Experimental competition release in a community of web-weaving spiders

Abstract

Spiders are generalist predators, abundant in terrestrial ecosystems, and hence good candidates for interspecific competitive constraints. Contrary to these premises, evidence of interspecific competition in web-weaving spiders is ambiguous, and new experiments on natural communities are interesting. This paper reports on an experiment designed to test competitive effects in a natural community of web-weaving spiders, accomplished by selective removal of each of the three locally dominant species: Araneus marmoreus, Argiope bruennichi, and Agelena labyrinthi-ca. No statistically significant difference was found among control plots and the plots where one of the three species had been removed, for any of the variables measured, but the small number of replicates limited statistic power. Considering only effect direction, and not statistical significance, these removal experiments supported competitive effects on community structure, habitat overlap, and spider body size. On the other hand, effects were contrary to competitive expectations as regards abundance and habitat breadth. These results are in line with the findings of the other removal experiments conducted so far on spiders, that uncovered small intra- and interspecific competitive effects for some variables but not for others. So far, there is no evidence that interspecific competition may affect spider communities, although a low-level competition remains possible, but difficult to test statistically for its evolutionary consequence. © 1999 Taylor & Francis Group, LLC.