Aug. 22, 2002 CORVALLIS – A new study of marine ecosystems suggests that the preservation of biodiversity is more than just a lofty goal - it's an absolute necessity to keep the system healthy and prevent both local or regional extinction of multiple species.
The population balance between various fish species, their competitors and their predators are all essential to the proper functioning of the ecosystem, the study showed, and overfishing of any one species can have ripple effects that destabilize the whole fishery.
The study was conducted by marine zoologists at Oregon State University and published this week in a professional journal, the Proceedings of the National Academy of Sciences. It was done on coral reefs in the Bahamas, which allowed the type of experimental manipulations that are usually impossible to do in a marine ecosystem.
"The research showed that all fish species within a food web are connected with one another, and the removal of any one species can cause whole populations to break down," said Mark Hixon, an OSU professor of zoology. "This is especially true when you take away the predatory species, which are a key to the natural balance and health of marine ecosystems." The study is particularly relevant to the global problems now being experienced in many commercial fisheries, Hixon said, because many of the fish species most commonly targeted by fisheries are marine predators.
In particular, this study confirmed the operation in a marine ecosystem of a concept that has long been recognized with animals in terrestrial ecosystems – that of "density dependent mortality." Basically, when a species population size is low, the mortality rate will also be low – predators tend to target species that are more abundant. And when a species population is high, the mortality rate will increase, as predators take advantage of the easy availability of food.
Other than the interaction between prey and predator, Hixon said, there is also an important role for competitors – two or more species that fight for the same resources. The competing species have negative effects on each other's population, and distracted competitors can be more vulnerable to predation.
"This interaction has been known for some time in land ecosystems," Hixon said. "But it's been much more difficult to demonstrate in the open ocean."
With experimental manipulations on coral reefs in the clear waters of the Bahamas, OSU scientists were able to isolate some reefs from others and selectively remove certain fish, their competitors or predators to observe the effect.
"We found that the removal of any one species can have ramifications for the whole ecosystem," Hixon said. "Without predation, a fish species can increase its population to an unsupportable size. Lacking food, fish become vulnerable to disease, changes in water conditions and ultimate collapse of that species or the whole fishery. Everything is connected to everything else."
In the Pacific Northwest, some of the key predatory species are lingcod, some larger rockfish and other groundfish.
The findings may help explain why some fish populations undergo such dramatic changes either naturally or when pressured by external forces such as fishing, Hixon said. It's not unusual for fish populations within a certain species and location to vary by as much as 10 to 100 times.
However, when there's a proper and natural balance between a species, its competitors and its predators, Hixon said, there is much less risk of population collapse or regional extinction.
The study was funded by a 4-year, $400,000 grant from the National Science Foundation, and also the National Undersea Research Program of the U.S. National Oceanic and Atmospheric Administration.
In continuing research, OSU scientists hope to study these processes within the context of marine reserves, and see if the maintenance of balanced and healthy fish populations in such reserves can have a positive influence on the availability of species elsewhere, including those sought in commercial or recreational fisheries outside the reserves.
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