Data from: Density-dependent habitat selection predicts fitness and abundance in a small lizard

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.