Vernal Pools – Ephemeral Hot Spots of Biodiversity

posted in: Frogs, Local Conservation, Turtles | 0
A shallow pool of water at the base of a hill, with floating leagues, bare shrubs, and a few fallen logs in it.
They may not look like much at first glance – but there’s a lot going on beneath the surface!

On the night of March 17, 1991, I was ready to begin my Ph.D. research into the ecology and conservation of salamanders and frogs that breed in vernal pools. What I saw that memorable night stunned me. Despite having already spent several years studying vernal pools, I had never before fully appreciated the incredible abundance of life that can be concentrated in these wetland wonders. 

Spotted salamanders caught in a bucket trap. On a rainy spring night, this is just a fraction of the animals we might find!

With several friends who were helping me, we had recently encircled a large, ½ acre vernal pool in Concord, MA with plastic silt fence partly buried into the ground. Together with 33 pairs of 5-gallon buckets set at even intervals around the fence and buried flush with the forest soil, we had created a large pitfall trap system to catch and release nearly all the frogs and salamanders that headed from the woods into the pool to breed and then back home again. By 7PM the forecasted cold rain began and my friends and I spent that entire night circling the pond and counting and releasing, on the other side of the fence, the frogs and salamanders that had just popped up through the leaf litter and poured downhill to breed in the vernal pool, only briefly detained in our buckets. By the time the sun rose on a mild early spring day, we had counted and released an astounding 5,300 wood frogs, 780 spotted and blue-spotted salamanders, 1,200 spring peepers, and several hundred individuals of other amphibian species. This remains one of the most incredible wildlife spectacles that I have witnessed in a career as a biologist. 

Among the vast range of wetlands that we New Englanders are blessed with, vernal pools seem modest. Most are small and shallow, less than ¼ acre in size and 4 feet deep even in spring, and many have dried completely before the end of summer. These small wetlands are, in fact, best defined physically by what they lack: flow, permanence, and, most importantly, populations of fish. But, freed from fish predation, amphibian larvae can develop in extraordinary numbers, feasting on the equally prodigious diversity of algae, fungi, crustaceans, and insects.

A brown wood frog sits on a branch below the surface of the water, a jelly-like mass of grayish-white eggs immediately below her.
A female wood frog sits near her mass of clear eggs, recently laid in the waters of a vernal pool.
Eastern spadefoot toads (top) and blue-spotted salamanders (bottom) are just two of the rare amphibians that are totally dependent on vernal pools to successfully reproduce.

This tremendous productivity is driven by the seasonal cycles of flooding and drought, which allow decomposers to break down the nutrients that wash into the pools from the surrounding forests far more efficiently than permanent ponds.  Vernal pools also provide the specialized breeding habitats that many rare species depend upon. In Massachusetts, eastern spadefoot toads, marbled salamanders, and the salamanders of the Jefferson – blue-spotted complex, all listed under our state Endangered Species Act, usually breed in vernal pools along with several rare species of fairy and clam shrimps. Several species of rare turtles also frequent vernal pools to fatten themselves on the rich bounty of aquatic animals. 

At Zoo New England, our Field Conservation Department staff spend a great deal of time in and around vernal pools. From radio-tracking spotted turtles in Boston, to releasing headstarted Blanding’s turtles in suburban wetlands, to collecting and raising marbled salamander larvae that we reintroduce to vernal pool-studded habitat in the Middlesex Fells, our team visits dozens of vernal pools each year. In areas that we work, we help protect these vital wetlands by collecting the data needed to register or “certify” vernal pool habitat in the database maintained by Mass Wildlife, a process that  is largely driven by citizen scientists (see: Mass Wildlife Vernal Pool Certification.) 

A boldly-patterned black and white marbled salamander crawls across a bed of lichen.
An adult marbled salamander. This species was totally extirpated from the Middlesex Fells, but ZNE’s Field Conservation team are headstarting and releasing larval salamanders from a donor population to help bring them back. Photo by Tigran Tadevosyan.

As you read this blog post, in late May or June, we are well past the explosive breeding migrations of frogs and salamanders in early spring. Yet this is a great time to observe the abundant biodiversity of vernal pools. With literally tens of thousands of vernal pools in Massachusetts, there are pools near you, which you can learn about by speaking to local naturalists or using “Oliver”, online GIS exploration software from MassGIS (Oliver). Bring a dip-net, a dish pan, and a handlens, if you have one, if you wish to explore hands-on. In early summer, vernal pools are often soup-thick as they begin to dry up, brimming with tadpoles, insects, and more. Try searching the shores of  a vernal pool after rainy late spring and summer nights for tiny newly-metamorphosed frogs and salamanders, as they leave the pools where they were born and head into the surrounding forests and wetlands. Remember to please explore gently and responsibly – ALWAYS release the animals you catch as soon as possible in the same spot that you caught them. Perhaps you will find the evidence needed to certify a new vernal pool. Maybe you will even find a new locality for a rare species. These small wetlands are full of wonder.