Amphibians, such as frogs, toads, salamanders and newts, in particular, have become the iconic example of human-induced extinctions, given that their declines exceed those of any other group of animals on Earth.
The risk of extinction among amphibians – the most endangered animals on the planet – increases for species of smaller body size as their females produce fewer babies per birth, new research shows.
Biologists have long believed that animals of larger body size are more vulnerable to extinction – an idea supported by the widespread endangerment faced by iconic large animals such as rhinos or whales.
This study, led by Queen's University Belfast, is the first to suggest resetting the theory to focus on reproduction levels of animals rather than body size when calculating extinction risk.
The research has been published today in the journal Global Ecology and Biogeography and was conducted in collaboration with Nottingham Trent University, Tel Aviv University, the University of Exeter and the University of Lincoln.
Amphibians, such as frogs, toads, salamanders and newts, in particular, have become the iconic example of human-induced extinctions, given that their declines exceed those of any other group of animals on Earth. Over 40 per cent of amphibians currently face extinction risk.
A rapidly growing extinction hypothesis suggests that larger body size increases the risk of extinction – this theory has been derived fundamentally from research on mammals.
This study is the first to investigate the causes behind extinction in amphibians based on the theory that it is not body size, but the number of babies a female produces per clutch that determines extinction risk.
Lead author Dr Daniel Pincheira-Donoso, of Queen's University, said: "More babies per clutch or birth means more variety among the babies.
"To some extent, it is like playing the lottery. The more tickets you play, the higher your chances to win.
"In this case, more numerous and diverse babies increase the chances that at least some can survive the stress of environmental alterations, such as progressive climatic changes."
Professor Dave Hodgson, Director of the Centre for Ecology and Conservation on Exeter's Penryn Campus in Cornwall, said: "Our findings demonstrate that there is much more to learn about the reasons why some species are endangered and others are thriving.
"Size matters, but does not tell the whole story. In amphibians, reproduction really matters."
The research team looked at amphibians from all over the world, and from their entire diversity (frogs, salamanders and caecilians – which look like big worms).
The team then matched the levels of endangerment of thousands of species (whether they are threatened with extinction or not) and analysed this information against their body sizes and the number of babies they produce per clutch.
They found strong evidence that extinction risk increases towards species that produce fewer offspring, such as the "rain frogs" (Eleutherodactylus), whereas extinction risk decreases towards species that produce more, such as different species of American water frogs (Lithobates) or the large-bodied "bufonid" toads, which can lay up to 80,000 eggs in one clutch.
In contrast with the widely held view about animal size as a driver of extinctions, they found that the risk of extinction of a species did not match its size as accurately as it matched its reproduction.
Dr Pincheira-Donoso added: "Our findings explain the discrepancies in the body size-extinction risk relationship observed between mammals and amphibians, given that larger mammals have fewer babies per birth and therefore extinction increases with larger body size, whereas in amphibians, larger females produce more babies, thus reducing extinction risk with larger body size."
This new research suggests that placing an emphasis on the number of offspring that species produce, rather than simply on their large body sizes, could guide more effective human actions about what species or areas to protect during the fight against the loss of biodiversity.
