Amphibians are integral components of many ecosystems, in some constituting the highest fraction of vertebrate biomass (Burton and Likens 1975 as cited in Blaustein et al. 1994). Their populations play an important role in structuring communities of forest floor decomposers and affecting nutrient cycling rates and energy flow. Amphibians also are important food sources for avian and mammalian predators; because of their small body size and physiological characteristics they are able to exploit prey that larger animals cannot (Pough et al 1987), thus serving as an important link in the food chain. Amphibian populations are, however, declining, some to the point of extinction, and adversely impacting other organisms and forest ecosystems.

Habitat destruction and fragmentation are major causes of amphibian decline, but others include chemical pollution, acid precipitation, increased ultraviolet radiation, exotic species, pathogens, harvesting by humans, and natural population fluctuations (Blaustein and Wake 1990; Phillips 1990; Wyman 1990; Pechmann et al. 1991 as cited in Blaustein et al. 1994). Amphibian mortality also is attributed to habitat fragmentation by roads and highways.

Barriers to Migration and Dispersal

Many amphibians have annual life cycles requiring migration between habitats with different ecological properties, making the effects of barriers (like roads) profound (Reh 1990). Whether they have explosive breeding migrations or less conspicuous movements, populations depend on dispersal connections and "landscape linkages," often in a human-altered landscape (Gibbs 1998). Simple linear landscape structures including roads, levees, and ditches may act as physical and psychological barriers for amphibian movement (Mader 1984 as cited in Gibbs 1998) and substantial mortality agents (van Gelder 1973, Cooke 1988 as cited in Gibbs 1998). Other landscape features like streambeds may act as conduits for amphibian movement. Gibbs' study (1998) found that amphibians will attempt to traverse open land lacking roads, particularly where streambeds occur. Accommodating amphibians in human-dominated environments may require the identification of key landscape components acting as filters and conduits for dispersal (Harris and Scheck 1991 as cited in Gibbs 1998).

Amphibian populations unable to disperse because of barriers may experience genetic isolation resulting in reduced heterozygosity. In one study, populations isolated by highways had only 0-4% variation in gene loci, suggesting highly inbred populations (Reh 1990). In addition, natural population fluctuations influenced by drought, rainfall, predation, and breeding impacts combined with anthropogenic effects could result in local extinction more easily than either factor alone. Fragmenting habitat through human intrusions like roads makes populations less resilient to natural population declines (Pechman et al. 1991).

Alteration of Microhabitats and Microclimates

Changes in forest structure and vegetation (including logging and road-building) alter microhabitats and microclimates. Amphibian populations correlate with the following microhabitat characteristics: quantity and quality of coarse woody debris, litter depth and moisture, understory vegetation density, and overstory canopy closure (Demaynadier and Hunter 1998). These structural and vegetative characteristics influence temperature and moisture regimes on the forest floor (Pough 1987). Roads alter these characteristics and result in edge effects likely to impact local amphibian populations. There is documented behavioral avoidance of road edges due to elevated mortality associated with road crossings (van Gelder 1973, Cooke 1988 as cited in Gibbs 1998). Forest edges associated with open land are more permeable to amphibian movement than road edges (Gibbs 1998). Maintaining some portion of the structural and vegetative microhabitat and microclimate elements will help minimize edge effects and hasten the rate at which logged stands are colonized by forest interior species (Demaynadier and Hunter 1998). Road construction in a forest environment, however, permanently alters these components.

Physiological Constraints

Amphibians are less adaptable to environmental changes because of physiological constraints. While human alteration creates landtypes that are dry and open, many amphibian species depend on moist refugia to maintain close contact with forest floor substrates. The maintenance of highly permeable skin that is cool and moist allows for efficient respiration. In addition, small size and slow movement result in relatively poor dispersal capabilities and small home ranges. These physiological factors cause amphibians to be especially sensitive to the abrupt transitions created by roads impacting their microclimate and microhabitat (Demaynadier and Hunter 1998).

