Aaron W. Reed

Demography and Environmental Stochasticity: Empirical Estimates of Cotton Rat Survival

Abstract Modern demographic models in conservation biology incorporate demographic and environmental stochasticity. Variability in vital rates, relationships among vital rates, and relationships between vital rates and the environment can affect projections from demographic models. We used a 30-year data set to estimate monthly mass-specific survival in hispid cotton rats (Sigmodon hispidus). Survival probabilities were estimated for both sexes and 3 mass classes using a multistate model and program MARK. We tested for differences in monthly survival among 3 mass classes, relationships between survival and environmental variables, and autocorrelation in survival. We found lower survival in summer for the smallest and medium mass classes. Survival in the largest mass class was positively autocorrelated during February and March, indicating that winter survival in large cotton rats is characteristic of a season, but that other monthly survival rates could be treated as unrelated to each other or to environmental variables.

Detection and Plant Monitoring Programs: Lessons from an Intensive Survey of Asclepias meadii with Five Observers

Monitoring programs, where numbers of individuals are followed through time, are central to conservation. Although incomplete detection is expected with wildlife surveys, this topic is rarely considered with plants. However, if plants are missed in surveys, raw count data can lead to biased estimates of population abundance and vital rates. To illustrate, we had five independent observers survey patches of the rare plant Asclepias meadii at two prairie sites. We analyzed data with two mark-recapture approaches. Using the program CAPTURE, the estimated number of patches equaled the detected number for a burned site, but exceeded detected numbers by 28% for an unburned site. Analyses of detected patches using Huggins models revealed important effects of observer, patch state (flowering/nonflowering), and patch size (number of stems) on probabilities of detection. Although some results were expected (i.e. greater detection of flowering than nonflowering patches), the importance of our approach is the ability to quantify the magnitude of detection problems. We also evaluated the degree to which increased observer numbers improved detection: smaller groups (3–4 observers) generally found 90 – 99% of the patches found by all five people, but pairs of observers or single observers had high error and detection depended on which individuals were involved. We conclude that an intensive study at the start of a long-term monitoring study provides essential information about probabilities of detection and what factors cause plants to be missed. This information can guide development of monitoring programs.

Effects of habitat disturbance on a Peromyscus leucopus (Rodentia: Cricetidae) population in western Pennsylvania

Abstract Many species of wild mammals occur in habitats that have been disturbed by fragmentation or degraded in quality. Previous researchers have hypothesized that demographic characteristics of populations may shift with changes in environmental conditions, with self-regulatory ability increasing with environmental suitability. We studied responses of white-footed mice (Peromyscus leucopus) to habitat disturbance. Given that optimal habitat for this species is deciduous woodland, we predicted that populations in habitats disturbed by cutting woody vegetation would be lower and more variable in density than in undisturbed habitat, density and stability of populations in disturbed habitat would increase over time, survival would be higher in undisturbed than in disturbed habitat, and populations in undisturbed habitat would show a greater degree of self-regulation. This 6-year study in western Pennsylvania involved 3 replicated study sites (each 3.8 ha), with woody vegetation removed on half of each site prior to beginning the study. Density in disturbed treatment averaged 65% of density in undisturbed habitat. There were no differences between treatments in survival or in population growth rates over time. Population trends over time were similar between treatments, populations in disturbed habitat did not become more stable with time, and density did not converge with that of undisturbed habitat. Although populations in the undisturbed habitat were clearly self-regulating, those in disturbed habitats were not. Despite expectations that demographic performance will align with environmental suitability, it may be difficult to ascribe a particular demography to a habitat generalist such as P. leucopus.

ENVIRONMENTAL CORRELATES OF SURVIVAL AND REPRODUCTION IN OLD-FIELD RODENTS

Abstract We used regression trees to assess seasonal relationships between vital rates (survival and reproduction) and a suite of environmental variables for hispid cotton rats (Sigmodon hispidus) and prairie voles (Microtus ochrogaster). We found only 2 relationships between vital rates and environmental variables for M. ochrogaster, whereas we found relationships in all seasons for S. hispidus. The majority of recovered regression trees contained multiple variables, indicating that the vital rates of S. hispidus were affected by interactions among environmental correlates. Vital rates of S. hispidus were affected most by precipitation, temperature, and snowfall; but the direction of effects of these environmental variables was not consistent among seasons. Our analyses indicate that populations of M. ochrogaster in northeastern Kansas are relatively insensitive to environmental variability, whereas survival and reproduction in S. hispidus are influenced by the environment throughout the year.

