Lizards Reduce Spider Species Richness by excluding Rare Species

Abstract

To assess the impact of lizards on species richness and properties of individual species of web spiders, we conducted a 4.5-yr field experiment on Staniel Cay, Exumas, Bahamas. Spider populations were censused at ∼2-mo intervals in lizard-removal enclosures and in control enclosures with lizards present at natural densities. Lizards reduced total number of individuals, species richness (number of species), and composite diversity of web spiders. The differential absence of rare 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 species, was weaker than the impact on the abundance of all rarer species combined. Mean body lengths of Argiope argentata, a large, rare spider, 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 two commoner species and thereby were potentially more vulnerable to lizard 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 impact of lizards on spiders was more devastating in the island experiment. Other experimental field studies that had terrestrial animals as subjects and that separately analyzed at least all common species in a prey assemblage demonstrated that predators reduced or had no effect upon prey species diversity. Each study in which predators reduced diversity indicated that local extinction of rare species in experimental areas with predators was common.