As a grad student—at a time when journals were rapidly disappearing from print in favor of online-only formats—I set two pseudo goals for myself: one was to publish a paper (period!) in an actual, printed journal, and the other was to have a paper featured on a cover. As a tremendous amount of luck would have it, Jonathan Richardson and I recently managed to pull off this feat. This past month, in a paper in Frontiers in Ecology and the Environment, we advocate for evolutionary perspectives in the ever-growing field of road ecology.
|Roads as drivers of evolutionary change! With a paper hot off an actual press and featured on a cover, the author of this post is overly giddy to tick off two bucket list items.|
The motivation for this paper (besides our personal view that road-induced evolution is really fascinating and probably pervasive the world over) is that evolutionary perspectives have generally been lacking in road ecology. While road ecologists have furnished a wealth of knowledge about the vast suite of road effects impacting the planet, insights into evolutionary consequences have typically been overlooked. And from what we can tell, that vast suite of effects is a recipe for evolutionary change.
But before we dive into the topic of road-induced evolution, let's first consider why roads matter from a conservation perspective. As always, it seems prudent to turn to a passage from Tolkien for an inspiring prelude. Thankfully, in The Hobbit, Bilbo Baggins returns from his journey singing a song about roads:
Roads go ever ever on,
Over rock and under tree,
By caves where never sun has shone,
By streams that never find the sea… (Tolkien 1938)
True to form, Tolkien's words speak as much about present day as days of yore. Roads most certainly do go ever ever on, and indeed can be found among the most remote and picturesque places. Said to be the largest human artifact, roads traverse some 64,000,000 km across the planet. When we consider the numerous negative ecological effects caused by roads (see below), it is easy to see why this global network of pavement and dirt has earned the nickname 'the giant embracing us'.
|The global road network -- an intertwining matrix of highways and byways -- is well described by its (underutilized) nickname 'the giant embracing us'.|
In the United States (which hosts about 10% of the global road network), one percent of the landscape is occupied by road infrastructure. At first blush, this may not sound like much; after all what could one percent really amount to? Well, in this case, quite a lot. In terms of shear magnitude, this one percent constitutes 14,000,000 lane km of roads. Said another way, the U.S. has enough road materials to pave a 25-lane highway (with a 12-lane unpaved shoulder) from here to the moon.
|In the United States alone, there are enough roads to pave a 25-lane highway (and 12-lane soft shoulder) from Earth to the Moon.|
Across this massive network, roads cause numerous, negative ecological effects. From road kill to contaminant laden runoff to fragmentation effects; from the spread of invasive species to the spearheading of land development to impacts on gene flow; roads are something of a one-stop shop for human impacts on the environment.
Among the various ways we utilize and modify the landscape – from agriculture to urbanization to forest harvest – roads are rather unique in that their effects are far-reaching despite having a relatively small footprint. Indeed, road effects extend well beyond the road surface and verge. For example, in the United States, that one percent of the area covered by roads is estimated to ecologically influence 20% of the landscape.
Roads are also unique in that their impacts can be substantial in ares that are otherwise undisturbed. Whether in contexts of highways bisecting large tracts of undeveloped land between distant destinations, or forest roads zigzagging through protected landscapes, roads and their impacts can be found among the most beautiful places on the planet. Indeed, while the impacts of roads may seem obvious in urban places, the aesthetic of our beloved country roads may belie their insidious nature (sorry, John Denver). All of this is to say that there are an awful lot of roads out there causing an awful lot of ill effects in an awful lot of places.
|Road effects are many. And many of those effects likely act as agents of natural selection (making roads ripe for evolutionary study!).|
Before I go any further on this road-bash, I have to make a confession: I love roads (and John Denver, just to be sure). As much as there is to hate about roads and their nasty ecological effects, there are easily as many things to love about them. Roads are the great journey-makers, the great soul-soothers, the great inspirers. Roads welcome all traveler alike. Roads do not judge, they do not play favorites, and they rarely let us down. Roads have shaped our economies, our cultures, and arguably our collective conscience. I might even go so far as to say that roads are the veins on the planet through which we humans course—with all of our hopes, dreams, and love.
Personally, I’ve put more miles on my cars than the ecologist in me cares to admit (although decidedly fewer miles than the road-tripping climber in me would prefer). Few feelings can compete with the first glimpse of an iconic destination after hours or days on the road. Whether rounding the corner to that first view of a massive granite cliff or cresting a hill to that first sight of the ocean, roads are a path to something that resonates deep inside us. Roads can also be great places to think. I find few things as mind-cleansing or idea-provoking as a long, quiet drive (or a bike ride for that matter). Undoubtedly (and surely out of necessity at times), some of my best ideas have come to me on long drives.
|Yet another hazard of roads: the out-the-window selfie. The author, boat in tow, blazes across the country for the umpteenth time, chasing the sun, the open road, and the promise of possibility that lies ahead.|
Long before roads appealed to my adventurous side (and about the time when my siblings and I lived in fear of the phrase 'scenic route'), I was struck by the potential for roads to impact nature. I can remember the uneasy feeling I would get as a child watching a plow truck pass by, leaving a blanket of salt in its wake.
