Amphibian Movement Patterns and the Effects of Roads

Life History

Other amphibians, such as the eastern red-backed salamander (Plethodon cinereus), have entirely terrestrial life histories. These amphibians are characterized by direct development; eggs are deposited on land and hatch into miniature adults, or they give birth to live young (Dodd and Smith 2003). As is true for any amphibian species, they are prone to desiccation and therefore require a humid environment, such as is provided by the moist microclimates under deciduous forest leaf-litter and coarse woody debris (Pough et al. 1987; deMaynadier and Hunter 1998). They may also inhabit subterranean or arboreal habitats, and may move between subsurface, surface, and arboreal habitats depending on season, life stage, and/or reproductive needs (Dodd and Smith 2003). Although they may be driven to cross roads periodically, this is probably restricted mostly to juvenile dispersal or other density-dependent causes, as roads have been found to be the most important factor hindering amphibian movement. In fact, a study by Gibbs (1998), in which the red-backed salamander was one of the species of interest, demonstrated that forest-road edges are substantially less permeable than forest-open land edges and even forest-residential edges. This indicates that entirely terrestrial amphibians probably are not inclined to leave the forest to cross a road unless some underlying factor necessitates it for survival.

Groups that may be forced to cross roads more frequently therefore, are the amphibians exhibiting a biphasic life cycle, in which primarily terrestrial adults must move to a water body to breed and deposit eggs that eventually hatch into aquatic larvae (tadpoles). These species may be vulnerable to roadway mortality repeatedly throughout their lives if their habitat has been fragmented by a road network. For example, terrestrial adults may have to cross a road to access an aquatic breeding site, and then cross back again to return to terrestrial foraging habitat. Some species may need to cross again to find overwintering habitat (Hels and Buchwald 2001). Upon completion of metamorphosis, dispersing juveniles may be forced to mass migrate across roadways in pursuit of suitable habitat. The juveniles may disperse hundreds to thousands of meters from their natal pools, increasing the odds that they will encounter a road at some point (Dodd and Smith 2003). As this is the group of amphibians most at risk for roadway mortality, the remainder of this paper will focus primarily upon those with the biphasic lifestyle.

Aquatic Breeding Habitat

A discussion of vernal pools is necessary when considering where to place road crossings, for these pools should be prioritized in any road-crossing program. They may possess a more diverse array of amphibian species than larger water bodies; the species may also be of more threatened status (Dodd 1992 as cited in Semlitsch 2003). However, vernal pools are often overlooked, even in wetland protection regulations (Preisser et al. 2000). This is an unfortunate oversight, for considering the irreplaceable function that they provide as breeding habitat for so many amphibian species, they should be of primary concern. For example: 14 species of frogs were found breeding in temporary ponds within upland areas in north-central Florida (Moler and Franz as cited in Dodd and Cade 1998), 20 amphibian species were discovered in a temporary pond on the coastal plain of Georgia (Cash 1994 as cited in Dodd and Cade 1998), 15 amphibian species were noted in a temporary pond in north-central Florida (Dodd 1992 as cited in Dodd and Cade 1998), and 16 species of amphibians were documented in temporary ponds in northern Florida flatwoods (O’Neill 1995 as cited in Dodd and Cade 1998). In New England, vernal pool obligates include the wood frog (Rana sylvatica), eastern spadefoot toad, spotted salamander (Ambystoma maculatum), marbled salamander (Ambystoma opacum), and Jefferson salamander (Ambystoma jeffersonianum) (Preisser et al. 2000).

Metapopulation Dynamics

The wide spatial distribution of suitable terrestrial and aquatic habitats for biphasic amphibians, especially the vernal pool specialists, often leads to the development of metapopulation structure (Dodd and Smith 2003).

“A metapopulation is a set of local populations… connected by processes of migration, gene flow, extinction, and colonization. Two primary factors control amphibian metapopulation dynamics: 1) the number or density of individuals dispersing among ponds, and 2) the density and distribution of wetlands in the landscape that determine dispersal distances and the probability of successfully reaching ponds” (Semlitsch 2003).

Taking into account the factors noted by Semlitsch, loss of wetlands, and particularly vernal pools, increases the distances between ponds and leads to the isolation of populations. This is particularly true for amphibians, as they cannot migrate long distances due to the physiological limitations of desiccation. This can lead to loss of gene flow or extirpation of local populations if the remaining wetlands are too far apart to allow for recolonization (Semlitsch 2003).

General Effects of Roads on Amphibian Populations

One can extend this reasoning to include the effects of roadways on metapopulations. Roadways may fragment habitat, making it more difficult to migrate to breeding sites. Also, high mortality incurred upon roadway crossing could serve to decimate an already stressed population. Vos and Chardon (1998) discovered that road density has a negative effect on the occupation probability of a pond by moor frogs (Rana arvalis). In areas with high road densities, occupation probability was reduced to 55 %. In the part of the study adjacent to a highway, occupation probability was lowered to less than 30 %. They point out that although the size and availability of breeding habitat is generally thought to be the limiting factor determining population size in amphibians, there are now indications that the distribution of suitable terrestrial habitat is becoming more critical due to high mortality in the terrestrial phase.

