- Jim Julian
Vernal Pools—Seasonal Ponds for Special Amphibians
Dynamic Wetlands in Time and Space: Vernal pools are wetlands that experience alternating periods of flooding and drying, and are synonymous with animal species that exploit these seasonal waters that are free of predatory fish. Fish have a difficult time colonizing vernal pools because they dry up during seasons (or years) with moderate rainfall, or lack stream connections through which fish can immigrate. For these reasons, vernal pools are more broadly called seasonal pools or isolated depression wetlands.
Populations of frogs, salamanders, and aquatic invertebrates in vernal pools cannot thrive without the surrounding forest. Trees within 100 feet of a vernal pool are particularly important because they moderate water temperatures and slow down pool drying. Furthermore, trees deposit leaf litter into the pool that is colonized by the bacteria and fungi, which are then eaten by aquatic invertebrates. Forested uplands as far as 1,000 feet away provide the summer, fall, and winter habitats for amphibians that breed in a vernal pool. These amphibians need shady, forested habitats to maintain their moist skin surface that they use for cutaneous respiration, the process by which their skin exchanges oxygen, carbon dioxide, and electrolytes with the environment.
Animals That Fascinate Us: The animals that breed in vernal pools rapidly complete their larval periods before ponds dry in the summer. Because food resources in pools can be scarce, their larvae are active in the water column, which makes them vulnerable to predation by fish. Small crustaceans called fairy shrimp, in addition to the marbled salamanders, lay their eggs in dried portions of vernal pool depressions during the Fall, and their aquatic larvae spend the winter in the bottom of ponds. Species such as the wood frog, Eastern spadefoot toad, spotted salamander, and Jefferson salamander begin to lay egg masses in vernal pools as soon as the ice around their edges begin to thaw. In the Spring Creek Watershed, these species migrate to pools from the surrounding forest on the first rainy evenings in mid-February and March. Some vernal pools will attract amphibians from over 1,000 feet away, with hundreds of animals migrating to a vernal pool in a single evening. Since many individuals breed in the same pond every year, we can find a reliable location to watch migrations, as well as provide opportunities for conservation. The fist-sized, gelatinous egg masses from amphibians will be attached to submerged sticks and vegetation, and will remain visible in pools into April (sometimes May for salamander egg masses). The adults, however, retreat into the surrounding forest within weeks of laying eggs, and are rarely seen until next year’s migration. By getting an early jump on the breeding season, these tadpoles and larval salamanders can complete their metamorphosis into terrestrial juveniles by July if ponds begin to dry. If water persists, however, some species can delay metamorphosis into August or September, emerging at a larger size that makes them less vulnerable to predators.
Three amphibians that breed in the vernal pools of Spring Creek’s watershed are among Pennsylvania’s 18 amphibians of greatest conservation need  (see list of references). Both the Jefferson salamander and the marbled salamander have populations that are vulnerable to decline, with conservation threats that include the destruction of their upland forested habitats, as well as illegal collecting for the pet trade. The spadefoot toad has a conservation status of “Threatened” in Pennsylvania, with only 8 counties documenting its occurrence since 2013 . Spadefoot toads are difficult to encounter because they only emerge from their underground burrows during the heaviest of rains; individuals may spend up to 200 consecutive days in their burrows, and their populations may not breed every year [3,4]. As a result, extensive biological surveys near vernal pools are a necessary to ensure that proposed development projects will not harm spadefoot toad populations.
Where We Find Vernal Pools:
Vernal pools can form in a variety of topographic settings: toe-of slope, ridge tops, topographic benches, saddles, flood plains, and occasionally on gentle slopes. Precipitation and surface water runoff have a strong influence on the water budget of many vernal pools . However, significant contributions can be made by the seasonal rise of the water table, as well as groundwater discharges from springs and seeps. Many vernal pools retain water because they often possess impermeable layers of rock and clay-laden soils within the first several feet below ground. Pools with less permeable layers, however, are able to contribute to the recharge of groundwater. In the northeastern U.S., the flooded areas of vernal pools are often less than two feet deep and 0.25 acre in area (about the size of two football end zones) . Natural resource inventories that catalogue wetland acreage often fail to detect vernal pools because of their small size and seasonal drying. The National Wetlands Inventory is our nation’s largest effort to monitor changes in wetland acreage, but the aerial imagery used to detect wetlands has difficulty identifying wetlands smaller than 1.0 acre in area . While some vernal pools can exceed this size threshold (referred to as a minimal mapping unit), our study of hundreds of amphibian breeding pools in Pennsylvania’s Delaware Water Gap National Recreational Area suggest there is only a 25% chance that a 0.25-acre pool would be included on National Wetlands Inventory maps .
