A vernal pool is a temporary or semi-permanent body of water, typically filled in the spring by snow melt and spring rain, and holding water for two or three months in the spring and summer. Vernal pools form in contained basin depressions, meaning that while they may have an inlet, they have no permanent outlet forming a downstream connection to other aquatic systems. They are typically small, rarely exceeding 50 m in width, and are usually shallow. While most are filled with meltwater and spring rains, others may be filled during the fall or with a combination of seasonal surface runoff and intersection with seasonally high groundwater tables. Typical substrates are formed primarily of dense leaf litter. While most vernal pools are found in upland forest, several types have been identified, including floodplain basins, swamp pools and marsh pools.
Periodic drying is a key feature of the ecology of vernal pools. Drying precludes the establishment of permanent fish populations, which would otherwise act as predators on the eggs and larvae of species that live or breed in the pool. While a typical vernal pool is dry during at least part of the year, others may contain some water throughout the year (or for several years), but a combination of shallow water, summer heat, winter freezing, and periodic oxygen depletion prevent the establishment of fish populations.
Vernal pools support the ecologies of a diverse number of organisms that depend on temporary waters for reproduction. These wetlands are an important and fragile component of the landscape and a limiting resource for a number of species whose local persistence depends entirely on the continued availability of appropriate breeding habitat. Vernal pool communities are composed of a group of obligate invertebrate and amphibian species and a larger group of facultative species, including several plants. Obligate vernal pool species must live or breed in vernal pools, while facultative species may be found in vernal pools, but can reproduce in other aquatic habitats where they are available. The presence of obligate species helps to define a particular body of water as a vernal pool. Of the facultative species, some have life histories and ecological requirements that may indicate conditions suitable for obligate species even when the obligate species themselves are not detected.
Obligate vernal pool species in western New York and Pennsylvania include several species of fairy shrimp, the Wood Frog (Rana sylvatica) and several species of mole salamander (genus Ambystoma), including the Blue-spotted Salamander (A. laterale), the Jefferson Salamander (A. jeffersonianum), the Spotted Salamander (A. maculatum) and the Marbled Salamander (A. opacum). In our area, the Marbled Salamander is only found in Pennsylvania from Westmoreland and Indiana Counties to Crawford County -- it is absent from western New York.
There are many additional species you may encounter in and around vernal pools that don’t depend on them for reproduction—the facultative species. Amphibians include the Four-toed Salamander, Red-spotted Newt, Spring Peeper, Gray Tree Frog, Green Frog, and American Toad. Facultative vernal pool reptiles include the Painted Turtle, Spotted Turtle, and Snapping Turtle. Facultative vernal pool invertebrates include freshwater fingernail clams, aquatic air-breathing snails, leeches and insects such as water scorpions, predaceous diving beetles, whirligig beetles, dobsonflies, caddisflies, dragonflies, damselflies, mosquitoes, springtails and water striders.
Plant communities in vernal pools are composed of facultative and obligate wetland species, typically including submergent and floating plants, but rarely emergents. Typical vascular plants include mannagrasses, Spikerush, Water Purslane, Duckweed, and Waterhemlock. Characteristic mosses include several sphagnum mosses.
To establish that a particular body of water is a vernal pool, in addition to establishing the physical characteristics of the pool (e.g. that it is seasonally flooded and contained) and that it is fishless, you may also want to establish that one or more obligate species is using the pool for reproduction—the mere presence of adult animals doesn’t indicate a vernal pool. Most of the evidence you will encounter will be in the form of eggs and larvae—you will rarely observe adult amphibians, as most of their activity takes place at night and only for a short period of time. Outside of the breeding season for obligate species (i.e. late May through February), a combination of several features may establish a pool as suitable habitat for obligate species: seasonal flooding, breeding or presence of certain species of facultative amphibians or invertebrates, presence of certain plant species, and a lack of fish. If obligate species are present, you don’t need to observe the pool dry or to establish that it is free of fish or seasonally occupied by facultative species.