Competition is at the root of diversity in rainforests: study

January 26, 2012, Ludwig-Maximilians-Universität München

Another attractive theory falls foul of the facts. A census of trees in rainforests on three continents has confirmed that competition plays a central role in structuring communities. This contradicts the so-called neutral theory in ecology, which views random fluctuations as the decisive factor.

Ecologists are still arguing about the nature of the factors that determine the composition of ecological communities. On the one hand, there are those who view interspecies competition as the key element. A second group of influential ecologists postulates that in and rates of species dispersal play the dominant role, particularly in the biological communities found in species-rich . LMU biologist Professor Susanne Renner, who is Director of the Botanic Garden and herbaria in Munich, and Professor Robert E. Ricklefs of the University of Missouri in St. Louis have now analyzed data from censuses of tree species in rainforests around the globe and also taken advantage of , allowing them to chart diversity in both space and time. Their findings show that variation in species richness among families is very similar in all in spite of millions of years of independent evolution and diversification. This correspondence strongly suggests that community structure in rainforests cannot be attributed to the action of stochastic factors. "The high degree of similarity was a surprise even to us," says Renner. "The results can be regarded as a nail in the coffin of the neutral theory."

In even the best habitats, resources are inevitably limited. This means that species must compete with each other for access to them. And for many ecologists, interspecies competition for resources is the critical factor that determines the composition of the community found in a given environment. According to the principle of competitive exclusion, two species that depend on the same vital resource or for their survival cannot stably coexist. The better adapted species will ultimately displace its competitor.

In contrast, what is known as "neutral" theory postulates that stochastic variations in factors such as the rate of dispersal and extinction of species determine the patterns of species abundance in different communities. The American ecologist Stephen Hubbell is the leading proponent of neutral theory, which he developed to explain species-rich communities, such as tropical rainforests.

In these environments it is not uncommon to find hundreds of tree species growing close together. Hubbell contends that this makes it very unlikely that segregation of ecological niches and the principle of competitive exclusion are the overriding forces that determine community structure. His neutral theory has received a great deal of attention in recent years.

LMU biologist Professor Susanne Renner and her American colleague Professor Robert Ricklefs have now challenged the theory with the help of quantitative data. In Central and South American, African and Asian rainforests, the two researchers compared the abundance patterns of different growing in plots of between 25 and 55 hectares. In addition, they compared the relative abundance of different families of trees in a 55- to 65-year-old fossil flora from tropical Colombia with their representation there today.

On the basis of the neutral theory, which assigns a leading role to stochasticity, one would not expect to find much similarity in over such a wide area and such a long span of time. However, the results of the new study show that when families are arranged in order of , the rankings that emerge are very similar on all three continents.

"The correlation is statistically highly significant," says Renner. "So we have uncovered a very substantial degree of agreement between the seven forest plots; even the numbers of trees per unit area that belong to a given taxonomic family are similar in all three regions. Moreover, the families with the highest species diversity in the Colombian rainforests today were already dominant 50 million years ago. The findings are astonishingly clear-cut, and should suffice to rule out the neutral theory."

Explore further: Why more species live in the Amazon rainforests

More information: Global correlations in tropical tree species richness and abundance reject neutrality, Ricklefs, R. E.; and S. S. Renner, Science Express, January 26, 2012.

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3.7 / 5 (3) Jan 26, 2012
yep, that seems obvious to me. Rare niche species never stay around for long, on geological time scales. Any species that's relatively rare compared to similar species in the same niche has already lost and will eventually either adapt to out-compete or it will be gone in short order.

Sharks are a good example. Some modern sharks are very similar to some sharks that existed millions of years ago, while specialized types of sharks have come and gone repeatedly in various time spans. Bears are the same, as well as many other species all the way up and down the food chain.
3 / 5 (4) Jan 26, 2012
Works for prosperous economies, too.

Rare niche species never stay around for long

Like big black and white bears that only eat bamboo or cute little bears that only eat leaves from one kind of tree?
2.3 / 5 (3) Jan 27, 2012
Like big black and white bears that only eat bamboo or cute little bears that only eat leaves from one kind of tree?

And like big white ones that depend on ice age climates.
1 / 5 (2) Jan 27, 2012
Like big black and white bears that only eat bamboo or cute little bears that only eat leaves from one kind of tree?

And like big white ones that depend on ice age climates.

Do they?
not rated yet Jan 28, 2012
Like big black and white bears that only eat bamboo or cute little bears that only eat leaves from one kind of tree?

And like big white ones that depend on ice age climates.

Do they?

Yes they do.
1 / 5 (1) Jan 28, 2012
Polar bear ancestors adapted to the climate, unless you believe in creation.
Polar bears, like all animals, either adapt to change or die.
1 / 5 (1) Jan 31, 2012
Yes, Polar bears are an adaptation from Brown bears, such as the grizzly or kodiak. Polar bears are known to mate with all types of Brown bears, and a polar-grizzly hybrid is called a Grolar bear. This happens both in nature and in captivity.

When Polar bears come into competition with any other Brown bear variety, the Polar bears are driven out, as is happening in Alaska and Canada right now (probably Siberia too, but who knows?). It is excessively difficult to get accurate population data. Even in the Central USA, wildlife expets are frequently surprised by things they didn't know about. One such example is the return of Mountian lions to the Kansas City area. Witnesses had been reporting them for decades but State and Federal officials dismissed the claims. Then with the introduction of 24/7 highway traffic webcams and in-car police video, there were an undeniable number of sightings indicating a substantial population. They didn't even know.
not rated yet Jan 31, 2012
One such example is the return of Mountian lions to the Kansas City area.

Adapt or die.
Why should pumas fear man if men don't hunt them?

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