Print PageIndividuals faced with altered thermal regimes and habitat structure due to broad-scale disturbance may respond in three ways: 1) shift in distribution, 2) plastic or evolutionary responses (behavioral compensation, phenotypic plasticity, or response to selection), or otherwise 3) fail to adapt and go extinct. Most often, species must attempt to maintain populations in a shifting environment by relying on the second choice. In particular, population persistence may be facilitated if individuals within the populations differentially use available resources (i.e., exhibit resource polymorphisms) and minimize potential competition. The success of this strategy however is contingent on external factors including both spatial and temporal resource availability and disturbance frequency. Here, we assess the demographic, performance, and behavioral components of tree lizard (Urosaurus ornatus) populations across sites in southeast Arizona that differ significantly in resource availability and disturbance frequency. Two sites were surveyed differing in historic burn frequency and thus composition of vegetation functional groups (i.e., grass, forb, and shrub categories). Morphometric, performance (endurance and dominance trials), and throat patch coloration (spectrometer reflectance curves) measurements were made on all tree lizards collected. Data on structural and resource (vegetation and arthropod) availability across habitats were recorded as well, at both random and lizard collection sites.
Results/Conclusions
The more-frequently burned site had greater grass cover (91 to 84%, respectively), and a reduced diversity of arthropod prey (by family: Sorenson’s D = 0.833) than the less-frequently burned site. Across both sites, male lizards exhibited greater endurance than female lizards (t = 2.439, df = 84, P = 0.017), and males were largest in the more-frequently burned site. This site was also marked with a greater frequency of yellow- and orange-throated lizards overall (> 80%). Based on mixing-model simulations, resource polymorphisms exist among focal tree lizard populations, with yellow-throated lizards foraging at a higher trophic step than orange- or blue-throated lizards (χ2=7.869, N=18, P = 0.02). These polymorphic differences were more evident in the site burned more frequently. Overall we show that variation in base-resources influences (arthropod) prey diversity, trophic positioning, and phenotypic frequency of our focal species. This study builds upon and extends past studies of resource-use polymorphisms by using isotopic analysis to model detailed nutrient dynamics and energy flow, which facilitates understanding how disturbances actually affect populations at a functional level.
