Getting Started
An ecosystem is a complex web of relationships, not just between organisms and their physical environment, but among the organisms themselves. At every scale, from a single rotting log to a vast forest, the availability of essential resources like food, water, and space is finite. This scarcity is the fundamental driver of the diverse and intricate interactions that shape ecological communities.
What You Should Be Able to Do
After completing this section, you will be able to:
Explain how the limited availability of resources creates different types of interactions between species.
Distinguish between predation, competition, and the three forms of symbiosis.
Describe how species can coexist by dividing resources, a process known as resource partitioning.
Identify real-world examples for each major type of species interaction.
Key Concepts & Mechanisms
The ways in which species interact are often defined by the outcome for each participant. We can classify these relationships based on whether a species benefits (+), is harmed (-), or is unaffected (0) by the interaction. The most common interactions driven by resource needs are predation, competition, and symbiosis.
| Interaction Type | Effect on Species A | Effect on Species B | Example & Explanation |
|---|---|---|---|
| Predation | + (Benefit) | - (Harm) | A predator is an organism that hunts, kills, and consumes another organism, the prey. This is a direct transfer of energy. For example, a lion (+) hunting a zebra (-) or a hawk (+) catching a mouse (-). |
| Competition | - (Harm) | - (Harm) | Occurs when two or more organisms require the same limited resource. Both are negatively impacted because access to the resource is reduced for each. This can be interspecific (between different species) or intraspecific (within the same species). For example, lions and hyenas fighting over a kill, or two oak trees competing for sunlight and water. |
| Mutualism | + (Benefit) | + (Benefit) | A type of symbiotic relationship where both interacting species benefit. For example, bees get nectar (food) from flowers, and in the process, they transfer pollen, allowing the flowers to reproduce. |
| Commensalism | + (Benefit) | 0 (Unaffected) | A symbiotic relationship in which one species benefits and the other is neither helped nor harmed. For example, barnacles attach themselves to whales. The barnacles gain a home and transportation to nutrient-rich waters, while the whale is unaffected. |
| Parasitism | + (Benefit) | - (Harm) | A symbiotic relationship where one organism, the parasite, lives on or inside another organism, the host, causing it harm. The parasite gains nourishment while the host is weakened. For example, a tick (+) feeds on the blood of a dog (-), potentially transmitting disease. |
Reducing Competition: Resource Partitioning
When two species compete for the same limited resource, they cannot coexist indefinitely if their needs are identical—one will eventually outcompete the other. However, competition can be reduced through resource partitioning, where species evolve to use the same resource in different ways, at different times, or in different places. For instance, several species of warblers may live in the same spruce tree but feed on insects in different parts of the tree—some on the outer branches, some on the inner branches, and others at the very top. This division of the "resource space" minimizes direct competition and allows for coexistence.
Key Models & Diagrams
The three types of symbiosis—a close, long-term interaction between two different species—can be modeled based on their outcomes. This matrix provides a clear visual summary of these relationships.
| Type of Symbiosis | Outcome for Species 1 | Outcome for Species 2 |
|---|---|---|
| Mutualism | Benefits (+) | Benefits (+) |
| Commensalism | Benefits (+) | Unaffected (0) |
| Parasitism | Benefits (+) | Harmed (-) |
Key Components & Evidence
Predation: The dynamic relationship between Canadian lynx and snowshoe hares, whose populations famously rise and fall in a cyclical pattern, demonstrates how predator and prey numbers influence one another.
Interspecific Competition: The struggle between native red squirrels and introduced grey squirrels in the United Kingdom for food and habitat has led to the decline of the native red squirrel population.
Intraspecific Competition: As a dense stand of pine seedlings grows, the trees compete with each other for light and soil nutrients, leading to the eventual death of weaker individuals in a process called self-thinning.
Resource Partitioning: Different species of Anolis lizards in the Caribbean islands avoid competition by occupying different perches, such as tree trunks, canopy leaves, or twigs.
Mutualism: The relationship between coral polyps and zooxanthellae algae is essential for reef ecosystems. The algae live within the coral, providing it with energy from photosynthesis, while the coral provides the algae with shelter and compounds needed for photosynthesis.
Commensalism: Cattle egrets are birds that follow grazing livestock. As the cattle move, they stir up insects from the grass, which the egrets then eat. The egrets benefit, while the cattle are unaffected.
Parasitism: A tapeworm lives in the intestines of a mammal, absorbing nutrients directly from the host's digested food. The tapeworm benefits from a constant food supply, while the host suffers from malnutrition.
Symbiosis: This is the broad category that encompasses mutualism, commensalism, and parasitism, defined by the close and prolonged association between the organisms.
Skill Snapshots
Causation
Cause: A sudden decline in the wolf population (predator) on an island.
Effect: The moose population (prey) increases dramatically, leading to overgrazing and eventual starvation.
Cause: The introduction of an invasive plant species that requires the same soil nutrients as a native plant.
Effect: Intense interspecific competition occurs, potentially leading to the displacement of the native species.
Cause: Two bird species evolve different beak shapes.
Effect: They can feed on different types of seeds (resource partitioning), reducing competition and allowing them to coexist.
Comparison
Predation vs. Parasitism: In predation, the predator kills and consumes its prey; in parasitism, the parasite typically lives on or in its host for an extended time, weakening but not immediately killing it.
Mutualism vs. Commensalism: In mutualism, both species gain a clear benefit from the interaction; in commensalism, only one species benefits while the other experiences no significant effect.
Interspecific vs. Intraspecific Competition: Interspecific competition is a struggle for resources between members of different species, whereas intraspecific competition is the same struggle among members of the same species.
Change and Continuity Over Time
Baseline: A forest ecosystem contains one species of finch that feeds on a variety of seeds.
Change 1: A new, competing finch species is introduced. Intraspecific and interspecific competition for seeds intensifies.
Change 2: Over many generations, one or both finch species may evolve different beak sizes to specialize on different types of seeds, an example of resource partitioning and character displacement.
Continuity: The fundamental need for energy (food) remains the constant selective pressure driving these ecological and evolutionary changes.
Common Misconceptions & Clarifications
Misconception: Symbiosis always means that both organisms benefit.
- Clarification: Symbiosis is a broad term for any close, long-term interaction. It includes mutualism (+/+), but also commensalism (+/0) and parasitism (+/-), where one organism is harmed.
Misconception: Competition always involves direct physical conflict.
- Clarification: While some competition is direct (lions and hyenas fighting), much of it is indirect. For example, two plants compete simply by trying to access the same limited water and sunlight, without ever physically interacting.
Misconception: A "successful" parasite quickly kills its host.
- Clarification: The most evolutionarily successful parasites are those that can live and reproduce without killing their host, as the parasite's survival is dependent on the host's survival.
One-Paragraph Summary
The availability of resources is the central force shaping how species interact within an ecosystem. These interactions range from predation, where one organism consumes another, to competition, a mutually detrimental struggle for limited resources. Organisms also engage in symbiosis, a variety of close, long-term relationships that can be mutually beneficial (mutualism), beneficial to one and neutral to the other (commensalism), or beneficial to one at the expense of the other (parasitism). To reduce the negative impacts of competition, species can engage in resource partitioning, dividing a resource to allow for coexistence. Understanding these interactions is fundamental to explaining the structure, function, and flow of energy in all ecological communities.