Coexistence of a generalist owl with its intraguild predator: distance-sensitive or habitat-mediated avoidance?

Intraguild predation is increasingly reported as a population-limiting factor for vertebrate predators. However, long-term coexistence of the intraguild prey with its predator is a common occurrence usually maintained by some form of predator avoidance, which may be achieved through distance-sensitive avoidance (selection of sites as far as possible from the intraguild predator), and/or habitat-mediated avoidance (avoidance of habitats associated with high predation risk). The former is expected when the distribution of the predator is heterogeneous, leaving gaps which can be exploited by the prey, while the latter is expected at high predator densities, when few predation refugia are available. To date, few studies have focused on such switch in predator avoidance under changing scenarios of intraguild predator density. To test this hypothesis, we censused tawny owls (Strix aluco, body mass similar to 0.4 - 0.7 kg) and their intraguild predator, the eagle owl (Bubo bubo, similar to 1.5 - 4 kg), in 12 areas of the Alps. As predicted, tawny owls were indifferent to predator distance in an area of low predation risk, they switched to distance- sensitive avoidance in an area of medium predator density and to habitat- mediated avoidance in an area of high predator density with few available refugia. Actual predation rates were low, but increased with proximity to the intraguild predator nest. Similarly, tawny owl breeding output declined with closeness to an eagle owl nest. Habitat loss associated with predator avoidance translated into population effects, leading to a negative relationship between the densities of the two owl species. The spatial gaps in tawny owl distribution caused by eagle owls indirectly favoured other owl species, resulting in higher diversity of the overall owl community and suggesting that eagle owls acted as keystone predators. Our results suggest that intraguild predation may alter habitat choices and affect density, productivity and guild structure of vertebrate mesopredators. Such effects are probably more common than previously thought.(c) 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.