Dr. Erika Sullivan

August 2015


Kangaroo Island

Kangaroo Island is an island off the mainland of South Australia. It is known for its diverse wildlife and nature reserves. It is a popular vacation destination amongst Australian residents and tourists, that visit to marvel the rugged natural landscapes and gaze at local wildlife including kangaroo, wallaby, koala, fur-seal, and bird populations. Besides tourism, the island is centered around agriculture, including honey from Ligurian honey bees and wool from grazing sheep. It is also home to South Australia's only eucalyptus oil distillery with oil distilled from the endemic Kangaroo Island Narrow Leaf Mallee.


Native island wildlife is varied and specifically includes the Kangaroo Island Kangaroo, Rosenberg’s Sand Goanna, Southern Brown Bandicoot, Tammar Wallaby, Common Brushtail Possum, Short-beaked Echidna, six species of bats, six species of frogs, and New Zealand Fur Seals. As it is naturally isolated from the mainland, introduced species such as foxes and rabbits are prohibited from entering, but some species like the Koala, Common Ringtail Possum, and the Platypus were once introduced and still thrive here.


Introducing non-native species to any environment poses the risk of disrupting symbiotic relationships between flora and fauna, by introducing competition for food sources. In the 1920’s, eighteen koalas were introduced to KI due to a concern that they could become extinct on the mainland from hunting, disease, and habitat loss. The introduced koalas have since flourished on the island, so much so that their preferred food source, the Manna Gum, is now at risk of local extinction. Tree damage from overbrowsing occurs when the koalas consume leaves to the extent that the canopy is devoid of leafage, causing death to the trees. Tree destruction on KI has been noted since the 1960s and has continued to threaten Manna Gums on the island as koala populations thrive.


Understanding Koalas

The koala is an arboreal herbivorous marsupial native to Australia. Three subspecies have been recognised, and debated, namely the Queensland koala (Phascolarctos adustus), the New South Wales koala (Phascolarctos cinereus), and the Victorian koala (Phascolarctos victor), which can be differentiated by pelage colour and thickness, body size, and skull shape. They are categorized by state borders with northern populations being smaller and lighter than their counterparts to the south. Genetic diversity between populations is low and high levels of inbreeding occur.


Koalas have stout bodies that lack tails, round furry ears, and spoon-shaped noses. They range in from four to fifteen kilograms in weight and their coat colour varies from silver-grey to chocolate-brown. Males are twice the size of females and contain scent glands on their chest (visible as a hairless patch). The fur on their backs is highly evolved to protect them from wind and rain, while the fur on their ventrum reflects solar radiation, making them evolved to survive some of Australia’s harsh climates.


Anatomically, they are designed to live in the trees. They have curved, sharp claws adapted for climbing. Their front feet have two opposable digits allowing them to grasp small branches, while their hind feet have two fused digits used for grooming. Koalas have a cartilaginous pad at the end of the spine that is thought to provide comfort when perching at the fork of a tree. Their handprints are unique, just like human fingerprints. They have proportionately more cerebrospinal fluid surrounding their primitive brains, which is thought to aid as a shock absorber should they fall from a tree. Having small brains, their ability to perform complex unfamiliar behaviours is limited. They use their large leathery noses to smell oils in branches while assessing palatability, but their vision is poor. They have unique vocal organs located in the soft palate called velar vocal cords, which produce low-pitch bellowing sounds that can travel far through air and vegetation. Bellows are used as mating calls to attract females, or as warning calls to announce their presence to neighbours upon entering a new tree. Females emit softer sounding bellows, in addition to making snarls and screams, in response to defensive threats. Although pregnant and nursing females are territorial and have been known to attack invaders, most koalas avoid energy-consuming behaviours.


