Contrasting responses of insects and vertebrates as seed consumers of two neotropical oak species: The interactive effects of individual crop size and seed mass

Tree species often exhibit considerable variability among individuals in seed crop size and averaged seed mass within the same year. However, very little is known about the consequences for seed consumers' preferences of this potentially large between-individual variability. In this study we quantified seed production and seed manipulation rates by animals over three years in two coexisting oak species of southeast Mexico (Quercus germana and Q. xalapensis) with the principal aim of evaluating the influence of two relevant plant traits (individual crop size and seed mass) on the responses of two guilds of acorn consumers with contrasting foraging behaviors and dietary breadths (vertebrate versus granivorous insects). We detected interactive effects of these two plant traits on seed consumers' preferences, with important differences between the two groups of acorn-feeding animals. In general, high densities of large-sized acorns triggered a negative density-dependent response (i.e. satiating effect) in granivorous insects and a positive response (i.e. attractive effect) in vertebrates, whereas the opposite occurred when considering the fraction of small-sized acorns. The potential consequences of producing bigger seeds will partly depend on the relative abundance of the two guilds of acorn consumers. Thus, in plant populations with overabundance of vertebrates, the higher attraction of large-sized seeds for these generalist consumers could counteract the satiating effect exercised on granivorous insects through multi-infestation. However, in forest sites with less abundance of vertebrates, the risk of seed predation (mostly by insects) could be reduced in those trees producing huge quantities of large-sized seeds. In summary, we found clear evidences that the direction and magnitude of density-dependent seed removal can differ not only between different groups of seed consumers but also among different fractions of seed size, which highlights the importance of considering this plant trait to better understand the complexity of mechanisms operating in these plant-animal interactions.