Abundance, diet and prey selection of arboreal lizards in a grazed tropical woodland

Abstract

The diet of predators is a critical determinant of their ecological effects. Small vertebrate predators of invertebrates are often characterized as diet generalists based on diet descriptions, but few studies examine prey availability to determine whether prey choice occurs. We studied the prey availability in relation to the diet of two common and abundant, but understudied small vertebrates: <i>Gehyra dubia</i>, an arboreal nocturnal gecko, and <i>Cryptoblepharus australis</i>, an arboreal diurnal skink. We sampled lizards in two major woodland habitat types, Reid River box (<i>Eucalyptus brownii</i>) and Silver-leaf ironbark (<i>Eucalyptus melanophloia</i>) and among four cattle grazing regimes (ranging from moderate – heavy stocking). <i>Cryptoblepharus australis</i> were more abundant in the Silver-leaf ironbark habitat, but there was no effect of grazing regime on their abundance. In contrast, <i>G. dubia</i> did not differ significantly in abundance in relation to habitat type, but were more abundant in paddocks with heavier stocking rates. We quantified invertebrate prey available to lizards in these habitats using defined-area searches and light trapping. Invertebrate community composition did not differ significantly between the habitat types or among the four grazing regimes. Although <i>G. dubia</i> and <i>C. australis</i> both occupied the same microhabitats, they were temporally segregated based on their activity times. While both species are apparently habitat generalists, we found that <i>G. dubia</i> and <i>C. australis</i> are selective in their diets. Only half of the invertebrate groups available in the environment occurred in the diets of either lizard species. Both species positively selected Coleoptera (beetles), Araneae (spiders), and Scorpiones (scorpions), and they exhibited high dietary niche overlap (<i>O</i> = 0.97). We suggest the increased availability of the top three preferred prey groups (beetles, spiders, and scorpions) may contribute to the high abundances of <i>G. dubia</i> in heavily grazed areas.