It’s not (only) a numbers game: where species live matters

All species differ in terms of their commonness. Some are rarely seen; others are everywhere you look. But as it turns out, those that fall somewhere “in-between” have the greatest influence on shaping patterns of biodiversity and also deserve our attention.

When we think about which species play the most important role in shaping biodiversity and ecosystem composition, we often assume that the most common species are the key drivers—due to their larger workforce, as it were. But what if this isn’t the case? A new study led by Dr David Deane and SAEF researcher Professor Melodie McGeoch suggests it is not quite so simple.

To understand their findings, one needs to appreciate that ‘commonness’ includes two different components: how numerous (abundant) that species is and how widely distributed it is in space. These two facets of commonness act together to determine patterns of biodiversity. It turns out that species with the least likely combination of these two components have the greatest influence on shaping patterns of biodiversity.

To put this into context, imagine you’re walking through a forest filled with different birds. The birds you see around you for about half the time have the greatest influence on overall membership of the bird community, even though they aren’t necessarily the most numerous or widespread. Other species—those that are found in very few places or those that are found everywhere—can still have an important influence on biodiversity patterns. But to do so, they must be very abundant, about twice as numerous in our imaginary forest as the species you encounter around half the time.

Why This Matters for Conservation

Understanding which species drive these spatial biodiversity patterns is essential for conservation efforts. If conservation strategies focus only on the most abundant or on the most rare and threatened species, they will overlook species with key contributions, including to ecosystem dynamics. Species that have the unusual combination of moderate-high numbers and intermediate distribution sizes need particular attention.

As ecosystems continue to change due to climate shifts and human activity, recognising the hidden influence of these “in-between” species will be vital for preserving biodiversity. Rather than focusing solely on the most common or rare species, conservationists should consider the broader network of species that help shape our planet’s rich and ever-changing ecosystems. All species matter – rare, common – and now we know – also those in between.

Read more

Deeane, D.C., Hui, C.H. & McGeoch, M. (2025) Species that dominate spatial turnover can be of (almost) any abundance. Ecography, e07733. https://doi.org/10.1111/ecog.07733