Documenting Road Impacts

Several studies examine the impacts of road induced mortality on amphibians. A study in 1987 by Kuhn documented that 20-40 cars on a road per hour killed 50% of migrating common toad (Bufo bufo) individuals (Reh 1990). Another study on Bufo bufo conducted in Britain discussed the impacts of traffic on the toads' breeding migration of up to 1.5 kilometers. This study attributed only 4% of mortality to roads. Although road casualties were not thought to affect significantly overall breeding potential, measures were taken to protect the toads from road mortality. Warning signs were posted encouraging motorists to slow down, and toads were collected in containers and released during times of heavy traffic (Gittins 1983). A study conducted on highway related toad mortality in a state nature reserve in the Netherlands reported 29% mortality for females crossing an asphalt road during breeding migration (van Gelder 1973).

A long-term study of the impacts of highway mortality on the largest known breeding migration of the flatwoods salamander (Ambystoma cingulatum) took place over a 22-year period. The study documented nightly migrations of 200-300 adults crossing a 4.3 km stretch of paved highway in 1970-1972, but these numbers dwindled to less than one individual per night in 1990-1992. Although this decline could be caused by both anthropogenic and natural factors, researchers were skeptical that highway mortality played a significant role. They based this assumption on documentation of few road kills and calculation of fewer than 5 cars/hour (Means 1996).

The continued survival of the larch mountain salamander (Plethodon larselli), a species endemic to the Columbia River gorge in Washington and Oregon, is dependent on unaltered talus slopes. Their populations are impacted by removing gravel from slopes for road building and maintenance, as well as clearcutting. Primary and logging roads follow the natural contour of the land, frequently bisecting major talus fields. Rocks from these fields are crushed to a size suitable for road surfaces, causing severe erosion and drastic changes in the soil properties of the slope. The resulting altered slopes are inhospitable to P. larselli. Examples of slope modifications by road-building are visible from major highways on both sides of the Columbia River (Herrington 1985).

In a study in Southern Appalachia where salamanders in the family Plethodontidae make up a large proportion of forest amphibian fauna (Demaynadier and Hunter 1998), the Red-backed Salamander (Plethodon cinerus) appeared more sensitive to clearcutting and forest edge effects than most anuran (frog or toad) species. This family of salamanders is affected by forest canopy loss due to its almost complete reliance on cutaneous respiration, a form of respiration limited by diffusion and most efficient when skin is moist and ambient temperatures are cool (Feder 1983 as cited in Demaynadier and Hunter 1998). This makes them particularly vulnerable to population declines due to clearcutting and canopy removal (Demaynadier and Hunter 1998, Pough et al. 1987). The biology of this species and its sensitivity to dry conditions makes the red backed salamander susceptible to population fluctuations (Pough et al 1987).

Protection from Road Impacts

These studies just begin to examine traffic and road induced mortality on amphibians. Ideas for protecting amphibians against these road threats on a landscape level emphasize habitat connectivity. Researchers suggest the conservation of meadow land as a conduit for genetic transfer and for its favorable microclimate; linking nature reserves with moist, water-filled corridors such as ditches; preventing new barriers such as roads, highways, and railways; and providing spawning sites with summer, and winter habitat in close proximity (Reh 1990).

Some managers have implemented mitigation measures protecting amphibians from existing roads and barriers. In Switzerland, road blocks were erected to stymie motorists during toad migration. In other locales, traffic signs warning of crossing amphibians alert drivers. Some amphibians were helped across roads as they encountered barriers, and diverted into a tunnel or a pitfall in which toads were collected and transported to the other side of the road (Van Gelder 1973).

Conclusion

While it is apparent that amphibian population decline is attributable in part to anthropogenic causes, further research on the impacts of roads, and ways to mitigate the damages is needed. The preceding protective measures only begin to combat road-induced amphibian mortality. Preventing additional habitat degradation and restoring road-fragmented habitat can help amphibian populations stabilize and recover.

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