Species interactions and thermal constraints on ant community structure

Patterns of species occurrence and abundance are infl uenced by abiotic factors and biotic interactions, but these factors are diffi cult to disentangle without experimental manipulations. In this study, we used observational and experimental approaches to investigate the role of temperature and interspecifi c competition in controlling the structure of groundforaging ant communities in forests of the Siskiyou Mountains of southwestern Oregon. To assess the potential role of competition, we fi rst used null model analyses to ask whether species partition temporal and/or spatial environments. To understand how thermal tolerances infl uence the structure of communities, we conducted a laboratory experiment to estimate the maximum thermal tolerance of workers and a fi eld experiment in which we added shaded microhabitats and monitored the response of foragers. Finally, to evaluate the roles of temperature and interspecifi c competition in the fi eld, we simultaneously manipulated shading and the presence of a dominant competitor (Formica moki). Th e foraging activity of species broadly overlapped during the diurnal range of temperatures. Species co-occurrence patterns varied across the diurnal temperature range: species were spatially segregated at bait stations at low temperatures, but co-occurred randomly at high temperatures. Th e decreased abundance of the co-occurring thermophilic Temnothorax nevadensis in shaded plots was a direct eff ect of shading and not an indirect eff ect of competitive interactions. Th ermal tolerance predicted the response of ant species to the shading experiment: species with the lowest tolerances to high temperatures showed the greatest increase in abundance in the shaded plots. Moreover, species with more similar thermal tolerance values segregated more frequently on baits than did species that diff ered in their thermal tolerances. Collectively, our results suggest that thermal tolerances of ants may mediate competitive eff ects in habitats that experience strong diurnal temperature fl uctuations. A major goal of ecology is to identify the processes that con trol patterns of community structure. At large spatial scales, historical processes determine properties of the regional species pool (Ricklefs 2004), which in turn infl uence the composition of local communities (Webb et al. 2002, Wiens and Donoghue 2004). At smaller spatial scales, both abiotic fi ltering (pH, temperature, water availability, etc.), and biotic interactions (interspecifi c competition, parasites, herbivory, etc.) may operate to determine which species occur, and cooccur, together within the local community (Weiher and Keddy 1999, Cavender-Bares et al. 2009).