The hummingbird and the hawkmoth: Species diversity, competition and niche partitioning across the United States

Background/Question/Methods

Competition between hummingbirds and hawkmoths is likely, yet that potential remains understudied. While differing in taxonomy and ontogeny, hawkmoth and hummingbird adults fill functionally similar niches as nectarivores. In this study, we analyzed the distribution of hawkmoths, Sphingidae, and hummingbirds, Trochilidae, in the continental United States, correlating their patterns of diversity with environmental variables. We tested for latitudinal and longitudinal biases in their species richness (S) and the potential role of 10 environmental variables. Finally, to establish the generality or uniqueness of these taxa's patterns, we also sought out the latitudinal and longitudinal biases of 7 different families of insect and 12 different families of birds.

Results/Conclusions

Hawkmoth diversity increases sharply with longitude (eastward-bias) while hummingbird diversity shows the reverse trend (westward-bias). Analyses of the environmental variables showed that hawkmoth diversity increases with July minimum temperature and declines with elevation; hummingbird diversity increases with elevation, greater winter precipitation compared to summer, and higher maximum July temperatures. We initially hypothesized that hummingbirds and hawkmoths may show a new type of niche partitioning according to elevation. Three various hypotheses were developed to explain the results including inter-taxon competition. Results from the other bird families showed that all were significantly westward-biased, making the case for competition weaker and coincidence stronger. It remains an open question whether hawkmoths and hummingbirds influence each others' continent-wide diversity patterns. We suggest other potential avenues to studying competition between these two taxa.