Many creatures that take refuge in tree cavities. But the number of tree hollows is shrinking. Scientists are investigating how the loss of tree hollows affects wildlife — and what to do about it.

There is no single way to make a tree hollow. Woodpeckers are a group of birds found on all continents but Australia and Antarctica. A woodpecker hammering the wood with its beak can gouge a hole in a tree within a few weeks or months. Some cavities form when a tree is damaged by lightning or wind. Fungi, bacteria and termites may eat away at the damaged site, creating a hollow. This process can take hundreds of years.

Once a hole forms, many animals can use it. Bats squeeze into small crevices. Birds nest in the hollows. Some cavities are even big enough for bears to enter and hibernate.

Tree hollows are particularly important in Australia. More than 300 bird, mammal, reptile and frog species need the holes to survive. "They can't live anywhere else," says David Lindenmayer. He is an ecologist at Australian National University. "They use them for everything."

But Australia doesn't have any woodpeckers. All the cavities have to form the slow way. And they're not being created fast enough to replace the homes being lost.

People have tried to replace tree hollows with artificial homes called nest boxes. Typically made of plywood, these can be attached to a pole or the side of a tree. Animals can enter the box through a hole and live inside. But nest boxes are not tree hollows. They may not always work as well as the real thing. They may not protect animals from heat and cold as effectively, and won't last as long as tree's hollow.

許多动物以树洞作为栖身之所，但是树洞的数量正在减少。科学家们正在研究树洞减少对野生动物的影响，以及人们该做些什么。

树洞形成的方式多种多样。啄木鸟遍布除澳洲和南极洲之外的所有大洲，一只啄木鸟凭借其鸟嘴的啄击，能在几周或几个月的时间里凿出一个树洞。当树被闪电或大风损坏时会在树上形成一些洞。真菌、细菌和白蚁可以在损坏部位进行侵蚀，形成树洞。这一过程往往需要上百年时间。

一旦树洞形成，许多动物就可以使用它。蝙蝠会挤进小小的树缝，鸟儿会来洞中筑巢，有些足够大的树洞甚至给熊提供了冬眠的场所。

在澳大利亚，树洞显得尤为重要。鸟类、哺乳动物、爬行动物以及青蛙等300多种物种为了生存都需要树洞。“除了树洞它们无处生存。”澳大利亚国立大学的生态学家戴维·林登迈尔说。

但澳大利亚没有啄木鸟，所有树洞都是在漫长过程中形成的，而现在树洞的形成速度远低于树洞被毁灭的速度。

人们已经尝试使用称为“巢盒”的人造住所来代替树洞。这些巢盒通常由胶合板制作而成，且可以挂在杆子上或者树的一侧。动物们可以通过盒子上的孔洞进入盒子里面栖息。但巢盒不是树洞，它们不会总是与真正的树洞同样有效。它们可能不会同样有效地庇护动物免于严寒和酷暑，而且不会像树洞那样长久。