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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).

Aim A major endeavour of community ecology is documenting non-random patterns in the composition and body size of coexisting species, and inferring the processes, or assembly rules, that may have given rise to the observed patterns. Such assembly rules include species sorting resulting from interspecific competition, aggregation at patchily distributed resources, and co-evolutionary dynamics. However, for any given taxon, relatively little is known about how these patterns and processes change through time and vary with habitat type, disturbance history, and spatial scale. Here, we tested for non-random patterns of species co-occurrence and body size in assemblages of ground-foraging ants and asked whether those patterns varied with habitat type, disturbance history, and spatial scale. Location Burned and unburned forests and fens in the Siskiyou Mountains of southern Oregon and northern California, USA. Methods We describe ground-foraging ant assemblages sampled over two years in two discrete habitat types, namely Darlingtonia fens and upland forests. Half of these sites had been subject to a large-scale, discrete disturbance ‐ a major fire ‐ in the year prior to our first sample. We used null model analyses to compare observed species co-occurrence patterns and body-size distributions in these assemblages with randomly generated assemblages unstructured by competition both within (i.e. at a local spatial scale) and among (i.e. at a regional scale) sites. Results At local spatial scales, species co-occurrence patterns and body-size ratios did not differ from randomness. At regional scales, co-occurrence patterns were random or aggregated, and there was evidence for constant body-size ratios of forest ants. Although these patterns varied between habitats and years, they did not differ between burned and unburned sites. Main conclusions Our results suggest that the operation of assembly rules depends on spatial scale and habitat type, but that it was not affected by disturbance history from fire.

We investigated the effects of the abiotic environment, plant community composition and disturbance by fire on ant assemblages in two distinct habitat types in the Siskiyou Mountains in northern California and southern Oregon, USA. Sampling over 2 years in burned and unburned Darlingtonia fens and their adjacent upland forests, we found that the effects of disturbance by fire depended on habitat type. In forests, fire intensity predicted richness in ant assemblages in both years after the fire, and plant community composition predicted richness 2 years after the fire. No factors were associated with richness in the species-poor fen ant assemblages. Speciesspecific responses to both habitat type and disturbance by fire were idiosyncratic. Assemblage composition depended on habitat type, but not disturbance by fire, and the composition of each assemblage between years was more dissimilar in burned than unburned sites.