Wildlife Crossing - Roads and Habitat Fragmentation

Roads and Habitat Fragmentation

Habitat fragmentation occurs when human-made barriers such as roads, railroads, canals, electric power lines, and pipelines penetrate and divide wildlife habitat (Primack 2006). Of these, roads have the most widespread and detrimental impacts (Spellerberg 1998). Scientists estimate that the system of roads in the United States impacts the ecology of at least one-fifth of the land area of the country (Forman 2000). For many years ecologists and conservationists have documented the adverse relationship between roads and wildlife. Jaeger et al. (2005) identify four ways that roads and traffic detrimentally impact wildlife populations: (1) they decrease habitat amount and quality, (2) they increase mortality due to wildlife-vehicle collisions (road kill), (3) they prevent access to resources on the other side of the road, and (4) they subdivide wildlife populations into smaller and more vulnerable sub-populations (fragmentation). Habitat fragmentation can lead to extinction or exterpation if a population's gene pool is restricted enough.

The first three impacts (loss of habitat, road kill, and isolation from resources) exert pressure on various animal populations by reducing available resources and directly killing individuals in a population. For instance, Bennett (1991) found that road kills do not pose a significant threat to healthy populations but can be devastating to small, shrinking, or threatened populations. Road mortality has significantly impacted a number of prominent species in the United States, including white-tailed deer (Odocoileus virginianus), Florida panthers (Puma concolor coryi), and black bears (Ursus americanus) (Clevenger et al. 2001). In addition, habitat loss can be direct, if habitat is destroyed to make room for a road, or indirect, if habitat quality close to roads is compromised due to emissions from the roads (e.g. noise, light, runoff, pollution, etc.) (Jaeger et al. 2005). Finally, species that are unable to migrate across roads to reach resources such as food, shelter and mates will experience reduced reproductive and survival rates, which can compromise population viability (Noss et al., 1996).

In addition to the first three factors, numerous studies have shown that the construction and use of roads is a direct source of habitat fragmentation (Spellerberg 1998). As mentioned above, populations surrounded by roads are less likely to receive immigrants from other habitats and as a result, they suffer from a lack of genetic diversity. These small populations are particularly vulnerable to extinction due to demographic, genetic, and environmental stochasticity because they do not contain enough alleles to adapt to new selective pressures such as changes in temperature, habitat, and food availability (Primack 2006).

The relationship between roads and habitat fragmentation is well documented. One study found that roads contribute more to fragmentation in forest habitats than clear cuts (Reed et al. 1996). Another study concluded that road fragmentation of formerly contiguous forest in eastern North America is the primary cause for the decline of forest bird species and has also significantly harmed small mammals, insects, and reptiles in the United States (Spellerberg 1998). After years of research, biologists agree that roads and traffic lead to habitat fragmentation, isolation and road kill, all of which combine to significantly compromise the viability of wildlife populations throughout the world.