This high mortality is being attributed more frequently to road kill as more roads are built and traffic densities continue to increase. Aquatic breeding amphibians are most vulnerable to roadway mortality due to their movement patterns, population structures, and preferred habitats (Hels and Buchwald 2001). Amphibians are small-bodied (which makes them less visible to drivers) (Semlitsch 2003), slow-moving, and non-cognizant of the dangers posed by vehicles (Ashley and Robinson 1996). Generally, two factors interact to determine where, when, and how severe roadway mortality of amphibians occurs: the movement patterns of the species and the intensity of the traffic.

Movement Patterns

Amphibians most often encounter roads during seasonal migrations between breeding sites, summer habitat, and hibernation sites, or during dispersal to new habitat patches (Ashley and Robinson 1996; Vos and Chardon 1998). Ashley and Robinson (1996), in a study conducted along a causeway on Lake Erie, observed seasonal highs in spring and autumn, consistent with life history patterns of reproduction and dispersal. Depending on the species, unimodal (leopard frog) and bimodal (bullfrog) patterns of road crossing were detected between April and October, again consistent with the life history pattern of the species. Amphibian mortality was significantly associated with the adjacent roadside habitat and vegetation communities preferred by various species. This indicates that some species were attempting to cross to desirable habitat on the other side of the causeway. The amphibians crossed mostly at night when traffic flow was light, which may have reduced the overall mortality (Ashley and Robinson 1996). Movement patterns may also be influenced by environmental factors, such as temperature and moisture (Sinsch 1988; Bayliss 1995; Bartlet 2000; as cited in Muths 2000).

Roadway Mortality

The Ashley and Robinson (1996) study along a Lake Erie causeway documented that short-lived species, which produce many young and are among the more common and abundant amphibian species, constituted the largest numbers of road kill: northern leopard frog (Rana pipiens), American bullfrog, green frog (Rana clamitans), and American toad (Bufo americanus americanus). It has been noted that a species often found killed along a road may simply reflect the presence of a large and thriving population (Huijser and Bergers 1997; Mallick et al. 1998; both as cited in Hels and Buchwald 2001).

Hels and Buchwald (2001) describe three factors they believe affect a species vulnerability to road mortality: 1) velocity of the species, 2) diurnal movement pattern of the species, and 3) diurnal movement pattern of vehicles. In their study, activity patterns of amphibians were concentrated at night, when traffic flows were lowest, yet they still report annual mortality of 25 % of the reproductively active adult population of spadefoot toad (Pelobates fuscus) and 21 % of the combined reproductively active populations of the common frog (Rana temporaria) and moor frog.

The probability of being killed increases with increasing traffic intensity (e.g., 34% - 61% probability of being killed crossing a road with 3,207 cars per day, increasing to 89% - 98% on a busy highway). Survival increases exponentially with velocity of the amphibian, and decreases exponentially with increasing traffic intensity. Up to a traffic intensity of 625 vehicles per hour (15,000 vehicles per day), the velocity of the amphibian has a large influence on its probability of being killed. Above this traffic intensity, the probability of being killed is very close to 100% for all species regardless of velocity (Hels and Buchwald 2001).

Hels and Buchwald (2001) additionally note several other interesting observations. For example, the angle of crossing also affects probability of being killed. Survival is greater with perpendicular crossing, with survival steadily decreasing as the crossing angle deviates from perpendicular. Unlike most animals, amphibians, due to their small body size, are killed mostly through direct hits by a vehicle’s wheel and may remain unharmed if they stay still under a passing vehicle, although some larger vehicles may kill through wind speed alone. Finally, they also note that estimates of amphibian road kill are often underestimated, as small body sizes may mean they go undetected, repeatedly being run over may obliterate the corpse, or they may be eaten by scavengers.

Roadway Mortality Impact

But how much of an impact does road mortality actually have on amphibian populations? Fahrig et al (1995) found that total number of dead and live frogs per kilometer (km) decreased with increasing traffic intensity. The proportion of dead frogs and toads increased with increasing traffic intensity, and the density of live frogs and toads decreased with increasing traffic intensity. Collectively, these results indicate that traffic mortality has a significant negative effect on local density of amphibians. At a rate of 24 – 40 vehicles per hour, Kuhn (1987, as cited in Clevenger et al. 2001) found that 50 % of a cohort of migrating common toads (Bufo bufo) was killed. A study by van Gelder (1973) on common toad breeding migration estimated that 60 cars per hour would kill 90 % of the adult toads. In yet another study, Heine (1987, as cited in Clevenger et al. 2001) predicted that at 26 vehicles per hour, the estimated survival rate of the common toad is zero. Ehmann and Cogger (1985, as cited in Fahrig et al. 1995) conservatively estimated that 5,480,000 reptiles and frogs are killed annually in Australia. Ashley and Robinson (1996) documented 27,846 road-killed young-of-the-year leopard frogs, while total amphibian road mortality averaged 11.65 amphibians per km per day between the months of April and October. Clearly, road mortality may have long-term impacts on amphibian populations (Semlitsch 2003).

