simple yearly difference equation model and a more realistic simulation incorporating seasonal dynamics of ticks and hosts. Both reservoir decay and demographic turnover caused (1) specific infectivity (proportion infected 3 reservoir competence) of host populations to vary with host community composition, (2) tick infection prevalence and the specific infectivity of reservoirs to be highly sensitive to the abundance of questing nymphs, and (3) specific infectivity and the infection prevalence of ticks to decrease at high host densities. Reservoir competence decay had similar effects in both model formulations, but host turnover had less effect than reservoir decay in the seasonal model. In general, exponential reservoir decay and abrupt loss of reservoir competence had similar effects, although exponential decay caused greater sensitivity to tick density and host community composition. Reservoir decay may explain the observed variability in published field measurements of reservoir competence of a host species. Our results illuminate mechanisms by which host diversity can dilute the impact of a highly competent reservoir and suggest that management to reduce nymphal tick abundance may reap an added benefit by reducing nymphal infection prevalence

Comparative Predation on Naturally Occurring Gypsy Moth (Lepidoptera: Lymantriidae) Pupae and Deployed Freeze-Dried Pupae

Predation is an important factor in the dynamics of gypsy moth (Lymantria dispar L.) populations, yet predation rates can be difficult to estimate accurately in the field. Biased estimates can result from spatial heterogeneity in risk or from artifacts associated with deploying prey. Here we compare predation rates on freeze-dried gypsy moth pupae affixed with beeswax to pieces of burlap with predation rates on naturally occurring live pupae in the same sites. Daily predation rates, primarily by small mammals, were two to eight times greater for freeze-dried deployed pupae than natural pupae, depending on the year. These results indicate apparent predation rates can be substantially biased by artifacts associated with deployed prey, such as human scent, artificial substrates, or freeze drying. Results from studies using similar methods may provide qualitative comparisons of relative predation risk, but their estimates of absolute predation rates should be interpreted with caution, and attempts should be made to quantify and correct for any resulting bias

Comparing Permeability of Matrix Cover Types for the Marsh Rice Rat (Oryzomys palustris)

Context Matrix land cover types differ in permeability to animals moving between habitat patches, and animals may actually move faster across lesssuitable areas. Marsh rice rats are wetland specialists whose dispersal crosses upland matrix. Objectives Our objectives were to (1) compare matrix permeability for the marsh rice rat among upland cover types, (2) compare permeability within versus outside perceptual range of the wetland, and (3) explore intrinsic and extrinsic features influencing matrix use and permeability. Methods We quantified permeability of grassland, crop field, and forest to the marsh rice rat during 2011–2012, by marking rats in wetlands and estimating the slope of capture rate versus distance (0–95 m) into the matrix. We also compared permeability within (0–15 m) and beyond the perceptual range of rice rats, and tested whether age, sex, time, water depth, rice rat abundance, and vegetation density influenced matrix use and permeability. Results Permeability was greater for soybean fields than grassland or forest but did not appear to differ within versus beyond rice rats’ perceptual range. Matrix capture rates were higher early in the study and in times and locations with thick ground vegetation and high rice rat abundance in the wetlands. Rice rats captured in the matrix were younger than those in wetland patches. Conclusions Our findings expand known matrix use by marsh rice rats, and support permeability being high in matrix types dissimilar to suitable habitat. Studying individual movements will help identify mechanisms underlying enhanced permeability in crop fields

Relative Preference and Localized Food Affect Predator Space Use and Consumption of Incidental Prey

Abundant, localized foods can concentrate predators and their foraging efforts, thus altering both the spatial distribution of predation risk and predator preferences for prey that are encountered incidentally. However, few investigations have quantified the spatial scale over which localized foods affect predator foraging behavior and consumption of incidental prey. In spring 2010, we experimentally tested how point-source foods altered how generalist predators (white-footed mice, Peromyscus leucopus) utilized space and depredated two incidental prey items: almonds (Prunus dulcis; highly profitable) and maple seeds (Acer saccharum; less profitable). We estimated mouse population densities with trapping webs, quantified mouse consumption rates of these incidental prey items, and measured local mouse activity with track plates. We predicted that 1) mouse activity would be elevated near full feeders, but depressed at intermediate distances from the feeder, 2) consumption of both incidental prey would be high near feeders providing less-preferred food and, 3) consumption of incidental prey would be contingent on predator preference for prey relative to feeders providing more-preferred food. Mouse densities increased significantly from pre- to post-experiment. Mean mouse activity was unexpectedly greatest in control treatments, particularly \u3c15 m from the control (empty) feeder. Feeders with highly preferred food (sunflower seeds) created localized refuges for incidental prey at intermediate distances (15 to 25m) from the feeder. Feeders with less-preferred food (corn) generated localized high risk for highly preferred almonds \u3c10 m of the feeder. Our findings highlight the contingent but predictable effects of locally abundant food on risk experienced by incidental prey, which can be positive or negative depending on both spatial proximity and relative preference