Every year thousands of wild animals come into conflict with us, the self-proclaimed rulers of the planet. When faced with our motor vehicles and domestic pets, they almost always come off second best with trauma accounting for well over 80% of the cases admitted to veterinary clinics for care. Unfortunately over 50% of these are euthanased more or less on admission because of the severity of the trauma that has been inflicted upon them.
The rest are assessed, treated and, in the best case scenario, released back into the wild. In general of all the wild animals admitted to veterinary hospitals less than 40% will be released. But what happens then? The assumption is that, once released, the individual will go forth and multiply, resuming its role in society and living happily ever after in the natural world. But is this what really occurs?
More and more follow up studies are being done to determine the actual fate of these released animals and it is becoming increasingly apparent that mortality rates of rehabilitated animals can be very high.
A 1992 review of macropod reintroductions produced some extremely gloomy results. Over 670 quokkas were reintroduced to the University of Western Australia field station at Jandakot over a period of 16 years (1972-1988). Despite supplementing the population with an average of 55 animals per annum from 1972 to 1983 the population had dropped to only nine individuals five years after supplementation was stopped. Similarly 85 tammar wallabies were introduced to the same field station between 1971 and 1981. No tammars were found after 1982. In 1972 24 parma wallabies were released onto Pulbah Island in Lake Macquarie. All wallabies were gone within 10 weeks of release. Another group of 45 parma wallabies was released near Robertson in NSW in 1988. All were dead within three months of release. The failure of all these populations to establish was predominantly due to predation and competition from rabbits (Short et al 1992).
A 1988 study looked at the fate of 31 captive reared malleefowl that were returned to the wild. Of these 52% were dead after seven days, 71% after 11 days and all were gone by 107 days, the majority being killed by introduced predators, principally foxes (Priddel and Wheeler 1994).
Other studies compared deaths of released animals with that of the natural population.
A Melbourne investigation compared survival of a resident population of brushtail possums with a translocated group over a period of 13 weeks. During this time, no resident possum carcasses, partial remains, or patches of fur were found. After the release of relocated possums in the same area, researchers found numerous patches of possum fur and the remains of eight possums likely killed by foxes.
Another 12 possums fitted with radio-collars were released, but only two were alive after 10 weeks. Seven possums died within the first week: four killed by predators, probably foxes; two died of stress-related causes; and one was hit by a car. Two of the collared possums could not be found (Pietsch 1994).
In Spain there was a significantly higher mortality of rehabilitated and captive bred barn owls compared with wild birds during the first four weeks after release. Of 41 released birds which died 51.2% were struck by motor vehicles and 26.8% starved. Interestingly owls that had live prey training prior to release had a significantly greater chance of survival than those that did not (Fajardo et al 2000).
A 1996 study of ringtail possums released into Ku-ring-gai Chase National Park north of Sydney found that hand reared and relocated possums survived an average of 101 days compared with wild ringtail possums that survived 182 days. Fox and cat predation was the overwhelming cause of death in both groups, but there was no significant difference in survival between the two groups (Augee et al 1996).
A study of timber rattlesnakes in Pennsylvania found that of eleven translocated snakes six died during the study period. This compared with only two of 18 resident snakes that died during the same period. Translocated snakes travelled much more extensively than did the resident snakes (Reinert and Rupert 1999).
A study examined post release survival of little penguins following the Iron Baron oil spill in Tasmania. Over a period of 20 months survival of oiled birds was 59% for penguins from Ninth Island and 44% for birds from Low Head, compared with non-oiled birds 77% of which survived from Ninth Island with 50% surviving from Low Head. Survival was directly related to the condition of the bird on arrival and release and the degree of oiling (Goldsworthy et al 2000).
A study of koalas following the Port Stephens fires of 1994 found a survival rate of 58% of 16 injured, rehabilitated and released koalas compared with 67% for 23 uninjured koalas. Predation by dogs was the main cause of mortality (Lunney et al 2004).
In 2006 and 2007 46 western ringtail possums that were orphaned or displaced from building development sites near Busselton Western Australia were translocated to Leschenault Peninsula, Martins’ Tank and Preston Beach Road near Bunbury. Of the 46 possums 29 died, 12 were lost because of radio-collar malfunction and five were still alive in mid-2007. Deaths were mostly attributable to predation: cats, carpet pythons or raptors. A further 23 possums were radio-collared near Busselton and not translocated. Of these eight died, two were lost to follow up and 13 remained alive. Deaths were mostly due to malnutrition rather than predation.
A similar study was done on common brushtail possums. Of 21 translocated possums five died compared with the non-translocated possums where nine of 22 possums died mostly due to fox predation (McCutcheon et al 2007).
A more successful study took place in Queensland. Two koala releases in the Gold Coast region were done in 1995-96 and 2002-03. In the first study 16 koalas were radio-collared and translocated with a further 11 taking part in the second study. No confirmed mortalities occurred in the first study but three collars failed. In the second study one koala was found dead three months after release and two collars failed.
