Sunday, April 22, 2012

Hydroperiod

A steady rain has fallen since mid-day, with heavier rain forecasted for tonight and tomorrow. This will help alleviate the 2012 spring drought. The dry spring had me thinking about frogs and salamanders and the wetlands that they require each spring to breed. The water in their breeding pools was getting a bit thin. The rain arrived none too soon, for amphibians, for our garden, to prevent forest fires, and to replenish water supplies.

Each spring wood frogs and spotted salamanders travel from their upland habitats to "vernal pools" to breed and lay eggs. After hatching the frog tadpoles and salamander larvae race against time to develop and transform (through metamorphosis) into their terrestrial adult forms. Once laid, wood frog eggs take 10 to 30 days to hatch and then another 42 to 105 days to mature into the adult frog. Spotted salamanders take a bit longer--31 to 54 days to hatch and 60 to 110 days for the larval period. For both species, this typically takes them into June or July at the earliest. You can see their problem this year--the wetlands were beginning to dry up far too soon. 

Wood frog eggs in a vernal pool, April 17, 2008
Wetlands come in various shapes and sizes and types and include lakes, ponds, emergent marsh, scrub-shrub, forested, wet meadows, peatlands, and vernal pools. Each of these has a different "hydroperiod,"  meaning the length of time in a given year that the wetland holds water. Vernal pools by their definition have short or medium hydroperiods, since they dry up in less than four months (short) or within 4 to 11 months (medium) and typically do not hold water year-round. Permanent water bodies, such as lakes and ponds, have long hydroperiods.

All of New Hampshire's 6 salamanders and 10 frogs will breed in wetlands that hold water year-round. However, some species prefer wetlands that dry up each year including the wood frog and spotted salamander. What is the advantage of this strategy, given that in some years, like 2012, the pond dries up too soon? Fewer predators. Short-hydroperiod pools have the fewest predators and long-hydroperiod wetlands have the most predators.

One can picture a frog on a lily pad zapping a fly with its tongue. But a more common occurrence is the insect eating the frog. Diving beetles, caddisfly and dragonfly larvae, giant water bugs, and water scorpions prey on tadpoles and young salamanders, as do fish and red-spotted newts. There are fewer of these predators in the medium hydroperiod and far fewer in the short hydroperiod wetlands. So breeding in pools that might dry up too soon is still a better risk than laying eggs in a permanent wetland that is for sure full of predators.

Green frogs and bullfrogs almost always breed in permanent wetlands as their tadpoles take one year and two years, respectively, to develop. Thus, a vernal pool that dries up each year doesn't work for them. They have to risk the high predator wetlands. But they have other adaptions, including toxic skin that tastes bad to fish.

A Bullfrog in a permanent wetland
Spotted salamanders and wood frogs spend a relatively brief time in the wetland each year. The rest of the year they live in the uplands around the wetland. Hence the absolute need to protect both the wetlands and the surrounding uplands. In addition, frogs and toads and salamanders disperse to new areas when the local population reaches a certain limit. Getting to the nearest vernal pool or wetland requires safe passage through uplands. An interconnected network of wetlands with a mix of hydroperiods is the best for all amphibians. In wet years the short hydroperiod wetlands remain wet long enough to hatch a batch of frogs and salamanders. In dry years, amphibians will shift to wetter sites, but with the risk of more predation.

Fortunately the rain has finally come, although it is unclear whether it was in time for the wood frogs and spotted salamanders that bred in the quick-drying pools. The plants and wetlands are soaking up the rain. I can see immediate effects on the vegetation, they seem to grow before my eyes with this welcome flush of precipitation. The effects on amphibians will take longer to evaluate. 

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