Over the past several decades, the field of road ecology has documented many of the negative effects of roads on plants and animals. Roadkill, contamination from runoff, and habitat fragmentation all challenge the success of populations in road-adjacent areas.
But these are just the immediate impacts of roads. Recently, researchers have begun considering how these same impacts can spur evolutionary changes in organisms.
A paper by Steven Brady (who was a post-doc at Dartmouth College and is now a biologist in the Department of Water and Land Resources at King County in Seattle, WA) and Jonathan Richardson of Providence College describes how roads can cause rapid evolutionary change in wild populations of plants and animals. It was published in the March 1 edition of the journal Frontiers in Ecology and the Environment.
It is critical to understand how roads are impacting wildlife due to their pervasiveness. Said to be the largest human artifact on the planet, road networks cover some 64 million km of Earth’s surface. Roads impact nearly 20% of the U.S. landscape, and it is estimated that around the world, roads will increase 60% in length by 2050.
Only a handful of studies have examined the evolutionary consequences associated with roads. But the data so far indicate both positive and negative effects are possible; that is, evolution can increase or decrease the fitness of road-affected populations.
Sometimes the evolved changes are adaptive. Several species living in road-adjacent areas, like the common grass Anthoxanthum odoratum, are evolving a higher tolerance to pollutants like road salt runoff. In another example, the number of roadkill cliff swallows (Petrochelidon pyrrhonota) decreased over a period of 30 years, despite an increase in overall population size. At the same time, the wing length of roadkill swallows increased while that of the overall population decreased. This suggests that evolution is selecting for shorter wings, which aid in maneuverability and vertical takeoff and likely help swallows avoid vehicle collisions.
Road-adjacent populations can also evolve in maladaptive ways, becoming less resilient to negative road effects.
Adaptation and maladaptation can even occur simultaneously in different species in the same road-affected habitat. Earlier work by Brady found that this was the case for the spotted salamander (Ambystoma maculatum) and the wood frog (Rana sylvatica), which breed in identical roadside habitats. The survival rate of wood frogs living by the road decreased compared to populations not living near roads. But for spotted salamanders, the fitness of the roadside population increased compared to other populations.
Brady says it is surprising that some populations appear to be evolving maladaptively right alongside populations that are evolving adaptively. But such maladaptive outcomes may become increasingly common in response to human modification of the environment.
“We think maladaptation may be an emerging consequence of global change,” says Brady. “As environments become more dynamic with global environmental change, it becomes harder and harder for organisms to adapt.
“There’s only so much genetic variation out there. If the rate of environmental change is really fast relative to that genetic variation, it can be difficult for adaptation to occur. We might expect maladaptation to be on the rise as a result.”
Brady says that from a conservation perspective, it will be important to predict which species or populations will become adapted and which will become maladapted. That kind of knowledge can be used to triage conservation priorities and determine tactics to improve specific populations’ success.
For instance, well-adapted road-adjacent populations could be used to supplement struggling populations by promoting gene flow or through assisted migration.
The study of road ecology is still in its infancy, and we are only beginning to appreciate the evolutionary impact of roads on wild plants and animals. In the future, a better understanding of how different populations are evolving will hopefully help scientists, conservationists, and policy makers predict and mitigate the negative consequences of roads.
Reference: Brady, S. P. and Richardson, J. L. (2017). Road ecology: shifting gears toward evolutionary perspectives. Frontiers in Ecology and the Environment 15(2): 91-98. doi: 10.1002/fee.1458.