Symbiotic Relationships in Ecology: mutualism, parasitism, and commensalism

What is a symbiotic relationship?

A symbiotic relationship is a long-term interaction between two species. Did you know lichens are two species in a symbiotic relationship? A lichen is a fungi and an algae or bacteria living together. This is just one of many examples of a symbiotic relationship! Symbiotic relationships typically benefit at least one of the two species involved. Symbiosis is a common topic covered when teaching students about ecology.

brown hummingbird selective focus photography
The relationship between flowering plant and pollinator is one example of mutualism. Photo by Philippe Donn on


Mutualism is a type of symbiotic relationship in which both species benefit. An example of mutualism is a squirrel eating nuts or fruit from a tree, then dispersing the tree’s seeds in its scat. The squirrel benefits because it receives food, and the tree benefits because its seeds are spread and can grow into more trees.

Examples of Mutualism:

  • A bee drinks nectar from flowers, while spreading pollen from plant to plant.
  • A squirrel eats the nuts of a tree and disperses the seeds in its scat.
  • Bacteria and fungi live in a songbird’s intestines. The bacteria break down and eat parts of food that are difficult for the songbird to digest.
  • An algae and fungi live together as a lichen. The algae produces food through photosynthesis and the fungi provides a thick, protective layer.
Close up of two small gray caterpillars have eaten a leaf until only the fibrous leaf veins remain.
Caterpillars devour leaves.


Parasitism is a type of symbiotic relationship in which one species benefits and the other species is harmed. An example of parasitism is a group of insects invading a tree. The insects benefit because they live in and feed on the tree, but the tree is harmed because its important tissues are getting eaten and destroyed.

Examples of Parasitism:

  • A group of insect larvae eat most of the leaves off a tree, which harms the tree’s ability to make its own food through photosynthesis.
  • Beetles cut off a tree’s circulation by feeding on the tree’s phloem, a tree part that moves sugars made during photosynthesis.
  • Ticks drink the blood of a rabbit and pass diseases to the rabbit.
  • A worm lives in the intestine of a deer, consuming some of the food the deer eats and causing the deer to not get enough nutrients.
shallow focus photography of a spiderweb with raindrops
A spider building a web between two plants could be an example of commensalism, as long as the plant isn’t harmed. Photo by NastyaSensei on


Commensalism is a type of symbiotic relationship in which one species benefits and the other species is not affected. An example of commensalism is a bird living in a tree. The bird benefits because it gets a safe place to live. The bird neither helps nor harms the tree.

Examples of Commensalism:

  • A bird builds a nest in a tree, providing the bird with a safe home, and neither harming nor helping the tree.
  • A chipmunk hides from the hot sun in the shadow of a tree. The tree is not affected.
  • A turkey hides from predators in shrubs, The turkey does not harm the shrubs in any way but does not help them either.
  • A spider builds a web between two ferns. The ferns are not impacted in any way.
An example of predation. Photo by Tina Nord on

Is predation a symbiotic relationship?

Predation is when one animal hunts and eats another animal as food. Although this is not always considered a type of symbiotic relationship (because it is usually short-term), predation is still an important interaction between animals in an ecosystem!

Examples of Predation:

  • A badger digs up and eats earthworms.
  • A fox catches a rabbit and carries it back to a den to feed to its pups.
  • An owl hunts for a mouse to eat.
  • A tree frog catches and eats a fly.
An example of herbivory.

What about herbivory?

Herbivory is a short-lived relationship between species, similar to predation. However, herbivory is an interaction in which a plant-eating animal eats a plant. Herbivory is an important ecosystem interaction because it allows energy to flow up the food chain from producers (plants) to primary consumers (herbivores & omnivores). However, herbivory is not typically considered a symbiotic relationship.

Examples of Herbivory:

  • A deer eats some grass.
  • A moose munches the leaves off of a willow.
  • A minnow nibbles on some algae.
  • A caterpillar devours a leaf.

Teaching Students about Symbiosis and Ecology:

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References and Further Reading

  1. Michigan Tech University. (n.d.). Wildlife Ecology Basics. Michigan Forests Forever Teachers Guide. Available:
  2. Lord, J. (n.d.). Exploring Symbiosis. Oregon Institute of Marine Biology. Available:
  3. National Geographic Society (n.d.). Symbiosis: The Art of Living Together. Available:

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