11. Keystone Species
Outcomes:
At this point, let's go back to out conceptual inquiry question, "How do changes in or shifts in species' functions and roles or interactions between species, influence the balance and stability of an ecosystem?' and Paine's experiments and claims about the role of a top predator in maintaining the stability of an ecosystem. We saw from processing and graphing Paine's data that removing the top predator, Pisaster, from the rocky shoreline ecosystem resulted in an unbalanced ecosystem with less species diversity. But why did that happen? Lets first take a look at the interactions of some of the organisms in a healthy, balanced rocky shoreline ecosystem.
As we can see, removing snails does not cause the balance of the ecosystem to be disrupted. Try it on your own, and you will find that the removal of any one other species will not cause the ecosystem to collapse in the way that removing Pisaster does.
Paine called species like Pisaster - species on which the health, balance and diversity of an ecosystem depends - keystone species, because the 'keystone' is the stone in a stone arch that holds the arch together.
Each ecosystem has its own keystone species: a species, often but not always a top predator, that is necessary in order to maintain a healthy balance of species and interactions. Beavers are the keystone species in a wetland ecosystem; prairie dogs are the keystone species in a grassland ecosystem; and grey wolves are the keystone species in the Yellowstone ecosystem.
There are many factors that may cause disruption to the natural balance and interactions between organisms in an ecosystem. Paine and his scientific research demonstrate one example of how and ecosystem can be disrupted due to the death or removal of some organisms from the ecosystem.
Some other historic and current examples are:
powerful storms, such as tornadoes, hurricanes or monsoons
the overhunting or animals, such as bison or racoons, for food or fur
the killing of predators, such as bear, lions and wolves, that are considered dangerous to human settlements
geological events, such as earthquakes, volcanic eruptions and tsunamis
the accidental poisoning of pants and animals due to chemicals from farming and industry.
As we can see, some of these factors are natural, and uncontrollable, while others result from human decisions and actions. With a partner, sort the factors into natural or human-initiated factors, and add some of your own ideas; then write your thoughts for why the disruption could have negative consequences on the balance of the ecosystem. Finally, offer some suggestions to lessen the damage to the ecosystem's balance.
