333 – Biodiversity models in question
November 2009
Many species manage to coexist in the same ecosystem. How do they do this and share the available resources? Since Darwin and his species theory 150 years ago, ecologists have been seeking to determine the factors favouring diversity in nature. In almost a century of research, the most strongly supported hypothesis has been the niche theory. This model postulates that a species occupies an ecological niche( 1)
of its own, to which it is well adapted. However, a second hypothesis, the neutral theory, with diametrically opposing principles, has come to the fore since the early 2000s. According to this the various species in the ecosystem have equivalent roles.
A population study on marine phytoplankton( 2), led IRD researchers and their partners( 3) to call this dichotomy into question. They put forward a new intermediary model, combining the two main theories, to explain biodiversity and, in the long term, help preserve it.
One living species disappears every 20 minutes. This rate is 100 to 1000 times greater than any recognized for pre-human times on Earth. An increasing number of scientists consider our century to be the time of the 6th great extinction, by reference to the cataclysms that triggered the disappearance of the dinosaurs, 65 million years B.P., or the earlier extinction of the Precambrian fauna, 500 million years B.P. The current extinction is affecting particularly strongly the tropics, the zone where the most species-rich communities occur. Conservation of the richness of the living world for a sustainable management of natural resources is one of the major challenges for the development of the countries of the South. Improved protection of this diversity requires understanding of the basic ecological principles that govern them. How can species live in cohabitation? How do the different species divide up the space and resources? After more than a century of research, scientists still do not have a good understanding of how diversity is maintained in the same ecosystem.
The niche theory: where adaptation involves domination
Since Darwin’s species theory of 1859 and his principle of natural selection which would favour the species best adapted to the habitat, ecologists have been asserting the niche theory. For them this hypothesis explained the irregular distribution of species in an ecosystem. This model supposes that a species possesses its own specific ecological niche1 in which it is dominant. This means it occupies a habitat and fulfils a trophic function, in other words a well-defined role in the ecosystem’s food regime (as prey or predator). According to this theory, two species with identical niches cannot coexist. Only species that are sufficiently different, notably in the use of resources to avoid competition, can share the same habitat.
Nevertheless, in nature, especially in high-diversity ecosystems, coexistence can be observed between two, three or even more competing species.
The neutral theory: perfect harmony
To explain how this biodiversity is maintained, researchers therefore opted for a completely opposing principle: the neutral theory. This is based on the hypothesis that all species, situated in the same environmental conditions, show the same performances in birth rate, death rate, dispersion and speciation. Overall, none of them is dominant over the others. There is no competition, no interspecific exclusion. Only chance brought by colonization-extinction processes could induce the observed disparities in distribution.
Do these two models that represent opposite ends of the spectrum, between them embrace a real biological situation? Previous studies have shown that they are not incompatible: nature corresponds to a combination of the two theories.
The two processes combined
IRD researchers and their partners( 3) are providing the first empirical proof, based on a real population, that on their own neither of these two models can explain the diversity within a particular community. Simulation of the distribution of different species of phytoplankton( 2)
within an ecosystem was performed using 12 years’ microorganism abundance data collected off Plymouth in the English Channel by the Western Channel Observatory in England. Phytoplankton is a good example for studying the combined influence of the two types of processes because the various constituent species appear to have similar functions in the ecosystem (the same nutrients, predators, hydrodynamic conditions and so on) and respond to their environment in a predictable way. Competition for nutrients depends totally on the species size: small organisms dominate in conditions of low nutrient concentrations, the larger ones in high concentration environments. The simulation showed the distribution of the different species in the ecosystem to be dependent at once on neutral processes and on other ecological niche-based ones.
This research work revealed clues to a better understanding of how different species coexist in a community. In a changing global context, they could in the long term help elaborate scenarios for possible changes and developments in ecosystems and thus offer countries of the South better possibilities for preserving their natural heritage with the aim of assuring sustainable management of their resources and development .
1. The ecological niche of a species defines its habitat --in other words the environmental conditions it requires--, its role in the ecosystem and position in the food chain.
2. Phytoplankton consists of the plant microorganisms in a plankton community which live in suspension in the surface waters. They feed off nutrients carried in the water.
3.This work was conducted jointly with researchers from the University of Sheffield in the United Kingdom and Simon Fraser University at Burnaby in Canada.
Redaction DIC – Gaëlle Courcoux
