The Effect of Road Mortality on Aquatic Turtle Populations

By: David Steen

Background:
Roads and roadsides cover approximately 1% of the land in the United States, an area equal in size to South Carolina (Forman 2000). These roads are estimated to have an ecological effect on 20% of the entire land area of the country (Forman 2000). Roads can generate a suite of effects onto the surrounding landscape, including altered streams and wetland drainage, road salt reaching surface-water bodies, habitat invasion by exotic species, microclimate changes and vehicular induced wildlife mortality, among others (Forman & Deblinger 2000).
The extent that roads may affect populations of vertebrate species has not been extensively studied. Studies on a 3.6 km road running through a wetland in Ontario identified 32,000 road-killed animals covering one hundred species in a total of four years (Ashley & Robinson, 1996). Road mortality of snake populations in Everglades National Park is estimated to have reached unsustainable levels (Bernardino and Darymple 1992)
Turtles are a useful study focus because their life history traits make them potentially sensitive to road associated mortality. Turtles are known for their longevity; the characteristics that often accompany this trait include delayed sexual maturity and high neonatal mortality. Together, these factors limit the ability of turtle populations to rebound from an increase in mortality above natural levels. (Congdon et al 1993).
Once a female reaches sexual maturity, her annual nesting migrations may require her to cross many roads adjacent to the wetland of origin in order to reach an area suitable for laying eggs. The subsequent road mortality of breeding females may be more than the population can sustain while maintaining viable levels. It is suspected that highways constructed along bodies of water can extirpate turtle populations in only a scant years (Ernst 1994). In addition, a 275-meter buffer zone around wetlands where no development occurs is likely necessary to ensure the integrity of turtle communities (Burke & Gibbons 1995).

In order to identify combinations of road density and traffic volume that would result in 10% annual road mortality for turtles in New York State, simulations of nesting turtle migrations were incorporated with maps of road networks and traffic volume data. A 10% increase in total mortality likely greatly exceeds what can be endured by turtle populations while maintaining positive growth rates (Doroff & Keith 1990, Brooks et al. 1991, Congdon et al 1993, 1994).

Method:
(Derived from James P. Gibbs and W. Gregory Shriver, in preparation)

The annual road mortality rate of aquatic turtles was estimated as:
droad = 1 - (1 - pkilled)ncrossings

where pkilled = the probability of a turtle being struck by a vehicle on any given road crossing and ncrossings = the number of road crossings an individual turtle undertakes annually. This equation was derived from binomial probabilities (Zar 1984:370-375) and estimates the likelihood that a turtle is struck (and presumably killed) at least once each year during the road crossings undertaken in their annual nesting migration.
To parameterize the model, the relationship between ncrossings and road density was determined based on simulations of nesting migrations of individual turtles inlandscapes along a regional urban-rural gradient of road density. The gradient extended across nine counties in New York State from the New York City metropolitan area north to the Adirondack Mountain region. Althoughthe distance between these two areas is only 300 km, itencompasses the extremes of road density and configuration found in theUnited States.
Simulated nesting migrations were basedon known nesting migrations of small aquatic turtles, such as painted turtles, and large bodied aquatic turtles, such as snapping turtles. The expected nesting migration of the smaller turtles was determined to be 100 m (Burke & Gibbons 1995) and the expected nesting migration of the larger bodied turtles was determined to be up to 5 km (Obbard & Brooks 1980). These simulated nesting migrations (one each of 100 m and 5 km) were initiated at 25 random bodies of water within each of the nine counties sampled.

These nesting migrations were overlain on US Geological Survey 1998 TIGER/Line Files of roads within ArcView G.I.S. 3.2 to determine the number of intersections between roads and each turtle's nesting migration. The relationship between road density and ncrossings was then estimated via least squares linear regression analysis (Zar 1984).
We estimated pkilled as the probability that a vehicle would pass while a turtle attempted to cross a road based on an equation derived by Hels and Buchwald (2001)

P = e-Na/v

where N = vehicles/min, a = kill width (m), and v = velocity (m/min) with twice the tire width plus twice the turtle length.
The crossing interval was considered to last approximately five seconds per lane based on the voluntary locomotion speeds of .109 m/sec of painted turtles (Zani & Claussen 1994). Roads were conservatively estimated to consist of only two lanes. Traffic volumes were derived from the number of motor vehicles registered in New York State (FHA 1999) multiplied by the annual miles traveled by an average vehicle in the United States in 1998 (FHA 1999) divided by the lane "mileage" in the state (FHA 1999).

Results:
Estimated annual road crossing frequency in the nine counties sampled ranged from 0-2 for 100 m nesting migrations and from 0-55 for 5 km nesting migration. (linear relationships from spreadsheets).
Our data show that 15 of the counties in New York State have a combination of road density and traffic volume that would result in annual road mortality in excess of an estimated 10% additive threshold for large bodied pond turtles. In contrast, we estimated that there are no counties in New York State that have traffic volumes and road densities that together produce road mortality of this extent for smaller bodied turtles.

Discussion:
The data analyses demonstrate that many counties in New York may not be capable of maintaining healthy populations of large bodied turtles, such as snapping turtles. In contrast, the relatively short nesting migrations of painted turtles do not expose them to a sufficient amount of road crossings to significantly affect their populations.
It is important to note that the mortality of all species included in the study may be underestimated. This model assumes that all highways consist of only two lanes, while this is assuredly not always the case. A turtle on a 5 km nesting migration is likely to cross at least several multilane highways. Also, many turtles nest during the hours surrounding dawn and dusk, which corresponds with "rush hours" (Ernst 1976, Legler 1954, Festin 1996).
To effectively conserve healthy complements of turtle species within wetlands, it is clear that the effects of roads on the native wildlife must be taken into account when considering development. Even without expanding the road base, the current road density and traffic volume is predicted to be high enough to lead to a population decline in several turtle species in many areas across the state and may be effective at limiting population growth in rare species, such as the spotted and blanding's turtles. We are currently undertaking a field study to directly test the predictions of these models in the central New York area.

About The Author: David Steen
B.S. in Zoology, University of New Hampshire; currently M.S. student concentrating in Ecology at SUNY-ESF.
My primary research interests concern the ecology and the conservation biology of terrestrial vertebrates. Past projects have endeavored to show the effect of habitat fragmentation on the abundance of northern water snakes and the effect of habitat disturbance on the species composition and richness of the herpetofauna in Ometepe, Nicaragua. This spring I will be initiating a field study to determine the effect of road density on turtle population structure. This will entail trapping turtles in areas of varying road density and revealing any correlations in population parameters between the different sites. After obtaining a Ph.D. in a related field, I would like to pursue a career in international conservation research in an attempt to determine how to effectively preserve intact ecosystems in a human dominated landscape.

Contact Information:
David A. Steen
350 Illick Hall
SUNY College of Environmental Science and Forestry
1 Forestry Drive, Syracuse NY, 13210
SwampWalker@hotmail.com

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