Data gathering: most species are medium-rare

Species abundance – the number of individuals in a given species – has a big influence on biodiversity and conservation.

But ecologists don’t have a firm grasp on the pattern of species’ abundance. Now an international team of researchers has drawn on more than a billion observations to figure out the global abundance of flora, fauna and fungi.

Publishing their findings in Nature Ecology & Evolution, the researchers’ conclusion is that most species are rare, but not too rare.

The researchers used records from the Global Biodiversity Information Facility (GBIF) for their analysis, collecting data from 1900 to 2019.

“The GBIF database is an amazing resource for all sorts of biodiversity related research, particularly because it brings together both data collected from professional and citizen scientists all over the world,” says first author Dr Corey Callaghan, now at the University of Florida, US.

They divided this data into 39 groups (including things like “birds” or “insects”), aiming to calculate the “global species abundance distribution” (gSAD).

Distribution of species showing most are rare but few are very rare or common
The global species abundance distribution (gSAD) has been fully unveiled for birds and shows a potentially universal pattern: There are a few very rare species like the Sri Lanka Hanging-Parrot, many rare species like the Northern Goshawk, and few common species like the House Sparrow. This pattern was first proposed by F. W. Preston in 1948. Credit: Gabriele Rada (illustration), Corey Callaghan (photos)

They found it mirrored the predictions of engineer and ecologist Frank W. Preston, who proposed in the mid-20th century that global species follow a “log-normal” model, where a few species are common or extremely rare, and most are in the middle.

“If you don’t have enough data, it looks as though most species are very rare,” says senior author Professor Henrique Pereira, research group head at the German Centre for Integrative Biodiversity Research and the Martin Luther University Halle-Wittenberg, both in Germany.

Sri lanka hanging parrot
The Sri Lanka Hanging-Parrot (Loriculus beryllinus) lives only in Sri Lanka. It is globally a very rare species, meaning there are few individuals. Credit: Corey Callaghan

“But by adding more and more observations, the picture changes. You start seeing that there are, in fact, more rare species than very rare species. You can see this shift for cycads and birds when comparing the species observations from back in 1900, when less data was available, with the more comprehensive species observations we have today.

“It is fascinating: we can clearly see the phenomenon of unveiling the full species’ abundance distribution, as predicted by Preston several decades ago, but only now demonstrated at the scale of the entire planet.”

The researchers now have a new question: how many species are there? For some things like birds, most species have already been named and recorded, but this isn’t true of insects or many marine creatures.

The leopard (Panthera pardus) is a rare to intermediate species. Credit: Corey Callaghan

“Even though we have been recording observations for decades, we have only lifted the veil for a few species groups,” says Callaghan.

“We still have a long way to go. But GBIF and the sharing of data really represents the future of biodiversity research and monitoring, to me.”

The researchers also hope that their data will help to understand which evolutionary and ecological processes influence species abundance, and how human activity is affecting it.

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