City-Friendly Eucalyptus Species Could Help Save Australia’s Koalas

Researchers explore use of endangered dwarf trees to provide both food and habitat to metropolitan marsupials.

Tucked away in an isolated pocket of tropical northeastern Australia is a dwarf eucalyptus tree that looks similar to its lofty 30-meter-tall cousins, but unlike them, grows to be just 5 meters tall. It also has another stark difference. This city friendly tree could be the key to securing the survival of Australia’s beloved koalas. In the process, it could also ensure a future for itself, or so experts hope.

photo of koala
Experts hope that urban parks and wildlife corridors could help ensure survival of Australia’s iconic marsupial. Photo by A. v. Z. / Flickr.

Koalas, which are listed as vulnerable on the International Union for the Conservation of Nature (IUCN) Redlist, are on track to become extinct in parts of Australia by 2050, in part because of the recent impact of bushfires — more than 5,000 individuals died as a direct result of fire earlier this year. Koalas rely exclusively on eucalyptus trees for both food and shelter and expansive tracts of koala habitat were obliterated by the fires. Populations already vulnerable to climate change, land clearing, and prolonged drought were left starving and homeless.

Urban koala populations, which experience comparatively low stress levels, could be key to conservation of the species. Unlike their forest and rural dwelling counterparts, they don’t face risks from bushfires or livestock conflict. Living in and around towns or cities, their main concerns are finding food and other koalas to mate with. However, habitat fragmentation can make them vulnerable to inbreeding and traffic conflict. Given the right environmental tools, though, koalas can navigate the urban jungle with ease.

Chrissy Joester, a zoologist in the nursery industry who works closely with koalas and wildlife caretakers to preserve populations in and around the Moreton Bay area in coastal Queensland, says that connecting habitat fragments is vital for the future of the species. “With habitat loss high on the list of threats for koalas, we need to be providing safe corridors for them to use in addition to large housing trees.”

Not all trees are created equal when it comes to koala compatibility and urban plantings. One common koala housing tree is the blue gum eucalyptus, which also happens to be a favorite food tree for koalas. Reaching 20 meters or more in height, blue gums are unpopular in urban centers simply because of their sheer size. Prone to ‘sudden limb failure,’ the shedding of apparently healthy branches, they also pose significant risk to people and property.

In contrast, the closely related Mount Beerwah mallee is just a third of the size but still shares the palatability characteristics of its taller sister species. Of the 900 eucalypt species in Australia, koalas will only eat leaves from between 40 and 50 species. “Depending on locality of koalas, they have a selection of approximately 10 species that they will feed on,” Joester says. “Generally, they have a few select primary species and then more secondary species when those are not available.”

Reaching its stunted size in just six years, the Mount Beerwah mallee (Eucalyptus kabiana) is ideal for providing habitat highways for koalas to get about town. The tree, which is listed as endangered on the IUCN Redlist and is found in only two naturally occurring isolated populations in the Glasshouse Mountains National Park in Queensland, also responds well to propagation.

It’s because of its palatability, shorter height, and high propagation success, that researchers have honed in on the species as a top contender for urban plantings.

Still, there are potential challenges with the use of Mount Beerwah mallee trees to support koala populations. For one, while eucalypt trees are a must for Australian revegetation programs, the dwarf tree isn’t extensively available in nurseries. “I’m yet to see E. kabiana widely available,” Joester says. “Its status is vulnerable, so it may be limited as to who can produce them.”

Joester also has reservations about the effectiveness of E. kabiana as a holistic solution to providing koala habitat in towns and cities. While koalas may use them for food, or stay in the smaller trees for a few days, she’s not sure that they would use the dwarf species “as a roosting or housing tree,” she says. “They generally prefer something a bit higher.”

But she still thinks they could serve an important role, particularly to improve habitat connectivity.

Lack of corridors exacerbates habitat fragmentation problems caused by land clearing for housing and urban development, which further compounds threats to metropolitan marsupials. A single koala can occupy an area up to 50 hectares in urban and peri-urban settings. “It can take 25-30 years for a good housing tree and 2-7 years for a koala to use a tree as a food source and temporary housing as they move from tree to tree”, says Joester. Without suitable green spaces, the risk of death and local extinction as urban koalas try to navigate their habitat remains an ever-present threat. “Connection is life,” explains Joester.

“Corridors need to be at least 100 meters wide to be most beneficial for wildlife. Providing these around new developments will aid wildlife in adjusting to habitat clearing,” she adds.

Planting koala food trees both in wildlife corridors and parks can encourage movement, reduce risk of injury or death, and help to alleviate the risks of inbreeding, a big problem for isolated koala populations. Though koalas prefer to roost in tall forest trees, they will frequently make use of smaller or juvenile trees, rehabilitated sites, and trees in urban streets and gardens for short periods of time. In other words, the tiny but mighty mallee tree could offer a bed and breakfast for city koalas on the move.

For all these reasons, researchers with the Urban Koala project have partnered with several groups to plant more than 600 Mount Beerwah mallee trees in Moreton Bay parks, gardens, and wildlife corridors. The number of Mount Beerwah mallees released to the community is likely to overtake the number of trees naturally occurring in the two wild populations. Koalas are responding well to the new habitat, and while other local governments have not yet taken up the initiative, Joester says local koala groups are monitoring progress with respect to population growth and habitat use. “We keep in touch with groups monitoring koalas in the area with collars and tagging,” she says.

This is a unique situation where the conservation of a threatened plant has been assisted by its use in conserving an iconic marsupial. As undisturbed natural spaces continue to shrink, we might look to solutions like this to aid the wildlife we share our urban spaces with.

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