Reduced predation on roadside nests can compensate for road mortality in road-adjacent turtle populations

Turtles are killed on roads, yet there is little evidence of negative road effects on their abundances. We hypothesized that this could be due to reduced predation of turtle nests laid along roadsides, which could compensate for the effects of higher adult mortality on turtle populations near roads. To test this, we quantified the relative differences in predation of artificial painted turtle (Chrysemys picta) nests near and far from roads, in a field experiment controlling for potentially confounding differences between sites. The field experiment tested the predictions that (1) nest predation rates are lower for roadside nests than nonroadside nests, (2) nest predation rates are lower for linearly placed than clumped nests, and (3) nest predation rates are lower for roadside, linearly placed nests than any other road-adjacency and configuration combinations. We then estimated how much adult roadkill could be compensated for by reduced roadside nest predation using population viability analysis (PVA). Linearly placed roadside nests had a 26% lower predation rate than nonroad nests in a natural (clumped) configuration. This result, combined in a PVA with life-history information for painted turtles, led to an estimate that approximately 3%-6% annual adult roadkill can be compensated for by reduced nest predation near roads. This suggests that compensation for adult roadkill via predation release is a plausible explanation for the previously documented lack of road effects on turtle population abundance. These results highlight the importance of considering species interactions when evaluating the effects of roads on wildlife populations.

Automated summary

Turtles killed on roads may not experience negative population effects due to reduced predation of nests near roads, compensating for higher adult mortality. Field experiments showed that linearly placed roadside nests had lower predation rates than nonroad nests in a natural configuration. Population viability analysis estimated that reduced nest predation near roads could compensate for approximately 3%-6% annual adult roadkill. This highlights the importance of considering species interactions when evaluating road effects on wildlife populations.