AP Biology

## Learning Outcomes

- Define symbiosis and distinguish it from other community interactions such as predation and competition.
- Identify mutualism, commensalism, and parasitism from written scenarios, graphs, or experimental data.
- Explain how symbiotic relationships can affect survival, reproduction, population growth, and natural selection.
- Use examples such as lichens, remoras and sharks, gut bacteria, ticks, and tapeworms to support explanations.
- Predict how changes in one species may affect another species in a symbiotic relationship.
- Design or evaluate experiments testing the effects of one organism on another in a symbiotic interaction.

## AP Biology Syllabus

In AP Biology Unit 8, community ecology is tested through the broader theme of systems interactions: organisms interact with other organisms and with their physical surroundings, and those interactions can affect population size, behavior, biodiversity, and evolution. Symbiotic relationships are especially likely to appear in scenario-based multiple-choice questions or free-response prompts that ask you to classify an interaction, interpret data, justify a prediction, or design an experiment with controls.

- Symbiosis describes close, long-term interactions between different species.
- Mutualism benefits both species involved.
- Commensalism benefits one species while the other is not significantly affected.
- Parasitism benefits the parasite and harms the host.
- Effects should be described in terms of survival, reproduction, energy gain, nutrient access, or population growth.
- AP questions may use unfamiliar organisms; the stem will provide the needed background.
- Experimental questions may ask for independent variables, dependent variables, controls, null hypotheses, and evidence-based conclusions.
- Graphs and data tables may show changes in abundance, growth rate, mortality, reproductive success, or fitness.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What is the difference between mutualism, commensalism, and parasitism?
2. Why is lichen often used as an example of mutualism?
3. A fish attaches to a shark and eats scraps of food without affecting the shark’s survival or reproduction. What type of relationship is this?
4. Why is parasitism different from predation, even though both benefit one organism and harm another?
5. In an experiment testing whether gut bacteria improve mammal digestion, what could be the independent and dependent variables?

## Introduction

A biological community contains populations of different species that live in the same area and interact with one another. Some interactions are brief, such as a predator catching prey. Others are close and long-lasting. These close relationships are called symbiotic relationships, and they include mutualism, commensalism, and parasitism.

For AP Biology, you need to do more than memorize examples. You should be able to read a new scenario, decide which species benefits or is harmed, and connect the relationship to survival, reproduction, population growth, and natural selection. Many exam questions describe organisms you have never studied before, but they give enough information for you to apply the core ideas.

> **Key Term: symbiosis**
> A close, long-term interaction between individuals of different species.

### Classifying Symbiosis by Fitness Effects

The simplest way to classify a symbiotic relationship is to ask two questions:

- Does species A benefit, suffer harm, or experience no significant effect?
- Does species B benefit, suffer harm, or experience no significant effect?

In ecology, “benefit” and “harm” usually refer to biological fitness rather than comfort or intention. A species benefits if the interaction increases access to energy, nutrients, protection, habitat, mating success, survival, or reproduction. A species is harmed if the interaction decreases survival, reproduction, growth, or resource availability.

Mutualism is a $$+/+$$ relationship: both species benefit. Commensalism is a $$+/0$$ relationship: one species benefits, while the other has no clear positive or negative effect. Parasitism is a $$+/-$$ relationship: the parasite benefits, while the host is harmed.

> **Key Term: fitness**
> The relative ability of an organism to survive and produce viable, fertile offspring in a particular environment.

For example, if bacteria in a mammal’s gut help break down food and gain nutrients and shelter from the mammal, both partners benefit. If a remora attaches to a shark and gains transport and food scraps while the shark is not measurably affected, the relationship is usually described as commensalism. If a tick feeds on a mammal’s blood and reduces the host’s energy or transmits disease, the tick benefits while the host is harmed, so the relationship is parasitism.

> **Test Tip:**
> On AP questions, do not classify the relationship based only on the names of the organisms. Classify it based on the evidence in the question stem: who gains resources, who loses resources, and whose survival or reproduction changes?

Symbiotic interactions often affect population dynamics. If a mutualistic partner becomes rare, the other species may have lower survival or reproduction. If parasite abundance increases, host mortality may rise or host reproductive success may fall. Because these effects influence fitness, symbiosis can also shape natural selection over generations.

### Mutualism: Both Species Benefit

> **Key Term: mutualism**
> A symbiotic relationship in which both interacting species benefit.

Mutualism is common because organisms often gain energy or resources more effectively by interacting with another species. A classic example is lichen. Lichen may look like one organism, but it is an association between a fungus and a photosynthetic partner, such as an alga or cyanobacterium. The photosynthetic partner produces organic molecules using light energy. The fungus provides structure, protection, and access to water and minerals. In many AP Biology contexts, lichen is treated as a mutualistic relationship because both partners gain resources or conditions that improve survival.

