Diversity is better than monotony. But what difference does the presence of one, six or twelve tree, bird or bacterial species make to an ecosystem and its ongoing processes?
In a study published in the Proceedings of the National Academy of Sciences, centre researcher Ingo Fetzer and colleagues from the Helmholtz Centre for Environmental Research (UFZ) in Leipzig, Germany have discovered that species that play a minor role in how an ecosystem functions in favourable environmental conditions may assume key roles in unfavourable environments.
Ingo Fetzer explains that some species perform functions in the ecosystem that are often only required if harsh environmental conditions occur.
"We have been able to demonstrate in microbial communities that the species' level of interdependence varies in terms of prevalent environmental conditions. In our studies, this interdependence increased with worsening conditions."
Ingo Fetzer, co-author
Impaired through absence
It has been known for quite some time that biodiversity facilitates processes in an ecosystem, e.g. biomass production or metabolic processes. It is also known that several species may overlap in terms of the tasks they perform in the ecosystem, so they are functionally redundant. Therefore, if only redundant species are lost, the ecological processes can continue to be sustained.
However, if essential species are absent from the ecosystem and their role cannot be compensated by others, these processes will be impaired.
"We now know that this crucially depends on the respective environmental conditions," Fetzer explains.
If species were redundant in good environmental conditions, their loss may even be considered bearable. Yet this situation could change very quickly if the prevalent conditions worsen. Individual species would suddenly assume completely new roles in the ecosystem to keep the processes going.
In extensive laboratory experiments, Fetzer and his colleagues exposed almost 900 microbial mixtures with various numbers and compositions of species to three different environmental conditions: good conditions in which many of the organisms thrive; less favourable conditions; and bad conditions with a very high salt concentration.
They observed how much biomass the individual communities of species produced as an indicator of ecosystem functioning.
The three scenarios revealed that the higher the biodiversity in the "model ecosystem", the more biomass was formed. Dr. Chatzinotas, leader of the Microbial Systems Ecology working group at the UFZ, explains one particularly exciting discovery:
For an ecosystem to function key species strongly depend on certain specific partners.
Dr. Chatzinotas explains that "Under bad conditions, none of the species could grow alone, but production suddenly began when certain other species were added to the ecosystem."
The reason for this may be that a diverse ecosystem has more species on hand to perform key roles after environmental changes. Above all, biodiversity also provides the basis for new and crucial interactions between the species. For example, one species' metabolic products could be used by another species.
Time for more extensive testing
Laboratory test systems with microorganisms are particularly suitable for investigating these kinds of fundamental ecological issues. Researchers can use them to perform a large number of experiments with many repetitions on a very small scale. It is also much easier to control the experimental conditions and keep them comparable than in a field experiment.
Ingo Fetzer concludes that more research now needs to be conducted to reveal whether these findings can be applied to higher organisms, animal and plant communities.
"It is crucial that the value of biodiversity must not only be assessed in favourable external conditions, but also in harsh conditions."
Fetzer, I., Johst, K., Schäwe, R., Banitz, T., Harms, H., Chatzinotas, H. 2015. The extent of functional redundancy changes as species’ roles shift in different environments. Proceedings of the National Academy of Sciences PNAS online, November 17, 2015, doi:10.1073/pnas.1505587112
Ingo Fetzer is a postdoctoral researcher at the centre. He has strong interest in whole system processes and investigates the interaction of multiple stressors across scales - global to local - and their interactions on systems' stabilities. This includes identifying relevant factors, examination and visualization of large datasets, and investigation of system dynamics applying various statistical methods.
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