Saving biodiversity: Scientists discover hidden pattern guiding species distributions

When naturalists tally the frogs in a farm pond or the orchids in a mountain meadow they always encounter the same rule of thumb: when you survey a larger range, the more species you find. The relationship seems simple, yet ecologists have puzzled over an odd signature hidden in species distribution data for nearly a century.

As one zooms out from local plots to regional landscapes. the tally of new species first soars. Then, it plateaus, taking off again at continental and global scales. That curious “S-shaped” curve is technically, the three-phase species–area relationship.

It has appeared in everything from beetles to birds and from coral reefs to desert shrubs. Until now, no single mathematical framework explained why the curve changes its slope where it does.

Predicting species distributions

A collaboration of researchers spanning Portugal, Germany, the United States and South Africa has now filled that theoretical gap.

The experts combined basic geometry with giant biodiversity datasets to show that the three phases arise inevitably from the way individual species spread their geographic ranges across Earth’s surface.

The team’s model not only reproduces the classic patterns, it also generates equations that predict how many species should occur at the precise break points between phases – a feat with direct relevance for conservation planners who must estimate extinctions as habitats are lost.

Range size affects species distributions

Ecologists formalized the species-area relationship (SAR) in the early twentieth century, discovering that a log-log plot of cumulative species versus area usually forms a straight line. Yet when studies started to cover larger domains, investigators found the line was actually composed of three segments.

In the first segment, from local quadrants to perhaps a few hundred square kilometers, adding more land brings a flood of new species because each little patch contains slightly different microhabitats. In the middle segment, spanning thousands of square kilometers, the pace slackens; the same bird species, for instance, occupy most forests inside a biogeographic region.

Beyond that, when continents are added, evolutionary history enters the picture and the slope shoots up again as entirely different faunas come into view. Empirically describing those bends was straightforward; explaining them from first principles proved harder.

Individual ranges of species

“We demonstrated that the individual geographical ranges of all species within the studied areas shape the typical species distribution patterns we observe across the globe,” said lead author Luís Borda-de-Água of CIBIO in Portugal.

By mathematically stacking these ranges like overlapping tiles on a floor plan, the team showed that the SAR’s inflection points fall where the average range size of resident species equals the sampling area. At small scales most ranges eclipse the plot, so each extra hectare introduces new species.

At intermediate scales the plot begins to rival range size, so fewer novelties appear. At the largest scales even the widest-ranging species occupy only one continent, restoring the steep climb.

Testing patterns and crunching records

To test the idea the researchers turned to the Global Biodiversity Information Facility, a compendium of citizen-science sightings and museum records. They extracted roughly 700 million geo-referenced observations spanning birds, amphibians, mammals and vascular plants.

For each taxonomic group they generated empirical SARs and compared them with the theoretical predictions. The correspondence was striking: predicted transition points – where the slope of the SAR changes – matched the observed curves within confidence bounds for all groups.

Borda-de-Água noted that this is a major step forward in ecology because it links a macroecological pattern to a simple, measurable property – range size – without invoking mysterious external factors.

Senior author Henrique Pereira is a research professor at the German Center for Integrative Biodiversity Research (iDiv) and Martin Luther University Halle-Wittenberg.

Pereira emphasized the significance of the research. “Discovering fundamental principles in ecology is just as thrilling as breakthroughs in physics.”

Practical stakes for a shrinking planet

Beyond intellectual satisfaction the model carries weighty consequences for biodiversity assessments. International panels such as IPBES rely on SARs to translate lost habitat into predicted extinctions.

If analysts know where a landscape sits on the three-phase curve they can apply the correct slope and avoid over- or under-estimating risk. The new equations provide exactly that guidance.

Suppose a logged forest fragment still covers more area than the average range of its endemic beetles; species losses may be modest. But once destruction pushes the fragment below the first transition threshold, disappearing hectares cause a much steeper decline in species count.

Because the theory is scale-free it can also help conservationists decide whether sampling has been adequate. If observed species richness does not yet reach the slowdown phase, additional survey work is warranted before managers proclaim an inventory complete.

Shifting species distributions

The authors acknowledge that species’ ranges are not static. Climate change, invasive species, and land-use shifts continually redraw distribution maps.

Incorporating dynamic range models into the three-phase framework will be a priority, as will extending tests to marine organisms and microbes, whose ranges are harder to delineate.

Still, the study offers a rare optimism amid grim extinction headlines. By revealing the geometry behind one of ecology’s most robust laws, scientists gain a clearer lens through which to view and perhaps mitigate humanity’s impact on life’s diversity.

As Pereira puts it, advances like these “unveil hidden patterns that have been shaping life on Earth for millions of years,” providing the knowledge base needed to steer those patterns, responsibly, into the future.

The study is published in the journal Nature Communications.

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