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.”
Posted By: -
Debajyoti Kalita
R. G. Baruah Road, Guwahati
Source: - Science News
Originally Written By: - Janet Raloff
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