Meeting Abstract
The role of hypoxia tolerance in limiting species elevational distributions is poorly understood. Distributions are predicted to shift upslope with warming, causing exposure to hypobaric hypoxia. Hummingbirds exhibit exceptional O2 consumption rates which make them particularly susceptible to changes in PO2. The lowland Black-chinned Hummingbird and montane Broad-tailed Hummingbird are elevational replacement species that compete for nectar resources at mid-elevations where they overlap. We captured adult males of both species in the zone of overlap and we used a hypobaric chamber to compare their responses to hypoxia. Specifically, we evaluated the effects of reduced pressure on activity levels and inter-specific competitive dominance. We asked 1) Do behavioral responses to experimentally reduced pressure differ between high and low elevation species? 2) Is competitive dominance by the low elevation species diminished at high simulated altitudes? We quantified activity level by examining the proportion of time spent in flight versus perching at multiple simulated elevations. To measure competitive dominance, naive adult males of each species were allowed to compete for perches of differing heights. Activity levels dropped at high altitude in both species, but the reduction was greater in the low elevation Black-chinned Hummingbird. Competitive dominance of Black-chinned Hummingbirds at simulated low elevations did not persist at simulated high elevations. These results suggest that differential adaptation to atmospheric pressure contributes to the maintenance of stable elevational replacement distributions in hummingbirds.
