Consistency and reciprocity of indirect interactions between tree species mediated by frugivorous birds

Despite the recognized importance of indirect plant-plant interactions for community structure, we still need to improve our current knowledge on how their outcomes are consistent in space and time, as well as reciprocal between participating species. These caveats are especially relevant in the case of indirect interactions mediated by animals, whose behavior may show high variability. We studied consistency and reciprocity of frugivore-mediated interactions between fleshy-fruited trees. For three years we examined the influence of crop size and neighborhood characteristics (con- and heterospecific fruit abundance and forest cover) on frugivory rates on Crataegus monogyna and Ilex aquifolium, two coexisting species in the secondary forests of the Cantabrian range that share a guild of frugivorous birds. Crop size and neighborhood characteristics influenced frugivory on C. monogyna and I. aquifolium. Both con- and heterospecific fruit abundance affected frugivory, evidencing the occurrence of indirect interactions between trees, although the strength and sign of these effects varied between tree species as well as across years within species. By showing complex temporal patterns in the consistency and reciprocity of indirect interactions, this study emphasizes the need for multispecific, long-term studies to assess the actual contribution of animal-mediated plant-plant indirect interactions to community dynamics.