It’s almost spring, and soon the woods will be full of the calls of frogs and toads of all shapes and sizes as they look for love. Salamanders and newts slither through the undergrowth with breeding on their minds. But for an amphibian to successfully find a mate, first they need to get to the breeding ground.
While many species of amphibians, such as bullfrogs, spend most of their lives in a pond, many others live primarily terrestrial lives in forests or other habitats. American toads and many salamanders spend their days in dank, dark areas, underneath leaf litter or beneath dead trees. Others, like the tiny Spring Peeper, spend most of their lives in trees. But all amphibians must breed in water.
Breeding is a matter of delicate timing. Many species breed in vernal pools, or temporary wetlands that form during early spring but dry up by summer. These temporary wet areas are free of fish and other threats to eggs and juveniles, but the ephemerality presents a tradeoff. Amphibians must arrive at the breeding pool as early as possible, usually as soon as snow and ice melt, to give eggs and juveniles as much time to develop as possible before the water dries up. Amphibians are prone to drying up, so they move on wet nights. The timing means that many of these small critters migrate at exactly the same time. On the right damp April night, hundreds of thousands may be on the move.
Unfortunately, migration usually means crossing roads, and each spring scores of amphibians are crushed by traffic as they laboriously make their way across. A smaller number are hit as the adults make their way back from the breeding pools, and when the young leave the pools on their way to adult habitats. In certain areas, road mortality can be incredibly high.
Preventing amphibian road deaths is difficult. There’s little support for widespread road closures, but a 2009 study aimed to predict those areas where risk is highest. Surprisingly, mortality was lower near major roads and in areas with a lot of roads; mortality was highest on more isolated roads. This may result from lower amphibian densities in these more developed areas. In rural areas, a wetland less than 100 meters from a road increases the risk for amphibians. Causeways, or areas where wetlands occur on both sides of a road, are the most deadly for reptiles and amphibians (but not necessarily vernal-pool breeders).
Knowing where the worst trouble spots are means that mitigation is possible. Culverts or other amphibian crossing infrastructure can be added. Many states have programs where volunteers can help steward migrating amphibians safely across the road on rainy nights. (Such as New York State’s volunteer program). It’s never wise to speed on a dark, wet road, but educational campaigns encourage drivers to slow down during migration season.