Tree hollows are naturally-occurring holes in living or dead trees. Hollows form in many species but are most abundant in eucalypts. Termite activity, storms and fire contribute to both the formation and destruction of tree hollows.
Termites attack live eucalypts, increasing the number and size of hollows as tree ages. So the oldest, largest trees also have the most and biggest hollows. Continuing termite activity can clog hollows with termite nest material, even more so after the tree stops growing and dies. So the number and size of available hollows decreases as dead trees begin shedding limbs and rotting away. The opposite is true for Ironwood, a Top End tree that has few hollows when growing. Live Ironwood is resistant to termite attack, so tends to have fewer and smaller hollows than would be expected in a similarly-sized eucalypt. After death, Ironwood becomes more susceptible to termite attack, and hollows start to form.
Storms also form hollows by tearing branches from trees and making them more vulnerable to termite attack. However limbs hollowed out by termites, and trees containing them, are most susceptible to storm-damage, so storms also destroy hollows. Fire has a similar ability to both create and destroy hollows. Fires lapping up the sides of trees destroy the bark and provide an entry point for termites; and fires are more likely to destroy trees or damage limbs that have been damaged by termites. In a healthy landscape, all these processes are in balance and there are plenty of tree hollows.
Tree hollows are usually abundant on fertile soils, as these support the tallest trees. Riparian areas, often the most fertile parts of the landscape, have the most hollows and the largest eucalypts. River Red Gums growing around billabongs, where nutrients settle, are larger, and therefore have more hollows, than those found along stretches of creeks, from which the nutrient-rich soils are frequently scoured away in floodwaters. In the Top End, more hollows are found in eucalypts growing on ridges and slopes than in trees on sandy flats.
Importance to wildlife
Hollows are an important resource for wildlife in the Northern Territory, with a wide range of animals using them as nests or for dens in which to shelter during the day. In the Top End, 41% of mammals, 16% of birds, 21% of reptiles and 13% of frogs are known to use hollows. Twelve of the Northern Territory's threatened species use tree hollows. More than half of these are small to medium sized mammals: Northern Quoll, Northern Brush-tailed Phascogale, Common Brushtail Possum, Arnhem Leaf-nosed Bat, Bare-rumped Sheath-tailed Bat, Brush-tailed Rabbit-rat and Golden-backed Tree-rat. The threatened birds that rely on tree hollows are Princess Parrot, two subspecies of Masked Owl and Gouldian Finch. The only threatened reptile that uses hollows is Oenpelli Python. Only two of the threatened animals that use hollows - the Princess Parrot and the Common Brushtail Possum - are from the arid zone. The remainder are from the Top End.
An animal may use a hollow for an occasional visit or repeatedly return to it. Many animals use the same hollow every breeding season and may continue to use it and the surrounding environment for much of the year. In some species, the number of hollows can limit the number of animals that can establish territories in an area. Some animals spend their lives confined to a small area close to their hollow. Animals, such as possums and gliders, that use hollows as dens will often move between a collection of hollows within their territory and may share the hollows with a colony or family group.
Many animals are particular about the type of hollows they use, making their selection on the basis of size or orientation and whether the hollow offers adequate protection from the weather and potential predators. Eucalypts seem to be preferred over ironwood by most tree-dwelling animals in the Top End. Gouldian Finches nest preferentially on rocky hills in the branches of smooth-barked gum trees, such as Salmon Gum. In the tropics, Common Brushtail Possums select the older, larger hollow-bearing trees that offer the best insulation from daytime heat. Some animals may prefer hollows in dead trees over hollows in live trees, as these greatest allow vigilance against predators. Others may prefer the camouflage provide by the canopy.
Access to food resources and water is also important, with many animals being limited in the distances they can travel from the hollow, especially when breeding. Gouldian Finch nest hollows are always close to seed reserves and waterholes. Some species are not faithful to a single hollow and may shift between den trees in response to changes in food resources, such as in response to the species of eucalypt that are in flower. This den-shifting may also help to reduce vulnerability to predators and parasites.
