Average Animal Sizes Will Shrink As Bigger Species Face Extinction – And It’s Our Fault

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Animals are shrinking – and it’s our fault.

According to a study published in Nature Communications, the average (median) body mass of mammals will collectively decline 25 percent over the next 100 years as many bigger species face extinction. Compare that to the last 130,000 years, when body mass fell 14 percent – a figure roughly equivalent to 0.001 percent per century.


Shockingly (not), it seems that humans are to blame, what with our wanton destruction of the planet’s natural habitat, our penchant for hunting, and human-caused climate change.

"By far the biggest threat to birds and mammals is humankind – with habitats being destroyed due to our impact on the planet, such as deforestation, hunting, intensive farming, urbanization and the effects of global warming," lead author Rob Cooke, a postgraduate researcher at the University of Southampton, UK, said in a statement.

"The substantial 'downsizing' of species which we forecast could incur further negative impacts for the long-term sustainability of ecology and evolution."

The team came to this conclusion after looking at five characteristics in 15,484 birds and land mammals – that is, body mass, litter (or clutch) size, diet, habitat, and the length of time between generations.


Using the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, they also predicted the species most likely to die out within the next 100 years. Species listed as critically endangered, like the Sumatran orangutan, were given a meager 1 percent survival rate. Animals like the Amur tiger, listed as endangered, were given a two-in-three chance of making it to the end of the century, while those listed as vulnerable, like the giraffe, were given a 90 percent survival rate.

Computer models were then used to forecast the loss of biodiversity we can expect to see over the next century.

"We have demonstrated that the projected loss of mammals and birds will not be ecologically random," explained Felix Eigenbrod, a professor at the University of Southampton.

"[R]ather a selective process where certain creatures will be filtered out, depending on their traits and vulnerability to ecological change."


The results suggest the small and swift (think: rodents and songbirds) have better odds of persisting, while the larger, less adaptable species (rhinos and eagles, for example) are at greater risk of extinction. Factors that put a species in good stead included small size, shorter lifespans, insect-heavy diets, and a "generalist" approach. (That is, they can survive in a wide range of environments.) As previous studies have shown, an ample frame is a detriment to species survival. 

The study's conclusion echoes those of the UN's biodiversity report and signs that we are in the midst of a sixth mass extinction. But hope is not lost yet.

"Extinctions were previously viewed as tragic, deterministic inevitabilities, but they can also be seen as opportunities for targeted conservation actions," Amanda Bates, Research Chair at Memorial University, Canada, added.

"As long as a species that is projected to become extinct persists, there is time for conservation action and we hope research such as ours can help guide this."