By Dr. Becker
About a decade ago in India, researchers made a surprising discovery: living inside tree hollows high in the air, a species of frog thought to be extinct was re-discovered.1 The animals not only depend on the tree hollows for shelter but also use them to lay eggs, which stick to the sides the hollow. The tree hollows also fill with water during rains, which provides the perfect "pond" for the newly hatched tadpoles to swim in as they grow.
They're one of many animal species that depend on the specialized nooks for survival, nooks that are both in high demand and disappearing far too rapidly from the face of the Earth. Tree hollows form in a number of ways, often after a damaged area (from a lightning strike or broken branch, for instance) is eaten away by fungi, bacteria and termites.2 The problem is this process takes hundreds of years, which means once tree hollows disappear, it's difficult to get them back.
Woodpeckers can help — their pecking can create new hollows in weeks or months, but they don't live everywhere. In Australia, for instance, where more than 300 species depend on tree hollows, there are no woodpeckers, just a lot of animals looking for a limited number of tree hollows to call home.
Animals Fight Over Tree Hollows in Urban Areas
In urban areas, trees are commonly cut down to make way for housing, leading to fewer hollow-bearing trees. Researchers set out to determine whether this, in turn, would increase competition for this limited resource, particularly among parrots, a cavity-nesting species in Australia.3 They installed motion-activated video cameras at 61 tree hollows in urban regions and monitored them over two years.
Compared to tree hollows in forested areas, the urban tree hollows had a higher visitation and usage rates and significantly more aggressive interactions among the parrots seeking to use the hollows. As Science News reported:4
"Birds called rainbow lorikeets defended their hollows against other birds, such as sulphur-crested cockatoos and laughing kookaburras. In one video, the cockatoos attacked a common brushtail possum that had entered their hollow. 'There were feathers and fur actually flying,' [ecologist Adrian] Davis [at the University of Sydney in Australia] says.
The animals might visit suburban hollows and fight more often over them because they are scarcer. If there are not enough cavities for everyone, some animals might try to steal other animals' hollows. All that fighting could stress out the animals, Davis suggests."
In another study looking into how valuable tree hollows are in providing habitat for cavity-nesting birds, it was revealed that both tree hollows in logged forests and farming areas have important conservation value, but few remain.5 Birds fared well using the tree hollows at the various locations, with hollow characteristics proving to be more important in nest survival than the surrounding habitat. In particular, birds fared better when the tree hollows were high up in the tree with small entrances and in those found in living trees.
The researchers suggested that limits set on minimum diameter cutting in forests are not enough and policies should be put in place to retain large old trees. In Australia, environmental officers worked with arborists to move large trees that had been cut down because they were deemed unsafe to a new location where a forest had been cleared. Using steel anchors, the trees were resurrected and are now being monitored to see if wildlife will use the hollows.6
How to Protect Hollows as Homes
According to Australia's National Parks & Wildlife Service (NPWS), "There needs to be enough hollow-bearing trees per hectare to meet the current wildlife requirements, as well as sufficient maturing trees to provide replacement hollows in the future. As a general guide, [three to] 10 hollow-bearing trees, with as many as 30 hollows, may be needed per hectare to support a rich mix of species."7
Because tree hollows rarely form in young trees, mature trees are of utmost importance. If you have such trees on your private land, retain and protect them (and be aware that many hollows exist high up in trees, where you can't see them from the ground). NPWS recommends keeping all trees with hollows standing — even if they're dead — and planting local species known to produce hollows while allowing trees already in the ground to grow to maturity.
In Australia, you can also take part in the Hollows as Homes citizen science project, which is aiming to identify hollow locations as well as document their usage.8 Along with providing valuable data about hollow resources, the project can provide immediate benefits. "For example," Science News reported, "if someone sees an endangered owl using a hollow, the city wouldn't be allowed to cut down that tree."9
It's one more environmental area that deserves urgent attention. Like the many species that live in specialized bromeliad plants, or arboreal folivores, animals that live in trees and eat leaves, depending on tree-top living for their survival, the importance of protecting the environment as a whole, including its seemingly innocuous tree hollows, is crucial for the ecosystem to go on and thrive.
One final way you can help is by installing nest boxes on trees in your backyard. While they don't always provide as great of protection from predators and the elements as an actual tree hollow, they can make a suitable alternative for animals in need.