Symbionts: Organisms that live in or on other organisms. More than half of Earth’s species are symbionts. Our own bodies can be a home to many other species.

A parasite consumes the tissues or body fluids of the organism on which it lives (the host).

Enslaving parasite: control the host in a way to aid in its dispersal (i.e drowning grasshopper)

Pathogens are parasites that cause diseases (an abnormal condition affecting the body of an organism).

Parasites typically harm, but don’t immediately kill, the organisms they eat (unlike predators).Degree of harm varies widely.

Parasites typically feed on only one or a few host individuals.

Include herbivores such as aphids or nematodes that feed on one or a few host plants.

Parasitoids: Insects whose larvae feed on a single host and almost always kill it.

Macroparasites: Large species such as arthropods and worms.

Microparasites: Microscopic, such as bacteria.

Ectoparasites live on the outer body surface of the host. (Many fungi are ectoparasites) [ex. Aphids, whiteflies, scale insects, nematodes, beetles, and juvenile cicadas] They can be thought of as both herbivores and parasites.

Animals also have many ectoparasites.


  • Athlete’s foot fungus, fleas, mites, lice, and ticks.
  • Some of these parasites also transmit disease organisms.

Endoparasites live inside their hosts, within cells or tissues, or in the alimentary canal.

Many disease organisms are endoparasites.

  • The alimentary canal is excellent habitat for many parasites. Most do not eat host tissue but rob the host of nutrients. Tapeworms attach to the host’s intestinal wall and absorb digested food.
  • Many endoparasites live in the host’s tissues or cells.
  • Mycobacterium tuberculosis, the bacterium that causes tuberculosis.
  • Plants also have endoparasites.
  • Ectoparasites can disperse more easily than endoparasites.
  • Ectoparasites are more exposed to predators, parasites, and parasitoids.
  • Endoparasites are protected from the external environment and have easy access to food. But they can also be attacked by the host’s immune system.

Defenses and Counterdefenses

Hosts have adaptations for defending themselves against parasites, and parasites have adaptations for overcoming host defenses.

  • Parasites exert strong selection pressure on their host organisms, and vice-versa.
  • Host organisms have many kinds of defense mechanisms.
  • Protective outer coverings include skin and exoskeletons.
  • Many parasites that do gain entry are killed by the host’s immune system.
  • Vertebrate immune systems have “memory cells” that can recognize microparasites from previous exposures.
  • Plants also have defense systems: Resistance Genes, Nonspecific immune response, chemical signals that “warn” nearby cells of attack, chemicals that stimulate deposition of lignin (barrier to stop invader’s spread).
  • Hosts can regulate biochemistry to deter parasites.
  • Plants have many chemical weapons called secondary compounds.
  • Some animals eat specific plants to treat or prevent parasite infections
  • Some hosts can encapsulate endoparasites, or their eggs, to make them harmless.
  • Some insects have lamellocytes—blood cells that can form multicellular capsules around large objects such as nematodes.

Parasite–Host Coevolution

Host and parasite populations can evolve together, each in response to selection imposed by the other.

When parasites and hosts each possess specific adaptations, it suggests that the strong selection pressure they impose on each other has caused both populations to evolve.

The rabbit and myxoma virus illustrates coevolution: When populations of two interacting species evolve together, each in response to selection imposed by the other.

  • Some plant genotypes have resistance genes to specific parasite genotypes: gene-for-gene interactions.
  • Wheat has dozens of different genes for resistance to fungi such as wheat rusts.
  • Different wheat rust genotypes can overcome different wheat resistance genes.
  • Ever-escalating “arms races” rarely occur.
  • As with the snails and trematodes, common host genotypes decrease in frequency because they are attacked by many; leading to an increase in previously rare genotypes.
  • An arms race may stop because of trade-offs: a trait that improves host defenses or parasite counterdefenses may reduce some other aspect of growth, survival, or reproduction.
  • Some rust genotypes are more virulent (can overcome more plant resistance genes).
  • Virulent rust genotypes were common only in host populations dominated by plants with many resistance genes.
  • The trade-off: virulent rust genotypes produce fewer spores than other genotypes

Ecological Effects of Parasites

  • Parasites can reduce the sizes of host populations and alter the outcomes of species interactions, thereby causing communities to change.
  • Parasites can reduce the survival or reproduction of their host.
  • Experiments with a beetle and a sexually transmitted mite showed a decrease in egg production by infected females.
  • At the population level, the harm done by parasites translates into a reduction of population growth rates.
  • Parasites can drive local host populations extinct and reduce their geographic ranges.
  • Parasites can impact host population cycles.
  • Parasites can alter the outcome of predator–prey interactions by decreasing the physical condition of infected individuals.
  • Predators may be less able to catch their prey, or prey less able to escape predation.

Physical Environment:

  • Ecosystem engineer species can change the physical characteristics of the environment, as when a beaver builds a dam.
  • The amphipod Corophium is an ecosystem engineer in tidal mudflats.
author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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