Uncover the secrets of species abundance

Sri Lankan hanging parrot (Lauriculus perellinus) lives only in Sri Lanka. It is a very rare species worldwide, which means there are only a few individuals. Credit: Corey Callahan

Over the years, global observations of biodiversity have revealed a potentially consistent pattern indicating how many species are common, extremely rare, or fall somewhere in the middle.

Natural observations over a century have revealed a consistent pattern in… Classify Abundance: Although most species are rare, they are largely not, with a few being very common. So-called global species abundance distributions have become completely exposed for some well-monitored species groups, such as birds.

As for other species groups, such as insects, the veil remains partly unlifted. These are the findings of an international team of researchers led by the German Center for Integrative Biodiversity Research (iDiv), the Martin Luther University Halle-Wittenberg (MLU), and the University of Florida (UF), published in the journal Nature ecology and evolution. The study shows how important it is to monitor biodiversity to discover the abundance of species on Earth and understand how they are changing.

“Who can explain why one species has a wide range and is so numerous, and why another similar species has a narrow range and is rare?” This question was posed by Charles Darwin in his groundbreaking book On the Origin of Species, published more than 150 years ago. A related challenge is understanding the number of common (multiple) species and the number of rare species, which is called the global species abundance distribution (gSAD).

Tiger (Tiger Bardus) It is a rare to medium species. Credit: Corey Callahan

Two main models of gSAD have been proposed in the last century: R.A. Fisher, a statistician and biologist, proposed that most species are very rare and that the number of species is decreasing relative to the more common ones (the so-called log series model). On the other hand, F. W. Preston, an engineer and ecologist, argued that only a few species are actually very rare and that most species have an average level of commonness (the so-called lognormal model). However, until now, despite decades of research, scientists have not known a model that describes the planet’s true gSAD.

Solving this problem requires huge amounts of data. The study authors used data from the Global Biodiversity Information Facility (GBIF) and downloaded data representing more than 1 billion observations of species in the wild from 1900 to 2019.

“The GBIF database is a great resource for all types of biodiversity-related research, especially because it brings together data collected from professional and citizen scientists from around the world,” says first author Dr Corey Callahan. He began studying while working at iDiv and MLU and now works at UF.

The global species abundance distribution (gSAD) has been fully revealed for birds and shows a potentially global pattern: there are a few very rare species such as the Sri Lankan hanging parrot, many rare species such as the northern goshawk, and a few common species such as the house sparrow. This pattern was first proposed by F. W. Preston in 1948. Source: Gabriel Rada (illustration), Corey Callahan (photos)

Callahan and his fellow researchers divided the downloaded data into 39 groups of species, for example, birds, insects or mammals. For each, they compiled the global species abundance distribution (gSAD).

The researchers discovered a potentially global pattern, which emerges once the distribution of species abundance is fully revealed: most species are rare but not very rare, and only a few species are very common, as would be expected in a lognormal model. However, the researchers also found that the veil has only been fully lifted for a few species groups such as cycads and birds. For all other species groups, data are not yet sufficient.

“If you don’t have enough data, it’s as if most species are too rare,” says lead researcher Professor Henrique Pereira, head of the research group at iDiv and MLU. “But by adding more and more observations, the picture changes. You start to see that there are, in fact, more rare species than very rare species. You can see this shift for cycads and birds when you compare observations of species since 1900, when data were available Less, with the most comprehensive species observations we have today. It’s remarkable: we can clearly see the phenomenon of revealing the entire distribution of species abundances, as predicted by Preston several decades ago, but only now demonstrated on the scale of the entire planet.

“Although we have been making observations for decades, we have only uncovered a few groups of species,” Callahan says. “We still have a long way to go. But the Global Biodiversity Information Facility and data sharing really represent the future of biodiversity research and monitoring for me.”

The results of the new study allow scientists to evaluate the extent to which gSADs are detected for different species groups. This allows another long-standing research question to be answered: How many species are there? This study found that while almost all species have been recognized for some groups such as birds, this is not the case for other taxa such as insects and birds. Cephalopods.

The researchers believe their findings may help answer Darwin’s question about why some species are rare and others common. The global pattern they found may indicate general ecological or evolutionary mechanisms governing the prevalence and rarity of species.

As more research is done, humans continue to alter the planet’s surface and species abundance, for example, by making common species less common. This complicates the researchers’ task: they need to understand not only how species abundances evolve naturally, but also how human impacts change these patterns simultaneously. There may still be a long way to go before Darwin’s question is finally answered.

Reference: “Uncovering Global Species Abundance Distributions” by Corey T. Callahan, Luis Borda de Agua, Roel van Klink, Roberto Rosi, and Henrique M. Pereira, September 4, 2023, Nature ecology and evolution.
doi: 10.1038/s41559-023-02173-y