Hollows for Wildlife

I was told an interesting “tree-fact” recently - that mountain coolibahs (Eucalyptus orgadophila) form hollows younger than most trees.
This is a matter of great interest to much of our wildlife. 17% of Australian bird species need hollows to breed. 42% of our native mammals cannot survive without them to shelter in. 28% of reptiles need them. So do tree frogs, and bees, both native and introduced. And our most drought-hardy orchid, the black orchid Cymbidium canaliculatum could not exist in the wild without them.
The hollows they will use range in size from ones with openings from 2cm to 75cm diameter, and useful depths might be anywhere from 10cm to 10m.
We modern Australians have been very hard on our hollow trees. Clearing them has been seen as part of good forest management, and, on a smaller scale, as good housekeeping. There is good reason to clear them, of course, where a weakened tree might fall on a roof, but it’s a pity to see them disappearing on the “acreage allotments” which are expanding the borders of our suburbs so far into the bush. Their perceived untidiness counts against them.
Yet even dead trees should be retained where possible because of their high habitat value. Some of our wildlife actually prefer them to live ones, and they can be statuesque and beautiful things, deserving of a place in a garden. A good many gardeners who have no naturally-occurring dead trees seem to wish they had them, as they choose to enhance their landscaping by careful placing of interesting and good-looking stumps which they have collected from somewhere in the bush. How much luckier are those who have naturally occurring dead trees on their places!
A healthy environment has about 30 hollows per hectare. It would be possible to reproduce these conditions in our suburbs by retaining existing trees with natural hollows, and by supplementing them with sturdy, well-designed nestboxes. Each species has very specific requirements about shape and size, so variety is important.
We can also think about planting for the future.
It is true that even something like a mountain coolibah is unlikely to form hollows for its first seventy years of life, and most other trees take from 120 to 300 years to do it - but good gardening is not just about short-term results, after all.
Here is a list of particularly good hollow producing local species: Angophoras, Eucalyptus albens, E. moluccana, E. camaldulensis, E. melliodora, E. microcarpa, E. orgadophila, E. populnea, E. saligna, E. tereticornis, E. viminalis, and Lophostemon confertus.
Rainforest and scrub species also make valuable hollows, and cater for a different range of wildlife. Some good ones are: Castanospermum australe, Flindersia species, Gmelina leichhardtii, Siphonodon australe and Ficus species of the strangling type.