Apr. 29, 2002 For more than half a century, ecologists have been aware of the devastating effects of species loss within an ecosystem. University of Chicago researchers have found that not only the number of species lost within the system, but also the identity of the species lost plays a vital role.
"How diverse the ecosystem is and how a particular species interacts with the rest of the system is perhaps more important than the actual number of species," said Mathew Leibold, an associate professor in the department of ecology and evolution, and co-investigator of the study, which is published in the April 25 issue of Nature.
Most previous studies have shown a "saturating" effect, which suggests that an ecosystem can lose a major portion of species before it harms the remainder of the biota. That is, the system doesn't break down until it's too late.
"But we found that there is a much bigger effect initially," Leibold said. "When you start losing species, you start losing productivity right away."
"This is not an extremely surprising result," said lead author Amy Downing, a former U. Chicago graduate student who currently is an assistant professor of zoology at Ohio Wesleyan University. According to Downing, most previous studies focused on the effects of biodiversity in much simpler ecosystems.
In this study, the researchers used a more complex system, involving a larger portion of the food web. On average, ponds have four main "trophic" levels: plants (such as algae), herbivores (such as zooplankton), carnivores (insects or fish) and top carnivores (predators of other carnivores).
In the past, scientists have studied single trophic-level systems, mainly focusing on plant communities. Leibold and Downing used three of the four trophic levels in their study.
"We added a little bit of realism to our study," Downing said. "We don't have worlds that just consist of plants. We have worlds that have plants, those plants have herbivores and those herbivores have predators. When we lose a species, we lose a species that's in a food web - one that interacts with a lot of other species. So we looked at the consequences of species loss in that broader context.
"It's like a house of cards," Downing added. "As you build more stories, you're going to have a more severe collapse when you finally pluck out a card. It all depends on how complicated your house is."
The researchers constructed 84 makeshift habitats using cattle tubs, well water and species from local natural ponds, manipulating the number and the identity of the species in the containers.
Every tank started off with a diverse mixture of pond water, including algae, zooplankton and bacteria. The scientists then altered the diversity and identity of macrophytes (rooted aquatic plants), herbivores (animals, such as snails and tadpoles, that feed on plants) and carnivores (insects, such as backswimmers and dragonflies, that feed on other animals). The tanks included seven variations for each trophic level within low-, medium- and high-diversity environments.
In the tanks with less diverse environments, the ecosystems showed less productivity. "We found less biological activity," Downing said. "The animals weren't reproducing as quickly. The plant communities weren't absorbing as much sun so they weren't growing as fast. Everything was slowing down."
"None of these ponds [that we studied] have species that are at risk or are near extinction," Leibold said. "But if we can understand how they work, we might be able to better understand what the consequences are and better predict the effects humans have on the Earth's ecosystems."
Most damage to ecosystems is caused by habitat destruction, pollution and the introduction of an invasive species - all of which contributes to overall species loss.
"All of this has an affect on ecosystem-level properties," Downing said. "It alters things like how much CO2 plants can absorb, how much biomass - the volume of living organisms in a particular habitat - is produced in the system." More biomass is sometimes a good thing, she added, if, for example, it serves as a sink for carbon dioxide.
But many effects of an ecosystem remain unknown. For example, in the summertime, Chicago's water quality is affected by algae that overpopulate the water supply because of a reduction of zooplankton, which eat algae. Scientists are not sure what causes this change in zooplankton population, but it results in drinking water that smells and tastes bad for the City of Chicago.
This is just one effect from a chain of events that happens in this particular complex ecosystem, Leibold said. "When you pluck a species out of an interacting food web, the results are going to have a cascading effect and be much more dramatic."
The research, which was funded by the National Science Foundation, was conducted at the Michigan State University's Kellogg Biological Station.
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