In the next 100 years, the average body mass of mammals on Earth may fall by 25% — a shocking prediction considering body mass fell by just 14% over the last 130,000 years.1 The dwindling in size is largely due to the potential extinction of large land animals and birds, which researchers expect to outpace that of smaller creatures.
If their predictions are correct, the study, published in the journal Nature Communications, suggests the pool of mammal and bird species on Earth will shift toward “small, fast-lived, highly fecund, insect-eating, generalists,” while large iconic species like rhinos and gorillas may slowly disappear.2
Large-animal extinctions may downsize average animal size
Researchers at the University of Southampton focused on 15,484 land mammals and birds for the study, honing in on body mass, litter/clutch size, breadth of habitat, diet and length of time between generations. They also consulted the International Union for Conservation of Nature (IUCN) Red List of Threatened Species to gauge which species are most likely to go extinct, then ran computer simulations to make projections about the loss of biodiversity in coming decades.
“We have demonstrated that the projected loss of mammals and birds will not be ecologically random — rather a selective process where certain creatures will be filtered out, depending on their traits and vulnerability to ecological change,” Felix Eigenbrod, Ph.D., professor at the University of Southampton, said in a news release.3
Larger animals like elephants, rhinos, deer and pelicans are slated to give way to smaller animals, including rodents like the dwarf gerbil and songbirds, such as the white-browed sparrow-weaver. The tawny eagle and black rhinoceros are among the slow-lived species that may prove to be less adaptable to environmental threats, which are primarily human-made. Lead author Rob Cooke explained:4
“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, urbanisation and the effects of global warming.
The substantial ‘downsizing’ of species which we forecast could incur further negative impacts for the long-term sustainability of ecology and evolution. This downsizing may be happening due to the effects of ecological change but, ironically, with the loss of species which perform unique functions within our global ecosystem, it could also end up as a driver of change too.”
Losing large animals will change ecosystems
Cooke is referring to some of the many benefits large animals bring to the environment. Elephants, for instance, convert woodland to shrubland, which allows impala and the black rhinoceros easier access to food. They also damage trees, which provides habitat for lizards, and, by making their way through the forest, open impenetrable thickets so large predators (like lions) can have an easier time hunting.
By traveling long distances, large mammals disperse seeds in their feces, and by grazing, they help keep grass patches short, benefitting food access for other species, like impalas and wildebeests.5 Thus, the predicted “downsizing” could have dramatic and far-reaching effects. According to the study:6
“Ecological downsizing can entail the loss of unique ecological functions and can impact ecosystem structure, function, and biogeochemical cycles. Hence, downsizing could be a driver, as well as a consequence, of global change with implications for the long-term sustainability of ecological and evolutionary processes.”
Will tiny and giant species go extinct?
Other research has suggested that animals outside of the “Goldilocks zone” — those either very big or very small, may face the highest risk of extinction. Analyzing more than 27,600 species, animals at both ends of the size spectrum were most at risk of extinction, with the breakeven point being 0.035 kilograms (0.077 pounds). The further in either direction from this body mass, the greater the risk of extinction became.
Among small animals, those less than 1.2 ounces (35 grams), the greatest risks came from loss or modification of habitat — especially that related to pollution, agriculture and logging.
For larger animals (and most of those greater than 2.2 pounds, or 1 kilogram), harvesting, including being killed and consumed by humans, was a major threat.7 Glimpsing the potential future of both large and small animals gives the potential for more targeted conservation efforts to take place.
While some efforts have been made to protect large mammals like whales, elephants, lions and rhinos, efforts are needed to protect other large creatures, including whale shark, Atlantic sturgeon, Somali ostrich, Chinese giant salamander and the Komodo dragon, from harvesting by humans.8
A future without giraffes, condors and caribou?
The featured study suggests some of the most iconic species, including giraffes, condors and caribou, as well as gorillas and eagles, could face extinction in the next century, while smaller mammals and birds flourish. It’s difficult to imagine a world without these magnificent species, and concerted actions taken now could change the outcomes for the better.
“Extinctions were previously viewed as tragic, deterministic inevitabilities, but they can also be seen as opportunities for targeted conservation actions,” study author Amanda Bates, Ph.D., said. “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.”
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