Increasing trait-matching with decreasing available resources: Morphological specialization in hummingbird networks
The vulnerability and fidelity of plant-animal interactions are shaped by evolutionary forces promoting specialization and generalization. The frequency of interactions among plants and pollinators are often related to their functional morphology, leading many to suggest co-evolutionary mutualism promotes the most effective pollinator. However, the majority of studies treat trait matching as a fixed property and use the abundance of resources solely as a null expectation for niche overlap. This framework ignores changes in resource availability and assumes that morphological tradeoffs are constant through time. We test two opposing theories for temporal niche breadth and trait matching based on available resources. Following optimal foraging theory, niche overlap should increase as available resources increase due to higher patch quality and decreased handling time cost of mismatched resources. Alternatively, niche partitioning reduces competition and pollinators should prefer to partition resources to decrease niche overlap when there are high available resources.
We test our predictions using bimonthly surveys of hummingbird flower visitation in a tropical montane cloud forest. Using time-lapse cameras and transects, we documented over 3,000 feeding events. We found that niche overlap increased as available resources increased. Hummingbirds more closely matched their bill lengths to flower corolla lengths during periods of resource scarcity. Our data suggest that morphological specialization is most crucial during low resource availability periods. The fidelity of plant-animal interactions through time depends both on their traits, as well as the surrounding biotic environment.