Many hollow-dwelling animals are restricted to areas of fertile soils, not only for the large tree hollows found there, but because these soils produce the most productive feeding areas. Common Brushtail Possums need both den sites and nutritious vegetation on the most fertile soils available in the arid zone. Both are found along drainage lines, which provide the core habitat for the Possums. Masked Owls in the Top End have a similar preference for fertile soils, needing both tall trees with large hollows and good views of the surrounding landscapes, and productive areas with an abundance of the small animals on which they feed.
The main issues affecting hollow-dependent wildlife are the loss of large numbers of tree hollows as a result of land clearance, logging, fire or storm-damage, or as a result of weed invasion; and the degradation of feeding habitat in the nearby territory. If an animal loses its hollow, there may be nowhere else for it to live. Once hollows are reduced in number, competition and predation also become a problem.
Vegetation clearance, mainly for the establishment of plantations or agriculture, is largely restricted to the Top End, where clearance is concentrated on the most fertile country that has the tallest forests, and therefore the most tree hollows. Clearance has destroyed habitat of the Masked Owl on the Tiwi Islands.
As Masked Owls are highly faithful to a breeding territory, and most available territories are occupied, Owls that lose their nests may fail to find another. The Northern Brush-tailed Phascogale is another species that could be affected by tree clearance on the Tiwi Islands, as it not only breeds in hollows in tall forests and woodland, but shelters in hollows throughout the year. Both species are also found in the Douglas Daly region, another area with high levels of vegetation clearance.
It may be possible to clear land and leave the tallest trees, particularly by leaving buffer zones along watercourses. However, land clearance not only destroys tree hollows, but the feeding habitat used by hollow's occupants. If too much of the feeding habitat is cleared, the animals may desert the hollow, and have difficulty establishing a new territory.
The number of dens used may be reduced by partial clearing of an animal's habitat.
Although logging of native forests selectively takes the larger trees, a preference for well-formed, single trunk trees means that well-managed selective logging is unlikely to reduce availability of hollows for wildlife.
Fires not only destroy hollows but also affect the surrounding feeding habitat. After the habitat of a re-introduced population of Brush-tailed Rabbit-Rats was burnt, every individual either deserted the area or died because they could no longer find food. Gouldian Finches have also been known to desert nesting sites after fires have affected food resources, even though the hollows remained undamaged.
Even though storms both destroy and create hollows, damage caused by severe storms, including cyclones, can be a problem for hollow-dwelling animals. In recent years, there has been an increase in severe weather worldwide, with four category five cyclones crossing the north Australian coast in the first decade of the 21st century (Cyclones Ingrid, Larry, Monica and Lawrence). Such an increase is a predicted effect of global warming, and so is likely to continue. It may increase the rate of both hollow creation and destruction. However, hollows created in the most severe storms are unlikely to survive long, especially if the whole tree has been damaged.
Some storms fell large tracts of trees, as Cyclone Monica did as it passed through Arnhem Land between Maningrida and Jabiru. It will take a long time for both the supply of tree-hollows and the feeding habitat to recover. Though less well-documented, similar damage was caused on Croker Island by Cyclone Ingrid. Even small storms can cause massive damage to hollow-bearing trees and nearby feeding habitat, such as the small cyclone that whipped through the Mary River regions of Kakadu National Park in 2007.
Weed invasion can directly replace hollow-bearing trees. This is the case in Central Australia, where hollow-bearing River Red Gums are being replaced by Athel Pine along the Finke River. Weed invasion can also reduce the availability of hollows by affecting fire patterns. Olive Hymenachne and Para Grass fuel intense and frequent fires in wetland environments compared to native vegetation. This process destroys canopy trees and their hollows. Gamba Grass does the same thing across the broader landscape. Invasion of some weeds, such as Parkinsonia and Mimosa, may suppress fires by replacing the naturally fire-tolerant vegetation with fire retarding shrubs and shading out grasses that form the main fuel in savanna fires. The impact of this change on availability of tree hollows is unknown.
