Terrapins and Tires
Every year in the early summer, thousands of diamondback terrapins crawl out of New Jerseys coastal salt marshes. Hundreds of these are killed on roads by speeding traffic. Why does this happen?
The carnage only occurs during the annual terrapin nesting season, a five-to-six-week period, which usually begins around the end of the first week in June and extends until the middle of July. All the terrapins that get squashed under the tires of vehicles are adult females that are looking for a place on the shoulder of roads above the high tide line to dig their nests and lay their eggs (typically 8 to 12 per nest).
Sadly, the embankments along the sides of the roads crossing and adjacent to salt marshes are the most accessible habitat left for these terrapins to nest. Nearly their entire original nesting habitat, sand dunes along the coast, has been destroyed by humans who have converted the barrier beach islands into summer resort communities. (Concerned people often suggest that terrapins would benefit from tunnels being dug under roads, allowing turtles to get from one side of the highway to the other without crossing the pavement. But terrapins are deliberately trying to find high ground to lay their eggs and are not simply attempting to get from one part of the marsh to another. There is no reason to conclude that a tunnel would be of any help to a nesting female.)
Even more sadly, adult females are by far the worst part of the terrapin population to selectively cull because terrapins have long life spans, probably exceeding 30 to 35 years. A female would ordinarily continue to reproduce for some two decades if allowed to live undisturbed. Female terrapins that are artificially removed from a population as the result of being run over take a long time to replace. The inevitable consequence is that the mounting numbers of adult female terrapins killed by vehicular traffic is contributing to a decline in the size of the overall terrapin population.
Just how serious is the problem?
Most drivers try very hard to avoid hitting terrapins that wander across coastal roads. In heavy, fast-moving traffic, even if the first driver has time to swerve and avoid hitting a hapless and confused terrapin, other drivers following closely behind often are not able to react to the small creature on the road, and another turtle will be inadvertently killed or maimed.
A further problem is caused by the fact that terrapin females nest around the clock. It is virtually impossible for a driver to see a terrapin on the road in the dark. Consequently, half of all the terrapins annually killed by traffic during the nesting season are run over at night. Moreover, some female terrapins (the actual percentage is unknown) double or triple clutch that is, they nest more than once in a single nesting season, exposing themselves to the dangers of road traffic more than once each year.
Researchers at the Wetlands Institute have discovered that it is sometimes possible to retrieve unbroken eggs from the carcasses of road-killed terrapins. These eggs are incubated and, after a period of two to three months, tiny hatchlings emerge from roughly one third of these orphan eggs.
A newborn hatchling has a soft shell the size of a quarter and is sometimes wryly referred to as a gull potato chip because birds happily vacuum up baby terrapins in one swallow! At the Wetlands Institute, hatchlings are kept in captivity for nearly a year, a process called head starting. During this time the terrapins grow enough (shell lengths typically reach 2 to 3 inches) so that when they are released, they are much more predator-proof than they would be if they were released as soon as they hatched.
Between 1989 and 2000, volunteers working at the Wetlands Institutes Diamondback Terrapin Conservation Project have salvaged more than 6,000 potentially viable eggs from road-killed terrapins. Over 3,500 of these have hatched. About 80 percent of these hatchlings survive the head-starting process and are released back into the salt marshes from which their mother emerged the previous summer.
With luck, a high percentage of head-started hatchlings will survive to adulthood. But not all of them will and, of those that do survive, not all will be females. Also, the number of head-started hatchlings released each year is far fewer than the number of adult females annually killed on roads during the nesting season. For example, in 2001, 215 head-started hatchlings were released while 513 adult females were traffic fatalities. So, even if all the head-started hatchlings survive (an unrealistic assumption), the terrapin populations along New Jerseys coast are still delining inexorably. Under current circumstances, therefore, human efforts can neither reverse nor even stem the terrapin population decline that is now taking place. At best, we are merely slowing the rate at which the population diminishes.