No abstract

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.

17Patterns of species occurrence and abundance are influenced by abiotic factors and biotic 18 interactions, but these factors are difficult to disentangle without experimental manipulations. In 19 this study, we used observational and experimental approaches to investigate the role of 20 temperature and interspecific competition in controlling the structure of ground-foraging ant 21 communities in forests of the Siskiyou Mountains of southwestern Oregon. To assess the 22 potential role of competition, we first used null model analyses to ask whether species partition 23 temporal and/or spatial environments. To understand how thermal tolerances influence the 24 structure of communities, we conducted a laboratory experiment to estimate the maximum 25 thermal tolerance of workers and a field experiment in which we added shaded microhabitats and 26 monitored the response of foragers. Finally, to evaluate the roles of temperature and interspecific 27 competition in the field, we simultaneously manipulated shading and the presence of a dominant 28 competitor (Formica moki). The foraging activity of species broadly overlapped during the 29 diurnal range of temperatures. Species co-occurrence patterns varied across the diurnal 30 temperature range: species were spatially segregated at bait stations at low temperatures, but co-31 occurred randomly at high temperatures. The decreased abundance of the co-occurring 32 thermophilic Temnothorax nevadensis in shaded plots was a direct effect of shading and not an 33 indirect effect of competitive interactions. Thermal tolerance predicted the response of ant 34 species to the shading experiment: species with the lowest tolerances to high temperatures 35 showed the greatest increase in abundance in the shaded plots. Moreover, species with more 36 similar thermal tolerance values segregated more frequently on baits than did species that 37 differed in their thermal tolerances. Collectively, our results suggest that thermal tolerances of 38 3 ants may mediate competitive effects in habitats that experience strong diurnal temperature 39 fluctuations. 40

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. Species‐specific 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.