Climate change has historically resulted in the expansion, contraction, and shift of species ranges. High-latitude range limits in areas where no physical barrier prevents dispersal are of particular interest in light of species range contraction or expansion because they represent limits that may inhibit occupancy. MacArthur proposed that abiotic factors have a greater influence on species distribution approaching high-latitude range limits. We examined aspects of environmental structure that influence distribution of Common Five-lined Skinks (Plestiodon fasciatus) and are expressed in that species' population dynamics. Eight sites in the southern Shield region of Ontario, Canada were surveyed during the active season for P. fasciatus. We collected data on the canopy cover, slope angle, slope aspect, substrate, cover type, and temperature at sites where P. fasciatus was observed (microsites) and compared these data to the general site conditions (macrosite). We assessed population size using capture–mark–recapture methods and compared population age structure at eight sites to detect if there was a correlation between the characteristics of the macrosites, population size, and age structure. Abiotic factors were the primary limiting factors affecting distribution at high-latitude range limits of the species. Spatial ecology at the individual scale was influenced by sites with suitable thermal conditions. Our results confirm the importance of microsites with suitable thermal profiles as key habitat for ectothermic vertebrates at high latitudes. The influence of temperature as a limiting abiotic factor is expressed in population density of P. fasciatus. Conservation and restoration of high-latitude populations of ectothermic vertebrates should focus on ensuring thermal requirements of the species are met before other factors are addressed, as temperature is likely the single most significant limiting factor at high-latitude range limits. [ABSTRACT FROM AUTHOR]