Symbiosis
Symbiosis describes close relationships between different species. Learn how mutualism, commensalism, and parasitism differ, how to identify each interaction, and why these relationships matter in ecological communities.
A conceptual model for understanding symbiosis in ecological communities.
In This Article
What Is Symbiosis?
Symbiosis refers to a close, often long-term relationship between organisms of different species. In biology, the term is commonly used for three major relationship types: mutualism, commensalism, and parasitism.
The key to identifying these relationships is to ask how each species is affected. Does each species benefit? Does one benefit while the other is essentially unaffected? Does one benefit while the other is harmed?
Identify symbiotic relationships by looking at the effect on each species, not simply by memorizing examples.
Mutualism
Mutualism is a relationship in which both species benefit. The benefits can include food, nutrients, protection, transportation, pollination, or access to resources.
A classic example is the relationship between flowering plants and many pollinators. The animal receives food such as nectar or pollen, while the plant gains assistance with pollen transfer. Another example is the relationship between mycorrhizal fungi and plant roots: fungi can receive sugars from plants while helping plants obtain water and mineral nutrients.
+
Receives a benefit from the relationship.
+
Also receives a benefit from the relationship.
Commensalism
Commensalism occurs when one species benefits while the other is not significantly helped or harmed by the relationship. The species that benefits may gain shelter, transportation, support, or access to food.
For example, epiphytic plants can grow on larger plants to reach light without necessarily taking nutrients from the plant they grow on. In another example, some animals may use another organism’s structure for shelter without substantially affecting its host.
“Unaffected” means there is no significant effect on the second species. If the second species is clearly harmed or benefits substantially, the interaction belongs in a different category.
Parasitism
Parasitism is a relationship in which one species, the parasite, benefits while the host is harmed. Parasites obtain resources from hosts without usually killing them immediately because the host is the parasite’s resource.
Examples include ticks feeding on mammals, tapeworms living in animal digestive tracts, and many disease-causing organisms that obtain resources from hosts. Parasites can affect host health, reproduction, behavior, and survival.
+
Obtains nutrients or another benefit from the host.
−
Experiences a cost such as nutrient loss, tissue damage, or disease.
How to Tell Them Apart
When a question gives you an unfamiliar example, do not depend on recognizing the organism. Instead, determine the effect on each species.
Common Misconceptions
If You See This on a Test, Think This
Quick Practice
1. What defines a symbiotic relationship?
2. What are the three major types of symbiosis commonly taught in high-school biology?
3. What are the signs for mutualism, commensalism, and parasitism?
4. A bird gains food from an animal while the animal is not significantly affected. What type of relationship is this?
5. Why should you focus on effects rather than memorized examples?
Key Takeaways
- Symbiosis describes close relationships between different species.
- Mutualism benefits both species (+/+).
- Commensalism benefits one species while the other is not significantly affected (+/0).
- Parasitism benefits one species while harming the host (+/−).
- Classify unfamiliar examples by determining the effect on each species.
- Symbiotic relationships can influence survival, reproduction, resource use, and community structure.
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