A symbiosis (siM-bie-OH-sis) is a close, long-term relationship between two organisms. Three examples of symbiotic relationships include: parasitism, mutualism, and commensalism. Parasitism (PAR-uh-SIET-IZ-UHM) is a relationship in which one individual is harmed while the other individual benefits. Mutualism (MYOO-choo-uhl-iZ-uhm) is a relationship in which both organisms derive some benefit. In commensalism (kuh-MEN-suhl-iZ-uhm), one organism benefits, but the other organism is neither helped nor harmed.


Parasitism is similar to predation in that one organism, called the host, is harmed and the other organism, called the parasite, benefits. However, unlike many forms of predation, parasitism usually does not result in the immediate death of the host. Generally, the parasite feeds on the host for a long time rather than kills it. Parasites such as aphids, lice, leeches, fleas, ticks, and mosquitoes that remain on the outside of their host are called ectoparasites. Parasites that live inside the host's body are called endoparasites. Familiar endoparasites are heart-worms, disease-causing protists, and tapeworms, such as the one shown in Figure 20-5. Natural selection favors adaptations that allow a parasite to exploit its host efficiently. Parasites are usually specialized anatomically and physiologically for a parasitic lifestyle.

Parasites can have a strong negative impact on the health and reproduction of the host. Consequently, hosts have evolved a variety of defenses against parasites. Skin is an important defense that prevents most parasites from entering the body. Tears, saliva, and mucus defend openings through which parasites could pass, such as the eyes, mouth, and nose. Finally, the cells of the immune system may attack parasites that get past these defenses.

figure 20-5

Tapeworms are endoparasites that can grow to 20 m or greater in length. Tapeworms are so specialized for a parasitic lifestyle that they do not have a digestive system. They live in the host's small intestine and absorb nutrients directly through their skin. Tapeworms reproduce by producing egg-filled chambers, which are released in their host's feces to be unknowingly picked up by a future host.

figure 20-6

Acacia trees in Central America have a mutualistic relationship with certain types of ants. The trees provide food and shelter to the ants, and the ants defend the tree from insect herbivores.

figure 20-6

Acacia trees in Central America have a mutualistic relationship with certain types of ants. The trees provide food and shelter to the ants, and the ants defend the tree from insect herbivores.


Mutualism is a relationship in which two species derive some benefit from each other. Some mutualistic relationships are so close that neither species can survive without the other. An example of mutualism, shown in Figure 20-6, involves ants and some species of Acacia plants. The ants nest inside the acacia's large thorns and receive food from the acacia. In turn, the ants protect the acacia from herbivores and cut back competing vegetation.

Pollination is one of the most important mutualistic relationships on Earth. Animals such as bees, butterflies, flies, beetles, bats, and birds that carry pollen between flowering plants are called pollinators. A flower is a lure for pollinators, which are attracted by the flower's color, pattern, shape, or scent. The plant usually provides food—in the form of nectar or pollen—for its pollinators. As a pollinator feeds in a flower, it picks up a load of pollen, which it may then carry to other flowers of the same species.


Commensalism is an interaction in which one species benefits and the other species is not affected. Species that scavenge for leftover food items are often considered commensal species. However, a relationship that appears to be commensalism may simply be mutualism in which the mutual benefits are not apparent.

An example of a commensal relationship is the relationship between cattle egrets and Cape buffaloes in Tanzania. The birds feed on small animals such as insects and lizards that are forced out of their hiding places by the movement of the buffaloes through the grass. Occasionally, the cattle egrets also feed on ectoparasites from the hide of the buffaloes, but the buffaloes generally do not benefit from the presence of the egrets.

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