Decoding Darwin’s Unsolved Query: Unveiling the Mysteries of Species Prevalence

The Sri Lanka Hanging-Parrot (Loriculus beryllinus) is exclusive to Sri Lanka and is globally an exceedingly rare species, characterized by its limited population. Credit: Corey Callaghan

Decades of global research into biodiversity have suggested a recurring pattern that delineates how certain species are either frequently found, extremely scarce, or occupy a middle ground.

Over 100 years of observational studies have discovered a recurring trend in species frequency: the majority of species are uncommon, but not exceedingly so, with only a select few being highly prevalent. This pattern, known as global species abundance distributions, has been completely elucidated for specific species categories that are well-documented, such as avian species.

In contrast, for other groups of species like insects, the complete picture has not yet been fully revealed. These results have been presented by an international group of scientists, spearheaded by the German Centre for Integrative Biodiversity Research (iDiv), the Martin Luther University Halle-Wittenberg (MLU), and the University of Florida (UF), in the esteemed journal Nature Ecology and Evolution. The research accentuates the significance of regular biodiversity monitoring in understanding the fluctuations of species prevalence on Earth.

Charles Darwin pondered more than a century and a half ago in his seminal work, “The Origin of Species,” why certain species are abundant and widespread, while others that are closely related are rare and restricted. This dilemma aligns with the endeavor to comprehend global species abundance distributions (gSAD).

Two prevailing gSAD theories have emerged over the past 100 years. R.A. Fisher, a statistician and biologist, posited that a majority of species are extremely uncommon, with declining numbers among more common species (known as the log-series model). Conversely, F.W. Preston, an engineer and ecologist, contended that only a minor proportion of species are truly rare, while the bulk exhibit a moderate level of abundance (described as the log-normal model). Despite extensive scientific scrutiny, the model that accurately represents Earth’s true gSAD remains unidentified.

To tackle this enigma, the researchers accessed a trove of data from the Global Biodiversity Information Facility (GBIF), incorporating over one billion recorded species observations from the period 1900 to 2019.

Dr. Corey Callaghan, the lead author, extolled the GBIF database as an invaluable asset for biodiversity investigations, as it amalgamates data amassed from both professional and amateur scientists globally. Callaghan initiated the study during his tenure at iDiv and MLU and presently works at UF.

The team categorized the amassed data into 39 different species clusters, such as birds, insects, and mammals. For each category, they plotted the respective global species abundance distributions (gSAD).

The researchers discerned a possibly universal model where most species are scarce but not extremely so, with a very limited number being highly prevalent, conforming to the log-normal model. However, they also found that complete data are only available for a small number of species clusters like cycads and birds. For the majority of other species groups, data remain inadequate.

Senior author Prof Henrique Pereira, the research group leader at iDiv and MLU, remarked that accumulating more data alters the initial perception that most species are extremely rare, revealing instead that there are more species that are merely rare. This trend becomes evident when contrasting historical observations with the more comprehensive contemporary data sets.

Callaghan added that while considerable strides have been made, much remains to be done, emphasizing the indispensable role that GBIF and data sharing play in advancing the future of biodiversity research.

The study allows scientists to evaluate the level of detail revealed for different species groups and potentially resolves another lingering research inquiry: the exact number of existing species. For instance, in the case of birds, nearly all species have been documented, which is not true for other categories such as insects and cephalopods.

The investigators speculate that their findings might offer insights into Darwin’s unresolved question concerning the commonness and rarity of species, suggesting the existence of overarching ecological or evolutionary mechanisms governing these attributes.

Ongoing human activities continue to modify Earth’s ecology and the prevalence of species, complicating researchers’ objectives. Not only must they understand the natural evolutions of species abundances, but they must also simultaneously consider how human actions are reshaping these patterns. Thus, the quest to definitively resolve Darwin’s query remains a challenging and protracted endeavor.

Reference: “Unveiling global species abundance distributions” by Corey T. Callaghan, Luís Borda-de-Água, Roel van Klink, Roberto Rozzi and Henrique M. Pereira, published on 4 September 2023 in Nature Ecology & Evolution.
DOI: 10.1038/s41559-023-02173-y

Frequently Asked Questions (FAQs) about species abundance

More about species abundance

• Global Biodiversity Information Facility
• German Centre for Integrative Biodiversity Research (iDiv)
• Martin Luther University Halle-Wittenberg (MLU)
• University of Florida (UF)
• “The Origin of Species” by Charles Darwin
• Nature Ecology and Evolution Journal
• Overview of Species Abundance Models
• History of Biodiversity Monitoring
• Global Species Abundance Distributions: Log-series Model
• Global Species Abundance Distributions: Log-normal Model