Lions, tigers, and bears, oh my! What is a keystone species and how can conservationists use them?

Lion (Panthera leo)

tiger cub

Tiger (Panthera tigris)

Grizzly bear (Ursus arctos horribilis)

Grizzly bear (Ursus arctos horribilis)

What do these three animals have in common?  No, it’s not that they’re all featured in a popular film.  It’s not even that they’re all at the top of the food chain, although that’s closer.  Lions, tigers, and bears are all keystone species.

In architecture, the keystone at the top of an arch holds the arch together.  Without the keystone, the whole arch and building surrounding it will collapse.  Similarly, “keystone species” are animals that have a disproportionately large effect on the ecosystems they live in.

For example, Robert T. Paine first tested the keystone species hypothesis empirically (scientifically) by removing a species of starfish (Pisaster) from its intertidal habitat and observing the dramatic changes this removal wrought on the starfish’s prey, mussels (Mytilus) and the ecosystem as a whole.  It turns out that the starfish helped to keep the mussels’ numbers in check, and without the starfish to eat the mussels, the mussel population exploded and took over the habitat, crowding out other species and decreasing biodiversity (the numbers of different species in an ecosystem).

Keystone species can fall under several different categories.  Top predators, like the starfish, are often keystone species because of the inherent role they play by eating lots of smaller animals.  That’s why it’s so important to conserve large mammalian carnivores like lions, tigers, and bears – all endangered species – which unfortunately attract a lot of poaching due to their magnificent furs and sought-after meat.


Another category of keystone species are mutualists, which are animals that help many other species for something in return (in a mutually beneficial relationship).  For example, fruit trees are keystone mutualists because they provide shelter and food for many animals like monkeys and birds, which repay the tree by dispersing their seeds far and wide.

Finally, ecosystem engineers are keystone species because they are animals that change the physical landscape in which they live.  Beavers, elephants, and prairie dogs are all examples of ecosystem engineers because they build dams, destroy trees, and make burrows, respectively.


Prairie dog (Cynomys)

Learning about these concepts helps scientists and environmentalists define categories in which to place animals and know which animals to target in conservation efforts.  It is far easier to conserve a whole ecosystem by protecting one species than by trying to protect the entire community of animals.  In this way, scientists use keystone species to protect Mother Nature, one animal at a time!



See No Ordinary World’s post on sea otters, another important keystone species:

Keystone species definition:


“Keystone Species.”

“Keystone Species.”