Eco-evolutionary dynamics of finch-plant interactions: Evaluating the effect of predation on community structure
The distribution and type of seeds in the Galápagos archipelago has shaped the evolution of Darwin’s finches and thus their adaptive radiation. Although the production of seeds is strongly influenced by abiotic factors, we suggest that predation by finches should also be important, especially during dry seasons. As a result, finches and plants should have reciprocal ecological and evolutionary effects on each other. We explore this possibility by examining the potential impact of seed predation on seed bank composition, plant assemblages, and plant phylogenetic community structure in the arid zone on Santa Cruz Island. To disentangle the effects of finch seed predation from other processes shaping plant assemblages, we use experimental plots from which finches were excluded. We compared 25 paired (exclusion and control) 1-m2 plots sampled over four years at each of two localities. First, we test the effect of predation on diversity using the Shannon diversity index for both seeds and plants. Second, we estimate the phylogenetic conservatism of seed defense traits (seed size, hardness, and spines) using Blomberg’s K statistic and the D statistic of Fritz and Purvis. Finally, we test the effect of predation on the phylogenetic structure of seeds and plants using two common indices of phylogenetic community structure: the net relatedness index (NRI) and the nearest taxon index (NTI).
Preliminary results suggest, first, that exclosure plots show a marginally lower diversity of plant species than do control plots (W = 547 P = 0.053). This result might suggest that finches regulate the abundance of dominant seeds, allowing rare species to persist. Second, seed size is conserved in plant communities (K = 0.34 P = 0.02), suggesting that if predation is strong it could drive phylogenetic clustering patterns within communities. Forthcoming analyses will test the prediction that exclosure plots will show more overdispersed phylogenetic patterns relative to the control plots. We discuss how these results fit with current understanding of plant-finch interactions in the Galápagos.