23 Aug. 2022 5 min read
by Koala Life Research Manager, Dr Ian Hough
All koalas throughout Australia are facing uncertain futures (National Recovery Plan for the Koala, 2022). Deforestation, habitat clearance, roads, dogs, disease, loss of genetic diversity, and natural disasters are affecting all populations.
In the north koala numbers are declining dramatically. Habitat clearance for agriculture and urban development, increased heat and drought from climate change, and contact with cars, dog and humans are the major drivers in the decline. In addition, infective diseases such as Chlamydia, retrovirus (KoRV) and mange all impact survival greatly. The highly selective nature of their diet (only a handful of eucalypt species are edible) and that their habitat exists prominently in urban and agricultural growth regions also contributes to the decline of the koala. In Qld, NSW and the ACT the koala is now characterised as endangered.
While land clearance is often considered the primary threat to koala habitat, the impacts of climate change are likely to be more devastating. The eucalypt forests that support koala population are going to undergo significant change as the effects of global temperature increases continue. Sadly, the active replanting programmes being undertaken by the Qld and NSW Governments may well fail to provide adequate long-term solutions to the shrinking eucalypt forests.
The situation in South Australia is ultimately no better, in fact being such a dry state, it may be worse.
Looking at the eucalypt tree family, a 2oC rise in ambient temperature will exceed most trees thermal tolerance. Currently the world is on track for a 3oC rise and is sitting at about 1.8oC now. This means most of Australia’s eucalypt forests will be under thermal stress from mid to later this century. This does not consider less frequent rainfall nor increased bush fire threat, which are also predicted as part of a changing climate. Eucalypts grow best at temperatures around 11oC; at temperatures above and below that threshold growth is slower. The predicted 3oC increase will see eucalypts grow at a 22% reduced rate, meaning less leaf and slower recovery from both browsing and fire.
Koalas are dependent on a small number of eucalypt trees overall for their feed, and water. These trees are already highly regional, having adapted to the conditions in which they currently grow. Changes in the survival and speciation of eucalypt forests will have an enormous impact on koala survival.
Tree plantings for koalas being undertaken now need to consider the long-term survival of those species, in their current ranges. Biome work done on koalas recently highlights that the species of gum consumed influences the gut biome and the biome in turn influences appetite and gum selection. The gut biome is critical in digesting and detoxifying the diet of koalas.
In South Australia, the primary koala food trees comprise a small number of species. These species being E. camaldulensis (River Red Gum), E. leucoxylon(SA Blue Gum), E. baxteri (Brown Stringy Bark), E. obliqua (Brown Top Stringy Bark or Messmate), E. globulus (Southern or Tasmanian Blue Gum), and E. viminalis (Manna Gum). All these species will be impacted by climate related temperature changes within the next 50 years, if not sooner. Shrinking of the range is normally toward the temperate coast occurring in the region.
The less commonly eaten species, or non-endemic to South Australia, species include E. ovata (Swamp Gum), E. goniocalyx (Long Leafed Box), E. macrocarpa(Grey Box), E. platypus (Playtpus Gum or Moort), E. occidentalis(Swamp Yate), and E. nicholii (Narrow Leafed Black Peppermint).
This species is predicted to shrink its current range by nearly 30% by 2085. It is likely to shift to the Southwest.
This species is predicted to slightly increase its range by about 2% in the same time frame. Changes will again be south-westerly. This species has a large water demand and may well experience range shrinkage with rainfall changes and permanent water changes, not factored into this study.
This species will shrink its range by nearly 40% by 2085. It will shift southerly by a significant distance.
This species will shrink its current range by a disturbing 45% in a southerly direction.
Messmate will shrink by almost 50%. It will also shift south-westerly a considerable distance.
Manna will shrink by 42% a considerable distance in a southerly direction.
Less common species:
This species will shrink by 44%, in a southerly direction and over a large distance.
This species will shrink considerably, by nearly 50%, in a southerly direction and over a large distance.
This species will shrink by a disastrous 62%, in a southerly direction.
This species will also shrink its range by nearly 50%, in a southerly direction.
Swamp Gum will shrink by nearly 40%, in a southerly direction.
Moorts will shrink by up to disastrous 65%, in a southerly direction and over 200km.
World temperature increases are likely to top 3oC by the mid to late 2000s. This will put significant pressure on the Australian eucalpyt forests as it will exceed the tolerance of many species. Growth rates will also be greatly affected, meaning grazing pressure from koalas will be an increasing threat.
The limited range of species eaten by koalas will need to expand if koalas are to remain in their current distributions. This mean planting of more drought tolerant species is a vital piece of strategy for the long-term survival of koalas. Recognising the shifting distribution of the preferred eucalypts means planning for a major shift in koala distribution needs to be undertaken sooner rather than later.
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Butt N, Pollock LJ, McAlpine CA. Eucalypts face increasing climate stress. Ecol Evol. 2013 Dec;3(15):5011-22. doi: 10.1002/ece3.873. Epub 2013 Nov 12. PMID: 24455132; PMCID: PMC3892364.
González-Orozco, C., Pollock, L., Thornhill, A. et al. Phylogenetic approaches reveal biodiversity threats under climate change. Nature Clim Change6, 1110–1114 (2016). https://doi.org/10.1038/nclimate3126
Hughes, Lesley, et al. “Climatic Range Sizes of Eucalyptus Species in Relation to Future Climate Change.” Global Ecology and Biogeography Letters, vol. 5, no. 1, 1996, pp. 23–29. JSTOR, https://doi.org/10.2307/299746....