Another major example is the relationship between mammals and gut bacteria. Humans and many other mammals provide bacteria with a nutrient-rich habitat. In return, the bacteria help break down substances the host may not digest efficiently on its own, contribute to nutrient processing, and can reduce the chance that harmful microbes will colonize the gut. This relationship connects community ecology with energetics because digestion affects the acquisition of matter and energy.

Plant-pollinator interactions are also common mutualisms. A bee, bat, bird, or other pollinator gains nectar or pollen as food. The plant gains increased transfer of pollen between flowers, raising the chance of successful reproduction. A prompt might describe one plant species whose seed production drops when pollinators are excluded by mesh bags. The correct explanation is not just that “pollinators help plants”; you should specify that pollinators increase reproductive success by moving pollen, while the pollinator gains food.

Some mutualisms are obligate, meaning one or both species depend on the relationship for survival or reproduction. Others are facultative, meaning the species benefit from the relationship but can survive without it. AP Biology generally focuses more on the direction of the effect than on these labels, but knowing the distinction can help you interpret experimental results.

> **Key Term: obligate mutualism**
> A mutualistic relationship in which at least one partner requires the relationship to survive or reproduce successfully.
>
> **Key Term: facultative mutualism**
> A mutualistic relationship that benefits the partners but is not strictly required for survival or reproduction.

Mutualism is not charity. Each species is favored when the benefits exceed the costs. For example, a plant produces nectar, which requires energy and matter, but the reproductive benefit from pollination may outweigh that cost. If a mutation caused a plant to produce no nectar and pollinators stopped visiting, the plant might have lower reproductive success.

> **Exam Warning:**
> Do not describe mutualism as “both organisms help each other because they want to.” Natural selection does not require intention. Traits that increase survival or reproduction tend to become more common if they are heritable.

### Commensalism: Benefit Without Measurable Harm

> **Key Term: commensalism**
> A symbiotic relationship in which one species benefits and the other species is not significantly helped or harmed.

Commensalism can be harder to identify than mutualism or parasitism because “no effect” is difficult to prove. In AP Biology, use the information given. If the prompt says one species benefits and the other is unaffected, classify the relationship as commensalism.

The remora and shark example is commonly used. Remoras, sometimes called sucker fish, attach to sharks. The remora may gain transport, protection, and access to food scraps. If the shark’s feeding, movement, survival, and reproduction are not significantly changed, the relationship is commensalism.

Other possible examples include birds nesting in trees or epiphytic plants growing on tree branches. The bird or epiphyte gains a place to live, while the tree may be unaffected if the relationship does not reduce light capture, nutrient access, growth, or reproduction. However, if the organism growing on the tree blocks light, damages tissue, or removes nutrients, the relationship may not be commensalism.

Commensalism shows why evidence matters. The same pair of species can sometimes be classified differently depending on measurable effects. A small number of attached organisms may not harm a host, but a heavy load could increase drag, reduce feeding efficiency, or damage tissue. If data show a negative effect on the host, the relationship shifts toward parasitism.

> **Test Tip:**
> If an AP question gives data, use the data rather than a memorized label. For example, if remoras reduce shark swimming speed or survival in the data provided, you should not call the relationship commensalism just because that is the usual textbook example.

A good free-response answer should explain both sides of the interaction. For a commensalism scenario, state which organism benefits, how it benefits, and why the other organism is considered unaffected. A weak answer says, “The fish benefits and the shark doesn’t care.” A stronger answer says, “The remora benefits by gaining food scraps and transport, while the shark has no measurable change in survival, reproduction, or energy intake.”

### Parasitism: The Parasite Benefits and the Host Is Harmed

> **Key Term: parasitism**
> A symbiotic relationship in which one organism, the parasite, benefits while the host is harmed.

In parasitism, the parasite obtains resources from the host. The host may lose blood, nutrients, tissue, energy, or reproductive capacity. Parasites can live on the outside of a host, such as ticks, lice, and fleas, or inside the host, such as tapeworms, some protists, and many disease-causing worms.

> **Key Term: parasite**
> An organism that lives on or in a host and benefits by obtaining resources at the host’s expense.
>
> **Key Term: host**
> The organism that provides resources or habitat to a parasite and is harmed by the interaction.

Parasitism differs from predation in its usual timing and degree of harm. A predator typically kills and consumes prey in a relatively short interaction. A parasite generally feeds on or uses the host over a longer period and often does not immediately kill the host. A tick feeding on a deer is parasitic. A wolf killing and eating a deer is predation. Both are $$+/-$$ interactions, but their ecological patterns differ.

