We conducted a field experiment to determine whether changes in the activity patterns of ants affect the distribution of spiders and centipedes. On the forest floor, 24 open 1x1-m plots were provided with baits for ants; 24 unbaited plots were controls. During the study, ant activity in baited plots was 28x higher than in control plots. At the end of the study, ant density was 2.5x higher in baited than control plots. Densities of spiders and centipedes that did not readily feed on ants in laboratory trials did not differ between experimental and control plots at the end of the experiment. However, densities of predators that did feed on ants in the laboratory showed a marginally significant increase (1.4x) in baited plots (p=0.057), with the spider Gnaphosa showing the strongest response, followed by the centipede Lithobius. Although the overall direct effect of ants on spiders and centipedes was not strong, multiple regression analyses revealed that ants may in fact influence the distribution of spiders and centipedes through a combination of direct and indirect effects of different ant species. Different species of the arthropods sampled significantly contributed to the different regression models with either positive or negative signs, suggesting that responses to increased ant densities depended on the species involved in each plot, and that the existence of counteracting direct and indirect effects is one possible explanation for the only moderate overall response of spiders and centipdes to baiting.
Keywords: distribution
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