Biology · Ecology and Evolution

Community Interactions

Every organism is part of a larger story.

Organisms in a community constantly interact. They compete for resources, consume one another, cooperate, and sometimes benefit from relationships that have developed over generations.

🌱PlantProducer
↔
🐝PollinatorConsumer
↔
🦊PredatorConsumer
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What Is a Community Interaction?

A community interaction occurs when organisms of different populations affect one another.

These interactions can help organisms, harm organisms, or have little direct effect on one of the organisms involved.

Big idea: Ecologists often describe interactions based on their effects on each population: positive (+), negative (−), or neutral (0).

− / −

Competition

Competition occurs when organisms attempt to use the same limited resource. Both populations can be negatively affected because access to the resource is reduced.

Example

Deer ↔ Deer

Two deer may compete for limited food, water, or territory. Competition can also occur between different species.

+ / −

Predation

Predation occurs when one organism kills and consumes another organism.

Example

🦅 → 🐇

The predator benefits by gaining energy, while the prey is harmed.

Predation is one of the major relationships connecting populations within food chains and food webs.

Long-Term Relationships

Symbiosis

Symbiosis describes a close relationship between organisms of different species. Three commonly studied forms are mutualism, commensalism, and parasitism.

Mutualism

+ / +

Both organisms benefit from the interaction.

Commensalism

+ / 0

One organism benefits while the other is not significantly affected.

Parasitism

+ / −

The parasite benefits while the host is harmed.

+ / +

Mutualism

In mutualism, both organisms benefit from the relationship.

Bees + FlowersBees obtain food while flowers receive pollination.
MycorrhizaePlants and fungi can exchange resources.
Cleaner FishOne organism obtains food while another benefits from removal of parasites.
+ / 0

Commensalism

In commensalism, one organism benefits while the other is not significantly affected.

A classic example is an epiphytic plant growing on a tree. The plant can gain access to light and support while the tree is not significantly helped or harmed by the relationship.

+ / −

Parasitism

In parasitism, one organism benefits while the host is harmed.

Parasites obtain resources from their hosts. Unlike typical predation, the parasite generally does not immediately kill the host.

Examples include ticks feeding on mammals and tapeworms obtaining nutrients from hosts.

Ecological Competition

Competition and the Niche

Competition is especially important when two populations depend on the same limited resources.

If two species occupy very similar niches and compete for exactly the same limiting resources, competition can become intense.

Species can sometimes reduce competition by using different resources, occupying different habitats, or using the same resource at different times.

Watch Out

Common Misconceptions

“Competition only happens between different species.”
Members of the same species can compete too.
“Predation means the predator always eats another animal.”
Predation is the killing and consumption of another organism; the prey does not have to be an animal.
“All symbiosis helps both organisms.”
Mutualism benefits both, commensalism benefits one without significant effect on the other, and parasitism benefits one while harming the other.
“Parasitism is the same as predation.”
Both can benefit one organism and harm another, but parasites typically live on or in a host and do not immediately kill it.
Test-Day Connections

If You See This on a Test, Think This

“Both species benefit.”
Mutualism (+ / +)
“One benefits; the other is unaffected.”
Commensalism (+ / 0)
“One benefits; the host is harmed.”
Parasitism (+ / −)
“Both populations are negatively affected by limited resources.”
Competition (− / −)
“One organism kills and consumes another.”
Predation (+ / −)
Check Your Understanding

Quick Practice

1. What is competition?
Answer: An interaction in which organisms use the same limited resource.
2. What type of interaction benefits both organisms?
Answer: Mutualism.
3. What type of interaction benefits one organism while the other is not significantly affected?
Answer: Commensalism.
4. What type of interaction involves a parasite and host?
Answer: Parasitism.
5. What happens during predation?
Answer: One organism kills and consumes another organism.
Remember These

Key Takeaways

  • Community interactions describe relationships among populations.
  • Competition can occur within or between species.
  • Predation benefits the predator and harms the prey.
  • Mutualism is +/+.
  • Commensalism is +/0.
  • Parasitism is +/-.
  • Interactions help shape population sizes and community structure.
Continue Learning
Ecosystems → Predator–Prey Dynamics → Food Webs →
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