The Horticulturalist Behind Devil's Gardens

Chris - Now tell us about your story.

Megan - I've been studying a phenomenon called the devil's garden.

Chris - That sounds scary. What's a devil's garden?

Megan - A devil's garden is a very unusual kind of growth of trees in the Amazon. They are very unusual because in this growth of trees there is only one, or at most a few species of trees. This is very unusual for the rainforest because usually there are many species of trees growing in a very small area.

Chris - So how big do these gardens get?

Megan - They get to be about 1300 square metres, which is about half the size of a soccer field.

Chris - That's a pretty big area to have just one kind of tree. What kind of thing was going through your mind when you were thinking what might be causing this?

Megan - Well there were basically two ideas about how it is that devil's gardens are formed, and how it is that there's only one kind of tree there. The first idea proposed was that maybe the tree that grows in devil's gardens produces some kind of toxin in its roots.

Chris - That kills everything else off?

Megan - Yes, that kills everything else off, and prevents other trees from growing in the area.

Chris - How did you prove that that wasn't the case then?

Megan - I did an experiment where I planted a bunch of saplings of a very common Amazonian tree species inside and outside devil's gardens.

Chris - So these are trees that are not normally found in devil's gardens?

Megan - That's correct. It was a different kind of tree species.

Chris - And don't tell me, they all died.

Megan - No actually they didn't. When I planted them inside devil's gardens I did two things. One was that I planted them as is, and I also planted some with a kind of sticky goo round the bottom to prevent insects from climbing up the trunks and getting at the leaves.

Chris - Right, and which ones survived.

Megan - Only the trees without the sticky substance around the trunk died after a couple of days in a devil's garden. So I showed that some kind of insect was attacking the trees in devil's gardens.

Chris - And what was that insect then?

Megan - It turns out that that insect is a species of ant.

Chris - So these ants are selectively homing in on plants they don't want and killing them.

Megan - That's correct.

Chris - How are they doing that?

Megan - That is something the is really fascinating. What they do is start by crawling up the tree, and then each worker ant starts by biting a small hole in the leaf tissue or the stem tissue with its mandibles, or jaws. Then it flips its abdomen underneath its body and it sticks the tip of its belly into the hole it's made and it releases a few drops of a very nasty chemical substance called formic acid.

Chris - And this just poisons the tree?

Megan - Yes.

Chris - But how does the ant know one tree species from another?

Megan - Well that's an excellent question. The tree species that they don't kill turn out to have a very special relationship with the ants. The ants live in the hollow stems of this tree species and make their nests in them. I first thought that they decided by seeing whether a tree had any good nesting space for them. And I did an experiment, and it turned out that that's not true.

Chris - So how do you think they do it then?

Megan - My best guess is that they distinguish between the tree that they live in and all the trees that they kill by some sort of chemical cue. They basically smell the difference between these types of trees.

Kat - So it's like recognising your favourite food by the smell of it. Can I also just ask really quickly, why are they called devil's gardens?

Megan - They're called devil's gardens after an Amazonian legend, which says that these gardens are cultivated by an evil forest spirit. So that's how they get their name.