give an example of how the change of one organism in a food chain could affect an entire food web.give an example that is good for a grade 9 stdent

There are two classes of organisms that are critical to
ecosystems in this way. Foundation species are species which are the dominant primary
producers in an ecosystem.

Kelp is a good example of a
foundation species; it is the basis of an extensive food web, and if the kelp is
suddenly removed from an area, the entire ecosystem will collapse in that place for lack
of food.

The second group is the keystone species. Keystone
species are not necessarily the primary producer in an ecosystem, but they do some
ecosystem service that all the other members of the system are dependent on in some way.
A good example of a keystone species would be beaver. Because of their land clearing and
dam building activities, beavers create ponds and marshy meadows where there would
otherwise just be a stream. The entire marsh ecosystem is dependent on the beavers water
management activities, and if the beavers are removed from an area, the dams will decay
and break up and the swamp will drain and turn back into a stream, causing many
creatures to move elsewhere or die off.