Experimental competition release in a community of web-weaving spiders

Spiders are generalist predators, abundant in terrestrial ecosystems, and h ence good candidates for interspecific competitive constraints. Contrary to these premises, evidence of interspecific competition in web-weaving spide rs 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 re moval of each of the three locally dominant species: Araneus marmoreus, Arg iope bruennichi, and Agelena labyrinthica. No statistically significant dif ference 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 direc tion, and not statistical significance, these removal experiments supported competitive effects on community structure, habitat overlap, and spider bo dy size. On the other hand, effects were contrary to competitive expectatio ns 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 fbr some variables but not for others. So Ear, there is no evidence that interspeci fic competition may affect spider communities, although a low-level competi tion remains possible, but difficult to test statistically for its evolutio nary consequence.