Mar. 14, 2008 The productivity and biodiversity of an ecosystem is significantly affected by the rate at which organisms move between different parts of the ecosystem, according to new research in Nature. Scientists hope that understanding the mechanisms which determine the diversity and productivity of ecosystems will help ecologists and conservationists to develop strategies to ensure that conservation areas are highly productive and rich in biodiversity.
The study used a lab-based artificial ecosystem of communities of bacteria to examine what happens when the bacteria move around and evolve to live in different parts of the ecosystem over the course of hundreds of generations. The scientists measured the effect this dispersal of species has on the productivity and biodiversity of the ecosystem over all.
'Productive' ecosystems are defined as those that support a large total amount of living matter, from tiny microbes up to plants and animals. Scientists refer to this measurement of the amount of life present as an ecosystem's 'biomass'. A number of studies in the last decade have shown that ecosystems that have a high biodiversity - meaning they are rich in variety of species - are also highly productive over short time scales, but until now the underlying processes creating this link between high levels of biodiversity and productivity over evolutionary time scales have not been understood.
The scientific team behind this new research found that both the biodiversity and productivity of an ecosystem are at a peak when there is an intermediate rate of dispersal of species - not too little and not too much - between different parts of the ecosystem.
When there is little or no dispersal, populations of species that remain in harsh areas of an ecosystem are unable to adapt to their environment due to a low population size and lack of genetic variation. Conversely, when there is too much dispersal in an ecosystem, species evolve to be 'generalists' that can survive in many habitats, but fail to thrive in any given one.
Dr Craig Maclean, one of the authors of the March 13 study at the NERC Centre for Population Biology at Imperial College London, explains that an intermediate rate of dispersal creates a 'happy medium' wherein species move around enough to ensure that harsh environments are adapted to, but not so much that they become generalists.
He says: "Dispersal constantly brings new individuals and new genes into harsh environments, which is essential for evolutionary adaptation to difficult environments. When species adapt to new environments it increases the productivity of the ecosystem and it can increase the biodiversity, as movement between different parts of an ecosystem provides more 'niches' for species to exploit."
To carry out the study, the research team created an artificial ecosystem for the bacterium Pseudomonas fluorescens. The ecosystem consisted of 95 different areas, each one containing a different food source. The scientists introduced the bacteria - which could eat approximately half of the 95 food sources - to the ecosystem, and then began to manipulate the rate at which the bacteria dispersed between the 95 different areas.
Every day during the experiment, the team measured the biomass in the ecosystem as an indicator of the ecosystem's productivity, and found that the levels of biomass were highest when there was an intermediate dispersal rate.
After 400 generations, the team isolated bacteria from the ecosystem and measured the ability of the bacteria to grow on each of the food sources. Using this data, the team were able to measure the diversity of the ecosystem, as it indicated how many different species had evolved from the bacteria which were originally introduced to the experiment, which could only eat half of the food sources available.
The research was carried out by an international team, led by Centre National de la Recherche Scientifique scientists at Montpellier 2 University in France, in collaboration with Imperial College London and the University of Liverpool.
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