Buffering and plasticity in vital rates of oldfield rodents

1. Under the hypothesis of environmental buffering, populations are expected to minimize the variance of the most influential vital rates; however, this may not be a universal principle. Species with a life span <1 year may be less likely to exhibit buffering because of temporal or seasonal variability in vital rate sensitivities. Further, plasticity in vital rates may be adaptive for species in a variable environment with reliable cues. 2. We tested for environmental buffering and plasticity in vital rates using stage-structured matrix models from long-term data sets in four species of grassland rodents. We used periodic matrices to estimate stochastic elasticity for each vital rate and then tested for correlations with a standardized coefficient of variation for each rate. 3. We calculated stochastic elasticities for individual months to test for an association between increased reproduction and the influence of reproduction, relative to survival, on the population growth rate. 4. All species showed some evidence of buffering. The elasticity of vital rates of Peromyscus leucopus (Rafinesque, 1818), Sigmodon hispidus Say & Ord, 1825 and Microtus ochrogaster (Wagner, 1842) was negatively related to vital rate CV. Elasticity and vital rate CV were negatively related in Peromyscus maniculatus (Wagner, 1845), but the relationship was not statistically significant. Peromyscus leucopus and M. ochrogaster showed plasticity in vital rates; reproduction was higher following months where elasticity for reproduction exceeded that of survival. 5. Our results suggest that buffering is common in species with fast life histories; however, some populations that exhibit buffering are capable of responding to short-term variability in environmental conditions through reproductive plasticity.

Simulation of oak early life history and interactions with disturbance via an individual-based model, SOEL

Early tree life history and demography are driven by interactions with the environment such as seed predation, herbivory, light availability, and drought. For oak (Quercus) in the eastern United States, these interactions may contribute to oak regeneration failure. Numerous studies have examined the impact of individual factors (like seed predation) on the oak regeneration process, but less information is available on the relative and combined impacts of multiple intrinsic and extrinsic factors on early oak life history. We developed an individual-based, spatially explicit model to Simulate Oak Early Life history (SOEL). The model connects acorn survival, acorn dispersal, germination, seedling growth, and seedling survival submodels based on empirical data with an existing gap model (JABOWA). Using SOEL, we assessed the sensitivity of several metrics of oak regeneration to different parameters associated with early oak life history. We also applied the model in three individual case studies to assess: (1) how variable acorn production interacts with timing of timber harvest; (2) the effect of shelterwood harvest-induced differences on seed predation; and (3) the consequences of interactions between drought, seedling growth and survival, and timber harvest. We found that oak regeneration metrics including percent emergence, seedling density, and sapling density were most sensitive to the amount of acorn production, acorn caching probability by scatterhoarders, and seedling growth rates. In the case studies, we found that timing harvest to follow large acorn crops can increase the density of oak regeneration in the short term following harvest, at least under some conditions. Following midstory removal, lower weevil infestation probability and lower post-dispersal acorn survival resulted in a modest decline in seedling density, but the decline did not persist to the sapling life stage class. Drought frequency had a powerful negative impact on both growth and survival for individual seedlings, which resulted in large reductions in both seedling and sapling density. The case studies presented here represent only a few examples of what could be accomplished within the SOEL modeling framework. Further studies could focus on different early life history parameters, or connect the parameter values to different predictor variables based on field data.

Detecting the signal of climate change in small mammal populations of Eastern Kansas

Sarah E. Meiners [1], Courtney Masterson [1], Aaron W. Reed [1], Norman A. Slade [2] [1] School of Biological Sciences, University of Missouri-Kansas City, Kansas City, MO 64110 USA [2] Department of Ecology and Evolutionary Biology & Biodiversity Research Center, University of Kansas, Lawrence, KS 66045 USA Introduction • Changes in climate are expected to have variable impacts on populations. • Many studies have projected the effects of climate change over the next century, but the effect of changes in climate may already be apparent in some populations. • Our goal was to determine if we could detect the signal of climate change in populations of Microtus ochrogaster, Sigmodon hispidus, and Peromyscus leucopus using long-term markrecapture data. • Based on previous studies we predicted: • P. leucopus would exhibit little or no change in abundance over time. • M. ochrogaster would not change in abundance, but display diminishing cyclicity over time. • S. hispidus would increase in abundance and stability over time.