Negative effects of the spatial clumping of thermal resources on lizard thermoregulation in a fragmented habitat

Abstract

In ecosystems threatened by the expansion of croplands, habitat fragmentation and climate change, two of the main extinction drivers, may have thermoregulation-mediated interacting effects on demographic trends of terrestrial ectotherms. We studied the thermal biology of a metapopulation of the widespread Mediterranean lacertid Psammodromus algirus in ten fragments of evergreen or deciduous oak forests interspersed among cereal fields. We obtained thermoregulation statistics (selected temperature range, body and operative temperatures, thermal quality of the habitat, and precision, accuracy, and effectiveness of thermoregulation) that could be compared among fragments and with conspecific populations living in unfragmented habitat. We also measured the selection (use vs. availability) and spatial distribution of sunlit and shaded patches used for behavioral thermoregulation in fragments, and we estimated operative temperatures and thermal habitat quality in the agricultural matrix surrounding the fragments. Variation of the thermal environment was much larger within fragments than among them, and thermoregulation was accurate, precise, and efficient throughout the fragmented landscape; its effectiveness was similar to that of previously studied unfragmented populations. The average distance between sunlit and shaded patches was shorter in deciduous than in evergreen fragments, producing a more clumped distribution of the mosaic of thermal resources. Consequently, in evergreen habitat the cost of thermoregulation was higher, because lizards were more selective in their choice of sunlit sites (i.e. they used sunlit patches closer to shade and refuge than expected at random, and the extent of such selection was larger than at deciduous habitat). Temperatures available in croplands were too high to allow lizard dispersal, at least in the post-breeding season. This result confirms the role of croplands as a thermal barrier that promotes inbreeding and associated fitness losses in isolated fragments, and it forecasts a dark future for populations of forest lizards in agricultural landscapes under the combined effects of habitat fragmentation and global warming.