DO ANIMALS AND PLANTS RESPOND TO DROUGHT?
by Whit Gibbons
September 3, 2006
a University of Georgia graduate student, studies the ecology of snakes
at the Savannah River Ecology Laboratory. I asked him to describe some
of the strategies that aquatic animals use to respond to the natural phenomenon
of drought. Here is his response.
with a lawn knows that when it doesn’t rain for a long time you
must water the grass to keep it alive. Have you ever thought about what
aquatic animals do when it doesn’t rain and their water source disappears?
The animals with the most extreme challenges may be aquatic species that
live in isolated wetlands, those not connected to a permanent water source.
Wild animals can’t turn a sprinkler on themselves or refill a pond
that has dried up. But some species do have various strategies for coping
with a severe drought. These fall into two main categories: skedaddle
or hunker down, more commonly known to scientists as migrate or estivate.
have long known about the ability of some aquatic species to migrate overland.
Perhaps the most striking example, familiar to anyone who has seen a nature
show on this topic, is the walking catfish, which uses its pectoral fins
to scoot out of a drying body of water and then travel overland to a deeper
pool. Numerous animals, including many species of semiaquatic turtles,
migrate to more permanent wetlands during drought, with some migrations
being well over a mile. Similar long-distance overland migrations occur
in some watersnakes, frogs, and salamanders. Aquatic mammals, such as
beavers or otters, will also travel to another area to avoid drying conditions.
Birds and winged insects dependent on water can obviously fly to more
permanent wetlands when the water disappears. Interestingly, some wading
birds actually prefer to forage in drying wetlands because aquatic prey
that are incapable of migrating make easy prey for them to feed on.
not all species are equipped to move overland for any distance. The most
aquatic species in a wetland, especially the great majority of fish, are
generally the least suited for overland migration. Some species of animals
have highly permeable skin that causes them to lose too much body water
if they venture onto land. Others have inefficient terrestrial locomotion
(most fish) or such small body sizes that they would be unable to reach
a faraway aquatic site. For such species the only possible outcomes are
to die (fish again) or to reduce activity and wait out the drought, that
is, to hunker down.
scientific term for the hunker down strategy is 'estivation,' defined
as a state of inactivity and reduced metabolism. (For those of you who
thought it meant the opposite of 'hibernation,' you're right. But English
is a living language and, like other living things, words evolve.) In
any case, some species, especially insects, fish, amphibians, turtles,
and snakes, use this strategy to survive droughts. Many invertebrate organisms
have life history stages that remain dormant during droughts and then
awaken when the environment becomes wet again. A few fish, such as the
lungfish of Africa and Australia, secrete a cocoon to protect them from
water loss and then reduce their metabolic rate.
of large-bodied salamanders are capable of secreting cocoons, reducing
their metabolism, and surviving for at least two years buried under dried
wetlands. Some freshwater turtles, such as chicken turtles and mud turtles,
burrow in the dried wetland or in the surrounding terrestrial uplands
to survive drought. They do not form cocoons but do decrease their metabolic
rate to conserve energy. Recent studies have demonstrated that some aquatic
snakes estivate during long-term droughts. Black swamp snakes, a southeastern
species and the most aquatic of U.S. snakes, were able to estivate and
survive the drought of 2000-2003.
can be tended and cared for by those who value them. Creatures in the
wild must rely on other tactics to ensure their survival. And they have,
indeed, developed some effective methods of self-preservation. For some
species, two such strategies--migration and estivation--often prove to
be successful means of surviving drought.
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