What follows is a guest essay from Stacy McNulty Associate Director and Research Associate at SUNY ESF’s Adirondack Ecological Center in Newcomb. Following last week’s story on the destruction of wetlands by ATVs at the 2011 SNIRT rally, the Almanack asked Stacy to provide some background on vernal pools, small intermittent wetlands that are important sources of Adirondack biodiversity.
On a proverbially dark and stormy night in mid-April I climb the hill, flashlight sweeping the ground for obstacles. The first warm, spring rain has been falling and snow piles lie here and there. Faintly I hear a quacking sound up ahead, signaling my target – but what I seek is not a duck, but a frog. Scores of wood frogs swim and call from the pool, their eyes shining in the beam of my light.
If not for the hoarse symphony emanating from the pools in which the frogs breed, one might not even notice these small forested wetlands. Vernal pools hold water from a few weeks to months and dry out at some point during the year. They fill with rain and snowmelt in spring (thus the name); some are fed by seeps or underground water sources as well. Though usually less than an acre in size, vernal pools harbor tremendous diversity, enriching the upland forest in which they are embedded.
Vernal pools are nurseries for a host of species. Big black and yellow spotted salamanders crawl silently along the pool floor to find a suitable place for egg-laying. Caddisflies, dragonflies, mosquitoes and other invertebrates abound in these small, still waters. All of these species evolved larvae that develop relatively quickly before the pool dries out.
Wood frogs and spotted salamanders are considered “obligate” vernal pool breeders, but research at SUNY ESF’s Huntington Wildlife Forest in Newcomb indicates beaver ponds are frequently used. In an average year, many shallow pools dry out too fast for larvae to complete development, whereas beaver ponds produce more (and larger) young. Vernal pools stand out in wet years, when thousands of metamorphs enter the forest to return as adults next year. If the cool, wet Adirondack climate changes, pool life may change as well.
Living in fishless waters, vernal pool dwellers may experience less predation than larval bullfrogs, newts and other inhabitants of more permanent waters. Still, mink, raccoons, green frogs and wading birds will visit vernal pools for a meal, and diving beetle larvae and other insects in the pools eat tadpoles.
There are other problems pool residents must overcome besides drying and predation. As with fish, when water pH drops below 4, amphibian eggs fail to hatch, larvae grow slowly, and adults are stressed. Presumably, vernal pools are recovering from acidification like their lake counterparts, but no one has studied this. Fertilizer and untreated wastewater promote algal growth that starves pools of the already limited supply of oxygen.
Many vernal pools are not hydrologically-connected to streams, so pollutants such as road salt do not readily flush out and concentrate in the small basins. In a controlled experiment by Karraker and coauthors, larvae experienced spinal deformities and death in water with salt levels comparable to pools near highways in the Adirondacks. It’s reasonable to draw parallels to impacts on drinking water and human health.
Because wetlands in the Adirondacks must be at least 1 acre to be regulated by law – larger than most vernal pools – loss of habitat is a serious concern. Pools may be threatened by development, or compromised by pollutants, invasive species and disturbance from vehicles. Skid trails and tire tracks filled with water are low-quality breeding sites compared to natural pools. Habitat guidelines recommend at least a 100-foot pool buffer where logging and disturbance are minimized – though this is a minimum as many adult amphibians migrate hundreds of feet into the woods. A key issue for vernal pools is simply lack of awareness of their presence and of the upland-wetland connection.
Frogs calling in spring may be the easiest way to find vernal pools, but the presence of indicator species tells the story all year. There is a way to get involved: the Center for Adirondack Biodiversity seeks volunteers to locate and monitor wetlands. Opportunities abound to discover and appreciate the diversity and denizens of vernal pools with the CAB; visit www.paulsmiths.edu/ATBI/amphibian.php for information.
When I revisit the vernal pools later in spring, a pair of wood ducks startle from the water, whistling and wings dripping. They nest and raise their chicks near pools – just one of a host of species using these wetlands during some part of the year. For me, the quacking of wood frogs (and wood ducks) signals the end of another Adirondack winter.
The Vernal Pool Association has an informative website, book and photos. For further reading, see The Amphibians and Reptiles of New York State: Identification, Natural History, and Conservation by James Gibbs and coauthors. Finally, relevant publications by Karraker and others can be found here.
Photo: A vernal pool at the Huntington Wildlife Forest in Newcomb. This vernal pool may contain over 200 wood frog egg masses in spring, yet dries out by August, if not earlier.