Since 2010, Clearwater Conservancy has collaborated with students and faculty from the Environmental Studies Degree Program at Penn State Altoona College to conserve, study, and map vernal pools in the Spring Creek Watershed. Our most thorough efforts have been within the “Scotia Barrens” landscape that contributes significantly to the recharge of the watershed’s groundwater. The Scotia Barrens is over 6,700 acres of highly porous soils dominated by mixed oak, pitch pine and scrub oak forest. This landscape includes public-use areas such as the “Scotia Range” State Gamelands 176 (6,200 acres) as well as Patton Township’s Haugh Tract Family Preserve (465 acres) and Grays Woods Wetlands (43 acres). Walking through this landscape is reminiscent of regionally-renowned natural areas such as New Jersey’s Pine Barrens and forested areas within Massachusetts’s Cape Cod National Seashore.
The Scotia Barrens is pockmarked with over 100 wetlands, many of which meet the criteria of a vernal pool according to either their drying cycle, their lack of stream connections, or the community of amphibians that use them for breeding. While studies on the water chemistry, hydrology, and amphibian communities at these vernal pools were well underway 25 years ago, the Western Pennsylvania Conservancy (WPC) was the first to include these wetlands in their inventory of vernal pools in Pennsylvania. Spurred by this research, Clearwater and Penn State Altoona began to evaluate 111 wetlands in the Scotia Barrens identified by the National Wetlands Inventory (NWI) to determine whether they met the criteria of a vernal pool. Of those 111 NWI wetlands, 42 were classified as vernal pools, but not all wetlands in the Barrens were identified in the NWI. Of the 71 known vernal pools in the Scotia Barrens, 29 were not detected by NWI maps, and 24 of them were not previously identified by WPC surveys. The Scotia Barrens has about 6.7 vernal pools per square mile, which is comparable to natural areas in New England (2.8 to 35 pools per square mile) where vernal pools are well-studied .
Suggestions to Enjoy Responsibly, Conserve Holistically:
Conservation must occur at several spatial scales to maintain the educational, recreational, and ecological values of a vernal pool. We can refer to these spatial scales as the life zones of a vernal pool. These zones include The Basin that floods each year, The Seasonal Envelope within 100 feet of the water’s edge, and The Terrestrial Habitat that extends an additional 900 feet .
The Basin (Depression): Leave all branches, sticks, and leaf litter in ponds because they are important egg mass attachment sites and they have an important role in the food web. Do not move animals or egg masses into, or out of, vernal pools. There is no guarantee the animals you move will survive in their new habitats, especially since requirements are needed at the spatial scales of the basin, envelope, and the terrestrial habitat. Avoid walking into ponds during the breeding season. Frogs and salamanders hide in waiting under the leaf litter in the shallows of these ponds – they can be easily stepped upon. If you want to hold some of these critters, handle enough animals to pique your curiosity, but not so many as to satiate it. Amphibians have thin skin that is easily damaged by excessive handling, petting/stroking, dry hands, and chemical residues like insect repellant and sunscreen.
Amphibians throughout Pennsylvania can carry chytrid fungus (Batrochochytrium dendrobatidis) and ranavirus viruses (typically Frog-virus 3). Handling infected animals can spread disease to the uninfected animals you handle, thus lowering their chances of survival . Always clean, disinfect, and dry personal gear like waders, boots, and nets before you move from one body of water to the next. Gear should be cleaned of mud and debris, washed down with disinfectants like a 1 part bleach to 9 parts water solution, and dried completely before exploring your next wetland. The national organization Partners in Amphibian and Reptile Conservation have several free educational materials on amphibian and reptile diseases at their website (http://parcplace.org/resources/herpetofaunal-disease-resources/ ), as well as a video on how to clean and disinfect your personal gear when exploring wetlands.
The Seasonal Envelope: Avoid cutting or removing vegetation, especially trees and downed logs. In this zone, the canopy cover from 20- to 30-foot-tall trees should exceed 75% . Trees provide leaf litter for pond basins, the shade from the canopy moderates water temperatures, and the water drawn by trees from the ground can help ensure that pool basins occasionally dry out. Downed trees and logs are great places to find salamanders and frogs during the daytime because the underside of logs provide a moist refuge. After looking under logs, be sure to roll them back into their original position to maintain moist conditions underneath them. Restrict vehicle usage in these areas, especially all-terrain vehicles whose tire tracks can alter the basin’s hydrology by changing drainage patterns. Vehicle use and timbering, if absolutely necessary, are best done in these areas during the late fall and winter months when amphibians have already dispersed into the upland terrestrial habitat and commenced spending most of their days underground.
