the soil and are taken up by plants. Nutrients can be brought in from other regions, perhaps
carried to a lake by a stream. When one organism eats another organism, it receives all of its
nutrients. Nutrients can also cycle out of an ecosystem. Decaying leaves may be transported
out of an ecosystem by a stream. Nutrients can blow out of an ecosystem on the wind.
Decomposers play a key role in making nutrients available to organisms. After scavengers
eat dead organisms, they almost always leave some parts of the dead animal or plant behind.
Decomposers complete the process of breaking down dead organisms. They convert dead
organisms into nutrients and carbon dioxide, which they respire into the air. These left over
nutrients are then available for other organisms to use. Without decomposers, life on Earth
would not be able to continue. Dead tissue would remain as it is and eventually nutrients
would run out. Decomposers break apart tissue and return the nutrients to the ground.
Without decomposers, life on earth would have died out long ago.
Relationships Between Species
Species have different types of relationships with each other.Competitionoccurs between
species that are trying to use the same resources. When there is too much competition, one
species may move or adapt so that it uses slightly different resources. It may live at the tops
of trees and eat leaves that are somewhat higher on bushes, for example. If the competition
does not end, one species will die out. Each niche can only be inhabited by one species.
Somerelationshipsbetweenspeciesarebeneficialtoatleastoneofthetwointeractingspecies.
These relationships are known assymbiosisand there are three types. Inmutualism, the
relationship benefits both species (Figure18.11). Most plant-pollinator relationships are
mutually beneficial. The pollinator, such as a hummingbird, gets food. The plant get its
pollen caught in the bird’s feathers, so that pollen is spread to far away flowers helping them
reproduce.
Incommensalism, the relationship is beneficial to one species, but does not harm or help
the other (Figure18.12). A bird may build a nest in a hole in a tree. This neither harms
nor benefits the tree, but it provides the bird and its young with protection.
Inparasitism, the parasite species benefits and the host is harmed (Figure18.13). Para-
sites do not usually kill their hosts because a dead host is no longer useful to the parasite. A
visible example of parasite and host is mistletoe on an oak tree. The mistletoe gains water
and nutrients through a root that it sends into the tree’s branch. The tree is then support-
ing the mistletoe, but the tree is not killed, even though its growth and reproduction are
slightly harmed by the parasite. Humans can host parasites, like the flatworms that cause
schistosomiasis.