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Effects of fire on desert tortoise (Gopherus agassizii) thermal ecology
AuthorSnyder, Sarah Jessie
AdvisorTracy, Richard C.
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Among the many threats facing the desert tortoise (Gopherus agassizii) is the destruction and alteration of habitat. In recent years, wildfires have burned extensive portions of tortoise habitat in the Mojave Desert, leaving burned landscapes that are virtually devoid of living vegetation. Here, we investigated the effects of fire on the thermal ecology of the desert tortoise by quantifying the thermal quality of above- and below-ground habitat, determining which shrub species are most thermally valuable for tortoises including which shrub species are used by tortoises most frequently, and comparing the body temperature of tortoises in burned and unburned habitat. To address these questions we placed operative temperature models in microhabitats that received filtered radiation to test the validity of assuming that the interaction between radiation and radiation absorbing properties of the model can result in a single, mean radiant absorptance regardless of whether the incident solar radiation is direct unfiltered or filtered by plant canopies, using the desert tortoise as a case study. We found that operative temperatures were nearly identical within microhabitats no matter the absorptance used in the model, which supports the use of a single mean absorptance in modeling operative temperature for animals in a variety of habitats. Using validated models, we calculated indices of thermal habitat quality, and also the hours tortoises could be active within their preferred body temperature range each day across the tortoise activity season. The thermal quality index was similar between burned and unburned habitat, but unburned habitat was more thermally heterogeneous, and it provided slightly longer activity times for tortoises within their preferred body temperature range as long as they could access all thermal microhabitats in their home range, and they were not constrained by habitat configuration or microhabitat abundance. However, the differences found were small and likely not biologically significant. Living Yucca species provided the best thermal microhabitats for tortoises during the summer and were used most frequently by tortoises in burned and unburned habitat. We found that burrows in burned and unburned habitats were of similar thermal quality regardless of whether shrubs were present near the burrow mouth. Finally, body temperatures of tortoises using burned and unburned habitat were similar, but tortoises in burned habitat had slightly higher minimum body temperatures. The small magnitude differences detected in minimum body temperature would likely not affect physiological performance. Taken together, these results suggest that burned habitat may be thermally suitable for desert tortoises but direct mortality from fire, and change in nutritional resources, should also be considered when evaluating the effects of fire, and the suitability of burned habitat for tortoise population persistence.