For over a century, scientists have embarked on a journey through the natural world, seeking to unravel the intricate tapestry of species abundances. This quest has uncovered a universal pattern: most species are rare, but not exceedingly so, while only a select few enjoy the status of being truly common. This intriguing revelation, known as global species abundance distributions (gSAD), has shed light on our understanding of biodiversity. However, the curtain of knowledge remains only partially lifted for certain species groups, leaving us with more questions than answers.

A multinational team of researchers, spearheaded by the German Center for Integrative Biodiversity Research (iDiv), Martin Luther University Halle-Wittenberg (MLU), and the University of Florida (UF), has delved into this enigmatic world. Their groundbreaking findings, published in the esteemed journal Nature Ecology and Evolution, emphasize the critical role of biodiversity monitoring in unraveling the mysteries of species abundances on our planet.

The inquiry into species abundance is not a recent endeavor, dating back to the time of Charles Darwin and his groundbreaking work, “The Origin of Species,” over 150 years ago. Darwin’s curiosity spurred him to ponder why certain species roam far and wide in vast numbers, while their close relatives confine themselves to narrow ranges, maintaining a state of rarity. This question gave birth to the concept of gSAD.

Throughout the last century, two main models have vied for supremacy in explaining gSAD. The first, proposed by R. A. Fisher, a pioneering statistician and biologist, posits that most specie are exceedingly rare, with a steady decline in numbers as species become more common (known as the log-series model). In contrast, F. W. Preston, an engineer and ecologist, contended that only a handful of species are genuinely rare, with the majority occupying an intermediate level of commonness (termed the log-normal model). Until now, despite decades of exploration, scientists have grappled with determining which model best characterizes our planet’s true gSAD.

Solving this riddle necessitates an abundance of data. The researchers harnessed the formidable power of the Global Biodiversity Information Facility (GBIF), amassing over a billion species observations from 1900 to 2019, contributed by both professional scientists and citizen scientists worldwide.

Dr. Corey Callaghan, the study’s lead author, enthuses about the invaluable resource that is the GBIF database. He notes its role in facilitating diverse biodiversity research, as it aggregates data from across the globe. Originally affiliated with iDiv and MLU, Dr. Callaghan is now based at UF.

To comprehensively tackle this complex puzzle, Callaghan and his colleagues categorized the data into 39 distinct species groups, spanning birds, insects, mammals, and more. Within each group, they meticulously constructed the corresponding gSAD.

The researchers unveiled a potentially universal pattern, which comes into focus when gSAD is completely disclosed: most species are rare, though not extremely so, while only a select few bask in the glory of true commonness—a phenomenon predicted by the log-normal model. Intriguingly, the researchers observed that this veil had been entirely lifted for only a handful of species groups, such as cycads and birds. For all other groups, the data remained insufficient.

As Professor Henrique Pereira, the senior author and research group head at iDiv and MLU, explains, insufficient data can mislead observers into believing that most species are exceedingly rare. However, with the accumulation of additional observations, a shift occurs, revealing a greater prevalence of rare species compared to the truly rare ones. This phenomenon, initially theorized by Preston decades ago, is now being demonstrated on a global scale.

While significant progress has been made, Callaghan reminds us that there is still much work ahead. He underscores the pivotal role of GBIF and data sharing as the future of biodiversity research and monitoring.

The study’s findings offer a tool for scientists to assess the extent to which gSADs have been unveiled across various species groups, leading to the age-old question: how many species inhabit our planet? While some groups, such as birds, have identified nearly all of their species, others, like insects and cephalopods, remain enigmatic.

The researchers believe that their discoveries may hold the key to answering Darwin’s inquiry regarding the commonness and rarity of species. The universal pattern they unearthed may point to overarching ecological or evolutionary mechanisms governing these distinctions.

As research continues, the impact of human activities on the planet’s surface and species abundance cannot be ignored. Human actions are reshaping the landscape, often diminishing the commonness of once-abundant species. This added layer of complexity challenges researchers to not only comprehend natural patterns but also understand how human influence is reshaping them. In the quest to answer Darwin’s age-old question, we find ourselves on a journey of discovery that may stretch far into the future.