Johanna Varner

The importance of biologically relevant microclimates in habitat suitability assessments.

Predicting habitat suitability under climate change is vital to conserving biodiversity. However, current species distribution models rely on coarse scale climate data, whereas fine scale microclimate data may be necessary to assess habitat suitability and generate predictive models. Here, we evaluate disparities between temperature data at the coarse scale from weather stations versus fine-scale data measured in microhabitats required for a climate-sensitive mammal, the American pika (Ochotona princeps). We collected two years of temperature data in occupied talus habitats predicted to be suitable (high elevation) and unsuitable (low elevation) by the bioclimatic envelope approach. At low elevations, talus surface and interstitial microclimates drastically differed from ambient temperatures measured on-site and at a nearby weather station. Interstitial talus temperatures were frequently decoupled from high ambient temperatures, resulting in instantaneous disparities of over 30°C between these two measurements. Microhabitat temperatures were also highly heterogeneous, such that temperature measurements within the same patch of talus were not more correlated than measurements at distant patches. An experimental manipulation revealed that vegetation cover may cool the talus surface by up to 10°C during the summer, which may contribute to this spatial heterogeneity. Finally, low elevation microclimates were milder and less variable than typical alpine habitat, suggesting that, counter to species distribution model predictions, these seemingly unsuitable habitats may actually be better refugia for this species under climate change. These results highlight the importance of fine-scale microhabitat data in habitat assessments and underscore the notion that some critical refugia may be counterintuitive.

Relating Sub-Surface Ice Features to Physiological Stress in a Climate Sensitive Mammal, the American Pika (Ochotona princeps)

The American pika (Ochotona princeps) is considered a sentinel species for detecting ecological effects of climate change. Pikas are declining within a large portion of their range, and ongoing research suggests loss of sub-surface ice as a mechanism. However, no studies have demonstrated physiological responses of pikas to sub-surface ice features. Here we present the first analysis of physiological stress in pikas living in and adjacent to habitats underlain by ice. Fresh fecal samples were collected non-invasively from two adjacent sites in the Rocky Mountains (one with sub-surface ice and one without) and analyzed for glucocorticoid metabolites (GCM). We also measured sub-surface microclimates in each habitat. Results indicate lower GCM concentration in sites with sub-surface ice, suggesting that pikas are less stressed in favorable microclimates resulting from sub-surface ice features. GCM response was well predicted by habitat characteristics associated with sub-surface ice features, such as lower mean summer temperatures. These results suggest that pikas inhabiting areas without sub-surface ice features are experiencing higher levels of physiological stress and may be more susceptible to changing climates. Although post-deposition environmental effects can confound analyses based on fecal GCM, we found no evidence for such effects in this study. Sub-surface ice features are key to water cycling and storage and will likely represent an increasingly important component of water resources in a warming climate. Fecal samples collected from additional watersheds as part of current pika monitoring programs could be used to further characterize relationships between pika stress and sub-surface ice features.

Long‐Term Patterns of Immune Investment by Wild Deer Mice Infected with Sin Nombre Virus

Immunocompetence of animals fluctuates seasonally, However, there is little consensus on the cause of these fluctuations. Some studies have suggested that these patterns are influenced by changes in reproductive condition, whereas others have suggested that differences result from seasonal variations in energy expenditures. The objective of our study was to examine these contrasting views of immunity by evaluating seasonal patterns of immune response and reproduction in wild populations of deer mice Peromyscus maniculatus exposed to Sin Nombre virus (SNV). Over three consecutive fall (September, October, November) and three consecutive spring (March, April, May) sampling periods, we used titration enzyme‐linked immunosorbent assay (ELISA) to quantify virus‐specific antibody production in 48 deer mice infected with SNV. Levels of reproductive hormones were quantified using ELISA. SNV antibody titers reached their lowest level during November (geometric mean titer [GMT] = 420) and their highest levels during September (GMT = 5,545) and May (GMT = 3,582), suggesting that the immune response of deer mice to SNV has seasonal patterns. The seeming decrease in antibody titer over winter coupled with the consistency in body masses suggests that during winter, immunocompetence may be compromised to offset the energetic costs of maintenance functions, including those associated with maintaining body mass. Deer mice showed distinct sex‐based differences in SNV antibody production, with males producing higher antibody titers (GMT = 3,333) than females (GMT = 1,477). Levels of reproductive hormones do not appear to influence antibody production in either males or females, as there was no correlation between estradiol concentrations and SNV antibody titer in female deer mice (r2 = 0.26), nor was there a significant relationship between levels of testosterone and SNV antibody titers in males (r2 = 0.28). Collectively, this study demonstrates that immunocompetence of wild deer mice is seasonally variable; however, reproduction is not the primary stressor responsible for this variation. Rather, the data suggest that deer mice may compromise immunocompetence during winter to offset other maintenance costs during this period.

Too hot to trot? Evaluating the effects of wildfire on patterns of occupancy and abundance for a climate-sensitive habitat specialist

Wildfires are increasing in frequency and severity as a result of climate change in many ecosystems; however, effects of altered disturbance regimes on wildlife remain poorly quantified. Here, we leverage an unexpected opportunity to investigate how fire affects the occupancy and abundance of a climate-sensitive habitat specialist, the American pika (Ochotona princeps). We determine the effects of a fire on microclimates within talus and explore habitat factors promoting persistence and abundance in fire-affected habitat. During the fire, temperatures in talus interstices remained below 19°C, suggesting that animals could have survived in situ. Within 2 years, pikas were widely distributed throughout burned areas and did not appear to be physiologically stressed at severely burned sites. Furthermore, pika densities were better predicted by topographic variables known to affect this species than by metrics of fire severity. This widespread distribution may reflect quick vegetation recovery and the fact that the fire did not alter the talus microclimates in the following years. Together, these results highlight the value of talus as a thermal refuge for small animals during and after fire. They also underscore the importance of further study in individual species’ responses to typical and altered disturbance regimes.

Estimating duration of infection with avidity assays: potential limitations and recommendations for improvement.

Recent infections often have higher pathogen loads. The number of recent infections can therefore be used to estimate transmission rates in a host population. Antibody avidity assays are an emerging technique to infer infection age in both domestic and wild animals. These assays have the potential to supplant intensive mark-recapture efforts for identification of recent infections, but their results may be confounded by antibody titer. We examined the effectiveness of an avidity assay for identifying recent infections of Sin Nombre virus, a hantavirus in North America that establishes a chronic infection in deer mice (Peromyscus maniculatus). We found that assay performance statistics such as sensitivity, specificity, and positive predictive value for low avidity scores were significantly improved when we accounted for antibody titer in the analyses. Without accounting for titer, avidity assays may classify samples with low titers as recent infections regardless of actual infection history, thereby overestimating the number of recent infections in a population and inflating estimates of transmission rates and/or human exposure risk. We recommend that antibody titers meet a minimum threshold for use in avidity assays, and we emphasize the importance of considering titer and dilution in the validation of newly developed avidity assays.