When Barriers Maintain Ecological Integrity

Connectivity maintains ecological integrity in several ways (see Connectivity). In most situations, it is much better to maintain connectivity than to accept barriers, because small populations have a high risk of ‘winking out’ or becoming locally extinct. However, certain situations indicate the need for separation of biological elements to maintain ecological diversity or integrity.

Barriers May Prevent the Spread of Exotic Organisms

One clear case where connectivity is likely to be detrimental is where human activities increase connectivity where historically it has been absent. The history of invasion of exotic species is almost entirely due to human-caused increases in connectivity.

Two of the first questions to ask when planning to increase connectivity are: 1) was the connectivity present historically and 2) could exotic organisms cross the barrier.

Increased connectivity between Eurasia and the Americas has allowed hundreds of undesirable organisms -- everything from zebra mussels to Dutch elm disease -- to flourish, largely to the detriment of native populations.

This unwanted colonization can occur at smaller scales as well. For instance, in Norway, seabirds nesting on islands separated from the mainland historically were safe from most mammalian predators. Bridges built in the last several years have allowed small predators such as martens, badgers and foxes to access the seabird nesting areas. These predators can rapidly eat through a colony of the ground-nesting birds. Efforts to stop them from accessing the islands have been less than fully successful to date (Biuell 2001). The long-term effects of exotic introductions can be devastating, and are largely irreversible.

The presence of new exotic species, or of elevated levels of generalist predators (ravens, coyotes, etc.) may fundamentally change the way some species disperse across the landscape. This may result in impacts when historical connectivity is restored. One of the better examples of this in the western U.S. is associated with the introduction of exotic fish species. Invasive aquatic species that were not present in historic periods may now be blocked by culverts from spreading into all parts of a drainage. As an example, on the San Diego State University Santa Margarita Ecological Preserve, a previously intermittent stream that is important for breeding amphibians has been converted to a perennial stream by irrigation runoff, thereby allowing fish to enter the previously dry upper reaches of the stream. However, a culvert prevents fish from reaching the important amphibian breeding area and eating the young amphibians (Claudia Luke, pers. comm. 2001).

In Montana’s Bitterroot Valley, feral cats have been documented using culverts to cross highways (Foresman 2001). Because vehicle-caused mortality is a major population control for feral cats, this reduction in mortality has the potential to impact potential native prey species. In both of these cases, the barrier is maintaining the natural biodiversity because of the restriction of the exotic organism.

In such cases, the potential benefits associated with increased connectivity need to be weighed against the risks associated with the increasing influence of exotics.

It may be possible to ameliorate these negative consequences by considering connectivity holistically. For instance, noxious weeds are a serious national problem. Native plant and animal diversity suffers greatly when noxious species squeeze out native species by aggressively spreading. Noxious weeds have an easy spreading medium along highway rights-of-way. A wildlife overcrossing may be a conduit for noxious weeds if care is not taken to include native species in the design.

Brown-headed cowbirds, a brood parasite that lays its eggs in the nests of other species, may also disperse more easily in forested areas with cleared rights-of-way because this species forages in open areas and seeks hosts along forest edges.

Building connectivity where these species are dense may favor their spread. Control of an invasive species may be required to avoid causing unintentional spread. The location of a connecting structure and the adjacent land management practices are important considerations.

Barriers May Prevent Disease Spread

In all of the cases mentioned to this point, the solutions lie in careful planning, and modification of the connective structures such that they favor particular species while discouraging the spread of undesirable species.

In the case of disease transmission, solutions are more difficult. This is because the undesirable organism (the disease or parasite) cannot be separated from the host, so if an animal crosses a highway its disease goes with it. Situations where one population has a disease and others don’t can occur by chance when diseases are part of the natural systems, but are much more likely to occur when diseases are exotic, or have recently emerged as problems. Chronic wasting disease in deer and whirling disease in fishes are good examples.

When faced with the spread of lethal diseases, increasing connectivity must be considered with the utmost care, and should be closely coordinated with ongoing disease control efforts. However, it should be noted that Hess (1996) showed in a series of papers that while connectivity is often a pathway for disease, the benefits of such connectivity often outweigh the costs.

…Sandra Jacobson, August 8, 2002 with contributions by Kevin McKelvey and Michael Schwartz, USDA Forest Service Rocky Mountain Research Station.