When Philip deMaynadier hears bells, he knows that spring has arrived.

Each April, the state wildlife biologist takes evening walks through the woods to listen for the song of the spring peepers, which he likens to the chiming of thousands of sleigh bells, and for the wood frog’s muted, birdlike call.

Tiny male frogs awaken from their winter hibernation and travel to vernal pools, ephemeral spring ponds born of melting snow, where they sing to charm the females and, eventually, mate and leave fertilized eggs in the pool.

Wildlife biologists are worried, however, that this spring, the frogs will sing in vain. Maine’s continuing drought has left the land parched and a mild winter offered little snow for moisture.

Smaller vernal pools didn’t form early this spring, and those that did are expected to disappear long before the amphibians have moved on.

If this week’s heavy showers don’t continue, thousands of juvenile frogs and salamanders are expected to die when their vernal pools dry out too early, biologists said.

“They’re in a race against time,” deMaynadier said. “Once you have learned about the life history of these organisms, the drama of life that occurs every spring, you can’t help but be impressed. It’s like each one is a character in a play.”

In the drama of the spring vernal pool, the wood frog and the spotted salamander are the stars.

Dan Vasconcelos, a graduate student in ecology and environmental sciences at the University of Maine, uses these common species to gauge the health of vernal pool ecosystems.

Since March 2001, Vasconcelos has visited natural and human-made sites on Sears Island every morning, checking his live traps for animals that traveled to or from the largest pools overnight.

Nylon fences have been placed around three pools. The amphibians search for an entrance until they fall into a series of buckets buried in the ground.

Hundreds of wood frogs have already made their annual journey, and luminous gobs of eggs float on the surface of every pool larger than 3 feet in diameter.

On this season’s peak day, April 10, more than 400 amphibians fell into Vasconcelos’ traps, he said.

The brown and red wood frogs rarely exceed about 3 inches long, and breed early, beginning as soon as the nighttime temperature exceeds 35 degrees in early April.

“The wood frogs don’t seem picky,” Vasconcelos said. “Yesterday, I saw three male wood frogs on a salamander. They just grab onto anything female and don’t let go.”

The eggs hatch after a few weeks, but the young frogs live three months as an aquatic species before they can leave the pond. Wood frogs instinctively breed in the same pond in which they were born, even if that pond is dry.

A researcher in Rhode Island has recently spotted wood frogs breeding and laying their eggs on dry ground that had supported a vernal pool last spring. The same could be happening in Maine’s forests.

“Some of them got confused and just sat there,” Vasconcelos said. “They probably dried out and died, waiting for water.”

Wood frogs will show the most immediate population reductions during a drought, because they live only a few years. If a drought extends through an entire frog’s lifetime, the population could crash, deMaynadier said.

Spotted salamanders, on the other hand, can live as long as 20 years, but their larvae depend on the vernal pool environment for much longer than a wood frog. Without five months of water, none of the eggs will develop into adults.

The shiny black salamanders sprinkled with yellow spots breed silently, but their courtship is every bit as elaborate as the frogs’.

By shining a flashlight on a vernal pool during the right early spring night, one can witness a salamander congress, a mating ritual, which begins the breeding season.

“It’s this frothing ball of dozens of salamanders all courting one another,” deMaynadier said.

Spring mating brings a flurry of activity to the vernal pools, but in an average year, most pools will naturally dry up after the new frogs and salamanders have left. Last summer, many disappeared by early August – too soon.

“There were just piles of dead salamanders,” Vasconcelos said.

This summer, vernal pools are starting out small, and some traditional pools never even formed.

“It’s all much drier than it was last year, it’s just parched,” he said. “I think the ground is so dry, as soon as we get any water, it just takes it right in.”

Only the large pools are expected to remain viable past June, putting most salamanders and many frogs at risk, Vasconcelos said.

The drought’s impact will extend far beyond these two species, however. Vernal pools tend to draw amphibians from acres of forestland, and half of all Maine’s amphibians breed in or near the pools, deMaynadier said.

Snakes, birds, turtles, coyotes, foxes and raccoons are also drawn to the pools to gorge themselves on the bounty of the amphibians.

“They’re really playing a big role at the base of the food chain for the forest,” he said. “Amphibians, as a rule, tend to be a very palatable group. There’s no feathers or fins. They’re just little protein packages.”

The full reach of vernal pool ecosystems will be damaged by the drought, and amphibian populations will plummet in many areas. But spring peepers, wood frogs and salamanders do not face immediate extinction this summer, deMaynadier said.

Development, however, destroys vernal pools every year, because the wetlands are too small to be protected under environmental laws.

Building a public awareness and appreciation of vernal pools is the wood frog’s best hope for the future, deMaynadier said.

“Amphibians have evolved for thousands of years, with or without drought,” he said. “What they haven’t come back to is a pool that’s been paved over with a parking lot.”