Size matters in battle against extinction, scientists find

An international collaboration of scientists have found extinction risk is the greatest for the world’s smallest and largest vertebrates, but for very different reasons.

By examining a database of more than 27,000 vetebrates, and analysing the relationship between extinction risk and body mass, scientists found that the heaviest vertebrates were most threatened by a rapidly growing population of human meat-eaters.

In contrast, the lightest vertebrates were most in danger from habitat loss and modification, stemming especially from pollution, agricultural cropping and logging.

Among the groups of animals evaluated were birds, reptiles, amphibians, bony fish, cartilaginous fish (mostly sharks and rays), and mammals, with more than 4000 listed as threatened or endangered.

The international research, led by Oregon State University in the US and supported by the University of Sydney and Deakin University, is published today in the journal Proceedings of the National Academy of Sciences.

Dr Thomas Newsome, a Research Fellow from the Faculty of Science at the University of Sydney and Deakin University, said the research put the spotlight on 241 threatened species in Australia.

“Fifty-five per cent of these species are threatened by biological resource use which includes hunting, logging, and fishing,” Dr Newsome said.

“As an example, the northern hairy-nosed wombat is one of the rarest land mammals in the world and is critically endangered. A key threat to this species is habitat loss.

“But Australia stands out in that a high proportion of species are also threatened by invasive species.”

Dr Newsome said an increased understanding of the relationship between body size and extinction risk would have practical implications for animal conservation, with different approaches needed for protecting larger versus smaller species.

“For the smaller species, freshwater and land habitat protection is key because many of these species have highly restricted ranges,” he said.

“For the large species there is an urgent need to reduce the consumption of wild meat to lessen the negative impacts of human hunting, fishing and trapping.

“But it’s ultimately slowing the human population growth rate that will be the crucial long-term factor in limiting extinction risks to many species.

“Based on our findings, human activity seems poised to chop off both the head and tail in the size distribution of life.”

Dr Newsome said such species loss would have a significant impact on the planet: “Large vertebrates play an important role in structuring ecosystems,” he said.

“Many large-bodied vertebrates shown to be at-risk in our analysis are also predators, which influence food webs from the top down, and can affect environmental processes like disease, wildfire and carbon storage, and may even buffer communities from climate change.

“The fact that our research shows extinction risk is highest for large animals adds to growing concern that the loss of top predators will disrupt to delicate balance of the ecosystem.”

Dr Newsome said smaller vertebrates also played an important role, particularly in ecological functions that required a small body size, including the pollination done by bats and hummingbirds.

“So our findings highlight the danger of disproportionately focusing on the conservation of larger species, often the face of major fundraising efforts,” he said.

“In the past, the vulnerabilities of smaller vertebrates has been underestimated, so this research shows the importance of conservation efforts for both the biggest and smallest members of the animal kingdom.”