The Ecological Effects of Roads

Busy highways like these are formidable barriers to wildlife and reduce landscape connectivity, isolating habitat patches. morgueFile

Ecologists studying plants and wildlife habitat have had concerns about roads for quite a while now. A number of them study the effects of roads on the environment, as a discipline sometimes called road ecology. In the context of land use, roads are often the first step in transforming a natural landscape. Once an expanse of habitat is dissected by a road, access is facilitated for activities like grazing, mining, or logging.

Depending on the geographic location, suburban or rural housing developments often follow, or agriculture. The initial landscape then becomes fragmented into smaller, disconnected pieces. There is a multitude of other problems that come along with roads:

  • In northern climes, road salt is often used as a deicer to make roads safer. This leads to the increased salinity of soils and waterways in proximity to roads, with negative consequences to terrestrial plants and aquatic ecosystems. Similarly, fuels, oils, and heavy metals from vehicles leach into the surrounding area.
  • An obvious negative consequence of roads is road kill. When it occurs at high enough frequency, road kill can seriously reduce an animal population. Many female turtles walk upland quite a bit in search of nesting grounds every year, making them more vulnerable to roads. This has for consequence a population’s sex ratio that becomes skewed towards males. And of course, road kill is a public safety issue with hundreds of millions of dollars lost annually due to collisions with vehicles, and numerous human fatalities.
  • Species who rely on sound to communicate, like many bird and frog species, can be affected by the noise associated with busy roads. Some studies have shown that these animals change the volume and pitch of their songs and calls in order to be heard above the traffic noise. In other studies, the diversity and density of birds and frogs was simply impoverished near roads.
  • For many animals, roads are barriers. Smaller snakes, frogs, salamanders, insects, and small mammals often cannot cross roadways because they could dry up in the sun, or would be exposed to predators out in the open. Some bird species which certainly have the physical capacity to cross a road right-of-way have been shown to be reluctant to fly across the gap. Similarly, badly designed culverts are well known for blocking migrating fish like salmon from their spawning grounds. Blocking access to parts of an animal’s home range can prevent it from completing their annual cycle. Further, these barriers prevent smaller habitat patches from being recolonized, and reduce the amount of genetic variation within a population.
  • While roads often act as barriers, sometimes they are effectively corridors for invasive species. The disturbed soil right on the road verges, and the drainage ditches dug alongside, are the perfect network for invasive plants to spread through. In arid regions, the paved roadway channels rainwater off onto the roadsides, creating zones that are better watered than the rest of the environment. Invasive plants are very effective at taking advantage of this resource. Similarly, the infamous cane toads currently invading Australia have been shown to disperse along the greener, wetter road corridors.

    Road ecologists, then, identify the populations, species, or ecosystems that are vulnerable to roads. Ecologists and managers then identify ways to minimize the risks through a variety of management tools like better transportation planning, the installation of wildlife passages under or over roads, or perhaps the use of appropriately positioned fences.