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Plant species can fulfil different functions within an ecosystem, even if they are closely related to each other. This surprising conclusion was reached by a global analysis of around 1.7 million datasets on plant communities. The study was led by Martin Luther University Halle-Wittenberg (MLU) and the University of Bologna. Their findings overturn previous assumptions in ecology. The study was published in "Nature Ecology & Evolution" and offers insights for nature conservation.
When a new plant species wants to populate a new ecosystem, it has to compete with other inhabitants for light, nutrients and water. It would therefore make sense for the different species to stay out of each other’s way so that they can fulfil different functions in the ecosystem. This variation in the plants’ functional diversity would presumably also be reflected in their phylogenetic diversity, i.e. how closely they are related to one another. "Until now, scientists have assumed that, in an ecosystem, there is a positive correlation between the plant species’ functional plant traits, such as height or leaf structure, and their phylogenetic diversity, in other words, the more distantly related the species are in the ecosystem, the more their functional traits should differ," explains Professor Helge Bruelheide, a geobotanist at MLU. One example of this is found in mixed forests, which contain coniferous evergreen tree species whose ancestors lived over 300 million years ago. Deciduous tree species, whose direct ancestors are not even half as old, live closely alongside them. Ferns, whose ancestors are even older, populate the ground below them. "In forests with such a high phylogenetic diversity, we would also expect to find a high functional diversity," says Bruelheide. However, the new study has found that this correlation most likely applies to mixed forests in Northern Europe, but not to the majority of terrestrial ecosystems.
The team of international scientists analysed 1.7 million datasets from the world’s most unique vegetation database, 'sPlot’. This database is hosted by the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig. It contains vegetation records of plants from 114 countries and from all climate zones on Earth. The researchers combined these data with a global phylogeny of all plant species and the world’s largest database of plant traits, 'TRY’. "The result came as a complete surprise to us as we discovered that there is no positive correlation between functional and phylogenetic diversity. In fact, the two are often negatively correlated," explains Georg Hähn from the University of Bologna, who began working on the study as part of his master’s thesis at MLU.
A more detailed breakdown of the results shows that more than half of the vegetation samples examined had a high functional diversity but only a low phylogenetic diversity. Only around 30 percent of the samples displayed either a high or low level of both types of diversity at the same time. It was particularly surprising that more than half of the plots had a higher level of functional than phylogenetic diversity. "Our study shows that plants in many ecosystems fulfil different tasks even though they are closely related. This has important implications for nature conservation," says Helge Bruelheide. An ecosystem could therefore be vulnerable to climate change if it has either an insufficient number of functionally diverse species or a lack of evolutionary diversity. "Therefore, effective environmental protection means more than just protecting the most species-rich sites. Instead, both functional and phylogenetic diversity must be considered," concludes Bruelheide.
The study was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation).
Study: Hähn G et al. Global decoupling of functional and phylogenetic diversity in plant communities. Nature Ecology & Evolution (2024) doi: 10.1038/s41559-024-02589-0
https://www.nature.com/articles/s41559-024-02589-0
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