ScienceDaily (Feb. 22, 2012)
— As scientists warn that Earth is on the brink of a period of mass
extinctions, they are struggling to identify ecosystem responses to
environmental change. But to truly understand these responses, more
information is needed about how Earth's staggering diversity of species
Curiously, a vexing modeling mystery has stymied research on this
topic: mathematical models have told us that complex ecosystems, such as
jungles, deserts and coral reefs, in which species coexist and interact
with another, cannot persist--even though they obviously do.
But now, Stefano Allesina and Si Tang, both of the University of
Chicago, have solved that vexing modeling mystery, and have thereby laid
the groundwork for improvements in the modeling of complex ecosystems
to environmental change.
The researchers' work, which was funded by the National Science Foundation (NSF), is published in this week's issue of Nature.
The tension between mathematical models of ecosystems and the
existence of Earth's rich biodiversity was first exposed about 40 years
ago by the development of a ground-breaking mathematical model that
represented the relationship between ecosystem stability and diversity;
the model was developed by Robert M. May of Oxford University.
According to May's model, ecosystems that harbor large numbers of
interacting species would necessarily be extremely unstable--so unstable
that even slight perturbations, such as variable weather and
environmental conditions, would be enough to trigger massive extinctions
within them. Therein lies a paradox: According to May's modeling, the
persistence in nature of the complex ecosystems we observe should be
Ever since May released his modeling results, scientists have been
attempting to identify factors that enable species to persist despite
the general tendency towards instability and extinctions highlighted by
May's results. Now, in their Nature paper, Allesina and Tang
explain why May's results do not accurately describe ecosystems in which
"Eat or be eaten," relationships (predator/prey relationships) are
prevalent. Allesina explains: "May's model assumes that any two species
in a large ecological network interact with one another at random, and
without any consideration of the specific type of interaction between
them, whether it is a predator-prey relationship, a mutualistic
relationship or a competitive relationship."
But in their recent research, Allesina and Tang modeled ecosystems in
which species consume each other in addition to interacting with one
another as competitors or mutualists. Their results explain why large
numbers of species do, in fact, thrive instead of necessarily going
extinct as predicted by May's model. This advance provides the
foundation for the development of increasingly sophisticated analyses of
ecosystem responses to environmental change.
Allesina believes that it is predator/prey relationships (not
competitor or mutualistic relationships) that provide the necessary
stability for almost infinite numbers of species to exist in ecosystems.
They do so by keeping the size of species populations in check at
supportable levels. Allesina explains, "When prey are high, predators
increase and reduce the number of prey by predation. When predators are
high, prey decrease and thus reduce the number of predators by
By contrast, mutualistic relationships may reinforce the growth of
large populations and competitive relationships may depress population
numbers to the point of ecological instability. Allesina says that May's
model mixed various types of species interactions but could not
represent these relationships accurately because of technical modeling
constraints that he and Tang overcame.
"The results of Allesina and Tang's network analyses are important,"
says David Spiller, an NSF program director, "because they show that the
stability properties of complex ecological systems are determined by
the type of interaction among species (predation, competition,
mutualism) and the strength of those interactions."
Allesina says that he and Tang intend to further improve their
ecosystem model by embedding into it well-known interactions that exist
between particular species. He also says that the insights gleaned
through this study may be used to improve models of other types of
networks that are unrelated to ecology, such as various types of gene
regulatory networks and chemical reactions.
Remarkably, Allesina says that he and Tang cracked the biodiversity
mystery without supercomputers or other high-tech instruments that are
so frequently at the core of current biological discoveries: "We did the
necessary calculations with just a pen and paper after finding a 1988
article on quantum physics that gave us the key to crack the problem."
The above story is reprinted from materials provided by National Science Foundation. Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Stefano Allesina, Si Tang. Stability criteria for complex ecosystems. Nature, 2012; DOI: 10.1038/nature10832