Interactions Among Organisms

In any given ecosystem, organisms have interactions that allow them greater access to resources. These interactions can lead to competition for resources. Consequently, relationships form that allow a greater number of species access to those resources. We call these relationships symbiotic relationships.

● Examples include competition, mutualism, commensalism, and parasitism.


● Competition is a relationship that occurs when two or more organisms need the same resource at the same time.

● Competition can be among the members of the same or different species and usually occurs with organisms that share the same niche.

● An ecological niche refers to the role of an organism in its environment including type of food it eats, how it obtains its food and how it interacts with other organisms.

● Two species with identical ecological niches cannot coexist in the same habitat. Competition usually results in a decrease in the population of a species less adapted to compete for a particular resource.


A symbiotic relationship exists between organisms of two different species that live together in direct contact. The balance of the ecosystem is adapted to the symbiotic relationship.

● If the population of one or other of the symbiotic organisms becomes unbalanced, the populations of both organisms will fluctuate in an uncharacteristic manner.

** Symbiotic relationships include parasitism, mutualism, and commensalism.


Parasitism is a symbiotic relationship in which one organism (the parasite) benefits at the expense of the other organism (the host). In general, the parasite does not kill the host.

● Some parasites live within the host, such as tape worms, heartworms, or bacteria. Some parasites feed on the external surface of a host, such as aphids, fleas, or mistletoe.

● The parasite-host populations that have survived have been those where neither has a devastating effect on the other.

● Parasitism that results in the rapid death of the host is devastating to both the parasite and the host populations.

● It is important that the host survive and thrive long enough for the parasite to reproduce and spread

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Mutualism is a symbiotic relationship in which both organisms benefit. Because the two organisms work closely together, they help each other survive.

● For example, bacteria, which have the ability to digest wood, live within the digestive tracts of termites; plant roots provide food for fungi that break down nutrients the plant needs.


Commensalism is a symbiotic relationship in which one organism benefits and the organism is not affected.

● For example, barnacles that attach to whales are dispersed to different environments where they can obtain food and reproduce; burdock seeds that attach to organisms and are carried to locations where they can germinate.

Predator- Prey relationships

Predation is an interaction between species in which one species (the predator) hunts, kills, and eats the other (prey). This interaction helps regulate the population within an ecosystem thereby causing it to become stable.

● Fluctuations in predator–prey populations are predictable. At some point the prey population grows so numerous that they are easy to find. A graph of predator–prey density over time shows how the cycle of fluctuations results in a stable ecosystem. ● As the prey population increases, the predator population increases.

● As the predator population increases, the prey population decreases.

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