New Research Suggests Conservation Biologists are Setting Minimum Population Size Targets Too Low to Prevent Extinction

A new study by University of Adelaide and Macquarie University (Australia) scientists has shown that populations of endangered species are unlikely to persist in the face of global climate change and habitat loss unless they number around 5,000 mature individuals or more.

The findings have been published in a paper entitled 'Pragmatic population viability targets in a rapidly changing world' in the journal Biological Conservation.

A long-standing idea in species restoration programs is the '50/500' rule. This states that at least 50 adults are required to avoid the damaging effects of inbreeding, and 500 to avoid extinctions due to the inability to evolve to cope with environmental change.

For this study, researchers investigated existing data regarding the minimum population size requirements for species based on empirical and theoretical estimates made over the past few decades. This literature was found to indicate that thousands, rather than hundreds, of individuals are required for a population to have an acceptable probability of riding out environmental fluctuation and catastrophic events, and ensuring the continuation of evolutionary processes.

"Often, [conservation biologists] aim to maintain tens or hundreds of individuals, when thousands are actually needed," says lead author Dr. Lochran Traill, from the University of Adelaide's Environment Institute. “Our review found that populations smaller than about 5,000 had unacceptably high extinction rates. This suggests that many targets for conservation recovery are simply too small to do much good in the long run."
"Our research suggests that the 50/500 rule is at least an order of magnitude too small to effectively stave off extinction," says Dr. Traill. "This does not necessarily imply that populations smaller than 5,000 are doomed. But it does highlight the challenge that small populations face in adapting to a rapidly changing world."

Team member Professor Richard Frankham, from Macquarie University's Department of Biological Sciences, says: "Genetic diversity within populations allows them to evolve to cope with environmental change, and genetic loss equates to fragility in the face of such changes."

Conservation biologists worldwide are battling to prevent mass extinctions in the face of a growing human population and its associated impact on the planet.

“We shouldn't necessarily give up on critically endangered species numbering a few hundred of individuals in the wild," says Dr Traill. "Acceptance that more needs to be done if we are to stop 'managing for extinction' should force decision makers to be more explicit about what they are aiming for, and what they are willing to trade off, when allocating conservation funds."

Other researchers in the study are Associate Professor Corey Bradshaw and Professor Barry Brook, both from the University of Adelaide's Environment Institute. Information from the University of Adelaide and EurekAlert.

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