Responses of seed size, ant worker size, and seed removal rate to elevation in Mediterranean grasslands

Elevational gradients are a powerful tool to investigate how abiotic factors affect ecosystems and to predict the possible consequences of climate change. Here, we investigated the assemblage-level responses of seed size, ant worker size, and rates of seed removal by ants along a 1700 m elevational gradient in Mediterranean grasslands of central Spain, taking into account abiotic factors and interactions between biotic variables. Our results showed that both seed size and ant size decreased with elevation. Structural equation models linking abiotic factors and biotic variables were built to describe these patterns. Two equally plausible models were selected, both based on temperature. In the first model, temperature directly affected seed size and granivorous ant size, and indirectly affected ant size (of both granivorous and non-granivorous species) through a bottom-up pathway. The second model included the direct effects of temperature on ant size and granivorous ant size, and indirect effects on seed size through a top-down pathway. In this model, the link between granivorous ant size and seed size was positive. Seed removal rates decreased with elevation and seed size, with smaller seeds being preferred with increasing elevation. Taken together, our results indicate that the observed patterns of ant size and seed size are ultimately controlled by abiotic factors, although the system is more satisfactorily explained if seed-ant interactions are incorporated into the models. We expect that climate change will have strong effects on this system, including mismatches between consumers and producers potentially altering the interspecific relationships among ants and plants.