Life's hidden pattern: Why most species are rare but a few rule the world

If you’ve ever noticed how squirrels vastly outnumber foxes in your neighborhood – or seen the same car brands dominate parking lots – you’re observing a pattern that governs not just human-made environments, but the natural world at every scale.

This pattern, where a few species are extremely common while most remain rare, is called the “hollow curve” species-abundance distribution.

For nearly a century, ecologists have sought a unified model to explain it. Now, researchers from the University of Virginia (UVA) believe they’ve found that model.

Universal pattern in nature

In a recent study, the team analyzed an enormous array of 30,000 datasets covering ecosystems from U.S. forests to the microbial colonies in the human gut.

The researchers discovered that a mathematical model known as the “powerbend distribution” consistently describes how species are spread out. This applies to birds, trees, bacteria, and other organisms in communities across the planet.

“This is the first study to comprehensively examine all types of organisms, large and small,” said Martin Wu, professor of biology at UVA and co-author of the study. “We found that the powerbend distribution consistently fits communities of all life forms, habitats, and abundance scales.”

The powerbend model shows most species are rare, while a few are extremely common – a core pattern in ecology.

Wu explains this through an everyday example: “If you walk around UVA, you might see a lot of squirrels and, only once in a while, a raccoon or fox. If you look at cars on city streets, you’ll see a lot of Hondas, Toyotas, or Fords, but there are many other brands you only see every once in a while.”

Estimating life we can’t see

The breakthrough isn’t just about better understanding nature – it also has practical implications for conservation and environmental monitoring. The powerbend model allows scientists to estimate the biodiversity of a region without needing to count every individual organism.

“Previously, if I collected one gram of soil, I had no way of knowing how many bacterial species it contained,” Wu said. “Now that we have the model, I don’t need to enumerate every single cell in the soil and I can have a pretty good estimate.”

This is particularly important when dealing with microbes, which are notoriously difficult to grow and study using traditional lab techniques. Prior to the development of DNA sequencing tools around 2008, scientists struggled to survey microbial communities effectively.

Since then, technological advances have given microbiologists access to vast genetic data, transforming our understanding of microbes.

“You can go to a forest to survey trees or use binoculars to look at birds, but until recent DNA sequencing technologies became available, we couldn’t easily survey microbes,” Wu said. “Since 2008, there’s been an explosion of microbial survey data, and we’re learning a lot.”

Ecology meets math

Though Wu is a microbiologist by training, the turning point in the research came when two math-savvy graduate students – Yingnan Gao and Ahmed Abdullah – joined his lab. Their expertise allowed the team to rigorously test the powerbend distribution against real-world datasets across ecosystems, organisms, and regions.

The model even made its way into the classroom. “Using a thumb drive-sized DNA sequencer and a laptop, we did a real-time demonstration in the class when we surveyed the gut microbiome of a beloved professor known to students,” Wu said. “It was a great way to show the students what kind of microbes live in the gut.”

The exercise gave students the chance to become “microbe detectives” in a hands-on lab course focused on sequencing-based ecological surveys.

Pattern stretches across life

One dataset in the study looked at the pattern of tree species across the United States. In Virginia, for instance, five species – red maple, loblolly pine, yellow poplar, sweetgum, and American holly – make up about 45% of the state’s 10.6 billion trees. The remaining 55% is spread across over 120 other species.

“Generally, you’ll find that 20% of tree species account for 80% of the total abundance,” Wu said. “This follows the Pareto Principle, a term coined by Italian economist Vilfredo Pareto, who found that 20% of the population controlled 80% of the wealth.”

A formula for nature

The discovery of the powerbend distribution as a consistent and predictive model offers scientists a new tool for tackling ecological questions.

More than that, it reaffirms that despite the dizzying complexity of life on Earth, there are underlying patterns that unify everything from backyard birds to invisible bacteria.

As ecologists confront biodiversity loss and work to restore ecosystems, understanding natural species distribution patterns could be a crucial step forward. Thanks to the UVA team’s work, that understanding just got a lot clearer.

The study is published in the journal Nature Communications.

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