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Aquatic predators affect carbon-storing plant life

Species at top of freshwater food web can indirectly limit buildup of carbon dioxide in the atmosphere

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In ecosystems around the world, big guys eat littler guys, who in turn eat plants and other organisms at the base of the food web. A study now finds that removing top predators in freshwater environments allows their prey to flourish — and overgraze on plants and algae. The result of the missing plant matter: a 93 percent reduction in uptake and storage of carbon dioxide.

Several research teams have explored the importance of predators in protecting organisms that store carbon, notes ecologist James Estes of the University of California, Santa Cruz, who was not involved in the new research. The new study is particularly strong, he says, because it demonstrates predators’ influence across a broad range of ecosystems. It therefore suggests “that the phenomenon may be fairly general.”

When pesticide runoff, overfishing or other human activities impact ecosystems, the first species to disappear are usually the bigger, top predators, notes freshwater ecologist John Richardson of the University of British Columbia in Vancouver and coauthor of the study, published online February 17 in Nature Geoscience. The new work shows that predator losses have effects beyond the loss of biodiversity: “We can see climate effects as well,” he says. “We start seeing a higher flux of carbon dioxide into the atmosphere.”

Study leader Trisha Atwood, then also at the University of British Columbia, and colleagues simulated three freshwater ecosystems outdoors to study the effects lower in the food web of predator loss at the top. They diverted water from streams near Vancouver into six channels they had constructed. Those channels accumulated critters and debris for about six weeks. To simulate ponds, Atwood’s team added water and sediment from ponds in Vancouver to 10 tanks, each about 2 meters across, and let them acquire organisms over 18 months. And to study the water-holding reservoirs among leaves of some plants, the ecologists went to Costa Rica and let the center well of 20 bromeliads — flowering plants found mainly in tropical regions — collect a little water and wildlife over a two-week period.

In half of the simulated ecosystems in each location, the researchers added top predators. For streams, that predator was the three-spined stickleback (Gasterosteus aculeatus), a 10-centimeter-long fish that feeds on zooplankton in stream water. Stonefly larvae served as the predator in the simulated ponds. And the researchers introduced damselfly larvae to feed on zooplankton in the bromeliads.

At the end of these accommodation periods, the researchers made daylong measurements of carbon dioxide in water. Then they compared the values for environments with and without their top predators.

Adding the top predators decreased the amount of carbon dioxide in the water by an average of 93 percent, Atwood and her colleagues report. When predators are absent, the researchers think the unchecked zooplankton aggressively feed on plants and algae in each ecosystem. Those photosynthetic organisms, had they not gotten eaten, would have used and stored carbon, removing it from the water. That in turn would have pulled more carbon dioxide from the atmosphere.

This predator effect on carbon dioxide has been reported in a few land-based environments, says David Butman of Yale University’s School of Forestry and Environmental Studies. But, he adds, “there have been few studies to explicitly suggest stream and pond systems may perform similarly.” As such, he argues, the new study is important in unraveling the complexity of natural environments. However, he cautions, scientists must recognize that the results come from artificial manipulations of ecosystems “until similar systems are identified in the wild.”

Correction: This story was updated on February 20, 2013, to clarify that predators decreased carbon dioxide levels in the water by 93 percent on average.

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