Do roads affect the abundance of garter (Thamnophis sirtalis) and redbelly snakes (Storeria occipitomaculata)?

Abstract

The greatest driver of the current biodiversity crisis is habitat loss. Roads are a major contributor to habitat loss because they destroy and fragment habitat, in addition to causing direct mortality. Animals may respond to roads either by avoiding them, thus leading to population isolation, or by attempting to cross them, thus potentially leading to increased mortality and, if so, also to population isolation. We studied the impact of road density on abundance of two snake species: redbelly snakes ( Storeria occipitomaculata Storer, 1839) and garter snakes ( Thamnophis sirtalis Linnaeus, 1758) around Ottawa, Canada. We hypothesized that roads are detrimental to snake populations due to road avoidance and mortality. Therefore, we predicted that snakes should be less abundant at sites with higher road density in their surroundings. We deployed cover boards at 28 sites along a gradient of road density in 2020 and 2021. We visited sites weekly, counted the number of individuals of both species, and measured snout–vent length (SVL) of all individuals captured. We captured fewer garter snakes at sites surrounded by more roads and fewer redbelly snakes at sites surrounded by more urban habitat. Snakes at sites surrounded by more roads were not smaller. The effects of roads and urbanization on the number of snakes were modest, but indicate decreasing population sizes that could lead to loss of ecological function.