How competitive trade-offs limit elevation ranges for temperate-breeding hummingbirds

Abstract

One intriguing hypothesis about range limits of species along environmental gradients is that interspecific interference competition limits the activity of the better exploitation competitor. The hypothesis works if the costs of interference and (or) exploitation vary along the gradient. However, in some systems, species turnover happens over gradients that may be too short to induce changes in costs associated with competition. An example is breeding Black-chinned Hummingbirds (Archilochus alexandri (Bourcier and Mulsant, 1846)) and Broad-tailed Hummingbirds (Selasphorus platycercus (Swainson, 1827)) in western Colorado, USA, where turnover happens over ∼400 m. We recorded foraging and chasing activity of the two species at feeders and found that their foraging activity changed with elevation but interspecific competition did not. Because the foraging activity of the two species changed inversely with each other, it may be the presence of Black-chinned Hummingbirds rather than active interference that limits the activity of Broad-tailed Hummingbirds. Importantly, the relationship between foraging activity and elevation depended on the distance between the two feeders, which shows that relationships with elevation are contingent on other factors such as spatial distribution of resources. Our results suggest that interspecific differences in flight performance are not manifested over the short elevation gradient and do not account for changes in activity. Flight performance may indirectly affect patterns in breeding-season activity by influencing how species interact with other competitors during the nonbreeding season.

Marginal sinks or potential refuges? Costs and benefits for coral-obligate reef fishes at deep range margins

“…However, these relationships were not apparent in the highly specialized territorial reef fishes at deep range margins. Therefore, our results do not concur with avian [23,30], mammalian [21,22], freshwater fish [57] and insect [24] studies conducted along elevation gradients. Similarly, our results are not consistent with investigations of butterflyfishes across horizontal resource gradients within their core shallow-water range [33,49,50].…”

“…Assessing individual performance among marginal populations requires detailed knowledge of changes in key ecological strategies and demographic traits between the range core and range-margin habitats. However, range margins are typically under-sampled, and detailed ecological assessments of individual space use, resource access and physiological condition at range margins constituting potential refuges are rare among animals [30]. Coral reefs provide an ideal model ecosystem for assessing the responses of populations across environmental gradients because they are highly diverse ecosystems that exhibit strong ecological gradients over small spatial scales (e.g.…”

Evaluating broad scale patterns among related species using resource experiments in tropical hummingbirds

“…). Although this example is an unusual case of extreme dimorphism on Caribbean islands, more modest sexual dimorphism could influence selectivity, especially if birds have increased energy demands during breeding or molting (Powers , Feldman and Mcgill ). In addition, we assumed that the high metabolic cost of hummingbird flight leads to high pressures on foraging success, and that the lack of resource acquisition due to interspecific aggression will lead to negative demographic consequences (Altshuler and Dudley ).…”

Retraction statement: ‘Species distributions shift downward across western North America’ by M.A. Harsch and J. Hille Ris Lambers

“…If the strength of such competitive interactions are influenced by changes in moisture availability (i.e., climate * competition interaction), then the downward distribution shifts we detected at lower distribution limits could also reflect climate change-mediated competitive release (Lenoir et al, 2010). Evidence for such climate change-mediated competitive release is lacking, likely because few studies whether climate or biotic interactions determine lower distribution limits (Ettinger et al, 2011;Feldman & McGill, 2013) and only one study, that we are aware of, jointly assesses the influence of climate and biotic interactions on lower distribution limit dynamics (Ettinger & HilleRisLambers, 2013). Our correlational approach does not allow us to assess whether downward distribution shifts reflect strong physiological responses to climate change or a weakening of competitive interactions mediated by climate.…”