Another study in the Gold Coast area monitored the release success of 13 brushtail possums for two months. Three possums died, two being eaten by carpet pythons and one due to trauma (Tribe et al 2005).
What most of these studies have in common is a high mortality rate of released animals (along with a not insignificant mortality rate of the resident population in many instances). But does this mean that rehabilitation and release are a waste of time; after all they almost all involve common species (Still, once upon a time, there was none commoner than the now extinct passenger pigeon)?
It is important to differentiate between rehabilitation and translocation. Most of the previous studies focussed on translocation, which involves the release of individuals into an area with which they are unfamiliar. Rehabilitation returns injured or sick individuals to their point of origin. The two are quite different and have vastly different outcomes. Translocated individuals often have to compete for territory and resources with established residents. As demonstrated by the rattlesnake study translocated animals travel more in order to find and establish their own niche which makes them more susceptible to predation and conflict with motor vehicles. It has been shown that kookaburras returned to familiar territory travel less and have better chances of survival than those that are released into unfamiliar areas.
While injured wildlife should always be released as close as possible to where it was found to maximise its survival chances, translocation does have its place and is frequently used for endangered species, such as orange-bellied parrots and brush-tailed rock wallabies, where individuals are introduced to historical colony sites.
It is important to realise that mortalities will be high and the focus needs to be on population rather than individual survival. Nevertheless in order to avoid the previous disasters highlighted by the aforementioned macropod and malleefowl studies a few principles need to be adhered to. Firstly, and perhaps self-evidently, it is important that the selected release site be in optimum condition. Secondly, the most common cause of failure is predation due mostly to introduced predators. While nothing can or should be done about python or raptor predation, sites must be adequately prepared prior to release by minimising the influence of introduced predators, generally by baiting or shooting. The idea of establishing neutered and vaccinated feral cat colonies is nonsense. Feral cats, dogs and foxes must be eliminated wherever possible. Thirdly it is important to prepare the release population as much as possible. As seen previously owls which had live prey training were less likely to fall victim to starvation than those that did not. A study of brown goshawks and peregrine falcons found an improved rate of survival if the birds were exercised to a reasonable level of fitness prior to release, rather than just turned loose without fitness training (Holz et al 2006).
Finally it is important to remember that “the wild” is not some kind of nirvana in which all animals live happily and have wonderful carefree lives. “The wild” is an exceedingly dangerous place and most animals are in peril of their lives every single day. Nevertheless, if done properly, wild animals can be successfully returned to their habitats to hopefully breed and survive and undo some of the damage we cause on a daily basis.
Dr. F. Bunny
Augee, M.L., B. Smith, and S. Rose. 1996. Survival of wild and hand-reared ringtail possums (Pseudocheirus peregrinus) in bushland near Sydney. Wildlife Research 23:99-108.
Fajardo, I., G. Babiloni, and Y. Miranda. 2000. Rehabilitated and wild barn owls (Tyto alba): dispersal, life expectancy and mortality in Spain. Biological Conservation 94:287-295.
Goldsworthy, S.D., M. Giese, R.P. Gales, N. Brothers, and J. Hamill. 2000. Effects of the Iron Baron oil spill on little penguins (Eudyptula minor). II. Post-release survival of rehabilitated oiled birds. Wildlife Research 27:573-582.
Holz, P.H., R. Naisbitt, and P. Mansell. 2006. Fitness level as a determining factor in the survival of rehabilitated peregrine falcons (Falco peregrinus) and brown goshawks (Accipiter fasciatus) released back into the wild. Journal of Avian Medicine and Surgery 20: 15-20.
Lunney, D., S.M. Gresser, P.S. Mahon, and A. Matthews. 2004. Post-fire survival and reproduction of rehabilitated and unburnt koalas. Biological Conservation 120:567-575.
McCutcheon, H., J. Clarke, P. de Tores, and K. Warren. 2007. Health status and translocation success of wild and rehabilitated possums. In: National Wildlife Rehabilitation Conference Proceedings.
Pietsch, R.S. 1994. The fate of urban common brushtail possums translocated to sclerophyll forest. In: Serena, M. (ed.) Reintroduction biology of Australian and New Zealand Fauna. Surrey Beatty & Sons, Chipping Norton.
Priddel, D., and R. Wheeler. 1994. Mortality of captive-raised malleefowl, Leipoa ocellata, released into a Mallee remnant within the wheat-belt of New South Wales. Wildlife Research 21:543-552.
Reinert, H.K., and R.R. Rupert, Jr. 1999. Impacts of translocation on behaviour and survival of timber rattlesnakes, Crotalus horridus. Journal of Herpetology 33:45-61.
Short, J, S.D. Bradshaw, J. Giles, R.I.T. Prince, and G.R. Wilson. 1992. Reintroduction of macropods (Marsupialia: Macropodoidea) in Australia – a review. Biological Conservation 62:189-204.
Tribe, A., J. Hanger, B. Nottidge, and T. Kawakami. 2005. Measuring the success of wildlife rehabilitation. In: National Wildlife Rehabilitation Conference Proceedings.