INVASION OF THE BODY SNATCHERS

The hit TV show The Last of Us imagined an apocalypse caused by a Cordyceps fungus turning humans into zombies. For many insects, real life isn't much different...

- ADAM HART

When we think about living things and where we find them, we tend to think at large scale. The word that comes to mind is ‘habitat’, which conjures up images of forests, deserts, oceans, swamps and the many other diverse places on Earth where life can be found. Of course, we might also be thinking at a smaller scale, focusing perhaps on our garden, a local hedgerow or a small pond.

If you think at the right scale, though, pretty much any physical space can be habitat to something. Anyone who has looked closely at mites living on a leaf, or at tiny springtails under the decaying bark of a dead tree, will have marvelled at the wonder of microhabitats. But when you start thinking at a really small scale, when a microscope is needed to see what’s going on, then habitat can become very interesting indeed – because once you think small enough, there’s no better habitat than that offered by other organisms.

Living in or on other organisms offers many advantages – if you can pull it off. Your host may not be entirely amenable to you taking up residence, but they can provide you with all the shelter, resources and travel you might require. You may even provide your host with some advantages in return: many of the bacteria that live within our guts, for example, help us with aspects of digestion. Such mutually beneficial relationships are known as mutualisms, and are quite common.

But not every organism that lives in or on another organism brings benefits – sometimes, these passengers can cause harm. When an organism is harmful to its host, we call it a parasite.