The world's growing extinction threats are typically worst for the largest and smallest creatures, a finding that should temper conservation efforts, new research has found.
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The study, published on Tuesday in the Proceedings of the National Academy of Sciences, examined 27,647 vertebrate species based on body mass as assessed by the Union for Conservation of Nature's Red List. About 17 per cent of all species, for which size data is available, are threatened with extinction.
The threat to larger animals has been relatively well known, not least because creatures, such as blue whales, elephants and tigers, are often highlighted by conservation groups.
For many species, the risks are often from growing populations of humans hunting or fishing wild meat, or the purported medicinal value of animal parts.
These "charismatic" species in turn often draw a relatively large share of research funding, Thomas Newsome, a research fellow with both Sydney and Deakin universities and an author of the paper, said.
By contrast, smaller species were often nocturnal with relatively few sighting by humans. They also typically had much more limited ranges than bigger animals.
As a result, the destruction of their habitat - particularly from land-clearing for agriculture or to house a growing human population - can wipe out the only ecosystem they have.
"We are currently poised to cut off the top and the tail" of the species chain, Dr Newsome said, adding that both ends of the spectrum played important roles as "ecological engineers".
Megafauna were often apex predators that played important roles in the abundance of species they preyed on, a cascading effect that reached all the way down to the quality of water and soil that human life depended upon, he said.
At the other end, the smallest creatures were often vital for the pollination of plants or spread of seeds, or of pest control.
The research included 117 threatened Australian vertebrate species that weigh less than one kilogram. Among the best-known animals at risk is the Leadbeater's possum, which has had its range in Victoria shrink rapidly because of logging and bushfires.
"Australia has the worst record for mammal extinctions in the world," Dr Newsome said, with the loss of 30 such species accounting for half the world's total. Some 241 Australian mammals are on the IUCN's threatened species list
The increased land clearing in Queensland in recent years has reintensified the threat, while the recent loosening in NSW of native vegetation laws allowing more deforestation by farmers could add to the risks in that state, he said.
The destruction Australia's woodlands not only removed the habitat of smaller creatures but also exposed them more to introduced pests, such as foxes and feral cats. Bushfires, which are likely to worsen along with climate change, are adding to the threats, Dr Newsome said.
While more conservation effort needed to be put into identifying and preserving the many lesser-known species in peril, resources that went to protect the megafauna had benefits for smaller creatures, too.
Since the larger animals tended to range over relatively large areas, creating sanctuaries for them often supported the ecosystems for much tinier ones, including invertebrates, he said.
Along with curbing habitat destruction, halting or reversing the extinction threat would also need a slowdown in the growth of human numbers, the paper said.
"Curtailing the human population growth rate may be crucial long-term factor in limiting extinction risks to species," the paper said.
