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An international scientific team from Sweden, Spain, and the United Kingdom has identified a simple rule that appears to explain how species are distributed on Earth on a large scale, regardless of whether they inhabit coral reefs, tropical rainforests, or African plains. The study, published in the journal Nature Ecology & Evolution, reveals that, in large geographic regions such as continents, despite differences in life forms and environmental conditions, global biodiversity follows a common pattern: most species are concentrated in a central "core" and biodiversity gradually decreases as one moves towards the periphery.
The research was led by Umeå University (Sweden) and included collaboration from the Doñana Biological Station-CSIC, the Institute for Global Change Research of the Rey Juan Carlos University (IICG-URJC), the Pablo de Olavide University, and the University of Seville. To conduct this research, the team studied bioregions around the world, analyzing very different life forms: amphibians, birds, dragonflies, mammals, marine rays, reptiles, and trees.
A universal pattern on a diverse planet
Life on Earth is distributed like a vast mosaic: different groups of species inhabit regions of the planet isolated by natural barriers such as oceans, mountains, or deserts. These regions, called bioregions, act as natural experiments, where each group of species has followed its own evolutionary path, adapting to its environment. Given the enormous differences between bioregions—both in their environmental and historical conditions, as well as in the species and life strategies they adopt, whether flying, crawling, swimming, or remaining fixed to the substrate—the scientific team expected to find highly variable distribution patterns. Surprisingly, the same pattern was found everywhere.
“We found that, in each bioregion, there is always a core area where the majority of species live,” explains Rubén Bernardo-Madrid, lead author and researcher at Umeå University (Sweden). “From this core, species expand toward peripheral areas, but only a few manage to establish themselves. These core areas appear to offer optimal conditions for species to evolve and survive, acting in a way as a source of life for the rest of the bioregion.”
This pattern highlights the disproportionate ecological role these small areas play in maintaining the biodiversity of the entire bioregion. As José Luis Tella, co-author of the Doñana Biological Station - CSIC, emphasizes, "protecting these core areas is therefore essential, as they represent critical priorities for conservation strategies."
The engine behind this mechanism
But what drives this pattern? The fact that it repeats itself in regions with such distinct histories points to a general process known as environmental filtering. “It doesn't matter whether the limiting factor is temperature, water availability, or salinity,” explains Manuela González-Suárez, a co-author from the University of Reading in the United Kingdom. “The result is always the same: only species capable of tolerating these environmental filters manage to establish and persist, generating a predictable distribution of life on Earth.”
Environmental filtering has long been considered a key theoretical principle in ecology to explain species distribution. Until now, however, there has been little empirical evidence on a global scale. This study provides broad confirmation across multiple branches of the tree of life and on a planetary scale.
A step forward in understanding biodiversity
The existence of a universal organizing mechanism has profound implications for understanding life on Earth. As Joaquín Calatayud, a co-author from the Global Change Research Institute at the Rey Juan Carlos University, points out, “this pattern suggests that life on Earth may be, to some extent, predictable.” This predictability can help scientists trace how life has evolved over time and offer valuable clues about how ecosystems might respond to global environmental changes.
This advance in our understanding of life on Earth is only possible thanks to sustained support for basic science—such as the discovery of new species and their distributions—and collaboration across disciplines. As Martin Rosvall of Umeå University concludes, “Continued investment in basic science and global collaboration allow us to test the ideas that underpin our knowledge and reveal how biodiversity is shaped.”