Indices can reveal biodiversity

“Biodiversity is extremely difficult to observe”, says CIRAD ecologist Antoine Becker-Scarpitta. “It is like a constantly moving black box, with a few fleeting openings that provide a partial glimpse of reality: a colour, a shape or a light.”

At the COP16 Biodiversity Conference in Colombia, beginning on 21 October, member States are supposed to reach agreement on indicators for monitoring their commitments in terms of biodiversity. The talks look like being far from simple, as CIRAD’s Deputy Director General in charge of Research and Strategy, Sélim Louafi, points out: “Indicators are never unbiased. In terms of pesticide use, for instance, depending on the measurement chosen, the figure may go up or down. And unlike with climate, for which a single metric can be used for greenhouse gas emissions, there is no single indicator for biodiversity as a whole that is capable of encompassing everything.” The effects of erosion of the living world are often highly localized, which means putting measurements into context. Adopting transverse, shared indicators will therefore be far from straightforward. However, this is a crucial task, since the chosen system of metrics will serve to assess the degree to which the Global Biodiversity Framework goals adopted at COP15 have been achieved.

To support policymaking, research needs to document that complexity more effectively and derive explanatory patterns from it. With several  Chinese scientific partners, Antoine Becker-Scarpitta has published an article in the Journal of Biogeography that analyses biodiversity measurement indices. Based on 67 datasets from around 50 sites worldwide, the team concluded that indices were not always aligned. This “technical” result could nevertheless have significant implications in terms of conservation strategies.

“The species richness index is the most conventional”, Antoine Becker-Scarpitta says. “It measures the number of species in a given area. The abundance index looks at the number of individuals per species across the area, and serves to determine which species are rare and which dominate the ecosystem. In some cases, while the richness index shows a high level of biodiversity, the abundance index may reveal uneven distribution, and potentially species under threat of extinction.”

Understanding biodiversity composition better 

Looking at the number of species within an area has its advantages. The index is easy to capture and compare, and provides an overall understanding of the degree of biodiversity. However, looking at this factor alone risks giving a very partial picture of reality. Using that index alongside the abundance evenness index provides an idea of the structure of ecological communities, in other words of how the different species within an ecosystem are organized.

For instance, a monoculture will have a low evenness index, with a single very dominant species and several very rare species. Conversely, certain primary tropical forests will have a higher evenness index, with more even distribution of the number of individuals per species.

Across the sites studied in the article, the scientists observed that the two indices were not correlated. Depending on the taxonomic group (amphibians, insects, mammals, plants, etc), the results were sometimes similar, sometimes opposing. “No index is better than any other”, says Antoine Becker-Scarpitta. “However, what we do know is that multiplying the indexes used provides a clearer understanding of biodiversity in action, and thus facilitates conservation strategies.”

Basing conservation policies on a complex, dynamic vision of biodiversity can prove difficult. However, it is a necessary step towards establishing effective strategies. Abundance reflects the presence of a dominant species, which may in some situations be generalist, that could potentially cope better with ecological change.

By associating this index with that for the indigenousness status of species, scientists can pinpoint whether an abundant species is local or not, and if not, whether it is an invasive exotic species. In this case, it is the combination of indices that provides a better understanding of the living world, and thus of what strategies would be appropriate.

The importance of international research partnerships

This novel work was facilitated by a scientific partnership with a Chinese team, and by open-access datasets shared worldwide. Antoine Becker-Scarpitta says this type of collaboration is vital: “This type of international research programme also enables exchanges between different cultures, and thus between different approaches to biodiversity. This is far from insignificant. It is a major asset for working towards the shared objective of improving our understanding of biodiversity”.

One of the co-author Prof. Jinliang Liu from Wenzhou University says: “My previous research mainly focused on understanding how species diversity is maintained on the islands of Thousand Island Lake in China. Through this research, we leveraged global data and cooperated closely with colleagues from CIRAD among others, combining our expertise. This allowed us to extend our research from a local scale focus to a broader global scale”.

Reference

Liu, J., Becker-Scarpitta, A., Wu, C., Liu, J. Species evenness-area relationships in fragmented landscapes. Journal of Biogeography