Competing for seed dispersal: evidence for the role of avian seed hoarders in mediating apparent predation among oaks

Abstract

Summary In communities of large‐seeded tree species, generalist seed hoarders often link the temporally variable dynamics of various species in ways that can result in indirect ecological interactions. A special case of such interactions is ‘apparent predation’ – when variation in seed production of one tree species changes the outcome of the interaction between seed hoarders and another tree species from seed dispersal mutualism to predation. We investigated how two species of avian hoarders responded to differences in acorn availability among individual valley oaks (Quercus lobata) and across a landscape dominated by blue oaks (Q. douglasii) to determine whether seed dispersal and predation dynamics result in apparent predation between these oak species. California scrub‐jays (Aphelocoma californica), high‐quality seed dispersers with large spatial scales of resource perception, showed strong numerical and functional responses to individually large Q. lobata acorn crops when landscape‐level acorn crops were low, but the numerical response was not observed when Q. douglasii produced a good acorn crop the subsequent year. In contrast, acorn woodpeckers (Melanerpes formicivorus), which are significant acorn predators, exhibited a consistent functional response to individual Q. lobata acorn crops independent of landscape‐wide availability. Consequently, Q. lobata experienced increased predation and a near absence of high‐quality seed dispersal when Q. douglasii acorns were abundant. Our results suggest that apparent predation among oak species can arise from contrasting responses of generalist seed predators and dispersers to acorn availability on different spatial scales. Thus, the spatial scale of resource perception, which can differ greatly among generalist seed hoarders, may provide a proximate mechanism underlying indirect, temporally variable ecological affects such as apparent predation among sympatric species of large‐seeded trees. A lay summary is available for this article.