A global analysis of more than 12,000 bird and mammal species reveals that ecosystems do not share energy evenly across body sizes. Small species dominate in numbers, but in high-productivity regions their abundance is spread so thinly across many species that larger animals end up capturing more energy per species. Human activity compounds these patterns by selectively eroding large-bodied diversity. The study has been published in the journal Frontiers of Biogeography.

How does an ecosystem distribute its energy across body sizes? A new study suggests the answer depends on where you are — and how much humans have altered the landscape. Analysing communities of birds and mammals worldwide, researchers show that larger-bodied species can, on average, capture more energy per species than smaller ones, particularly in highly productive environments. The work also reveals that human impacts restructure these patterns by disproportionately removing large-bodied species from local communities.

The research, led by Luis F. Camacho and Miguel B. Araújo is published in Frontiers of Biogeography, the journal of the International Biogeography Society.

Ecologists have long observed that larger species tend to have smaller populations: a bigger body requires more energy to sustain, so fewer individuals can be supported. A slope of −0.75 in the log–log relationship between body mass and abundance has been proposed as a null expectation - the point at which every species, regardless of size, commands an equal share of ecosystem energy. Yet testing this idea globally has been difficult because abundance data are patchy across regions and taxa.

This study tackles that limitation head-on. By combining large global datasets of abundance and distributions of species, the authors model local populations of birds and mammals across the planet and ask: is ecosystem energy concentrated in a few large organisms, or dispersed among many small ones - and how does that change from place to place?

Using extensive abundance datasets and species trait information, the authors modelled population densities (individuals per km²) for 12,057 terrestrial bird and mammal species on a global grid at one-degree resolution. This enabled them to reconstruct community structure across environments and to examine three complementary signatures of how body size relates to energy and diversity within communities: the body mass–abundance relationship across species (how abundance declines with body size), the individual mass distribution (how individuals are distributed across body sizes regardless of species identity), and the richness mass distribution (how species richness is distributed across body sizes). They then related these patterns to two major global drivers: ecosystem productivity (net primary productivity) and human pressure (human footprint index).

1. Productive ecosystems tilt energy capture towards larger species.

As productivity increases, the study finds that small-bodied species become more diverse without a matching rise in total abundance. Individuals are effectively “spread thinner” across more small-bodied species, reducing the average energy share per small-bodied species. Meanwhile, large-bodied species — fewer in number but less diluted by richness — end up capturing more energy on average. The body mass–abundance slope ranged from approximately −0.69 in the least productive environments to −0.35 in the most productive, consistently shallower than the −0.75 null expectation.

2. Human pressure reshapes community organisation, with lasting consequences.

Human activity produces a shift in these relationships that resembles - but is mechanistically distinct from - the productivity effect. Human impact operates primarily by reducing both the abundance and, more strongly, the species richness of large organisms. This likely reflects the well-documented disproportionate extirpation of large-bodied species by human activities, leaving a persistent imprint on how energy and diversity are distributed across body sizes.

3. Energy distribution and ecological opportunity vary independently.

Across the globe, patterns of energy flow across body sizes and the opportunities for diversification (how many species exist at each body size) can decouple. Interestingly, the distribution of species richness across body sizes was more stable across environments than the distribution of energy - yet it had a stronger influence on the body mass–abundance relationship. This helps explain why simple, one-size-fits-all expectations about body size and abundance often fail at large scales.

The authors argue that body mass–abundance–richness relationships can serve as a functional metric of ecological change - capturing not just how many species are present, but how ecosystems allocate individuals and energy across body sizes. This matters for biodiversity assessments because losing large-bodied diversity can reorganise ecosystems in ways that species counts alone cannot reveal. Specifically, these patterns may help improve biodiversity offset strategies and address the risk of underestimating the ecological importance of large-bodied species.

Small-bodied animals still dominate numerically, but in productive ecosystems they are divided among many more species. That dilution changes the per-species share of individuals and, by extension, energy.

— Luis F. Camacho, first author

Human pressure does not just remove species; it reshapes the functional organisation of communities. Looking at body size, abundance, and richness together reveals structural changes that standard indicators miss.

— Miguel B. Araújo, senior author

Original source:

Camacho LF, Araújo MB (2026) Body mass–abundance relationships reveal uneven global energy distribution across body size classes in vertebrates. Frontiers of Biogeography 19: e164408. https://doi.org/10.21425/fob.19.164408

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