Ecology's neural theory which sees random fluctuation as the decisive factor in structuring communities, has to be abandoned, as a census of trees in rainforests on three continents confirms that competition plays the key and central role in structuring communities. 

Ecologists are still arguing about the nature of the factors that determine the species composition of ecological communities. Those who view interspecies competition as the key element argue with ecologists who postulate that random fluctuations in population structure and rates of species dispersal play the dominant role, particularly in the biological communities found in species-rich tropical rainforests
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LMU biologist Professor Susanne Renner, (above right) who is director of the Botanic Garden and herbaria in Munich, and Professor Robert E. Ricklefs (left) of the University of Missouri in St. Louis have now analyzed data from censuses of tree species in rainforests around the globe and taken advantage of fossil evidence, to allow them to chart diversity in both space and time.

Their findings show that variation in species richness among families is very similar in all tropical forests in spite of millions of years of independent evolution and diversification.

This  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 ecological niche 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 (left) 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.

 Renner and 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 tree species growing in plots of between 25 and 55 hectares.  They also 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 community structure 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 species richness, the rankings that emerge are very similar on all three continents.

“The correlation is statistically highly significant,” says Renner. “So we have uncovered a verysubstantial 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.”

MYTH OF FLIGHT
It’s one of the most tenacious myths in physics and it frustrates aerodynamicists the world over. University of Cambridge’s Professor Holger Babinsky, from Department of Engineering, has created a 1-minute video that he hopes will finally lay to rest a commonly used yet misleading explanation of how wings lift.

“A wing lifts when the air pressure above it is lowered. It’s often said that this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air travelling along the lower, flat surface. But this is wrong,” he explained.

“I don’t know when the explanation first surfaced but it’s been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics.”

To show that this common explanation is wrong, Babinsky filmed pulses of smoke flowing around an aerofoil (the shape of a wing in cross-section). With a paused video, it is clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing, by the time the flow below the aerofoil reaches the end of the lower surface.

“What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface,” says Babinsky, from the Department of Engineering.

“This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”

Babinsky is quick to stress that he is far from the only aerodynamicist who is frustrated by the perpetuation of the myth: colleagues have in the past expressed their concerns in print and online. He hopes his video  will help debunk the myth once and for all,  by providing a quick, visual demonstration to show that the most commonly used explanation cannot possibly be correct.

The original video, he created using a wind tunnel, has been re-edited in high quality with a voice-over in which he explains the phenomenon as it happens.

Babinsky’s research projects revolve around applications such as Formula 1, micro-air vehicles (flapping wings), vertical axis wind turbines, shock control for transonic wings and supersonic engine inlets.

One of his visions is to design a wing that will enable aircraft to fly faster and more efficiently. Using a massive wind tunnel within the Department of Engineering, Babinsky and his team have been modelling the shockwaves that are created on aircraft wings and that restrict the plane’s top speed.

Babinsky will be giving lectures as part of a series of University of CambridgeSubject Masterclasses aimed at Year 12 school children: “It’s important to put out this video because when I give this lecture to school kids I start by giving the wrong explanation and asking who has heard it and every time 95% of the audience puts their hand up. Only a handful know that it is wrong.”