Habitat suitability of patch types: A case study of the Yosemite toad

Abstract

Understanding patch variability is crucial in understanding the spatial population structure of wildlife species, especially for rare or threatened species. We used a well-tested maximum entropy species distribution model (Maxent) to map the Yosemite toad (Anaxyrus (= Bufo) canorus) in the Sierra Nevada mountains of California. Twenty-six environmental variables were included in the model representing climate, topography, land cover type, and disturbance factors (e.g., distances to agricultural lands, fire perimeters, and timber harvest areas) throughout the historic range of the toad. We then took a novel approach to the study of spatially structured populations by applying the species-environmental matching model separately for 49 consistently occupied sites of the Yosemite toad compared to 27 intermittently occupied sites. We found that the distribution of the entire population was highly predictable (AUC= 0.95±0.03 SD), and associated with low slopes, specific vegetation types (wet meadow, alpine-dwarf shrub, montane chaparral, red fir, and subalpine conifer), and warm temperatures. The consistently occupied sites were also associated with these same factors, and they were also highly predictable (AUC= 0.95±0.05 SD). However, the intermittently occupied sites were associated with distance to fire perimeter, a slightly different response to vegetation types, distance to timber harvests, and a much broader set of aspect classes (AUC= 0.90±0.11 SD). We conclude that many studies of species distributions may benefit by modeling spatially structured populations separately. Modeling and monitoring consistently-occupied sites may provide a realistic snapshot of current species-environment relationships, important climatic and topographic patterns associated with species persistence patterns, and an understanding of the plasticity of the species to respond to varying climate regimes across its range. Meanwhile, modeling and monitoring of widely dispersing individuals and intermittently occupied sites may uncover environmental thresholds and human-related threats to population persistence.

Keywords

species distribution models / Maxent / habitat patch / patchy populations / Yosemite toad / Anaxyrus canorus / Bufo canorus

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Acknowledgements

Data were provided by the Inyo National Forest, Sierra National Forest, Stanislaus National Forest, Sequoia-Kings Canyon National Park, and Yosemite National Park. We thank R. Grasso for assistance in classifying Yosemite toad locations and S. Kumar for Maxent training. S. Kumar, T. Holcombe, A. Lind, S. Lawler, and J. Quinn provided helpful reviews on an earlier version of the manuscript. CTL funding was provided by the University of California at Davis. TJS contribution supported by US Geological Survey Invasive Species Program, USGS Fort Collins Science Program, and USDA CSREES/NRI 2008-35615-04666. The Natural Resources Ecology Laboratory at Colorado State University provided logistical support. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.

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