An experimental test for predator-mediated interactions among spider species

The primary objective of this study was to test the: hypothesis that lizard s intensify interactions among spider species. The most abundant lizard and web-spider species (Anolis sagrei and Merepeira datona, respectively), wer e manipulated within field enclosures over a 30-mo period. The design was a 2 x 2 factorial: A. sagrei present and removed, crossed with M. datona pre sent and removed. A. sagrei reduced abundance of Eustala cazieri, the secon d most abundant web spider, and tended to reduce abundance of all rare spid er species combined. M. datona had no significant effect on abundance of E. cazieri or of all rare species combined. In each statistical analysis of a bundance, the A. sagrei x M. datona interaction term was not significant, i ndicating that lizards did not intensify (or reduce) interactions among spi der species. Furthermore. data on spider food supply, body length, and web height showed no evidence for two proposed mechanisms in which interactions among spider species are stronger with lizards than without. Thus, this ex periment supports the hypothesis that direct predation is the major mechani sm by which lizards reduce the abundance and species richness of spiders.