Nineteen Years of Amphibian Counts in National Capital Region Parks

Frogs, toads and salamanders in the National Park Service's National Capital Region parks are holding steady. Monitoring by the region's Inventory and Monitoring Network (NCRN) found amphibian populations stable but dynamic in 10 parks, though urbanization and a changing climate put them at risk. The article was written by an NCRN science communication intern.

Wood frog (Rana sylvatica) (NPS).

Why watch amphibians

Amphibians start life in the water and move onto land as adults. They are sensitive to small changes in both wet and dry habitats, so they are called indicator species: their numbers say something about the health of the ecosystem. Tracking them also tells managers how the resources they depend on are doing.

The monitoring asks which species are present in each park and how their populations change over time. Researchers flip over mossy stones, look under leaves and use dipnets in streams and wetlands, then analyze the data for occupancy trends. Occupancy is the proportion of pools in a park where a species lives.

What the counts show

In 2023 the team surveyed 215 wetland pools and 26 stream sites. Since 2005 they have detected 15 frog, toad and salamander species in wetlands and 5 salamander species in streams, 20 species in all, across the 10 parks. Annual swings in occupancy are natural and expected. Over the past 19 years few species had severe declines or unexpected growth. The data come from a synthesis of 15 years of monitoring, plus preliminary data through 2023.

Threats

NCR parks are more forested and have more breeding habitat than their surroundings, so they may serve as refuges. Even so, several pressures bear on them.

Marbled salamander (Ambystoma opacum) (NPS).

• Water quality. Urbanization and agriculture raise dissolved solids and pollutants, which raises salinity and with it conductivity, the water's capacity to carry electricity. Amphibian skin is highly permeable to water and electrolytes, so small changes in quality matter to them.
• Roads. Roads can split wetlands apart. This fragmentation makes it harder for amphibians to find food and mates.
• Longer, hotter summers. More evaporation, drought and altered rainfall can shorten the time a wetland holds water.
• Warmer water. Storm water that rushes over paved and hardened surfaces heats up and carries pollutants into park waterways.

What parks can do

Wetlands that are larger, closer together and lower in conductivity are better for amphibians. They also do well where a wetland stays wet through the breeding season, from March to July, because a long hydroperiod lets tadpoles finish metamorphosis. The article's management suggestions:

• Prevent roads, trails or other interruptions that break up wetlands.
• Limit roads near wetlands to reduce contaminated run-off.
• Expand and deepen wetlands so they stay wet longer.

American toad (Bufo americanus) (NPS).

Extending a wetland's hydroperiod in dry years has been widely discussed. Researchers at Patuxent Research Refuge have installed pond liners. Supplying water from developed sources is another option. Sandbags around a pool are less intrusive, though the bottom changes to fine silt over thick mud, which can slow the breakdown of debris. In Oregon, people are turning to supporting beaver populations and to beaver dam analogs. The advice is to weigh the environmental and structural limits of each site before choosing.

Sandbags placed along a raised barrier to extend a wetland's hydroperiod, as at this wetland in Rock Creek (NPS).

With almost 20 years of data, the team says it has documented and measured trends that many had only noticed or assumed, and it expects continued monitoring to support recommendations for protecting amphibians.

Words used

• Hydroperiod: how long a wetland stays wet.
• Vernal pool: a seasonal wetland covered by shallow water for variable periods from winter to spring, possibly dry most of summer and fall.
• Water conductivity: how well water conducts electricity, a measure of dissolved salts and other materials, usually pollutants.
• Wetland connectivity: the ability of amphibians to move across the landscape to suitable habitat.

Sources

• An NCRN Inventory and Monitoring science communication intern, National Park Service, Amphibian Monitoring Update 2023; rewritten in hubnx's own words. Two of the article's photographs are left out.