University of Adelaide, Science Alert 13 Oct 09
Conservation biologists are setting their minimum population size targets too low to prevent extinction.
That's according to a new study by University of Adelaide and Macquarie University scientists which 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 5000 mature individuals or more.
The findings have been published online on 12 October in a paper 'Pragmatic population viability targets in a rapidly changing world' in the journal Biological Conservation.
"Conservation biologists routinely underestimate or ignore the number of animals or plants required to prevent extinction," says lead author Dr Lochran Traill, from the University of Adelaide's Environment Institute.
"Often, they aim to maintain tens or hundreds of individuals, when thousands are actually needed. Our review found that populations smaller than about 5000 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."
A long-standing idea in species restoration programs is the so-called `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.
"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 5000 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 a mass extinction event in the face of a growing human population and its associated impact on the planet.
"The conservation management bar needs to be a lot higher," says Dr Traill. "However, we shouldn't necessarily give up on critically endangered species numbering a few hundred of individuals in the wild. 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.
http://dx.doi.org/10.1016/j.biocon.2009.09.001
Conservation targets too low to save at-risk species
Nora Schultz, New Scientist 13 Oct 09;
Conservation biologists may be deluding themselves. An analysis of the minimum number of individuals needed for a species to survive in the long term has found that current conservation practices underestimate the risk of extinction by not fully allowing for the dangers posed by the loss of genetic diversity. If correct, it means the number of individuals in endangered species are being allowed to dwindle too far.
Lochran Traill at the University of Adelaide, Australia, and colleagues found that for thousands of species the minimum viable population size (MVP) – where a species has a 90 per cent chance of surviving the next 100 years – comes in at thousands rather than hundreds of individuals. Many biologists, Traill says, work with lower numbers and so allow unacceptably high extinction risks.
"If critically endangered species are not brought up to total population sizes of a few thousand, then we are simply managing for their short-term persistence," Traill says. He hopes his work will "encourage greater focus of scarce resources toward the populations that need attention now".
Reality check
Jean-Christophe Vié of the IUCN Species Programme says it is still worth investing in the protection of very rare species, since this can have broad positive implications for whole ecosystems even if the survival of the species is uncertain.
Traill's team hopes that an acceptance of how high population numbers should be for long-term survival will help conservation policy by providing a reality check, even if political or logistical constraints may make it unlikely that such targets can easily be reached.
"Explicit acceptance of the genetic and demographic requirements of vulnerable species may lead to an increase in the size or number of protected areas," says Traill.
"It's also important to remember that 5000 individuals do not need to occur in one large area, but can occur as a metapopulation [a collection of populations in a region] – so long as genetic transfer remains," he says. "This can be done through artificial dispersal, such as the translocation programmes in place for black and white rhino in southern Africa."
Journal reference: Biological Conservation, DOI: 10.1016/j.biocon.2009.09.001
