Ecology has got to be about the coolest subject around. It’s all about how organisms make their livings and how they interact with each other. Just about any time you study the subject, you’ll learn that most everything in nature is interconnected.
One of the most interesting species interactions that I have ever seen in person is the acacia tree – ant relationship. If I told you to picture a tree in Africa, I’d say there’s a pretty good chance that you’d think of something like an acacia. You may not know how the trees relate to ants, but you might know that many acacia trees have big spines around their leaves. Those sharp spines are a defense mechanism. They are meant to deter hungry herbivores who want to nibble the tree’s leaves.
Image by Lindsay Martinez
Acacia spines are great for the trees, but they are not a completely effective defense mechanism. An African elephant can completely destroy an acacia tree – spines or no spines – if it wants to. An elephant might eat the tree, or it might just knock it down because it can. Elephants have a huge effect on acacias and other trees across African landscapes. That’s why they’re sometimes referred to as “ecosystem engineers.”
Acacia trees must deal with the constant threat of murder by elephants. Since spines aren’t always enough to protect themselves, some acacias have found another defense strategy: a mutualistic relationship with ants. Tiny ants living on trees can be a huge annoyance for elephants looking for a snack. Elephant trunks are highly sensitive, so elephants do not want swarms of mean, biting ants getting into their trunks when they are trying to eat a tree.
Image by Lindsay Martinez
Acacia drepanolobium trees (let’s call them “dreps” for short), which I observed firsthand in Kenya, work with ants to protect themselves from elephants. Dreps don’t just grow trunks and branches and leaves and spines. They also grow little black bulbs called domatia that mutualist ants live in. In addition to providing ants with a home, the dreps also give them food – they make nectar around the domatia that the ants feed on.
Image by Lindsay Martinez
Each drep tree is home to a colony of ants. There are several different ant species that will live on dreps. In exchange for the shelter and snacks, ants guard the trees from elephants or other animals. As soon as a tree is disturbed, the ants begin swarming. They find the animal and begin crawling on it and biting it.
I can tell you from firsthand experience that these little bites are not very pleasant. I once brushed up against a drep through an open car window while driving by and the ants swarmed me and began biting. An ant attack should be enough to drive an herbivore away from a tree.
When the ants attack, they aren’t just doing something nice for the tree. They’re also defending their own home. And, while it costs a lot for the tree to produce houses and meals for the ants, the protection is likely worth it when there are elephants around.
The overall story about the relationship between a tree, an insect, and an herbivore is a pretty simple one. Trees provide food and shelter. Ants provide protection from destructive herbivores. If you love ecology like me, you’ll likely be interested in diving more into this relationship. Then, you can learn about the differences between different mutualistic ant species, how the ants affect the trees, and what dreps do when elephants are not around.
Very good info thanks so much!