Signs of Fall 8: Wood Frogs and Climate Change

Photo by D. Sillman

There is a population of wood frogs (Rana sylvatica) down at Ohiopyle, Pennsylvania that Deborah and I and Rob and Michele Bridges look in on during our annual, early March hike around the Ferncliff Peninsula. Most years there are only a few frogs quacking in the scattered vernal pools, although one year (2013) we hit the jackpot and timed our hike with the frogs’ mass emergence from hibernation into an abundance of water-filled pools. The past two winters, though, have been mild with very little snow, so the vernal, frog-breeding pools did not fill up or persist very long into the spring.

Wood frogs are found from northern Georgia all the way up to the Arctic Circle. It is, in fact, the only “cold blooded” vertebrate that lives north of the Arctic Circle! They utilize temporary pools formed by spring rains and snow melt as breeding ponds and then spend most of the rest of their active season away from standing water. Adult wood frogs feed opportunistically and extensively on small insects and other invertebrates. They use their long, sticky tongues to capture prey and are said to eat “anything that they can fit into their mouths.”

The ability of wood frogs to survive in high latitude ecosystems depends upon a number of specialized physiological adaptations that enable them to tolerate being almost completely frozen through the long winters. There is a cost to this extreme hibernation, though. The longer the frog is frozen, the more likely it is not to survive.

As soon as the wood frog thaws in the spring, it moves to its breeding pools which are free of fish and other potential egg and tadpole predators. These pools, though, are inherently transient and dependent upon unpredictable weather conditions. The use of these temporary pools, then, represents a very delicate, ecological “cost-benefit” balance for the species.

Photo by D. Sillman

In the mating pools, males call to females with their “duck-like” songs. An attracted female enters the pool and is quickly grasped on the back by the smaller male (this is called “amplexus”). The male may remain in place on the female’s back for 24 to 72 hours and will release his sperm into the pool water as the female ovulates and thus externally fertilizes the forming egg mass. A typical egg mass contains 1000 to 2000 eggs. The female moves the floating egg mass into the shallow areas of the pool in a large, communal raft. Counting these rafts in an area’s pools is an accepted, and highly efficient, way to determine the population density of the wood frog in a particular region.

Since mating and egg laying occur very soon after ice melt, the chance of seasonal, sub-zero temperatures re-occurring is quite high. The eggs and embryos of R. sylvatica have an interesting adaptation that enable them to survive both transient and sustained periods of freezing and sub-freezing temperatures. The melting point (i.e. temperature at which material changes from a solid (frozen) to liquid state) of the mucopolysaccharide and mucoprotein “jelly” that surrounds the eggs and the developing embryos is higher than that of the fluids inside the egg. As temperatures fall, then, the jelly freezes before the egg or embryo. This freezing osmotically draws water out of the egg into the jelly mass. The dehydrated egg and embryo, then, are more resistant to freeze damage and are thus able to better survive the early spring temperature fluctuations. Larger embryos in particular, are more tolerant of longer periods of freezing, so severe weather patterns may generate a selection pressure for faster growing, and, thus, larger and more resistant embryos.

Wood frogs are important models of some of the potential effects of climate change. Warming temperatures could, logically, reduce the hibernation stress on wood frogs but may also push the frogs past their upper limit of temperature tolerance during their spring and summer activity periods.  Also, changes in precipitation patterns, especially changes in snowfall and in spring rains, could have large effects on the size, availability and persistence of the vernal pools that are critical for the reproduction of this species.

Photo by D. Sillman

A consortium made up of the U. S. Geological Survey and fourteen universities (including Penn State) looked at the sizes and reproductive health of 746 populations of wood frogs scattered across North America from Tennessee up into Canada to try to determine what the effects of climate change might be upon this important, bell weather species. Their results were published in the August 19, 2017 e-journal, Global Change Biology.

Warming temperatures had a negative effect on wood frogs in the southern (warmer) areas of their range, but increased temperatures had a positive (stimulatory) effect on wood frog populations in the middle and more northern areas. Increased precipitation, though, even in areas that were normally moist, was found to consistently benefit wood frogs numbers and reproduction (as measured by egg counts) especially when this increased rainfall was coupled to increased temperatures.

This study, then, noted that the North American distribution of wood frogs is likely to shift northward as the result of climate change, but somewhat surprisingly also found that the impacts of warmer temperatures and increased rainfall in the middle and more northern regions of the species’ range were likely to have positive effects on the species’ numbers and reproductive rates.

So ongoing climate change is having a complex impact on wood frogs. Some populations of wood frogs in certain locations will be benefited by climate change while other populations in other locations will be negatively impacted. This is likely to be the model of response that we will see in other species whose physical environments are being altered by changes in seasonal temperature and precipitation patterns. The influences of the interactions of a species with other members of its unique biotic community, the influences of specific features of a species’ physical environment, and the potentials for local adaptations to the changing climate matrix will all combine to make each population’s response to climate change unique.

 

 

 

 

 

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