It is well known that koalas need eucalyptus (gum) trees. But how many gum trees do koalas need to survive, breed and thrive?
The answer depends on the location, but in most locations, koalas need more trees than you think.
In fertile, wet, coastal Queensland and New South Wales koalas might only need 400 trees each. In dry western Victoria koalas might need 20,000 trees each.
In the You Yangs, our research has found that wild koalas need 7,000 to 30,000 trees.
Whichever way you look at it, koalas need a lot more trees planted. They also need the trees to be in fertile soil with high moisture. Read why koalas need trees in rivers and drainage lines here.
How to estimate how many trees a koala needs.
Here’s a summary of the home range sizes and tree estimate* for wild koalas in the You Yangs in 2017:
*tree estimate is based on mature eucalyptus trees at a spacing of 5m x 5m (so one tree every 5 m). This gives 400 trees per hectare (ha). In fact there are more trees than that in most koala home ranges. Acacias, Cherry Ballarts, Melaleucas and other important koala roost trees are not counted.
Pat (older female 13 yo): 18.3 hectares = minimum 7,320 trees
Ngardang (young female): 30.9ha = min. 12,360 trees
Misty (mature female): 22.5ha = min. 9,000 trees
Winberry (older male): 46.8ha = min. 18,720 trees
Clancy (mature male): 47.4ha = min. 18,960 trees
Anzac (older male): 34.9ha = min. 13,960 trees
Cruiz (older male): 66.9ha = min. 26,760 trees
Average koala home range size in You Yangs = 38.2ha
(to calculate an average, add all the home range sizes you have together, then divide by how many home ranges you have)
To find the average home range size of koalas (in hectares) in your area check this information:
1.2 hectare (females) & 1.7 ha (males) on French Island, coastal east Vic
23.9 ha (females ) & 37.3 ha (males) in You Yangs, inland west Vic
15 ha (females) & 34 ha (males) at south-east Qld
8 ha (females) & 16 ha (males) at Brisbane, south-east Qld.
101 ha (females) & 135 ha (males) at Blair Athol, central Qld
NEW SOUTH WALES:
11ha (females) & 45 ha (males) at Pilliga Forest, north-west NSW
10ha (females) & 23 ha (males) at Coffs Harbour, coastal NSW
44ha (females) & 82 ha (males) at Lismore, inland north-east NSW
This excellent article gives a table of the known koala home range size in different parts of Australia, and details the different methods used to calculate them:
Goldingay, Ross L, and Barbara Dobner. “Home Range Areas of Koalas In an Urban Area of North-east New South Wales.” Australian mammalogy, v. 36,.1 pp. 74-80 http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.889.8407&rep=rep1&type=pdf
If in doubt, use the closest location, or use our You Yangs figures for anywhere west of the Great Dividing Range, and halve it for wet areas in coastal Queensland and New South Wales.
How many koalas share a territory / home range?
Koalas do share some of their home space with other koalas. This is also dependent on location and soil fertility & moisture. In the You Yangs the degree of overlap is small.
In 2017 Koala Pat shared around 80% of her home range with male Anzac, and around 20% of her home range with female Misty. Misty shared about 20% of her home range with Pat & Anzac, and about 50% with male Cruiz. This situation is typical in the You Yangs.
So home range overlap equals 2 (ie. 2 koalas are using the same area).
Finding the home range overlap in your area is even harder than estimating home range size. There is very little data. It could be as high as 4 or 5 in some rich fertile areas, or as low as 1 in dry areas.
If in doubt, assume it is 2.
How many koala trees need to be planted?
The simple answer is as many as possible! But it does help to have an idea of how many to aim for.
First consideration is that a koala cannot live alone. Viable populations number in the hundreds or thousands of animals. If you are planning for an entire koala population, you need to be planting hundreds of thousands of trees.
If you are designing for a development that is cutting down koala habitat trees you need to be retaining enough trees for the current koala population, or planting new trees for that entire population. Leaving 100 trees “for the koalas” is not enough for even one koala.
If you are planning to help out an existing koala population, then you should try to plant for at least 10 koalas.
To estimate how many koala trees need to be planted use this calculation:
(average koala home range size in ha) x 400* trees = A / (home range overlap) = number of trees per koala.
*400 trees per hectare. It is possible that this figure could be higher or lower depending on the fertility of the area and the size of the trees.
Around the You Yangs our calculation looks like this:
38.2 x 400 = 15,280 / 2 = 7,640 trees needed per koala.
7,640 x 10 koalas = 76,400 trees to be helpful.
And to provide for the entire koala population of the You Yangs:
127 koalas x 7640 trees = 970,280 trees.
Please don’t be put off by these numbers – any tree is better than no tree. But aim high. This is urgent.
Koala Clancy Foundation have very proudly planted nearly 8,000 trees in the last 3 years. It feels like a lot, but it is not nearly enough. Please help, by sharing this post, by alerting landowners, by attending one of our Koala Conservation Days, by donating.
Koala Clancy Foundation are running our second annual Koala Tree Planting Information Event on Thursday 28 November at 7.30pm at Balliang Hall, Balliang VIC. More information here:
NOTES & REFERENCES:
Ellis W. A. H. , Melzer A. , Carrick F. N. Hasegawa M. (2002) Tree use, diet and home range of the koala (Phascolarctos cinereus) at Blair Athol, central Queensland. Wildlife Research 29, 303-311. http://www.publish.csiro.au/WR/WR00111
Moore Benjamin D. Foley William J. (2000) A review of feeding and diet selection in koalas (Phascolarctos cinereus). Australian Journal of Zoology 48, 317-333.
Koalas Need Farmers & planting in drainage lines: https://koalaclancy.wordpress.com/2018/09/14/koalas-need-farmers/
Koala Population Decline in the You Yangs: https://koalaclancy.wordpress.com/2018/11/04/koala-population-decline-in-the-you-yangs-victoria/