Pollinator recognition by a keystone tropical plant.

ABSTRACT

Understanding the mechanisms enabling coevolution in complex mutualistic networks remains a central challenge in evolutionary biology. We show for the first time, to our knowledge, that a tropical plant species has the capacity to discriminate among floral visitors, investing in reproduction differentially across the pollinator community. After we standardized pollen quality in 223 aviary experiments, successful pollination of Heliconia tortuosa (measured as pollen tube abundance) occurred frequently when plants were visited by long-distance traplining hummingbird species with specialized bills (mean pollen tubes = 1.21 ± 0.12 SE) but was reduced 5.7 times when visited by straight-billed territorial birds (mean pollen tubes = 0.20 ± 0.074 SE) or insects. Our subsequent experiments revealed that plants use the nectar extraction capacity of tropical hummingbirds, a positive function of bill length, as a cue to turn on reproductively. Furthermore, we show that hummingbirds with long bills and high nectar extraction efficiency engaged in daily movements at broad spatial scales (∼1 km), but that territorial species moved only short distances (<100 m). Such pollinator recognition may therefore affect mate selection and maximize receipt of high-quality pollen from multiple parents. Although a diffuse pollinator network is implied, because all six species of hummingbirds carry pollen of H. tortuosa, only two species with specialized bills contribute meaningfully to its reproduction. We hypothesize that this pollinator filtering behavior constitutes a crucial mechanism facilitating coevolution in multispecies plant-pollinator networks. However, pollinator recognition also greatly reduces the number of realized pollinators, thereby rendering mutualistic networks more vulnerable to environmental change.