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F3.1 · Explain competition, predation, symbiosis, and other interactions

Learn to explain competition, predation, symbiosis, and other interactions through clear examples and targeted practice.

Ontario Grade 12 Biology

Population Dynamics

Competition, predation, symbiosis, and other ecological interactions

In SBI3U, you learned that organisms have needs and live in ecosystems with other organisms. Organisms can affect one another as they obtain food, space, shelter, or other resources. For example, two plant species may both need light, while a bird may eat insects living on a tree. An ecological interaction is a relationship in which one organism affects another. The effect may be helpful, harmful, or have little noticeable effect. Classifying an interaction is a useful first step, but it does not always explain every detail of what happens in nature.

What you will learn

Explain how competition and predation affect interacting organisms.
Distinguish mutualism, commensalism, and parasitism as types of symbiosis.
Recognize other interactions, including herbivory and facilitation.
Use effects on each organism to classify an interaction while noting what evidence is still needed.

1. From ecosystem needs to interaction patterns

A resource is something an organism needs and uses, such as food, water, light, or nesting space. When organisms depend on the same limited resource, they may compete. Competition is an interaction in which organisms’ use of a shared resource reduces its availability to one another. Competition can occur between members of the same species or between different species.

Predation occurs when one organism, the predator, captures and eats another organism, the prey. The predator gains food; the prey is harmed and is usually killed. A wolf eating a deer is a familiar example. The terms predator and prey describe roles in that interaction, not permanent categories for every situation.

A compact way to compare effects is to use signs. A plus sign means an organism benefits; a minus sign means it is harmed; and zero means no clear effect is identified. For competition, the effect on each participant is negative. For predation, the predator benefits and the prey is harmed. These signs summarize effects; they do not measure how large the effects are.

\text{competition }(-,-) \qquad \text{predation }(+,-)

Competition involves a resource needed by both participants.
Predation involves a predator consuming prey.
The signs $+$ , $-$ , and $0$ summarize benefits, harms, and no clear effect.

2. Symbiosis and other interactions

Symbiosis is a close, ongoing relationship between organisms of different species. Different kinds of symbiosis are distinguished by how each partner is affected. Mutualism benefits both partners. For instance, a pollinator may obtain food from a flower while helping transfer pollen. Commensalism benefits one partner while having no clear effect on the other. A small plant growing on a tree may gain support without noticeably affecting the tree. Parasitism benefits a parasite while harming its host. A parasite lives on or in its host and obtains resources from it; the host is not usually killed immediately.

The signs help compare these patterns: mutualism is positive for both partners, commensalism is positive for one and neutral for the other, and parasitism is positive for the parasite and negative for the host. A host is the organism that a parasite lives on or in. These descriptions identify general effects, not every possible change over time. The effect of a relationship may depend on conditions or on the organisms involved.

Not every interaction fits neatly into the main symbiosis categories. In herbivory, an animal eats all or part of a plant or alga. The animal benefits, and the plant or alga is harmed, though it may survive. In facilitation, one organism makes conditions more suitable for another. For example, shade from one plant may help a young plant avoid drying out. Facilitation is not the same as mutualism: the organism providing the benefit need not receive a clear benefit in return. Organisms can also interact indirectly. One species may change the availability of food or shelter and thereby affect another without directly touching it.

\text{mutualism }(+,+) \qquad \text{commensalism }(+,0) \qquad \text{parasitism }(+,-)

Symbiosis is a close, ongoing relationship between different species.
Mutualism benefits both; commensalism benefits one with no clear effect on the other; parasitism benefits one and harms the other.
Herbivory and facilitation are other interaction patterns. Identify the effects rather than relying only on a label.

3. Classifying interactions with evidence

To classify an interaction, first name the organisms and describe what each does. Next, identify the resource or relationship involved. Then consider the effect on each participant. Does either gain food, shelter, or another resource? Is either harmed, or is there no clear effect? Finally, choose the best-fitting category and explain the evidence behind the choice.

A direct observation can show behaviour, such as an animal eating a plant. It may not reveal the full effect on the plant or whether the animal benefits over time. Evidence about effects may come from observing organisms in their usual setting or comparing what happens when the interacting partner is present or absent. A comparison can help separate an interaction’s effect from other influences, but a single observation may not establish cause. Weather, food supply, or other organisms may also affect the outcome.

Use the signs as a model, not as a substitute for evidence. They are a simple way to organize the direction of effects. They do not give a numerical measure, prove that one species depends on another, or guarantee that the same relationship will have identical effects in every setting. If the effect on one partner is uncertain, say so rather than forcing a confident classification.

Name both participants and describe what happens before choosing a label.
Use evidence to support each effect; observation alone may not establish cause.
Treat the sign model as a summary, not a measurement or proof.

4. A practical comparison model

The table summarizes common patterns. The first sign refers to the first organism named in an example; the second sign refers to the other organism. For a predator–prey example, the predator is listed first. For a parasite–host example, the parasite is listed first. This order matters because reversing the organisms reverses the order of the signs.

The table gives a starting point for reasoning, not a list of rules that can classify an interaction without context. Competition can involve organisms of the same species or different species. A close relationship may also have effects that are difficult to detect. State what is known, and keep uncertainty visible.