Parasites may reduce host fitness in several ways. They may remove nutrients, damage tissues, cause immune responses that require energy, or transmit pathogens. A tapeworm living in the intestine absorbs nutrients from the host’s digestive tract, which can reduce the host’s energy gain. A tick feeding on blood can reduce host condition and transmit disease-causing microbes.

> **Key Term: pathogen**
> A disease-causing agent, such as a virus, bacterium, protist, fungus, or other biological agent.

Not all parasites cause obvious death, but even small reductions in growth or reproduction can matter. In AP Biology, be alert for evidence such as lower offspring number, reduced body mass, slower growth, decreased photosynthetic rate in an infected plant, or higher mortality. Those are signs that the host is harmed.

Parasitism can drive natural selection in both species. Hosts with traits that help them resist parasites may leave more offspring. Parasites with traits that help them attach, evade host defenses, or reproduce within hosts may also be favored. This can lead to coevolution, where changes in one species influence selection pressures on the other.

### Using Experiments and Data to Study Symbiosis

AP Biology often tests ecology through experiments rather than direct recall. You may be asked to design a study, identify variables, describe a control group, interpret a graph, or justify a claim using data.

Suppose researchers ask whether a gut bacterial species improves digestion in mice. One experimental group of mice receives the bacteria, while a control group does not. The independent variable is the presence or absence of the bacteria. Dependent variables might include mouse mass gain, amount of undigested food in feces, glucose absorption, or survival. Controlled variables should include diet, age, initial mass, temperature, water access, and length of the experiment.

A possible null hypothesis is that the presence of the bacteria has no effect on mouse digestion or growth. If mice with bacteria gain more mass on the same diet and have less undigested food in feces, the data support the claim that the relationship benefits the host. If the bacteria also gain nutrients and habitat, the relationship can be classified as mutualism.

> **Key Term: null hypothesis**
> A testable statement predicting no difference or no effect between groups in an experiment.

For a parasitism experiment, researchers might compare infected and uninfected plants. If infected plants have lower growth rate, fewer seeds, or reduced photosynthesis, the data support the claim that the parasite harms the host. For commensalism, the challenge is showing that one partner benefits while the other shows no significant change. Error bars or confidence intervals may be used to decide whether differences between groups are likely meaningful.

When interpreting graphs, check the axes first. A graph may show population size, growth rate, percent survival, mean number of offspring, or parasite load. If host survival decreases as parasite load increases, that supports a harmful effect of the parasite. If plant seed production increases when pollinators are present, that supports a benefit to the plant. If the pollinator also gains food, the relationship is mutualistic.

> **Exam Warning:**
> Avoid claiming that a relationship is commensalism just because harm is not described. If there are no data on the second species, say that more evidence is needed to determine whether it is unaffected.

Strong AP explanations usually follow this pattern:

- Identify the type of interaction.
- State which species benefits and how.
- State whether the other species benefits, is harmed, or is unaffected.
- Connect the effect to survival, reproduction, energy, nutrients, or population growth.
- Use data from the prompt if data are given.

For example:

> **Question:**
> Researchers find that plant species A produces twice as many seeds when fungal species B is present near its roots. The fungus receives carbohydrates from the plant. Identify the relationship and justify your answer.

A strong answer would be: “The relationship is mutualism. Plant species A benefits because the presence of the fungus increases seed production, which raises reproductive success. The fungus benefits because it receives carbohydrates from the plant as an energy-rich food source.”

That answer earns more credit than simply writing “mutualism because both benefit,” since it explains the specific evidence and links it to fitness.

## Key Point Checklist

This article has covered the following key knowledge points:

- Symbiosis is a close, long-term relationship between different species.
- Mutualism is a $$+/+$$ interaction in which both species benefit.
- Commensalism is a $$+/0$$ interaction in which one species benefits and the other is not significantly affected.
- Parasitism is a $$+/-$$ interaction in which the parasite benefits and the host is harmed.
- Benefits and harms should be described through survival, reproduction, energy gain, nutrient access, growth, or population change.
- Lichen is commonly treated as a mutualism between a fungus and a photosynthetic partner.
- Gut bacteria and mammals can have a mutualistic relationship involving habitat, nutrients, and digestion.
- Remoras attached to sharks are a common commensalism example when the shark is unaffected.
- Ticks, tapeworms, lice, and many disease-causing organisms are examples of parasites.
- Parasitism differs from predation because parasites usually live on or in hosts over time and do not usually kill immediately.
- AP questions may require experimental design, including controls, independent variables, dependent variables, and null hypotheses.
- Data should override memorized examples when classifying symbiotic relationships.

## Key Terms and Concepts

- symbiosis
- fitness
- mutualism
- obligate mutualism
- facultative mutualism
- commensalism
- parasitism
- parasite
- host
- pathogen
- null hypothesis


Community ecology - Symbiotic relationships (mutualism, commensalism, parasitism)