Hummingbird-plant interactions along an altitudinal gradient in northwestern Mexico

Abstract

Comparing ecological networks along environmental gradients can enable a better understanding of the environmental filtering and coexistence mechanisms that determine community assembly. In the present study, we assessed hummingbird-plant network metrics at three sites along an altitudinal gradient in the Sierra Madre Occidental corresponding to tropical forest (148 and 289 m a.s.l.), ecotone (1131 and 1423 m a.s.l.) and pine-oak forest (1800 and 2218 m a.s.l.). We evaluated variation in the specialization (d’) of the hummingbird species and their visited plant species across altitude. We determined if hummingbird species inside the modules of the general network were separated by biogeographical origin, migratory behavior, or morphology. We assessed whether the role in the modular partition of hummingbird and plant species in the general network was related to traits of species. Finally, determined which traits were associated with the importance of different species within the network. We recorded 1050 interactions between 20 species of hummingbirds and 64 species of plants. We found that network metrics in the ecotone differed from other sites, reflecting the convergence between temperate and tropical forest flora and the midpoint in hummingbirds' altitudinal migration. As in previous studies, we found that hummingbird specialization was related to the bill length and found that the migratory status, particularly the altitudinal migratory species, was associated with the hummingbird specialization. The abundance of hummingbird species and flowers (ornithophilous and non-ornithophilous) along the gradient plays a central role in the importance within the network and in the formation of the modules.