Assessment of Competitive Interactions between Two Iguanid Lizard Species

Abstract

In riparian mesquite acacia woodland in SE Arizona, we found tree lizards (Urosaurus ornatus) occupying trees with yearling spiny lizards (Sceloporus magister). To examine possible short-term effects of the larger spiny lizard on the smaller tree lizard, we analyzed the joint distribution of resident lizards, and dry season growth rates and residency times of tree lizards in the presence and absence of spiny lizards. Despite a significant positive association in the distribution of lizard species, we could detect no reduction in growth rate or residency time of male and female tree lizards that were resident with spiny lizards. To investigate changes in tree lizard behavior in association with spiny lizards we enclosed 1-m lengths of mesquite branches to confine lizards to a restricted area of their natural habitat. Control tree lizards were introduced to the enclosure and their occupation time on the branch was recorded. Lizards could leave the branch by attempting to climb the metal partition or by jumping from the branch. The same (control) tree lizard was introduced to the branch enclosure with a spiny lizard; however, for each tree lizard the order of control and treatment was randomized. In another set of trials we increased ant density on the branch enclosure and recorded the feeding rate of tree lizards. Spiny lizard yearlings had no effect on either the time tree lizards were on the enclosed branches or the feeding rate of tree lizards.