The Upland Terrestrial Habitat: Again, it is important to maintain tree canopies that provide enough shade to maintain cool, moist forest floors. Within the first 300 feet of this zone, canopy cover from 20- to 30-foot-tall trees should exceed 50% . Roads located in this zone can intersect annual migration pathways of vernal pool amphibians, and hundreds of animals can be killed in a single night on even moderately travelled roads. To prevent road-killed amphibians in the Scotia Barrens, the Pennsylvania Game Commission has allowed Clearwater to post road signage that warns drivers of crossing amphibians. Some communities conduct annual road-closings during migration periods. These closing need only to be during the evening hours (usually 9 pm to midnight), and only during the first couple of rainy nights of the year. Still other communities have built culvert-sized underpasses in sections of roads where amphibians can safely travel underneath roadways.
PGC-PFBC (Pennsylvania Game Commission-Pennsylvania Fish and Boat Commission). 2015. Pennsylvania Wildlife Action Plan 2015 - 2020 (C. Haffner & D. Day, Eds.). Pennsylvania Game Commission and Pennsylvania Fish and Boat Commission, Harrisburg, Pennsylvania.
Pennsylvania Amphibian and Reptile Survey. 2018. [Map of Eastern spadefoot toad accounts since 2013]. Reptile and amphibian atlas for Pennsylvania. Retrieved from http://paherpsurvey.org/taxon.php?taxon=622&all=1
Criswell, R. 2012. The eastern spadefoot. Pennsylvania Angler and Boater, March/April 2012: 52-53.
Overduijin, K. (nd) Eastern spadefoot toad (Scaphiopus holbrookii). Species profiles from University of Georgia Savannah River Ecology Laboratory. Retrieved from http://srelherp.uga.edu/anurans/scahol.htm
Colburn, E. 2003. Hydrology. Pp. 21-32 In Vernal Pools: Natural History and Conservation. The McDonald and Woodward Publishing Company, Blacksburg, VA.
Colburn, E. 2003. Vernal pools in the landscape: Origins, landscape positions, and habitat characteristics. Pp. 33-52 In Vernal Pools: Natural History and Conservation. The McDonald and Woodward Publishing Company, Blacksburg, VA.
Dahl, T.E. 2011. Status and trends of wetlands in the conterminous United States 2004 to 2009. U.S. Department of the Interior; Fish and Wildlife Service,Washington, D.C.
Julian, J. 2009. Evaluating amphibian occurrence models and the importance of small, isolated wetlands in the Delaware Water Gap National Recreational Area (Doctoral dissertation). Pennsylvania State University, University Park, Pennsylvania, USA.
Brown, L. and R. Jung. 2005. An Introduction to Mid-Atlantic Seasonal Pools, EPA/903/B-05/001. U.S. Environmental Protection Agency, Mid-Atlantic Integrated Assessment, Ft. Meade, Maryland.
Gray M., J. Spatz, E. Carter, C. Yarber, R. Wilkes, D. Miller. 2018. Poor biosecurity could lead to disease outbreaks in animal populations. PLoS ONE 13.
NEPARC Publication 2014-02. 2014. Disinfection of field equipment to minimize risk of spread of chytridiomycosis and ranavirus. Available from http://www.northeastparc.org/products/pdfs/NEPARC_Pub_2014_02_Disinfection_Protocol.pdf
DeMaynadier P. and J. Houlahan. 2008. Conserving vernal pool amphibians in managed forests. Pages 319-344 In Science and Conservation of Vernal Pools in Northeastern North America (A. Calhoun and P. deMaynadier, Eds.). CRC Press, Boca Raton, FL.
Dr. Jim Julian is an Assistant Professor of Biology at Penn State Altoona College, where his research focuses on the fungal and viral pathogens that cause disease in amphibian communities. He has partnered with the U.S. Fish and Wildlife Service's Northeast Fisheries Center to compare disease prevalence between natural and human-constructed wetlands, and he is currently evaluating methods to detect amphibian pathogen DNA in water samples from ponds. He enjoys engaging the public through tours of amphibian breeding ponds, and he participates in national and regional task teams on herpetological diseases through the organization Partners in Amphibian and Reptile Conservation.