Vernal Pool Ecology

Seasonal or vernal pools are found worldwide and are known by many names. For example, in the Midwestern United States ephemeral wetlands are referred to as prairie potholes, while in the Southeastern United States they are called seasonally ponded isolated wetlands. In Maine, the term vernal pool has come to mean a specific type of wetland defined as naturally occurring, seasonal bodies of water, free of predatory fish, in which one or more of four vernal pool indicator species are present—spotted and blue-spotted salamanders (Ambystoma maculatum and A. laterale), wood frogs (Rana sylvatica), and fairy shrimp (Anostraca). Vernal pools occur in a variety of landscape settings including the bottomlands of rivers, as part of wetland complexes, and as an isolated pool in the upland forest. These pools may be semi-permanent (never completely drying) or permanent. Permanent breeding pools tend to be shallow enough to exclude fish populations. This happens either because the pools have extremely low dissolved oxygen in the water by the end of summer and/or the fact that the pool completely freezes in winter. The lack of predatory fish is one of the features that make vernal pools extremely important habitat. Vernal pools typically hold water for three to five months beginning in early spring. Characteristics of a vernal pool, such as size, length of flooding, substrate, and vegetation are influenced by many factors including landscape setting, the geology of the surface, soil type, and surrounding vegetation. Furthermore, the size and duration of flooding for a particular pool may be quite different from year to year depending on local snow and rainfall patterns.

In some regions of Maine human-created pools and other wetlands, including beaver flowages and lakeshores, may provide important breeding sites for the indicator species. We encourage landowners to protect these sites as well. Vernal pool habitat may include non-vegetated pools, marshes, wet meadows, shrub swamps, and forested wetlands. Vernal pool characteristics vary widely but all are vital habitat in the landscape.