Science High School Reviewer | Grade 4 Science | Life Science | Exploring Ecosystems | Lesson 10: Interactions Between Organisms in an Ecosystem

Ecological relationships:

competition—when two or more organisms rely on the same environmental resource
predation—behavior of one animal feeding on another
symbiosis—the close relationship of two dissimilar organisms
mutualism—a symbiotic relationship where both organisms benefit
commensalism—a symbiotic relationship where one organism benefits and one does not benefit but is unharmed
parasitism—a symbiotic relationship where one organism benefits and one is harmed

Organisms in an ecosystem compete for and share resources.
They have other relationships too.
Change in Ecosystems
Populations in ecosystems change naturally as the amount
of resources changes. Think about the chipmunk at the
beginning of the chapter. A population of chipmunks will grow
where food is plentiful. As the population increases, more food,
more water, and more living space are needed. Eventually, the
population may use up these resources.
Then, as resources decrease, each chipmunk will have less
food to eat, less water to drink, and less space in which to
live. Some chipmunks will die or move out of the area. As the
population decreases, more resources will be available for the
remaining chipmunks. The population begins to grow, and the
cycle starts again.
Competing
Populations grow when their needs are met. But populations
that share an ecosystem may need the same resources.
When two or more species must use the same limited
resources, competition occurs. Every organism has
adaptations that help it compete for resources. An
organism that competes successfully is more likely
to survive and reproduce.
Living space is one cause of competition.
Plant species compete for light and
water. Bird species compete for
the same nesting site. Predators
compete for prey.

Sharing Resources
Certain behaviors help animals avoid or reduce
competition. For example, both owls and hawks
feed on some of the same animals. Because hawks
hunt during the daytime and owls hunt at night,
competition is reduced.
Living in groups can make it easier to obtain food
or stay protected. Wolves are predators that usually
hunt large herbivores, such as deer. Several wolves
will work together to hunt a deer. Deer protect
themselves from wolves by traveling in groups. With
many deer bunched together, it is hard for a pack of
wolves to attack any individual deer.

Helping Each Other
Two different organisms can live closely
together for most or all of their lives. This
relationship may be helpful to both
organisms, or it may help one organism but
not the other. These special relationships
exist between animals, plants, fungi, protists,
and bacteria.
Lichens are two organisms that help each
other. Lichens are combinations of fungi and
algae that live together, often on the surface
of rocks. The algae supply the fungi with
sugar, nutrients, and water. The fungi protect
the algae from too much sunlight and very
warm temperatures. Neither organism could
survive in its habitat without the other.
Living Side by Side
Moss and oak trees, as shown in the
picture, also live together. The tree provides
the moss with a sheltered living space, but
the moss does not help or harm the oak tree.
Some animals also have this kind of
relationship. Beetles, silverfish, and other
insects sometimes travel with army ants.
They eat whatever food is left as the ants
march along. They have no effect on the
army ants.

Causing Harm
Sometimes a close relationship between two
organisms helps one but harms the other. The
organism that is helped is called a parasite.
Parasites are organisms that live on or in
another organism. The organism that is
harmed by the parasite is called the host.
Both plants and animals can act as parasites
and as hosts. A parasite uses its host as a
source of food.
An insect called the balsam woolly adelgid
is a parasite that kills Fraser fir trees of the
Appalachian Mountains. As the insects feed,
they harm the trees.