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.