Some species have a big impact on the ecosystem they live in - from altering the physical structure of their environment to dictating the populations and diversity of other species; these species have a disproportionate effect on an ecosystem relative to their abundance. These species are called keystone species. The absence or removal of a keystone species may send the whole ecosystem into a diversity tailspin. Elimination of a keystone species will drastically reduce the number of species in an ecosystem because of the dependency amongst species. The namesake of keystone species is the keystone in an arch. This special stone is at the apex of the arch, locking all the stones into position, allowing the arch to bear weight. A keystone species is analogous to the keystone in an arch, without it, the system lacks integrity.
The idea of a keystone species was originally introduced by Robert T. Paine in 1969, a professor at the University of Washington. Paine observed that the diversity of organisms in the intertidal zone declined as predators were removed and hypothesized that some predators had a greater role in maintaining this diversity. He tested his hypothesis by keeping an 8x2 meter swath of the intertidal zone free of the ochre sea star Pisaster ochraceus, a predator on bivalves, gastropods, and barnacles. Whenever a sea star was found within the plot it was thrown into the ocean, thereby removing sea star predation pressure.
When compared with the control area (the area with sea stars), Paine observed dramatic changes in the ecosystem after removing the sea stars. Remaining members of the intertidal (e.g. mussels, barnacles, limpets, and chitons) almost immediately began to compete for the limited space. After 3 months, up to 80% of the available space was occupied by a single species: the acorn barnacle Balanus glandula. The acorn barnacle quickly took up all the unoccupied space, but within another nine months, it had been replaced by populations of the gooseneck barnacle Pollicipes polymerus and the California mussel Mytilus californianus. This continued until the system was completely dominated by mussels. Mussels also crowded out algae, which caused herbivores such as limpets to move out of the environment due to the lack of food. Within one year of sea star removal, the number of species decreased from fifteen to eight!
The ochre sea star was reducing the highly competitive populations of the mussels and barnacles. Without the sea star predation, the mussels completely dominated the intertidal. The ability for a small population of sea stars to control the local mussel population and, by proxy, increase the diversity of the system makes the ochre sea star a keystone species.
Ochre sea stars aren’t the only keystone species. There are a number of other well-described examples of keystone species such as the sea otter Enhydra lutris. Sea otters keep the population of certain benthic (ocean bottom) herbivores, particularly sea urchins, in check. Sea urchins graze on the holdfasts and stipes of large kelps causing them to drift away. Holdfasts keep kelps attached to the bottom of the ocean while stipes keep all the kelp blades attached to one another. These kelps normally provide a rich environment for hundreds of species of invertebrates and fishes, which utilize the structure and feed off the kelp. With sea otters absent, the kelp is quickly removed by sea urchins creating an urchin barren – a place inhabited by very few species, other than sea urchins, and devoid of most palatable algae.
Not all keystone species impact an ecosystem through predation. The acorn banksia Banksia prionotes (a shrub in Western Australia) is the sole source of nectar for honeyeaters (a group of birds). Without nectar being produced during the fall and winter months, honeyeaters wouldn’t survive and fulfill their crucial role as pollinators for a wide variety of other plant species during the rest of the year.
The North American beaver Castor canadensis is another important keystone species (see Alena’s post on the re-introduction of beavers here). By building dams, they flood large areas of forest creating an ecosystem that aquatic species thrive in (e.g. fish, frogs, and some insects). After several years, beavers move on and leave their dam unattended. The unattended dams eventually break down and the once flooded areas are cleared of water. This opens up large areas which quickly start to transition to forest again. Another group of animals and plants utilizes this early successional habitat, some of which cannot survive without it (see Alena’s post on the need for early successional habitat and preserving populations of the New England cottontail here). Without beavers modifying the contiguous forest, many species wouldn’t have their necessary habitats.
The concept of keystone species has been heralded within the field of ecology as a major breakthrough in the understanding of complex ecosystems. It has allowed ecologists to more easily communicate with conservation policy-makers on the best path of action in preserving particular systems. By protecting the keystone species and preserving the necessary environment it needs, you end up protecting a large amount of dependent species.
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