I wrote a short article about the genetic rescue of this critically endangered species when the initial success of the translocation program was reported. At that point the researchers were delighted to find juvenile hybrid possums. Not only did this prove that the introduced males from Mt. Hotham had successfully mated with resident females on Mt. Buller, the hybrid offspring were heavier and therefore more likely to survive the coming winter.
The latest news is even better: half of the Mt. Buller population now contains genes from the Mt. Hotham population, the genetic variation has doubled. In addition, or rather because of this genetic shift, the population size has more than tripled since 2009 and male survival has dramatically increased.
Why males matter
The mountain pygmy-possum (Burramys parvus) population at Mt. Buller was about 300 in 1996 but had dropped to 30 in 2009. The reasons for this include habitat fragmentation and pressure from introduced predators, but the population also had extremely low genetic diversity and few males.
The impact of population size on genetics is well understood: both genetic drift (the probability of losing alleles through chance) and inbreeding (mating between relatives) increase in small populations. These effects are magnified when there are few males in the population. Not only does this increase inbreeding because many individuals share the same father, there was always the chance that a female could miss out on mating for an entire season.
Mountain pygmy-possums have an interesting mating system called resource defence polygyny, in which the one sex sets themselves up the best habitat. In this case the females establish themselves in boulderfields at the top of the mountain. These sites are valuable because this is where the Bogong moths gather, an important source of fat and protein in the diet of these alpine possums.
With the need to build up enough fat reserves to sleep through winter, females vigorously defend their territories and males are driven from the nest at a young age. These animals are always acutely aware that winter is coming.
Males suffer higher mortality because they are forced to live in marginal habitat, and because they have to travel to find females during the breeding season. In a fragmented landscape such as Mt. Buller, searching for females means crossing roads and open grassy ski runs, increasing the chance of mortality from predator attacks and other mishaps.
The increase in male survival following the genetic rescue program indicates that male mortality must have had an additional cause, probably reduced fitness due to inbreeding depression. Here is a powerful demonstration of the importance of conserving the genetic potential of the species (a requirement of the Flora and Fauna Guarantee Act in Victoria).
Why do we fear outbreeding depression?
This program’s success goes beyond mere population recovery. The strategy of releasing a few animals each year is significantly cheaper than the now abandoned captive breeding program. So why don’t wildlife managers more regularly move animals from one population to another?
We have long recognised that populations differ genetically, and often those differences provide adaptations to local conditions. In these cases, mixing genes is not always the best solution, and early research told us that the Mt. Buller population was genetically unique. The best scenario is to preserve genetic diversity between populations, but in the case of Mt. Buller, this meant risking the loss of genetic diversity within a single population.
More importantly, wildlife managers are concerned about the risk of outbreeding depression – a reduction in fitness due to crossing populations that have diverged genetically. Outbreeding depression has been documented in Australian velvet worms and Corroboree frogs: both groups produced higher rates of abnormalities in hybrid offspring.
However, outbreeding depression is rare in mammals or birds. Most frequently it has been documented in cases of taxonomic confusion: when two different species are crossed, hybrid offspring are often unwell or malformed. Ensuring that the animals you are crossing are of the same species is critical, and it may be that the incidence of outbreeding depression is over-stated.
Outbreeding depression can also be temporary. When co-adapted gene complexes are disrupted, fitness declines are inevitable but short-lived. Natural selection soon acts on the new genetic opportunities and populations bounce back. The problem is that we cannot know in advance what is going to happen, and to date caution has been on the side of inaction or breeding in captivity.
More translocation in the future?
The rapid recovery of the mountain pygmy-possum at Mt. Buller is a good news story with a moral: perhaps we can help more endangered species in genetic decline. With this example to show the world, Australia could be part of a renaissance in threatened species conservation.
Obviously, genetic rescue is not the only way forward, but when habitat restoration fails, translocation in the wild, under certain conditions, may be a good option. Conservation genetics has always had a strong theoretical basis, but it is time to remember the practical applications.
In the meantime, I am just delighted to know that these lovely little animals, which have only three populations in the world, will be around for a few more years. I have had the privilege of trapping on Mt. Buller with Dean and I appreciate the hard work it takes to survey these nocturnal subterranean specialists. Also, it is impossible to overstate the beauty of these animals.
Once you have had the prehensile tail of a mountain pygmy-possum curl around your fingers, you can never forget them.
Susan Lawler is the Head of Department, Department of Environmental Management & Ecology.
First published on The Conversation on 16 January.