LIZARDS REDUCE SPIDER SPECIES RICHNESS BY EXCLUDING RARE SPECIES

To assess the impact of lizards on species richness and properties of individual species of web spiders, we conducted a 4.5-yr field experim ent on Staniel Cay, Exumas, Bahamas. Spider populations were censused at similar to 2-mo intervals in lizard-removal enclosures and in contr ol enclosures with lizards present at natural densities. Lizards reduc ed total number of individuals, species richness (number of species), and composite diversity of web spiders. The differential absence of ra re species was primarily responsible for the lower species richness in controls than in lizard-removal enclosures. The impact of lizards on the abundance of Metepeira datona, the numerically dominant spider spe cies, was weaker than the impact on the abundance of all rarer species combined. Mean body lengths of Argiope argentata, a large, rare spide r, were larger in removals than in controls, whereas mean body lengths of M. datona, a small, common spider, did not differ significantly in removals and controls. A. argentata were closer to the ground than tw o commoner species and thereby were potentially more vulnerable to liz ard predation. The present mainland-enclosure experiment and an island -introduction experiment both demonstrated the same general pattern of lizard predation: exclusion of rare spider species. However, the impa ct of lizards on spiders was more devastating in the island experiment . Other experimental field studies that had terrestrial animals as sub jects and that separately analyzed at least all common species in a pr ey assemblage demonstrated that predators reduced or had no effect upo n prey species diversity. Each study in which predators reduced divers ity indicated that local extinction of rare species in experimental ar eas with predators was common.