Fiordland Bat Diaries – Trees

We join the Te Anau bat monitoring team at the end of their Eglinton short-tailed bat season. They are pretty excited as they have just recorded a new population highest count of 4179, superseding the last record set in 2018 by over 1000 bats!

The saying goes ‘you can’t see the forest for the trees’ but often these days people don’t seem to see the trees for the forest. To the untrained eye a beech forest can look all the same, one tree the same as another, next to another just the same. Trees seem endless, expendable. But trees are not all the same, even when they look the same from the outside. In the forest old trees are the safe havens for many.

Meet M72. M72 is a red beech tree, Fuscospora fusca, found in the Eglinton Valley, Fiordland National Park. According to our records M72 was ‘discovered’ in 2009, is about 30m tall with a diameter at breast height of 140cm. This makes M72 about 700 years old give or take. That’s far older than the measly 200 years Europeans have been around in Aotearoa and only slightly younger than the time Maori have inhabited this land. From the outside M72 looks like any other grand old tree in the forest, however M72 holds a secret.

And what is that secret? A home big enough for over 4000 southern short-tailed bats! That’s about 60kg of ‘bat weight’ hanging from the inside the tree, rather a lot of bat to fertilise the tree from the inside…Homes for NZ bats are mainly tree cavities, the occasional cave if there happens to be one around, and sometimes just a piece of bark or vegetation for a single bat to hide under. Bats move home regularly so need to have an array of trees to choose from, spending as little as one night in some trees. Young trees don’t provide good roosting opportunities, the older the better to develop cavities and palaces inside.   

We only get to monitor this bat colony for one month each year so there is a lot we don’t get see or understand about their movements, but years of monitoring them generally gives us counts of 1 to 1600 bats in a tree. Which means there M72 is very unique, even amongst other large old trees, that allows it to be home to so many bats. Maybe M72 is the only tree in this forest with a large enough cavity to fit everyone in? That’s something to think about.

M72 it’s a great privilege to know you, hope you live for many years to come.