Road ecology of a Chihuahuan Desert snake community: size-based mortality sets the stage for evolutionary change in a widespread pitviper

TL;DR

Roads in the Chihuahuan Desert are causing high snake deaths, especially for larger Western Diamondback Rattlesnakes, potentially leading to evolutionary changes in their population.

Contribution

This study shows that road mortality can drive evolutionary changes in snake body size due to size-based collision risks.

Findings

34% of all snakes encountered were found dead on the road.

Larger Western Diamondback Rattlesnakes had a higher probability of road mortality.

Simulations suggest road mortality could reduce average body size by 12 cm in 50 generations.

Abstract

Snakes are significant predators in many ecosystems, but high rates of road mortality threaten to diminish their ecological contributions. Documenting species-specific and demographic patterns of road use is crucial for understanding potential impacts, information that can be leveraged for implementing mitigation measures. I investigated the road ecology of a snake community in the Chihuahuan Desert of southern New Mexico, USA. Nocturnal road-cruising surveys were performed three times per month from April to November 2017 along a 37-km stretch of road, which produced 101 snake encounters of 10 species. On average, 4.2 snakes were encountered per survey with no individuals recaptured, equating to a density of 0.057 snakes per kilometer. Seasonal activity patterns indicated a peak in snake encounters from August through October. Standardized data were combined with opportunistic surveys conducted along the same stretch of road from 2014 to 2016. Overall, road mortality was considerable, with 34% of all snakes found dead-on-road (DOR). Across all species, males and females exhibited similar proportions of DOR while juveniles had a lower DOR rate. The Western Diamondback Rattlesnake (Crotalus atrox) was the most frequently observed species across all surveys (48% of all encounters) and 33% of these observations were found DOR. Models for adult C. atrox revealed that longer body sizes were linked to road mortality, where every 1 cm increase in total length increased the probability of a vehicle collision by almost 10%. Preliminary simulations indicated that directional selection against larger body sizes could reduce the mean size in this population by nearly 12 cm in 50 generations (or about 165 years in this species). Road mortality may have set the stage for evolutionary change in a widespread and ecologically important pitviper. Because roads and vehicle traffic will continue to expand globally, efforts to mitigate road-based effects on snakes are essential to implement before major ecological and evolutionary impacts are felt.

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