Modelling for better management

Conservation management in New Zealand often involves translocating endangered species to predator-free sanctuaries.  These translocations are often not as successful as they should be, but it can be difficult to pinpoint the reason why.  A major problem for newly established populations can be the loss of genetic diversity that comes with establishing new populations from only a few founders.  Loss of genetic diversity can increase the risk of extinction by reducing a population’s ability to adapt to new threats or environmental changes.  Generally, conservation management programs for threatened species should aim to retain 90-95% heterozygosity over 100-200 years, but in reality management practices are often dictated more by convenience or by what can be realistically achieved in the field at the time. 

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Tuatara ready for release

New research (to be published soon in Molecular Ecology) by Victoria University PhD student Kim Miller has the potential to improve translocation planning for our native reptiles by offering guidelines for maximising genetic diversity and managing populations over time.  Kim compared levels of genetic diversity in newly established tuatara populations with that of the original populations, to measure how much diversity had been lost through the translocation process.  She then modelled what genetic diversity in the new populations would look like in another 400 years (10 tuatara generations) under a number of different scenarios.  Tuatara are currently restricted to a few dozen off-shore islands around New Zealand, so establishing new populations through translocation is an important part of ensuring their long-term survival. 

Kim’s study found that increasing the number of founder individuals for a new population results in higher long-term genetic diversity in that population, as you might expect.  However, populations founded from source populations with high diversity are likely to lose more diversity than those founded from populations where genetic diversity is already low.  For tuatara, there is an additional factor that can contribute to loss of genetic diversity – the fact that only a small proportion of males mate successfully, a phenomenon known as “reproductive skew”.   Competition between males in high density tuatara populations means that only 25-30% of males will successfully mate and pass their genes on to the next generation.  Kim’s research found that this can have a significant impact on how well genetic diversity is maintained when populations are small, and that the  current practice of translocating about 30 individuals when establishing new populations may be insufficient – releasing 70 would be better.  “For species with a low reproductive output, high mortality rates after release, highly polygynous mating systems and high levels of background in-breeding, releasing a larger group is more effective in improving the long-term maintenance of diversity,” she says.  

Using these models, conservation managers can determine how many individuals need to be released when establishing a new population to meet the genetic goals for management.   This study provides a tool for predicting what the likely outcomes of different translocation practices will be, enabling more informed decisions about population management to be made.

Reference: KA Miller, NJ Nelson, HG Smith, JA Moore (2009). How to reproductive skew and founder group size affet genetic diversity in reintroduced populations? Molecular Ecology, in press.

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