Effects of Top and Intermediate Predators in a Terrestrial Food Web

To determine the effects of lizards and web spiders on species in lower trophic levels, we manipulated their abundances within large field enclosures on Staniel Cay, Bahamas, from May 1989 to March 1992. The experimental design (2 x 2 factorial) measured the separate effects of lizards and spiders and compensatory predation (lizard x spider interaction). In treatments where web spiders were unaltered, mean number of web spider individuals was 1.4 times higher in enclosures with lizards removed than in those with lizards present at natural densities. Total biomass of aerial arthropods caught in sticky traps was 1.4 times higher in treatments with web spiders removed than in treatments with web spiders unaltered. Lizards had no significant effect on aerial arthropods. Total amount of herbivore damage on sea grape leaves was 3.3 times higher in treatments with lizards removed than in treatments with lizards unaltered. Web spiders had no significant effect on leaf damage. The lizard x spider interaction was not significant in each analysis, in- dicating that compensatory predation was weak. Our results support a model in which the interaction between top predators (lizards) and herbivores is strong, whereas the interaction between intermediate predators (web spiders) and herbivores is weak. Consequently, the net effect of top predators on producers is positive.