Study of the strength of interactions between predator and prey shows intermediate species have large effects
New research at Scripps Institution of Oceanography at the University of California, San Diego, has shown that in nature, size may not necessarily matter as much as we think.
Scripps scientists Enric Sala and Michael Graham have produced a new study showing that so-called "intermediate" players in natural communities can often have as much and greater impacts than larger species.
Sala and Graham conducted field experiments and lab studies evaluating interactions between predators and prey (also called "interaction strengths") in a kelp forest community. They found that previous conclusions about interaction strength in natural communities may not be as general as some believe, and that intermediate-sized predators, pound for pound, can represent the ecosystem’s most significant consumers.
The research is featured as the cover story of the March 19 issue of the Proceedings of the National Academy of Sciences.
"In this paper we have shown that species that were previously overlooked can also be important players," said Sala, deputy director of the new Center for Marine Biodiversity and Conservation at Scripps. "Although these species may be less important at certain time periods because of their size and interaction strength, they may have been important in the past or will be important in the future, as natural and unnatural shifts in density occur."
The research addresses the importance of biological diversity, or "biodiversity," the number and types of species in an environment. Some argue that only a few species, such as the "keystone predators," are most important in ecological communities because they impose the strongest effects. Other species, which are labeled as "redundant" under this idea, are not necessary in a given environment. If these species become extinct, the notion contends, other species will simply move in to take their place and function, and the ecosystem will not suffer significant changes.
Sala, Graham, and others, however, believe the opposite. They believe that the complexities of ecosystems are not immediately evident and that every species is important for the stability of ecosystems.
"The conservation of (marine) biodiversity should focus on all species, not only on the larger species or those we believe are more important," says Sala. "Of course, more urgent conservation actions should focus on species that are threatened (by fishing, for example), but let’s not forget the rest for the long term."
Although the new study focuses on the marine environment, Sala believes the findings can be extended to terrestrial systems.
In their study, Sala and Graham were faced with the historical problem that experimentally and logistically it is difficult to quantify the strength of the interaction between species. Because food webs are complex systems, Graham said their study concentrated on one layer in the kelp forest, the plant eaters, or herbivores.
In the first phase of the study, Sala and Graham conducted bibliographic research, field experiments, and observations in kelp forests off Point Loma (a coastal community in San Diego, Calif.) that helped identify various herbivores that feed on small kelps.
The research then moved into experimental aquariums at Scripps Institution, where Sala and Graham could control the behavior of nine herbivore species. By devising experiments with and without herbivores, they were able to observe the maximum effect of herbivores on small kelps. In the field herbivores can feed on a variety of choices, but in the controlled laboratory environment Sala and Graham were able to carefully track the full extent of the animal’s impact on the kelp, or the maximum effect of the predator on prey.
"We used information on the densities and abundances of all the species that can be found in California kelp forests and we estimated the effects of the total herbivore population," said Sala. "We saw that smaller species that previously were not considered strong interactors in the community can have similar and even larger effects than larger species."
In one example, Sala and Graham found that one kilogram of large red sea urchins removed up to approximately 12 percent of small kelp per square meter per day. A similar amount of purple sea urchins, a much smaller species, can remove up to 25 percent per square meter per day, or more than double.
"Being bigger means you will eat more, but you will not necessarily be able to effectively eat all the small stuff," says Sala. "This research emphasizes the importance of various species levels."
"Tiny crustaceans that you can barely see can seem trivial, but they can have a large role," said Graham, currently at the UC Davis Center for Population Biology. "One crustacean might not seem important, but en masse they can have a long-term ecological effect. So studying diversity is very important, especially in figuring out who is doing what within the community."
Funding for the research was provided by the Catalan Government (Commission for Universities and Research), a Professional Association of Diving Instructors Foundation grant, and a University of California Faculty fellowship.
Scripps Institution of Oceanography at the University of California, San Diego, is one of the oldest, largest, and most important centers for global science research and graduate training in the world. The scientific scope of the institution has grown since its founding in 1903.
A century of Scripps science has had an invaluable impact on oceanography, on understanding of the earth, and on society.
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Now plunging boldly into the 21st century, Scripps will celebrate its centennial in 2003.