Diet Selection and Depression of Prey Abundance by an Intensively Foraging Lizard

Abstract

Diet of the intensively foraging Puerto Rican lizard Ameiva exsul included most taxa of macroorganisms found in the leaf litter and soil surface at the study site. Nevertheless, certain items, Anolis eggs an extreme example, were overconsumed in relation to their abundance as determined by stratified random sampling of the leaf litter and soil surface where the lizards foraged. Other food items, such as land snails, were important in the diet but underconsumed relative to their high level of abundance in the leaf litter. Millipedes and termites were the only common litter organisms encounterd that were not found in diet samples. Effects of Ameiva predation on abundance of land snails were studied in two experiments with hardware cloth exclosures that permitted free movement of snails through the 1.3 cm x 1.3 cm mesh and entrance of Anolis and birds through the open tops. Adult A. exsul were the only predators known to be excluded. Protection from A. exsul was associated with increased abundance of the low mobility land snails. No consistent changes in other species were detected. Exposure to high quality prey items or high quality patches permits selectivity in the context of a broad diet, a tenet of foraging theory (Stephens and Krebs, 1986). The probing, investigative, energetic behavior of intensive foragers (Anderson and Karasov, 1981; Regal, 1983) may bring them into contact with a great number of potential prey including some items of high quality (Huey and Pianka, 1981). Moreover, the high level of movement of intensive foragers may increase the chance of encounter with rich patches of prey. Intensively foraging lizards obtain broad diets by searching large areas (Schoener, 1971; Regal, 1983). The Puerto Rican teiid Ameiva exsul, for example, has been reported to eat everything from other lizards and lizard eggs to frogs, arthropods, snails, fruit, fungi, and garbage as it probes the leaf litter and soil surface (Wolcott, 1924; Rodriguez-Ramirez, 1985; Lewis, 1986; Lewis and Saliva, 1987). Whereas emphasis sometimes has been placed on diet breadth giving the impression that such relative omnivores forage indiscriminately (Heatwole and Torres, 1967), intensive foragers avoid some potential prey types (Vitt and Cooper, 1986) and probably favor others. Activity of lizards over the spectrum of styles from sit-and-wait foraging to intensive foraging in both Brazil (Magnusson et al., 1985) and Africa (Huey and Pianka, 1981) is positively correlated with the number of termites, a patchy resource, in the diet. Exclosure and enclosure experiments with experimentally amenable aquatic organisms indicate that predators can modify prey abundance and thus, at least potentially, mold community structure (Morin, 1984; Quammen, 1984; Vanni, 1986). Experiments with sit-and-wait iguanids suggest that lizards using this low energy foraging mode are selective feeders that take disproportionate quantities of relatively rare items (Stamps et al., 1981) and influence the structure of arthropod communities in certain microhabitats (Pacala and Roughgarden, 1984). Intensively foraging lizards, particularly when present at high density, may similarly modify relative and absolute abundance of litter dwelling macroorganisms. Evidence is presented here to test hypotheses that 1) Ameiva exsul forages selectively and that 2) foraging by A. exsul has a significant impact on the leaf litter community.