Koalas eat leaves from eucalypt trees. Because their diet is naturally low in calories, they lead sedentary lifestyles, sleeping as much as twenty hours a day. They can eat up to a kilogram of leaves per day. Their herbivorous dentition is designed for their eucalypt diet and can finely grind leaves to aid digestion. Koalas store food in their cheek pouches before it is ready to be chewed, and sometimes regurgitate food to be chewed a second time, similar to rumination in cattle. They are hind-gut fermenters with large cecums obtaining ten percent of their energy from cecal fermentation; their cecum is also designed to store and conserve water in times of drought. Since eucalypt leaves are high in water content, female koalas (being smaller) do not need to drink often, however larger males require additional water sources found on the ground or in tree hollows.


Adults are solitary preferring to be alone, unless a mother is caring for dependent offspring. Males mark their presence via secretions from the scent glands located on their chest, and have been known to scent mark trees with urine. Koalas are seasonal breeders, giving birth from October to May. Due to long dependency periods, koalas usually breed in alternate years, unless favourable environmental factors like an abundance of high-quality food trees allows them to reproduce annually. Gestation lasts 33 to 35 days. Being marsupials, females give birth to embryonic offspring, which crawl into the dam’s pouch for further development. Twins are rare and baby koalas, called joeys, are fully attached to a teat in the pouch for up to six months. At this time, the dam will produce a faecal pap that the joey eats from her cloacum, in an attempt to offer supplemental protein while transitioning the baby from a milk to leaf diet. After seven months, the joey fully emerges from the pouch, weighing 300 to 500 grams, and clings to its mother for support while cautiously exploring the new environment. At one year of age, joeys are fully weaned.


Koalas in the wild live from twelve to fifteen years. With few natural predators, koalas are threatened by natural disasters such as bushfires and drought, and infectious pathogens such as Chlamydia and the Koala Retrovirus. In the 20th century they were hunted for their fur, and widespread cullings of populations in Queensland and New South Wales have left them listed as vulnerable. The largest threat to their existence is habitat destruction caused by agriculture and urbanisation, causing many to become injured or killed by vehicular accidents while searching for safety, water, or mates in urban areas.



Impact of Koalas in Nature

Koalas are herbivorous and only feed on certain Eucalypt species. Since limited areas of woodland contain the preferred trees, this highly selective browsing poses significant threat to Kangaroo Island’s unique vegetation. As a result, large areas of manna gum (Eucalyptus viminalis v. cygnetensis), the koala’s preferred tree, have been lost on KI through overbrowsing. Koalas show a strong preference for species containing a high protein content and low levels of lignin and fiber. Other native tree species preferred by koalas on KI include: red river gum (Eucalyptus camaldulensis), South Australian blue gum (Eucalyptus leucoxylon), swamp gum (Eucalyptus ovata), messmate stringybark (Eucalyptus oblique), brown stringybark (Eucalyptus baxteri), marsh gum (Eucalyptus paludicola), Port Lincoln mallee (Eucalyptus albopurpurea), and peppermint box (Eucalyptus odorata).


The consequences of overbrowsing on KI has resulted in the loss of some of these species in certain regions, and poor tree health across a broader region. Manna gums grow along riverways and provide important habitats for other native animals, including bees, pardalotes, and the regionally-rare White-naped Honeyeater. Loss of these trees affects the entire ecosystem. Firstly, the other species dependent on them risk survival. Along rivers, the loss of trees can result in erosion of stream banks and sedimentation of waterways. And the loss of shade and changes to ground litter (bark and leaves) affects smaller plants and ground-dwelling invertebrates.



Kangaroo Island Koala Management Program

Since culling koalas is not permitted under the National Koala Conservation and Management Strategy, a program was started in 1996 to protect habitat from overbrowsing. Ongoing management involving capture and sterilisation is important to maintain a smaller population and ensure that numbers do not increase again. Since 1996, over 11,000 koalas have been sterilised, many of which have been translocated back to native South Australian habitat. In densely populated areas on KI, it is estimated that 35% of the females have been sterilised, resulting in substantial declines in koala numbers born each year in one area.


