Herbivores as predators: how an arboreal herbivore encounters and impacts declining bird populations

Predation by carnivores is fundamental to ecological systems, however, not all prey are consumed by classic predators. Increasing evidence shows that seemingly obligate herbivores may supplement their diet with vertebrate prey, yet little is known about how they encounter or impact prey. Here we study the predation habits of an arboreal herbivore, the Common Brushtail Possum ( Trichosaurus vulpecula ), that has been recorded depredating multiple bird species, including the critically endangered Regent Honeyeater ( Anthochaera phrygia ). We conducted artificial nest experiments to test if possums seek out nests using odour and whether low predation rates were due to incidental predation or predation by specific individuals. We then used population viability analysis (PVA) to investigate the impact of this predation on extinction risk for a small declining population. We found no evidence that possums seek out nests using odour or of individual specialisation. Rather, our results suggest possums are incidental predators of birds' eggs, with possums only consuming eggs at 10% of sites. Regardless, PVA modelling demonstrated that low rates of predation by native arboreal mammals can substantially increase extinction risk by raising the threshold population density at which Allee effects cause negative population growth from 600 to 1000. These results have significant implications for conservation globally. They demonstrate how natural rates of incidental predation can become problematic for small populations, providing critical insights for conservation managers into the role of herbivores as predators and revealing how incidental native predation can act synergistically with other agents of decline to drive prey to extinction. • Arboreal herbivores can act as incidental predators of birds' eggs and nestlings. • Incidental predation can markedly increase extinction risk for small populations. • Managing this predation by native herbivores poses a wicked ecological problem.