Summary
The ideal free distribution concept predicts that organisms will distribute themselves between habi-tats in a density-dependent manner so that individuals, on average, achieve the same fitness in each habitat. In ecto-therms, environmental temperature has a strong impact on fitness, but temperature is not depletable and thus not density dependent. Can density-dependent habitat selection occur in ectotherms when habitats differ in thermal qual-ity? We used an observational study of habitat selection by small snakes in field and forest, followed by manipu-lative habitat selection and fitness experiments with com-mon gartersnakes in enclosures in field and forest to test this hypothesis. Snakes were much more abundant in the field, the habitat with superior thermal quality, than in the forest. Gartersnakes in our controlled experiment only used the forest habitat when snake density was highest and when food was more abundant in the forest; habitat selection was largely density independent, although there was weak evidence of density dependence. No female garter-snake gave birth in the forest enclosures, whereas half of the females gave birth in the field enclosures. Growth rates of females were higher in field than in forest enclosures. Overall, our data indicate that temperature appears to be the most important factor driving the habitat selection of gartersnakes, likely because temperature was more limiting than food in our study system. Snakes, or at least temper-ate snakes, may naturally exist at population densities low enough that they do not exhibit density-dependent habitat selection.
Methodology
We conducted an observational study of snake habitat selection at Queen’s University Biological Station (QUBS; 44°33′N, 76°21′W) in eastern Ontario, Canada. Although QUBS is home to nine species of snakes, we only obtained sufficient capture data for common gartersnakes (Thamno-phis sirtalis), Dekay’s brownsnakes (Storeria dekayi), and red-bellied snakes (Storeria occipitomaculata). All three species have relatively generalized diets, but T. sirtalis eats invertebrates and small amphibians, whereas S. dekayi and S. occipitomaculata only eat invertebrates.We set up five 50 × 100-m study plots that each encom-passed 50 % field and 50 % forest. All of the fields were cut once per year and were thereby maintained as a mixed grass and forb community. Forests were mixed hardwood, mainly sugar maple (Acer saccharum), ironwood (Ostrya virginiana), and American beech (Fagus grandifolia). We placed sixteen 60 × 60-cm plywood cover boards on each grid (4 rows of 4 boards spaced every 25 m) to act as snake refuges (Halliday and Blouin-Demers 2015), with half of the boards placed in the field and half placed in the forest. We surveyed field and forest habitats twice a day (0900 and 1400 hours) for 3 days every 2 weeks from 5 May 2013 to 16 July 2013. These 3-day periods represent one sam-pling period, for a total of six sampling periods throughout the study. During each survey, we walked across the plots at a constant pace and checked under every cover board. We hand-captured each snake that we encountered and gave each individual a unique mark by branding its ven-tral scales using a medical cautery unit (Bovie Aaron Low-Temp Reusable Cautery Unit, Bovie Medical Corp. Clear-water, FL; technique and rationale for branding described in Winne et al. 2006). We then released each individual at its point of capture.