I was fortunate to have been able to volunteer with the KI Koala Management Program in March 2015. Certain days of the week were dedicated to catching the koalas, where a team of SA government workers and I set out in utes with appropriate equipment for catching koalas. Resident landholders granted us access to their properties where high density koala populations had been noted to  impact vegetation. After finding forested areas containing the desired species of eucalypts, we surveyed the grounds on foot while looking high into the trees for koalas. Males found were marked with GPS coordinates for population-control research. Females found, if not already tagged, were caught. To catch a koala from a tree involved placing a flag on a lengthy pole and waving it in front of their face; a threatening gesture that resulted in them climbing down the tree. Once near the bottom, they were placed into a holding sack and transferred to a transportation vessel. Next, the koalas were transported to a local veterinary hospital which contained a natural outdoor enclosure for them to rest in, while awaiting surgery.


Being a veterinarian, I found the surgical sterilisation of female koalas fascinating. Just like our companion animals, the koalas were given sedative analgesics and general anaesthetics to avoid causing them any stress or pain. Once fully unconscious, the koalas were given a complete physical examination. By noting the ridges and wear on the cusps of their molars, their age was predicted up until five years. After six years of age, koalas’ chewing teeth begin to wear down and chewing efficiency decreases. Each koala was weighed and skull size was measured. Any signs of pathogens including sarcoptic mange, skin ulcers from Mycobacterium ulcerans, tapeworms and nematodes, Chlamydia, or Koala Retrovirus were noted and treated. Each koala had a tag placed in its right ear, to identify that it had been sterilised and avoid being caught a second time.


To perform surgery, the anaesthetised koalas were restrained on a surgical table and aseptically prepped for the procedure. Using surgical endoscopy allowed for small incisions and fast recovery times, with no requirement for suture removal. Female koalas have two lateral vaginas and two separate uteri. The uteri and oviducts were electrocauterized and skin incisions were closed with absorbable suture material. After uneventful recoveries, the koalas were returned to the wild, to the exact tree and forested region where they were caught.



Success For All

Since the implementation of the KI koala program, marked improvement in tree condition has been recorded and over 15,000 trees have been planted to restore over-browsed habitat. Koala management is essential to protect the unique environments of Kangaroo Island. The island has an extremely high conservation value, with large areas of native vegetation containing endemic species to KI, and even healthy populations of rare and threatened species to mainland Australia. Importantly, koala overbrowsing threatens the survival of the koalas themselves. There is no evidence suggesting that koalas can self-regulate their numbers like kangaroos do. Without effective management, koalas could consume all their available food sources and starve. The KI Management Program demonstrates that trees can recover when koala populations are controlled. It was an honour and rewarding opportunity to have participated in this worldwide leading population-sterilisation program, which offers an effective alternate solution to culling koalas, a species that is already listed as vulnerable.




  1. Koala management 2014 - 2015, accessed 7 August 2015, <http://www.naturalresources.sa.gov.au/kangarooisland/plants-and-animals/native-animals/koala-management>

  2. Koala Management Program Update 2014 - 2015 PDF; Natural Resources Centre Kangaroo Island, accessed 30 July 2015, <www.naturalresources.sa.gov.au/kangarooisland>

  3. Koala Fact Sheet, accessed 7 August, 2015, <www.environment.sa.gov.au>

  4. Koala, accessed on 30 July, 2015, <https://en.wikipedia.org/wiki/Koala>

  5. Lee, A. K., K. A . Handasyde., and G. D. Sanson (editors) 1991. Biology of the Koala. Surrey Beatty and Sons Pty Ltd, and World Koala Research Corporation Pty Ltd.

  6. Lee, A. and R. Martin. 1996. The Koala: A natural history. University of New South Wales Press, Sydney, Australia.

  7. Martin, R. and K. Handasyde. 1999. The Koala: natural history, conservation, and management Australian Natural History Series, 2nd Edition. University of New South Wales, Sydney, Australia.

  8. Masters, P., T. Duka, S. Berris, and G. Moss. 2004. Koalas on Kangaroo Island: from introduction to