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updated 10/23/2007 2:20:41 PM ET 2007-10-23T18:20:41

The skimmer and swimmer critters in ponds dried out by drought end up looking the same as each other when waters return, causing a decline in biodiversity, a new study finds.

In worst-case outcomes, drops in biodiversity—the variety and number of species, in a given locale can lead to more serious consequences, such as resulting in ecosystem collapses that affect the web of life and food that supports all animals and humans.

Scientists are more interested than ever in the effects of extreme climate swings, such as prolonged drought, because the computer models predict wilder extremes as one effect of the climate change now underway.

To learn how drought affects pond life, Jonathan Chase, an ecologist at Washington University in St. Louis, imposed drought conditions on 20 artificial ponds and investigated how the harsh conditions affected the species counts and varieties.

Each pond community had the same environmental conditions, but Chase varied the timing of the introduction of species, such as dragonflies, water-bugs, frogs, water fowl and algae, before letting the species naturally flourish.

As the communities began to thrive, the species took hold to varying extents pond by pond, with some harboring only 10 to 20 percent of species in common. Some of the variation was due to plants being randomly introduced as they fell from the feathers of a duck, for example.

After pond communities established themselves, Chase imposed the drought conditions on half. When those ponds were allowed to recover from drought and life moved back in, their species content looked much more similar to each other.

“Drought homogenizes the variance among communities,” Chase said. “It takes all these communities that used to be very different from each other and makes them very similar to each other.”

Why? Because certain species are much hardier than others and are quicker to re-establish themselves once the drought subsides.

"When it comes to drought, there are wimpy species and hardy species," Chase said. "Several types of zooplankton, many water-bugs and some frogs are the hardy ones. A wimpy species, perhaps surprisingly, is the bullfrog. Their tadpoles require two years to grow, so they often don’t rebound very well from drought. “

Zooplankton deposit their eggs in mud, so they lay low until waters return, whereas frogs leave the pond when it dries up. Algae and a few plant species that make lots of seeds also weather droughts fairly well, Chase said.

His study, detailed in the Oct. 15 issue of the journal Proceedings of the National Academy of Sciences, establishes an important distinction between local biodiversity (in one pond) and regional diversity (between several ponds), the latter of which is often overlooked, Chase said.

“I found drought had less than a 10-percent reduction on local diversity, but a nearly 50-percent reduction on regional diversity," Chase said. "This is important because if you just count the number of species in any given pond you might say that drought had little effect on species diversity. But if you take exact data and you ask: 'Did drought affect regional diversity?' I found it had a huge effect on regional diversity.”

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