A novel trophic cascade between cougars and feral donkeys shapes desert wetlands

Introduced large herbivores have partly filled ecological gaps formed in
the late Pleistocene, when many of the Earth’s megafauna were driven
extinct. However, extant predators are generally considered incapable of
exerting top-down influences on introduced megafauna, leading to unusually
strong disturbance and herbivory relative to native herbivores. We report
on the first documented predation of juvenile feral donkeys (Equus
africanus asinus) by cougars (Puma concolor) in the Mojave and Sonoran
Deserts of North America. We then investigated how cougar predation
corresponds with differences in feral donkey behavior and associated
effects on desert wetlands. Focusing on a feral donkey population in Death
Valley National Park, we compared donkey activity patterns and impacts
between wetlands with and without cougar predation. Donkeys were primarily
diurnal at wetlands with cougar predation, thereby avoiding cougars.
However, donkeys were active throughout the day and night at sites without
predation. Donkeys were ~87% less active (measured as hours of activity a
day) at wetlands with predation (p<0.0001).  Sites with
predation had reduced donkey disturbance and herbivory, including ~46%
fewer access trails, 43% less trampled bare ground, and 192% more canopy
cover (PERMANOVA, R2 = 0.22, p=0.0003). Our study is the first to reveal a
trophic cascade involving cougars, feral equids, and vegetation. Cougar
predation appears to rewire an ancient food web, with diverse implications
for modern ecosystems. Our results suggest that protecting apex predators
could have important implications for the ecological effects of introduced
megafauna.