Ecological correlates of the thermal quality of an ectotherm’s habitat: a comparison between two temperate lizard populations

Abstract

1. Although thermal factors are of paramount importance to the quality of the habitats occupied by small ectotherms (e.g. lizards), the hypothesis that the relative abundance of squamate populations is related to the thermal quality of their habitats remains untested. If differences in the availability of thermally suitable microhabitats was the primary determinant of the overall quality of a lizard’s habitat, population density in a habitat should be proportional to its thermal quality. 2. The influence of the thermal quality of a habitat on thermoregulatory behaviour, body temperatures, expected physiological performance and relative abundance was compared in two populations of the temperate lizard Psammodromus algirus separated by 700 m altitude in central Spain. Habitat thermal quality was estimated with an index of how closely the available operative temperatures in a habitat match the range of body temperatures that lizards attempt to maintain in a laboratory thermogradient (Hertz, Huey & Stevenson 1993). 3. Operative temperatures, measured using copper models, were higher at the lowland site, where a large proportion of readings fell within or above the preferred range, especially in the early morning. Thermal habitat quality was also higher at the low altitude site, with smaller absolute deviations from the preferred range throughout most of the morning hours. Moreover, lizard body temperatures, the accuracy of thermoregulation and the predicted running performance of lizards were somewhat higher at the lowland site, although differences were smaller than expected on the basis of operative temperatures. A log‐linear analysis showed that lizard selectivity for basking sites was more pronounced at the thermally inferior montane site. Thus, the observed differences in the thermal quality of habitats appeared to influence the behaviour of lizards. 4. Nevertheless, the relative abundance of lizards, as estimated by repeated transect counts, was higher at the montane site. This unexpected result suggests that thermal constraints may not be the most important factor determining lizard population densities on a regional scale. Alternatively, food availability and/or predation pressure might have a greater impact on the overall quality of a habitat for temperate lizards.