Lizard abundance in forest fragments: effects of patch size, patch shape, thermoregulation, and habitat quality

Abstract

The effects of forest fragmentation on ecosystems are pervasive, but little is known about the factors that influence lizard abundance in fragmented landscapes. We studied the roles of patch size and shape, thermal quality, and refuge availability as predictors of the relative abundance of Psammodromus algirus lizards at deciduous or evergreen forest fragments surrounded by cereal fields. Relative abundance, based on time-controlled counts, decreased from the northeast (dominated by deciduous habitat) to the southwest (with a higher cover of croplands, and dominated by evergreen habitat). Refuge availability was correlated with this gradient, decreasing from the northeast to the southwest and being larger in deciduous than in evergreen fragments. After controlling for the effects of this environmental variation, lizard abundance increased as perimeter-to-area ratio (P/A) decreased (and consequently as fragment size increased). Although the effects of thermal quality as such were negligible, our results can be interpreted in the light of thermoregulatory requirements; given the low temperatures available at shaded locations, lizards should actively select sunlit patches while they try to minimize predation risk by basking as close as possible to the nearest refuge. Although use of fragment edges as basking sites is expected to increase with P/A ratio, lizards should avoid using them as basking sites, because both exposure to predators and risk of overheating are expected to be higher at edges and croplands than inside fragments. We conclude that long and narrow forest strips with high P/A ratios could act as ecological traps rather than as dispersion-promoting corridors.