Demonstrating the benefits of biodiversity
or years, ecologists have debated whether complex ecosystems (i.e., with many interacting species) are more reliable or less reliable than simple ecosystems in their ability to produce biomass, recycle nutrients, absorb carbon dioxide or perform other "services." For example, how predictably would these services be performed by a banana plantation as opposed to a rain forest?
Now, University of Minnesota ecologists, working with communities of microbes, have shown that having more species at each level such as green plants, decomposers, herbivores and predators improves the reliability of ecosystem functioning. The research supports the idea that preservation of a diversity of species is important, even if certain species appear to have no direct benefit to humans. An account is published in the Dec. 4, 1997, issue of Nature.
In the research, Shahid Naeem, assistant professor of ecology, and graduate student Shibin Li filled petri dishes with single-celled green algae, which performed photosynthesis to provide basic food for Naeem's tiny ecosystems. Ecologists call such organisms primary producers. The researchers then added bacteria whose function was to decompose the algae and other dead matter. They also added microbes such as amoebae and paramecia, which acted as consumers of the algae and each other.
With these three functional groups producers, decomposers and consumers represented, the researchers varied the number of species in each group and watched for 57 days. They measured the performance of each micro-ecosystem by how easy it was to predict one result: the amount of green algae biomass in the petri dish after 57 days. They maintained replicates of each ecosystem under two levels of light and three levels of nutrients to ensure that the results were due to interactions among organisms, not to specific levels of nutrients or energy.
At the end of the experiments, the researchers found that the amount of green algae was most predictable in those dishes that had the highest number, namely three, species in each functional group. This, said Naeem, supports the idea that complex ecosystems are more reliable in their functioning than simple ones.
"It only takes three species in each functional group to get good reliability in these experimental systems," he said. "Therefore, if you tell me that your ecosystem has three species each of producers, decomposers and consumers, I'll be able to do a good job of predicting how well your ecosystem will provide services such as producing biomass, recycling nutrients, absorbing carbon dioxide or whatever you want. This work contributes to the growing evidence based largely on studies of plants such as those by David Tilman and researchers at Stanford and the Imperial College of London that biodiversity provides a number of benefits by improving ecosystem functioning."
Although a petri dish may not look much like the real world, micro-ecosystems allow researchers to uncover ecological principles by studying real organisms, a big step above using computer simulations, Naeem said. Also, with micro-ecosystems Naeem was able to study the roles of more functional groups present in natural ecosystems that is, decomposers, herbivores and predators as well as plants than is possible in field experiments. And, since microorganisms reproduce rapidly, Naeem's 57-day experiment covered a minimum of 57 generations (many more for bacteria) in the life history of each species.
Ecosystems around the world are becoming simplified as humans convert the natural landscape to cultivated fields, orchards, parks, golf courses, and so on. Naeem's results suggest that people will find those ecosystems less reliable producers of crops or other services, he said.
|Shahid Naeem, Ecology, Evolution and Behavior Department, University of Minnesota, (612) 624-6790.|