1. 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.2. 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 behaviour and associated effects on desert wetlands.3. Focusing on a feral donkey population in the Death Valley National Park, we used camera traps and vegetation surveys to compare donkey activity patterns and impacts between wetlands with and without cougar predation. 4. 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, R 2 = 0.22, p = 0.0003). 5. 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.

Introduced large herbivores have partly filled ecological gaps formed in the late Pleistocene, when many of the Earth's megafauna were driven extinct. However, surviving predators are widely considered unable to influence introduced megafauna, leading them to exert unusually strong herbivory and disturbance-related effects. We report on a behaviorally-mediated trophic cascade between cougars (Puma concolor) and feral donkeys (Equus africanus asinus) at desert wetlands in North America. In response to predation of juveniles, donkeys shifted from nocturnal to almost exclusively diurnal, thereby avoiding peaks in cougar activity. Furthermore, donkeys reduced the time they spent at desert wetlands by 87%: from 5.5 hours a day to 0.7 hours at sites with predation. These shifts in activity were associated with increased activity and richness of other mammal species and reduced disturbance and herbivory-related effects on these ecologically-distinct wetland ecosystems, including 49% fewer trails, 35% less trampled bare ground, and 227% more canopy cover. Cougar predation on introduced donkeys rewires an ancient food web, with diverse implications for modern ecosystems.

Much of the foothill and valley areas of coastal southern California are covered by stands of non-native grasses with occasional scattered patches of native bunch grasses. For decades, the dominant view held that these native bunchgrasses, particularly Stipa pulchra, once covered these lands and that grazing practices were a key cause of their decline. The so-called bunchgrass dominant paradigm (BDP) put forth by Clements influenced thinking on grassland ecology and biogeography for decades until it was discredited. Recent research suggests that grazing and related disturbances might have supported native bunchgrasses, but results are mixed and most studies cover only a short time frame. This research uses a long-term, repeat study design to analyze and compare data from three surveys of 15 permanent quadrats in La Jolla Valley, California, to determine changes in grassland cover over 34 years. A unique aspect of the study is that the site was selected by researchers precisely because it contained excellent 'relic' stands of Stipa pulchra (in accordance with the BDP) and the baseline study was conducted shortly after the area was released from grazing and placed under conservation management. We compared data on species frequency and percent cover collected using the same sampling routine for three periods: 1981, 1994, and 2015, to document the shifts in vegetation. We found that native grass cover decreased dramatically (especially on the valley floor), exotic grass cover fluctuated widely over time, while both native and exotic forb cover increased over time. The findings support the notion that prior grazing management practices may have supported the former stands of Stipa pulchra.