Hollows can only be an important wildlife resource as long as food is available nearby, so any degradation of the feeding habitat may reduce the usefulness of hollows for shelter or nesting. The same processes that lead to a loss of hollows can also lead to habitat degradation. As described above, vegetation clearance, poor fire management and weed invasion can all reduce the area and quality of feeding habitat. In addition, degradation can also be caused by grazing by introduced animals, both domestic and feral.
The areas that contain the most important hollows, those used by rare or threatened species, are often found in the areas under the most environmental pressures. This is particularly the case along drainage lines. Not only do these areas generally have the most hollows they also have more reliable, abundant and nutritious food resources than the surrounding landscapes, even in lean times. This subjects them to heavy grazing pressures by a range of introduced animals, such as rabbits in the arid zone and Buffalo in the Top End. Riparian areas are also prone to weed invasion because of their fertile soils and high levels of disturbances caused by irregular flooding, heavy grazing and high fire intensities.
In the arid zone, hollow-dwelling animals may have difficulty finding shelter and nest sites where hollow-bearing River Red Gums along watercourses have been replaced by Athel Pine. This process will lead to the long-term decline of tree hollow availability. Common Brushtail Possums have the additional problem of competition with rabbits for food.
In many environments, birds and mammals use only a small portion of the available hollows. However, where hollows are in short supply, most suitable hollows will be occupied and there may be competition for hollows. A shortage of hollows not only limits the ability of a species to breed, but can expose it to increased predation. Predators and aggressive animals that use hollows, such as Little Corella, Common Brushtail Possum or Oenpelli Python, may eat or evict existing occupants. European Honeybees can also compete for hollows, but are only likely to be a problem in areas where hollows are in short supply, such as in strips of remnant vegetation.
Any clearance of vegetation should consider the presence and importance of tree hollows. Actively used hollows should be retained wherever possible. Hollows used by Masked Owl should not be destroyed when native forest is cleared for plantations, particularly on the Tiwi Islands. Clearing too close to nest-bearing trees should also be avoided, as this exposes the tree to storm damage. Any clearance of tall forests and woodlands in the Daly River region could also reduce hollow availability for Northern Brush-tailed Phascogale and the mainland subspecies of Masked Owl. The best way to protect hollow-dwelling threatened species in these areas is not to clear their habitat.
While fire helps create some hollows, it destroys others. Hollow-bearing trees are particularly susceptible to fire, as pre-existing damage to these trees makes it easy for fire to get hold. Repeated late dry season fires throughout the Top End are therefore a problem for a number of relatively sedentary mammals and animals that return to the same nesting area each breeding season. In addition to Masked Owl and Northern Brushtail Phascogale, animals that may be displaced from burnt hollows include Golden-backed Tree-rat and Brush-tailed Rabbit-rat. While late dry season fires are a feature of the nesting habitat of hollow-nesting Gouldian Finches, there is no evidence that the species is threatened by a lack of hollows in the Northern Territory. Elsewhere, this may not be the case.
Not only is loss of hollows a concern, but animals sheltering in hollows may be killed by heat-stress during a fire. Fires that occur during the breeding season have probably contributed to the decline of Northern Quoll, and are likely to be a threat to other small mammals. Even species that use hollows opportunistically, such as Oenpelli Python, may be killed if sheltering in a hollow when a fire passes.
Implementing a fire regime that has a mixture of early dry season fires and fires lit after the first wet season rains, or burning small patches over an extended period, can reduce fire intensity and extent. Managing fire hazard to protect tree hollows also includes avoiding the use of high volume introduced grasses, or restricting them to intensively grazed areas.
- Hussey, P. 2005. Tree hollows and wildlife. <i>Western Australian Department of Conservation and Land Management Wildlife Notes</i> No 15. The publication is available for download from this site.
- The gradual loss and episodic creation of Palm Cockatoo (Probosciger aterrimus) nest-trees in a fire- and cyclone-prone habitatMurphy S.A. and Legge S.M. (2007). <i>Emu</i> 107, 1-6