Density‐dependent habitat selection predicts fitness and abundance in a small lizard

Abstract

Density‐dependent habitat selection has been used to predict and explain patterns of abundance of species between habitats. Thermal quality, a density‐independent component of habitat suitability, is often the most important factor for habitat selection in ectotherms which comprise the vast majority of animal species. Ectotherms may reach high densities such that individual fitness is reduced in a habitat due to increased competition for finite resources. Therefore, density and thermal quality may present conflicting information about which habitat will provide the highest fitness reward and ectotherm habitat selection may be density‐independent. Using ornate tree lizards Urosaurus ornatus at 10 sites each straddling two adjacent habitats (wash and upland), we tested the hypothesis that habitat selection is density‐dependent even when thermal quality differs between habitats. We first tested that fitness proxies decline with density in each habitat, indicating density‐dependent effects on habitat suitability. We also confirmed that the two habitats vary in suitability (quantified by food abundance and thermal quality). Next, we tested the predictions that habitat selection depends on density with isodar analyses and that fitness proxies are equal in the two habitats within a site. We found that monthly survival rates decreased with density, and that the wash habitat had more prey and higher thermal quality than the upland habitat. Lizards preferred the habitat with more food and higher thermal quality, lizard densities in the two habitats were positively correlated, and fitness proxies of lizards did not differ between habitats. These patterns are consistent with density‐dependent habitat selection, despite differences in thermal quality between habitats. We expect that density‐dependent habitat selection is widespread in terrestrial ectotherms when densities are high and temperatures are close to their optimal performance range. In areas where thermal quality is low, however, we expect that depletable resources, such as food, become less limiting because assimilating resources is more difficult.

Male throat colour polymorphism is related to differences in space use and in habitat selection in tree lizards

“…In general, tree lizards prefer the wash habitat (Paterson & Blouin‐Demers, ). We found evidence that habitat selection differed between throat colour morphs: green‐throated males were closer to the wash than blue‐throated males.…”

“…We found evidence that habitat selection differed between throat colour morphs: green‐throated males were closer to the wash than blue‐throated males. Since population density is higher in the wash habitat than in the upland (Paterson & Blouin‐Demers, ), green‐throated males displaying a reproductive parasite strategy (Lattanzio & Miles, ) may be more successful in crowded areas where female density is highest. We believe the estimated difference in distance to the wash habitat (5 m) is biologically significant because it is large compared to the mean home‐range size (31–70 m 2 ) of males (Zucker, ; Mahrt, ).…”

Tree lizard (Urosaurus ornatus) growth decreases with population density, but increases with habitat quality

“…The parameter g is the density‐dependent parameter and d is the mean density per habitat (lizards/ha). We calculated population size in each habitat at the beginning of the breeding season from open‐population mark‐recapture models with one estimate per site in each year (for detailed model descriptions and density data, see Paterson & Blouin‐Demers, ). One of us (J. E. P.) walked the boundary of each habitat with a handheld GPS unit and calculated the area in QGIS (QGIS Development Team, ).…”

“…The wash habitat had an open canopy and sparse herbaceous vegetation, and the upland habitat consisted of pine‐oak woodlands. The two habitats differed in quality relevant to lizard fitness; the wash had more arthropod prey and allowed lizards to achieve their T set (32.2–36.0°C) for a longer period in a day than the upland habitat (Paterson & Blouin‐Demers, ). Ornate tree lizards preferred the wash habitat and occurred at higher densities in that habitat (Paterson & Blouin‐Demers, ).…”

“…We tested the hypothesis that ornate tree lizard ( Urosaurus ornatus ) habitat selection is density dependent because growth is limited by intraspecific competition for resources and habitat quality. We have previously reported a negative relationship between ornate tree lizard survival and population density in two habitats (Paterson & Blouin‐Demers, ), but we found no relationship between growth rate and population density using mark‐recapture data. However, population density effects on body size and growth often manifest themselves as differences in asymptotic body size, rather than growth rates (Beverton & Holt, ; Lorenzen, ).…”