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Nature's puzzle: Why are some species rare and others are common?

At the core of Earth’s biodiversity lies a fundamental question: Why do certain species thrive in abundance, while other species are dangerously rare? A recent groundbreaking study has shed light on this mystery, offering new insights into a debate that has been ongoing since the time of Darwin.

A pattern, long observed in nature, has become more evident with over a century’s worth of data: Most species are, intriguingly, rare, yet not exceedingly so. Meanwhile, only a select few species dominate in numbers. 

Species distribution 

This phenomenon, known as the global species abundance distribution (gSAD), has been documented more conclusively for certain species, like birds. 

However, when it comes to insects and several others, the clarity fades. This was the primary discovery made by an international team led by the German Centre for Integrative Biodiversity Research (iDiv).

The research, published in the journal Nature Ecology and Evolution, highlights the vital role of biodiversity monitoring in capturing the diverse essence of life on our planet.

The enigma of species distribution is not new. Charles Darwin, over 150 years ago, pondered in his revolutionary work, “The Origin of Species”: “Who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare?” 

Two primary models

This query sparked the exploration of the gSAD, leading to the proposition of two primary models in the last century.

The first, introduced by R. A. Fisher – a renowned statistician and biologist – asserted that a vast majority of species are exceptionally rare. This perspective, known as the log-series model, implies that common species are a minority. 

On the other hand, F. W. Preston, both an engineer and ecologist, brought forth the log-normal model, suggesting that truly rare species are few and far between, with most exhibiting an intermediate level of abundance. Yet, until this recent study, the scientific community remained divided on which model truly depicted Earth’s gSAD.

Focus of the study 

To address this conundrum, the researchers analyzed data from the Global Biodiversity Information Facility (GBIF), which contains over a billion species observations spanning from 1900 to 2019. 

Dr. Corey Callaghan, the study’s lead author, extolled the GBIF database, acknowledging its significance in “bringing together both data collected from professional and citizen scientists all over the world.” The data was meticulously categorized into 39 species groups, from birds to mammals, each with its unique gSAD.

What the researchers learned 

The analysis revealed a potential universal pattern: most species are rare, but not exceedingly so, a finding consistent with Preston’s log-normal model. However, the complete gSAD picture emerged only for specific groups, like birds and cycads. For others, the data remained limited.

Professor Henrique Pereira, the study’s senior author, remarked on the intriguing shift witnessed with increasing data: “You start seeing that there are, in fact, more rare species than very rare species. This shift, evident for birds and cycads, illustrates the unveiling of the full species abundance distribution as Preston envisaged.”

Yet, as Callaghan emphasized, there’s a vast expanse of biodiversity yet to be charted. “Even though we have been recording observations for decades, we have only lifted the veil for a few species groups,” he said.

Study implications 

While the new study has made strides in unveiling gSADs for varied species, it has also opened doors to another profound question: Just how many species inhabit our planet? It appears that while some groups, like birds, have been exhaustively documented, others, including insects and cephalopods, remain shrouded in mystery.

These recent revelations not only bring us closer to answering Darwin’s question but also hint at the possible ecological and evolutionary mechanisms shaping species’ rarity and abundance. Yet, this challenge will become much more complicated as human actions continue to reshape Earth’s biodiversity.

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