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Ephemeral vegetation drivers of occupancy dynamics of Mojave desert tortoises (Gopherus agassizii)

Harju, Seth1; Cambrin, Scott2; Ferrazzano, Stefanie2

Published Jul 25, 2025 on Dryad. https://doi.org/10.5061/dryad.hdr7sqvws

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Abstract

Understanding the factors associated with the probability of a species occurring across the landscape has become a foundational component of wildlife management and monitoring. Static factors (e.g., plot characteristics that are consistent across time, such as topography or permanent woody vegetation)are often investigated, but dynamic vegetation factors may also drive apparent occupancy dynamics as energy-minimizing species respond to ephemeral changes in food resources. We analyzed the role of ephemeral forbs and grasses in the occupancy dynamics of Mojave desert tortoises (Gopherus agassizii), expecting higher occupancy when food plants (forbs) were abundant and lower occupancy when non-food plants (grasses) were widespread. We found that none of the static plot-level factors explained initial occupancy in the first year, but vegetation factors did influence interannual occupancy dynamics. Plots that were unoccupied in a given year were 14.4% less likely to become occupied in the following year for every 1% point increase in grass cover within the plot (i.e., a negative effect of grass cover on annual plot occupancy). Conversely, occupied plots were 72.5% less likely to become unoccupied in the subsequent year for every 1% point increase in the cover of forbs and we unexpectedly found that occupied plots were 76.9% less likely to become unoccupied for every additional grass species present in the plot. However, credible intervals were wide, indicating strong but inconsistent patterns of ephemeral vegetation driving occupancy dynamics of Mojave desert tortoises.