At that time (mid 1980s), ecologists had not paid much attention to road effects. In fact, despite the long history of roads on our landscapes, it was not until the mid 1990s that the ecological impacts of roads drew the eyes of many ecologists. As the story goes, it was 1994 when Richard T. T. Forman (the 'father of road ecology') was driving to the annual ESA meeting (via the scenic route, no less!). On that fateful drive, Forman—impressed by the scale of the canyon road before him—envisioned the critical need for an ecological understanding of road effects. (A perfect example of what roads can inspire!)
Contrasting Forman's vision, the ESA meeting left him disappointed: of the more than 2000 presentations that year, only one had the word ‘road’ in its title. This disappointment was apparently converted rapidly into inspiration and action. Less than a decade later, Forman and colleagues had amassed enough information on the ecological effects of roads to write the formative book on the subject (Forman et al. 2003). Since that time, the field has advanced steadily, unveiling the diverse ecological consequences of the net of roads blanketing modern landscapes. Road ecology institutes have formed, reviews have been published (e.g. Formanand Alexander 1998, Trombulak and Frissell 2000), and practical guides have been written (Andrews et al. 2015, van der Ree et al. 2015). Yet amid this wealth of knowledge, one insight remains conspicuous by its absence; namely, evolutionary change caused by roads. Indeed, my experience at a recent ESA meeting left me feeling disappointed much like Forman; among the dozens of talks on road effects, only one mentioned evolution.
|The field on road ecology has grown steadily since its inception in the 1990s (inset). While many effects are well described, road ecologists have only scratched the surface of evolutionary consequences of roads.|
This absence of the evolutionary consequences of roads is surprising for a couple of reasons. First, other contexts of conservation have been hot on the trail of human-induced evolution for some time now. And second, some of our earliest knowledge of human induced evolution was actually discovered in roadside habitats. Studies by Briggs (1972), Wu and Antonovics (1976), Atkins et al. (1982), and Kiang (1982) all demonstrated rapid, adaptive evolutionary responses to road runoff contaminants (road salt and lead). Strikingly, all of these studies occurred back in the 1970s and early 1980s! But as far as we can tell, there was little evolutionary road activity after this early group of studies.
Until now. In 2013, Brown and Bomberger Brown reported that during a 30-year period, the number of road-kill cliff swallows decreased despite an increase in overall population size. At the same time, wing length of road-kill swallows increased while that of the overall population decreased. Together, these patterns suggest an evolved response to selection favoring increased maneuverability and vertical takeoff achieved by shorter wings.
At about the same time as the cliff swallow paper, I was finding evidence in my own work suggesting that roadside populations of amphibians were evolving in response to road-adjacency and road salt. In reciprocal transplant experiments, roadside spotted salamander populations showed a local adaptation pattern whereas roadside wood frog populations (in the same suite of ponds as the salamanders!) showed a maladaptation pattern (Brady 2013; 2016). From my perspective, evidence for maladaptation in roadside wood frogs was both confusing and surprising (detailed in another Eco-Evo Evo-Eco post), particularly since it occurred in counterpoint to the adaptive response of the co-inhabitant salamanders.
These outcomes of adaptation and maladaptation occurring side by side are part of our argument for the need to consider evolutionary change in the context of road ecology. In essence, the severity of the consequences of roadside dwelling appear to be decreasing for some species but increasing for others. These contrasting patterns occurring in response to the same form of habitat conversion (ie roads) highlight the complexity induced by road effects that are only unveiled through the lens of evolution.
Our goal in writing this paper is to advocate for a new way of thinking about road effects. It is our hope that questions aimed at understanding road effects will be guided by a recognition of the evolvable nature of populations, both in adaptive and maladaptive directions. Indeed, shortly after our paper was published, I was heartened by an influx of emails by researchers who were prompted to share their anecdotal evidence of evolutionary change in a variety of roaded contexts. If those emails any indication, we appear to be on the right track toward our stated goal of shifting gears in road ecology. Hopefully, road ecology will soon yield insights into adaptation and maladaptation across suites of species and contexts, the genetic variation underlying those responses, and the likelihood for adaptive versus maladaptive outcomes.
|The future of road (evolutionary) ecology is wide open.|
As I wrap up this blog entry, something else occurs to me. That is, this call to shift gears should go out not only to road ecologists, but also to evolutionary ecologists by and large. So here is my pitch to those researchers already steeped in evolutionary biology: come over to the roadside! Roads are natural (ish) laboratories for studying evolution. The suite of potential selective agents associated with roads is diverse and seemingly strong. And, selection appears to be 'hard' in many cases, where populations decline from pressures such as road kill. Further, as Mary Rogalski keenly points out in a recent Eco-Evo Evo-Eco post, selection from pollution may pose a unique set of challenges compared to more typical forms of selection (such as from predation and competition). Not to mention, roads acting as impediments to gene flow can hasten responses to selection.
All in all, roads are ripe for evolutionary study. Hopefully, a decade from now, we will have a handle not only on the giant's embrace, but also the way that life squeezes back.