Exactly how much of an impact road mortality will have depends on the species. Some populations may be depressed by road mortality while others may experience little impact (Ashley and Robinson 1996). It mostly depends on whether road mortality is additive or compensatory for a given population. In general, for adults, density-independent mortality factors are more important, while for larvae both density-dependent and density-independent factors matter (Duellman and Trueb 1994 and references therein, as cited in Hels and Buchwald 2001). If a population is mainly regulated by density-independent factors, then road mortality would be an additive effect, and the impact to the population would be greater. If a population is regulated more by density-dependent factors, then road mortality would be compensatory, and the overall impact on the population would be much smaller. However, traffic intensity may eventually increase to the point where it has an additive effect on an otherwise density-dependent population (Hels and Buchwald 2001). For threatened species, road mortality often does have an additive effect that is detrimental to populations and metapopulations (Semlitsch 2003).

Management Considerations: Predicting Crossing Locations

An important management consideration is how to best predict where amphibian roadway crossings are likely to occur, so that measures may be implemented to reduce roadway mortality, especially for species experiencing additive effects. As mentioned previously, those species most likely to be crossing a road are those with the biphasic lifestyle. Therefore, two habitat requirements must be within migratory proximity of a road: terrestrial (for most species, preferably deciduous forest) and aquatic. The remaining questions then concern the distance an amphibian will be likely to travel to access a given habitat type and any evident patterns to their movement directions and pathways.

The answers to these questions vary quite widely based on species, although a good rule of thumb would seem to be a movement distance estimate of several hundred meters. A review of the literature offered the following wide-ranging figures, a summary of which appears in Table One. Semlitsch (2003) states that amphibians usually live within a few hundred meters (m) of their natal aquatic habitat, although some do disperse to new sites. Vernal pool specialists often breed in a single pool throughout their reproductive lives and live in the surrounding uplands, dispersing an average of 125 m into the uplands after breeding (Semlitsch 1998, as cited in Preisser et al. 2000). Juvenile salamanders may disperse up to 670 m and juvenile frogs up to 1 km from their natal pool (J. Victoria, personal communication; Berven and Grudzien 1990, both as cited in Preisser et al. 2000). The mean distance traveled from breeding ponds for boreal toads (Bufo boreas) is 721.46 m (females) and 218.15 m (males); 92 % of all movements are within 700 m of the breeding site (Muths 2003). Spadefoot toads move a maximum of 1200 m between their hibernation sites and their breeding ponds (Nöllert 1990 in Hels and Buchwald 2001). Moor frogs migrate a maximum distance of 350 m and common frogs a maximum distance of 600 m (Haapanen 1970 in Hels and Buchwald 2001). Striped newts (Notophthalmus perstriatus) will travel up to 709 m, while eastern narrow-mouthed toads (Gastrophryne carolinensis) will travel up to 914 m (Dodd 1996, in Dodd and Cade 1998). The average of all the above figures is 628 m. This indicates that amphibians will travel considerable distances for their small sizes to reach suitable breeding and foraging habitats. These numbers also indicate that any road within approximately 100 m to 1,000 m of a wetland could potentially become the site of an amphibian roadway crossing.

Movement directions and patterns also vary considerably between species, making the identification of corridors difficult, if even possible. Some species are reported to move in specific directions when entering or leaving a breeding site, and it has been suggested that some species follow migratory corridors (Stenhouse 1985; Verrell 1987; both as cited in Dodd and Cade 1998). For example, the mole salamander (Ambystoma talpoideum) travels to and from breeding sites in a nonrandom manner (Semlitsch 1981, as cited in Muths 2000). Certain toads may also move in linear patterns away from breeding sites (Bartlet 2000, as cited in Muths 2000). Shoop (1968, as cited in Dodd and Cade 1998) detected a travel corridor approximately 30 m wide for the spotted salamander in Massachusetts. Other studies have found less specificity. Stenhouse (1985, as cited in Dodd and Cade 1998) noted that individual spotted salamanders tend to enter and exit ponds at the same location each year, although no specific pathway is followed among a group. Dodd and Cade (1998) found that striped newts and eastern narrow-mouthed toads favor certain directions of movement over others even though no specific corridor is followed, indicating that they probably move directly to the breeding pond from their terrestrial habitat. It does appear from some of the studies that amphibians may follow some kind of general pathway or direction during breeding migrations, making it likely that an under-road passageway in association with appropriate funneling techniques could prove successful in channeling the amphibians away from death on the roadway.
Table X. Summary of distances traveled by amphibians between habitat types.

Species	Distance Traveled Between Aquatic and Terrestrial Habitats (meters)
Amphibians, general	400
Vernal pool specialists	125
Juvenile salamanders	670
Juvenile frogs	1000
Boreal toad, female	721
Boreal toad, male	218
Spadefoot toad	1200
Moor frog	350
Common frog	600
Striped newt	709
Eastern narrow-mouthed toad	914
Average distance traveled, all species	628

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