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.

許多動(dòng)物以樹洞作為棲身之所，但是樹洞的數(shù)量正在減少。科學(xué)家們正在研究樹洞減少對(duì)野生動(dòng)物的影響，以及人們?cè)撟鲂┦裁础?/p>

樹洞形成的方式多種多樣。啄木鳥遍布除澳洲和南極洲之外的所有大洲，一只啄木鳥憑借其鳥嘴的啄擊，能在幾周或幾個(gè)月的時(shí)間里鑿出一個(gè)樹洞。當(dāng)樹被閃電或大風(fēng)損壞時(shí)會(huì)在樹上形成一些洞。真菌、細(xì)菌和白蟻可以在損壞部位進(jìn)行侵蝕，形成樹洞。這一過程往往需要上百年時(shí)間。

一旦樹洞形成，許多動(dòng)物就可以使用它。蝙蝠會(huì)擠進(jìn)小小的樹縫，鳥兒會(huì)來(lái)洞中筑巢，有些足夠大的樹洞甚至給熊提供了冬眠的場(chǎng)所。

在澳大利亞，樹洞顯得尤為重要。鳥類、哺乳動(dòng)物、爬行動(dòng)物以及青蛙等300多種物種為了生存都需要樹洞?！俺藰涠此鼈儫o(wú)處生存?！卑拇罄麃唶?guó)立大學(xué)的生態(tài)學(xué)家戴維·林登邁爾說(shuō)。

但澳大利亞沒有啄木鳥，所有樹洞都是在漫長(zhǎng)過程中形成的，而現(xiàn)在樹洞的形成速度遠(yuǎn)低于樹洞被毀滅的速度。

人們已經(jīng)嘗試使用稱為“巢盒”的人造住所來(lái)代替樹洞。這些巢盒通常由膠合板制作而成，且可以掛在桿子上或者樹的一側(cè)。動(dòng)物們可以通過盒子上的孔洞進(jìn)入盒子里面棲息。但巢盒不是樹洞，它們不會(huì)總是與真正的樹洞同樣有效。它們可能不會(huì)同樣有效地庇護(hù)動(dòng)物免于嚴(yán)寒和酷暑，而且不會(huì)像樹洞那樣長(zhǎng)久。