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From the front garden to the continent, here's why biodiversity does not increase evenly from small to large

The number of species does not increase evenly when going from local ecosystems to continental scales—a phenomenon ecologists have recognized for decades. Now, an international team of scientists, including researchers from the German Center for Integrative Biodiversity Research (iDiv) and the Martin Luther University Halle-Wittenberg (MLU), has developed a new theory to explain the three distinct phases typical of species distributions across scales.

The theory, in the journal Nature Communications, may be crucial for estimating how many species are lost when habitats are destroyed.

As one moves from a small area to the continental scale, the number of species increases. For example, a village pond might host only a handful of amphibian species, but as the scale expands to include rivers and marshes, more frogs, toads or salamanders appear, reaching several hundred or thousand species at the continental or intercontinental level.

Three-phase pattern of species distribution across scale

These patterns are known as species-area relationships (SARS). Ecologists have long observed that SARS follow a characteristic three-phase pattern: In phase one (local to regional), the number of species increases rapidly. In phase two (regional to continental), the increase slows down. Finally, in phase three (continental to intercontinental), the number of species accelerates once again.

Researchers have now developed a universal theory to explain these three-phase patterns and estimate the number of species at key transition points between the phases.

"This is a major step forward in ecology," says first author Dr. Luís Borda-de-Água from the CIBIO research center in Portugal. "We demonstrated that the individual geographical ranges of all species within the studied areas shape the typical species distribution patterns (SARS) we observe across the globe. By combining these distributions in a novel way, we developed a formula to estimate the number of species at the transitions between different phases."

Conservation implications of new theory

Such estimates can be crucial for biodiversity conservation. For instance, identifying where the rate of new species appearances changes can help estimate how many species are lost when habitats are destroyed. Such figures form the basis of extinction rate calculations in international biodiversity reports.

To validate their theory, the researchers compared SARS based on observation data from various species groups, such as birds and amphibians, with their calculated estimates, utilizing around 700 million observations from a single dataset for their analysis. The strong agreement between data and theory gives scientists great confidence in their approach.

"Discovering fundamental principles in ecology is just as thrilling as breakthroughs in physics," says senior author Prof Henrique Pereira from iDiv and MLU. "New findings like ours unveil hidden patterns that have been shaping life on Earth for millions of years. Just as physics deciphers the universe's deepest mysteries, new ecological theory can reveal the fundamental forces shaping biodiversity on our amazing planet."