Tuesday, August 4, 2009

PS 40-176: Spatial variation in nectar production across the Southwest USA and its relationship to hummingbird distribution

Richard E. Feldman, McGill University and Brian J. McGill, University of Arizona.


In some animal communities, species co-exist via an interference-exploitation trade-off: species that are inferior exploitation competitors are superior interference competitors and vice versa. The trade-off is especially important in hummingbird communities. Some species defend flower clumps and exclude competitors while others efficiently forage on sparse flowers. To date, studies of hummingbird distribution have focused on how relative competitive ability changes along elevation gradients. To obtain a more complete picture of how abundance changes at the scale of species ranges, one must include spatial variation in resource density. Our goal was to examine variation in energy available from nectar plants across gradients of elevation, space and time. Our focal species were black-chinned (Archilochus alexandri) and broad-tailed (Selasphorus platycercus) hummingbirds and we considered variation across the breeding season in the region of the US where the two species overlap. We counted nectar flowers in 250m x 250m plots at 103 locations in a 350000 km2 rectangle covering parts of Colorado, New Mexico, Arizona and Utah. We used the literature to convert nectar flower density to energy available from sugar. We modeled energy availability as a function of climate averages, current climate and elevation. We used the model to produce a continuous map of nectar availability for the study area.


The result is a visual representation of spatial variation in nectar energy availability at two week intervals from May 1 – Aug 1. Since this map corresponds to the breeding ranges and season of black-chinned and broad-tailed hummingbirds, it becomes a tool by which to test they degree to which energy availability influences spatial variation in abundance. Moreover, it leads to the hypothesis that the interference-exploitation trade-off not only mediates co-existence and relative abundance at local scales but influences the distribution and abundance of hummingbirds across their species ranges.