Keep participant order consistent when assigning signs.
Similar sign patterns can occur in different interactions, so also describe how the organisms interact.

Common interaction patterns

Interaction	Effect on first participant	Effect on second participant	What the pattern describes
Competition	Harmed ( $-$ )	Harmed ( $-$ )	Both use a shared resource
Predation	Predator benefits ( $+$ )	Prey harmed ( $-$ )	Predator captures and eats prey
Mutualism	Benefits ( $+$ )	Benefits ( $+$ )	Both partners benefit
Commensalism	Benefits ( $+$ )	No clear effect ( $0$ )	One benefits; the other is not clearly affected
Parasitism	Parasite benefits ( $+$ )	Host harmed ( $-$ )	Parasite obtains resources from a host
Herbivory	Herbivore benefits ( $+$ )	Plant or alga harmed ( $-$ )	Animal eats plant or algal material

Worked example

Two plants sharing light

Two different plant species grow close together. Their leaves overlap, and each plant receives less light than a similar plant growing with more space. Classify the interaction and explain the evidence.

Identify the resource
Both species need light, and the overlapping leaves indicate that they use the same resource.
Consider each effect
The comparison suggests that each plant receives less light when growing near the other. This supports a harmful effect on both participants.
Classify
This is competition between different species. The conclusion is supported by the stated comparison, though other growing conditions could also affect plant growth.
$(-,-)$

Answer: Competition between different species, because both use light and the stated comparison indicates that each receives less when growing close to the other.

Check: The reasoning identifies both a shared resource and a negative effect on each species.

Worked example

A bird feeding on a tree’s insects

A bird eats insects found on a tree. The bird obtains food. Based on this information alone, can the relationship be classified as mutualism?

State the known effect
The bird benefits by obtaining food. The information does not tell us whether the tree benefits, is harmed, or has no clear response.
$(+,?)$
Check the evidence needed
To call the relationship mutualism, there must be evidence that both the bird and tree benefit. Eating insects on a tree does not by itself show that the tree benefits.
Give a careful conclusion
The bird’s benefit is clear, but the effect on the tree is unknown. More evidence is needed to classify the complete interaction.

Answer: No. The bird benefits, but the tree’s effect is not established. Mutualism should not be claimed without evidence that both benefit.

Check: A question mark marks an unknown effect; it is not the same as a zero effect.

Worked example

A parasite and its host

A parasite lives on an animal and obtains resources from it. The animal is harmed but is not immediately killed. Classify this interaction.

Identify the roles
The organism obtaining resources is the parasite. The animal it lives on is the host.
Compare effects
The parasite gains resources, while the host is harmed. Immediate death is not required for this interaction to be parasitism.
$(+,-)$
Classify
This is parasitism, a type of symbiosis. The label fits the close relationship and the stated effects on parasite and host.

Answer: Parasitism: the parasite benefits by obtaining resources, and the host is harmed.

Check: Parasitism does not mean that the host must be killed immediately.

Common mistakes and how to avoid them

Calling every interaction between different species symbiosis.

Correction: Symbiosis means a close, ongoing relationship. A one-time encounter between species is not automatically symbiosis.

Assuming commensalism just because one organism benefits.

Correction: Commensalism also requires no clear effect on the other organism. If that effect is unknown, describe it as unknown.

Assuming a parasite must kill its host immediately.

Correction: Parasitism describes the benefit to the parasite and harm to the host; immediate death is not required.

Using the signs alone to decide what interaction occurred.

Correction: Signs summarize effects, but the behaviour and evidence are also needed to distinguish interactions.

Lesson summary

Competition occurs when organisms use a shared resource and negatively affect one another.
Predation benefits the predator and harms the prey.
Symbiosis is a close relationship between different species; it includes mutualism, commensalism, and parasitism.
Herbivory and facilitation are other interaction patterns.
Classify interactions using evidence about both organisms, and state when an effect is uncertain.

Check your understanding

Question 1

Two species of birds use the same limited nesting sites. Each has fewer available sites when the other species is present. What is the best classification?

Competition
Mutualism
Commensalism
Parasitism

Show answer and explanation

Competition

Both species use a shared limited resource, and each is negatively affected. That is competition.

Question 2

A small organism benefits by living on a larger organism, which is harmed. Which interaction best fits?

Mutualism
Commensalism
Parasitism
Facilitation

Show answer and explanation

Parasitism

Parasitism benefits the parasite and harms its host.

Question 3

An animal eats part of a plant. The animal gains food, and the plant is harmed. What is the most specific interaction named in this lesson?

Herbivory
Commensalism
Competition
Mutualism

Show answer and explanation

Herbivory

Herbivory is an animal eating all or part of a plant or alga.

Key terms

Resource: Something an organism needs and uses, such as food, water, light, or nesting space.
Competition: An interaction in which organisms’ use of a shared resource reduces its availability to one another.
Predator: An organism that captures and eats another organism.
Prey: An organism captured and eaten by a predator.
Symbiosis: A close, ongoing relationship between organisms of different species.
Mutualism: A symbiotic relationship in which both partners benefit.
Commensalism: A symbiotic relationship in which one partner benefits and the other has no clear effect.
Parasitism: A symbiotic relationship in which a parasite benefits while harming its host.

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