Relocation - problems and possible mitigation
As long as the translocation of animals that are going to lose their habitat is a continuing practice, we need to concentrate on achieving a successful reintroduction of those animals into new habitat areas. However, more than 50% of Australia’s reintroductions fail and most of these failures (up to 80%) are usually attributed to fox or cat predation (Short et al, 1992, Short, 2009, Moseby and Hill, 2011), which is not surprising as cats and foxes are also implicated in the high extinction rate of Australian mammals since 1788. (Moseby and Read, 2015)
The naiveté of native wildlife that was isolated from predators over an evolutionary timeframe is an often cited reason. They fail to recognise a predator and/or fail to show an effective response. Even if a predator is recognised as such and there is a defence reaction, it is often inappropriate – such as freezing when threatened by a cat or fox. (Moseby and Read, 2015)
The idea of training wildlife is not new (Moro et al, 2015) and some fairly early research shows that predator response skills can be improved through training but this did not always lead to improved survival rates. The newly acquired awareness was also not transferred to the next generation. (Moro et al, 2015)
Nevertheless, exposing for instance bilbies to cat carcasses and their urine while acting in a threatening way (chasing, trying to catch) resulted in higher vigilance, better recognition of predators and in behavioural changes. (Moseby and Read, 2015)
Even if the threatening and unpleasant experience was completely unrelated to the behaviour a predator would show (e.g. flicking with a rubber band, being squirted with water), it seemed to work if it happened in combination with visual (stuffed or dead predator) or olfactory (usually urine) clues. (Moseby and Read, 2015) It is doubtful that this could be done in a field situation but animals to be relocated could be trained first. Reinforcement of the learned behaviour would again be close to impossible though. (Moseby and Read, 2015) New research suggests that predator naiveté will eventually reduce with coexistence through adaptation. Unfortunately they also proved in the case of bandicoots that 200 years were not enough – 4000 years were though. (Frank et al, 2016)
Predator exclusion is still the most successful method used to re-establish threatened species populations in Australia. Relocation on to predator free islands started in 1920s and many have been very successful.
The hardest to fulfil but most important requirement for successful reintroductions is clearly the selection of an appropriate release site of sufficient size and with high quality habitat providing abundant food and shelter.
Predation pressure increases dramatically in times of resource depletion by drought and the resulting increase in competition. Reproduction and recruitment would also be lower during severe drought. The suitability of a release site therefore needs to be evaluated with a long-term perspective.
Populations are often established with only a small number of founders. Small populations are as such vulnerable to extinction but as mainly threatened taxa are translocated, the number of individuals available is usually highly restricted. (Batson et al, 2015)
If the density is too low, the few individuals could even be unable to find each other to mate (Allee effect- after Allee, 1938)) which could lead to a quick repetition of the local extinction after the relocation. (Sinclair et al, 2006)
Also the composition of the release group could play a major role. The sex ration should be dictated by the social and mating system of the species. In reality, the composition of the group often just depends on the need to remove a certain number of animals and how many can be caught or what is available from rehabilitation.
Issues such as personality can rarely be taken into account and it is unclear for most species which traits would be beneficial for survival. There is some indication that mortality is higher in bolder animals than shy animals after release. (Bremner-Harrison et al, 2004, May et al, 2016)
Physical traits of released animals obviously influence outcomes and animals in good body condition and with a certain maturity might survive longer than others. The best season of the release is another possibly important parameter that is often ignored when animals become available for translocation.
The ultimate goal of every reintroduction program is a self-sustaining population that does not require ongoing management. However, intensive management during the introduction phase could achieve this earlier or at least could reduce the rate of failure.
In some programs short-term supplementary feeding has been used to assist with the acclimatisation after release. A different strategy however would be a relatively long-term supplementary food supply in order to increase success rates for populations under predation pressure. A review of studies concluded that in addition to better establishment success, support feeding led not only to better body conditions and advanced breeding but also to small home ranges and larger population sizes in a short timeframe. Larger populations with higher recruitment can tolerate higher predation pressure without the risk of quick population collapse (Boutin, 1990) – as long as the habitat values in general are acceptable.
With less acceptance of ’salvage’ translocations and less development funding for translocations, the majority of animals that need a new habitat in the future might be animals after rehabilitation. Behavioural changes through captivity might add another layer of problems to the already difficult process. Adaptation to captivity might result in lower fitness and in the long run in lower genetic diversity (Weeks et al, 2015).
So far zoo-based breeding programs are mainly initiated to establish insurance populations in captivity for species at imminent risk of extinction in the wild (Jakob-Hoff et al, 2015) and there is no current breeding program for western ringtail possums in place.
Animals from rehab facilities that are not part of a government-led program are usually not health screened before translocation and pose an increased risk of pathogen transmission between sites and species. (Clarke et al, 2013)
Disease could however also be a result of the reintroduction/translocation process itself, a risk factor that would have to be investigated by diligent and long-term monitoring and surveillance.
Monitoring would also safeguard that dangerous declines do not go undetected - as happened in Leschenault Peninsula and recently in Karakamia Sanctuary - and give valuable information about predator/prey reactions which would be particularly interesting during and after baiting programs.
As reintroduction usually involves small sample sizes and a variety of locations, collection and integration of data from multiple translocations to multiple sites would also improve the reliability of predictions and allow the evaluation of the importance of factors that vary between sites, such as habitat quality or predator abundance. (Chauvenet et al, 2015)
Long-term monitoring seems paramount for any successful reintroduction and the lack of it is maybe the most often criticised short-coming.
