The value of natural tree hollows for Australian fauna

Nothing beats the hollows of old alive or dead trees for offering natural habitat, safety, breeding and roosting space for any number of animals.

What makes a tree hollow?

Ageing, natural fungal decay, insect attack and bushfire create hollows in a handful of Australian tree species that become home to a range of Australian fauna.

Tree hollows take some 100 to 150 years to form in an Australian tree. Long-lived Australian eucalypts are some of the most likely species to form tree hollows.

Example species that form hollows include:
– River red gum (Eucalyptus camaldulensis)
– Manna gum (Eucalyptus viminalis)
– Mountain grey gum (Eucalyptus cypellocarpa)
– Yellow box (Eucalyptus melliodora)
– Brush box (Lophostemon confertus)*
– Antarctic beech (Nothofagus moorei)
– Flooded (or Rose) gum (Eucalyptus grandis)*
– White mahogany (Eucalyptus acmenoides)*
– Grey gum (Eucalyptus biturbinata)*.

National Parks and Wildlife Service NSW: Natural tree hollows: essential for wildlife (PDF)

*Found in the Tamborine Mountain Escarpment Protection Precinct

How do animals use tree hollows?

In Australia, some 17 per cent of bird species, 42 per cent of mammals and 28 per cent of reptiles use tree hollows.

The significant range of fauna using hollows includes bats, possums, gliders, owls, parrots, antechinus, ducks, rosellas, kingfishers, snakes, frogs and a range of lizards. In South East Queensland, up to 130 species need tree hollows to survive.

Land for Wildlife Queensland: The Value of Habitat Trees (PDF)

Hollows come in various sizes, from 2cm to 30cm. This size difference provides habitat for a range of animal species. The smaller entrances are used by animals, such as bats no larger than 10 grams, while the bigger entrances are used by larger animals, such as the Powerful Owl, the Glossy Black Cockatoo and possums.

Department of Environment and Climate Change: Hollow bearing trees (PDF)

There is a variety of reasons animals use tree hollows, including for shelter, nesting, roosting and foraging. Tree hollows are especially crucial for species that need them for nesting and roosting, including a range of threatened species.

The number of tree hollows required by animals

As a guide, there needs to be between 3 and 10 hollow-bearing trees producing up to 30 tree hollows per hectare to provide adequate habitat and guarantee the health and genetic diversity of a range of species.

National Parks and Wildlife Service NSW: Natural tree hollows: essential for wildlife (PDF)

The right landscape is vital to producing the necessary number of tree hollows and hollow varieties to house animals from different species as well as animals from the same species.

National Parks and Wildlife Service NSW: Natural tree hollows: essential for wildlife (PDF)

Threats to tree-hollow wildlife

A number of species use dead trees with livable hollows. Dead trees last only for a decade, sometimes a little longer, and provide short-term habitat for animals dependent on them. If human activity reduces forest area, fewer supporting trees are available to create future hollows. Reduction in hollow habitat puts dependent wildlife under stress, which can affect their numbers and lead to local species collapse / extinction.

Eco Magazine: Trees and non-flying mammals: a hollow understanding

Nesting boxes are often used as tree-hollow replacements. But are they adequate compensation for the loss of hollow-bearing trees, where the loss of the tree could be and should be avoided?

“Increasingly, artificial hollows (nest or roost boxes) are being installed to compensate for the loss of hollow-bearing trees. However, there is much debate about how effective nest boxes are as replacements for natural hollows … It is clear that nest boxes should not be used to justify the removal of hollow-bearing trees or unsustainable forestry practices.”

Eco Magazine: Trees and non-flying mammals: a hollow understanding

We’ve just scratched the surface of what tree hollows provide and how they’re used by Australian fauna.

We have a good understanding of the effects threats to hollow habitat can have on species numbers. This understanding makes the protection of good, healthy natural hollows vital for the safety and diversity of an amazing range of our native animals.

More information:

Wires Northern Rivers: Tree hollows and nestboxes

Western Australian Museum: Veteran and stag trees