It's there, and then it's gone. Vernal pools may appear to be little more than accidental puddles that fill in the spring and disappear by summer, but wherever they form they're an important part of the ecosystem.

In Pennsylvania, where 300 to 350 acres of wildlife habitat is lost each day to residential and commercial development, many vernal pools are disappearing, never to return.

Amphibians and many invertebrates rely on the temporary appearance of the pools for breeding purposes. In Pennsylvania, five species of amphibians and a crustacean use the pools for breeding, the hatching of eggs and development of young and larva. The spring peepers frequently heard in the spring are beneficiaries of vernal pools -- their mass migrations to and from the pools are a marvel of nature.

The national Vernal Pool Association reports that heavy rains in the second week of March brought large numbers of wood frogs, salamanders and other amphibians to vernal pools and other wetlands from Ohio to Massachusetts, with many crossing roads near the pools.

"Another storm has coaxed more amphibians from hibernation to migrate to vernal pools," reads a recent posting at VernalPool.org. "Migrations take place on many rainy nights in spring, not just on one night. Wood frogs are abundant in vernal pools, and in many pools have finished breeding and are, or will be, leaving the pool to head to the uplands for the rest of the year. Mole salamanders are just beginning to reach pools in large numbers."

The Nature Conservancy in Pennsylvania is also reminding people that although vernal pools may exist for only a few weeks, they're critical to forest health.

"One of the many reasons why vernal pools are important is because they support a distinct biological community that is adapted to a dry phase," said Conservancy scientist Tracy Coleman. "This seasonal drying allows for a fishless environment that is necessary for successful reproduction by several amphibians that would otherwise succumb to competition or predation."

The Nature Conservancy is a conservation organization that has protected more than 18 million acres in the U.S. and helped protect more than 117 million acres worldwide.

Looking for more from the Post-Gazette? Join PG+, our members-only web site. You'll get exclusive sports content, opinion, financial information, discounts from retailers and restaurants, and more. Our introduction to PG+ gives you all the details.