The spatial scale of plant-animal interactions: effects of resource availability and habitat structure

Plant-animal interactions are crucial nodes in the structure of communities and pivotal drivers of ecosystem functioning. Much of this relevance may depend on how animals cope with plant resources at different spatial scales. However, little is known about how and why different interactions perform at different scales in the same environmental setting. In this study we assess the spatial scales at which two plant-animal interactions operate and disentangle the environmental factors (plant resource availability vs. habitat structure) underpinning these operational scales. We studied two interactions with opposite (mutualistic vs. antagonistic) ecological effects on fleshy-fruited trees, frugivory and seed dispersal by birds, and the later predation by rodents on bird-dispersed seeds. Employing a standardized sampling, we covered three temperate ecosystems hosting structurally similar plant-frugivore-seed predator systems: Cantabrian forest, Mediterranean shrubland, and Patagonian forest. We sampled habitat structure (tree and understory covers), fleshy-fruit abundance, bird-dispersed seed occurrence, frugivorous bird abundance, and seed predation rate, along 15002500 m transects. Using a spatially explicit approach, we broke down the predictable spatial patterns of bird abundance and seed predation rate into patchiness at three consecutive spatial scales (broad, intermediate, and fine). The degree of patchiness and the allocation of spatial variability at different scales suggested a hierarchically nested structure in frugivory and seed predation, but a larger operational scale in seed predation than in frugivory. Scale-specific spatial distributions were explained by the response of animals to plant resource availability and habitat structure. Birds tracked fruits at large spatial scales in all systems and, within some systems, even across consecutive scales. Seed predation distribution was more responsive to habitat features than to resource availability. The reinforcement of resource tracking patterns across scales sometimes occurred simultaneously with the dilution of habitat effects, suggesting that scale dependence may emerge from trade-offs between resource acquisition and the effects of other factors, such as predation risk, on interacting animals. Our findings suggest that scale dependence in frugivory and seed predation may affect the balance of demographic effects of these interactions in plant populations. Moreover, the consistency of frugivory patterns within and across spatial scales may condition the redundancy of seed dispersal as an ecosystem function.