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

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.