Three years ago, That Blog wrote about functions – useful things that plants and animals do. Normally, in nature, each type of plant and animal performs many functions, and each function is performed by multiple plants and animals. This is why we see complex, thriving ecosystems, in which all the members are involved in a web of interactions.
But within an ecosystem, some members are especially important. Often, this extra importance comes from the fact that the plant or animal performs a function that isn’t duplicated by another plant or animal in the system. These critically necessary members of ecosystems are called keystone species.
Keystone species can be important for many reasons. Maybe a certain kind of small animal is the only prey of a larger animal. If the small animal disappears, the larger animal will too, because it has nothing to eat. Because the fate of the larger animal depends entirely on the fortunes of the smaller animal, the smaller animal would be considered a keystone species.
Conversely, if a large animal was the only predator of a smaller animal, then the large animal would be the keystone species, since without it, the smaller animal would multiply prolifically, decimate its own food sources, and unbalance the entire ecosystem.
As another example, an insect could be a keystone species, if it is the only animal that pollinates a certain plant. The disappearance of that insect would then spell trouble for the plant, as well as for every species that relies on the plant.
The name keystone – a reference to the single wedge that holds an entire arch together – might make it sound as though each ecosystem has only one centrally important species. But this is not the case. An ecosystem can have many crucial members. In fact, the more scientists learn about ecosystems, the more they think that every species can be described as a keystone species.
Plants and animals do all kinds of things – for each other, and for us – and we still don’t know how all of these interconnections work. Rather than treating uninteresting species as disposable, we should take the more cautious approach of assuming that every kind of plant and animal is important. We just don’t know which species, once it’s gone, will turn out to have been holding the whole system together.