Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

Interference competition is widely considered to structure ant communities. Competition’s effect, however, may be contingent upon disturbance or the abiotic environment. The interaction of temperature and competition is implicit in a wide body of ant community research; however, very few studies have experimentally manipulated these variables. To investigate the role of competition and temperature on ant communities, I (i) employed null models to investigate how species partition their spatial, temporal, and thermal environments in disturbed and undisturbed forests, (ii) used pairwise behavioral experiments to construct a Markov chain model to predict relative abundance patterns and correlated behavioral indices to species co-occurrence patterns, and (iii) conducted a shade, physiological thermal tolerance, and fully factorial shade and removal experiment to investigate the interaction of competition and temperature on ant community structure. The results of these studies are summarized below. First, I took advantage of a natural experiment, the 2002 Biscuit Fire, to investigate how species partition their temporal, thermal, and spatial environments in disturbed and undisturbed forests with null models. I found that most sites displayed a high degree of temporal niche overlap and species aggregation along the thermal axis. Half of the sites, however, had regular spacing of the temperature at which species obtain maximum activity. Species co-occurrence patterns in space modulated with diurnal temperature variations. Unburned sites had more spatial segregation of species than burned sites. Overall, it appears as though species activity is regulated, at least in part, by the thermal niche axis, and ant communities may repeatedly assemble and disassemble throughout the day. Second, I used data from pairwise behavioral experiment to generate transition probabilities for a Markov chain model. Assuming the landscape represents a large number of patches, the model predicted the relative abundance of an assemblage. I compared Markov chain predictions of relative abundance to relative abundance measurements on the local and regional scale. I used the same pairwise behavioral data to predict species co-occurrence values in three sites. Neither model accurately predicted community patterns. The only significant result was the Markov chain prediction of bait occurrence on the local scale; however, the relationship was opposite of the prediction. Finally, I conducted a shade experiment to investigate how communities respond to an altered thermal environment and associated their response to results from physiological thermal tolerance experiments. I then conducted a fully-factorial shade and Formica moki removal experiment to investigate if thermal responses were mediated by competitive effects. The addition of shade tables greatly reduced temperatures in the field, and Temnothorax nevadensis abundance was consistently lower in shade treatments. Decreased abundance at shade stations did not appear to be an indirect effect of F. moki activity. Physiological thermal tolerance was strongly associated with changes in abundance in shade treatments: the lower a species thermal tolerance, the greater its positive change in abundance after shade additions. The only species with a strong foraging response to F. moki removal was T. nevadensis, a species who was often cooccurred with F. moki on baits. I did not find evidence for the interaction of competition and temperature, and it appears as though physiological differences strongly influence the foraging activity of Siskiyou ant communities.

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