Conservation Biology: An Unflinching Vision
Road Building Leads to Extinction

by Michael Soulé

We live in two worlds, and both are under siege. One, the oldest, is the biological world; its remaining wild places are rapidly being fragmented, invaded by alien species, then destroyed.

As the human volume is turned up, wildlands fall silent, and the vital links that connect them—wildlife corridors—are being nibbled to death like babies attacked by rats.

The other world, the social world, is also under siege. Intimacy and community are being replaced by electronic surrogates. As the volume of digital and visual devices grow louder, people fall silent—their voices and literature replaced by a cacophony of beeps and the hypnotic sounds of commercialized violence. As nature flies apart, so does society.

Perhaps it is no coincidence that technology is undermining both the integrity of life in the biosphere and the dignity of human life in the social sphere, and that the loss of connectivity is not only an object of concern among conservation biologists, but is also eliciting alarm among social critics. My intention is not so much analytical as it is prescriptive: stopping and reversing these dissipative environmental and social trends.

Habitat Fragmentation and Networks of Reserves
The consequences of habitat fragmentation have been studied across the world, and the results of fragmentation are predictable, regardless of hemisphere, latitude or clime. Metaphorically, if untrammeled nature was one global container of species, then isolated remnants of wildlands are myriad leaky sieves. Species disappear from remnant patches of habitat, and the smaller the remnant, the leakier it is.

The disappearance of species from remnant habitat obeys certain rules. Most ecologists and biogeographers agree on the following important, broad principles.

The Area Effect
One of the principles of modern ecology is that the number of species an area can support is directly proportional to its size. A corollary is that if area is reduced, the number of species shrinks. Moreover, the rate of extinction in an isolated habitat remnant is inversely proportional to its area.

Even quite large habitat islands have observable rates of extinction. For instance, it is now recognized that most national parks in the western U.S. are too small to prevent the extinction of many medium-sized and large mammals on a time scale of a century or less. On a local scale, isolated patches of vulnerable habitat less than 100 hectares or so are too small to prevent catastrophic rates of habitat disturbance and the loss of many, eventually most, species of vertebrate animals and plants on a time scale of decades. Similar kinds of observations led to one of the first guiding principles of conservation biology: "bigger is better."

Edge Effects
Because the ratio of edge or circumference to habitat area increases geometrically as fragment size decreases, it is important to understand how boundaries affect wildlife. Edges occur where a habitat such as a forest meets a road, a clearcut, or some other habitat. Artificial edges, particularly recently created ones, benefit certain species such as deer.

But edges in general are harmful to the maintenance of native species diversity. Among some of the major categories of deleterious edge effects are: (1) higher rates of habitat desiccation and tree death, (2) higher frequency and increased severity of fire, (3) higher rates of predation by native and exotic predators (e.g., foxes, cats, crows and their relatives, and by human hunters), (4) higher probability of nest parasitism, (5) greater windfall damage, and (6) higher intensities of browsing, grazing and other forms of disturbance which favor the growth and spread of weedy and exotic species.

Roads, the most frequent source of new edges, also facilitate the movement of weeds and pests associated with disturbance or that spread along rights of way; roads also cause erosion, stream sedimentation, pollution, and cause increases in mortality rates of wildlife from collisions and hunting. Thus, a second guiding principle is to minimize roads and seek road closures including "ripping" up the road bed and restoring native vegetation.

The Distance Effect
The inverse relation between isolation and immigration is known as the distance effect. As habitat destruction spreads and the distance between remnant patches increases, plants and animals are less likely to disperse or migrate between remnants.

Dispersal of individuals helps protect against demographic "accidents," such as when an episode of unusually high mortality perturbs age structure and sex ratio. Immigrants can also "rescue" a population that is in genetic jeopardy because of inbreeding. Therefore, lower dispersal or migration rates increase the frequency and duration of local extinction; hence, distance works against persistence of species.

Thus, a third guiding principle is to minimize the distance between habitat islands. One caveat, though, is that some species (including many in tropical forests and Mediterranean scrub ecosystems) will not cross any barrier or gap between habitat islands, regardless of how trivial it may appear to humans.

Michael Soulé is the chair of the environmental studies department at University of California, Santa Cruz, and was the founding president of the Society for Conservation Biology.

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