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 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.
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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." |
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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.  |
