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Effectiveness of Amphibian Mitigation Measures Along a New Highway


In 2004-2005, a new highway bypass was constructed through an area of predominantly upland forest with many vernal pools in southern New Hampshire. The highway is complete but is not yet open to traffic. Potential im¬pacts to vernal pool amphibians (spotted salamanders (Ambystoma maculatum) and wood frogs (Rana sylvatica)) and their habitat include habitat loss, barriers to animal movements, potential mortality on roads, and changes in water quantity and quality in breeding pools. Measures to maintain viable vernal pool-breeding amphibian populations along the bypass were implemented and monitored. Effectiveness as used in this paper refers to the ability of the various mitigation measures to contribute to the overall goal of maintaining viable populations, as well as the ability of each measure to provide its specific functions. The mitigation measures and results of their effectiveness to date include:

• Bridges: Two bridges were constructed for general wildlife habitat connectivity.

• Wildlife crossing structure and diversion walls: A 1.2 m by 1.2 m (4’ by 4’), 17-m (55’) long concrete box culvert and diversion walls were installed. After three years of monitoring spring amphibian migrations, it appears the diversion wall is successfully diverting the few vernal pool-breeding amphibians that encounter it, but there is no evidence the crossing structure has been used.

• Seasonal pool construction: Two new pools were constructed in an effort to maintain viable amphibian habitat and populations on both sides of the new road. Post-construction monitoring shows the new pools are used by a relatively diverse community of amphibians (including spotted salamanders in one pool) and macroinvertebrates, although the pools’ long-term value to vernal pool amphibians is not yet certain.

• Drainage: Natural hillside drainage was maintained across the new roadway to maintain existing vernal pool hydrology to the extent feasible. Where possible, roadway drainage was routed to swales and detention basins that discharged outside of vernal pool watersheds. Based on two years of observations, vernal pools immediately adjacent to the roadway have been hydrologically altered, but other pools do not appear to have been affected by the changes.

• Habitat preservation: The land around the greatest concentration of existing vernal pools, all on one side of the new highway, was purchased to preserve habitat integrity. Six years of pre-construction and two years of post-construction monitoring show that spotted salamander breeding (as measured by egg mass counts) has not changed substantially compared to pre-construction levels. However, there is a great deal of variation in breeding activity from year to year and pool to pool, and longer-term monitoring may reveal different trends. Opening the highway to traffic may also affect populations.

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