Difficulty: Intermediate | Prerequisites: Introduction to Ecology and Population Growth study notes
Competition is one of the main biotic interactions that regulates population size and shapes community structure. This unit introduces the concept of the ecological niche, explains why two species cannot occupy the exact same niche indefinitely, and covers the mechanisms that allow similar species to coexist. If you are not comfortable with density dependence and r, review the Population Growth notes first.
Competition is a lose-lose interaction (–/–) that occurs within species (intraspecific) and between species (interspecific). Two species sharing the exact same niche cannot coexist indefinitely; the stronger competitor drives the other to extinction. Species avoid this through niche differentiation, resource partitioning, and character displacement, which allow them to coexist by using resources in slightly different ways.
Intraspecific competition
Competition between individuals of the same species. This is a key driver of density dependence. In simple terms, members of your own species are often your fiercest competitors because they need exactly the same resources.
Interspecific competition
Competition between individuals of different species. This shapes community structure and determines which species can coexist in an area.
Exploitative competition
Competition where individuals consume the same limited resource. Neither directly harms the other; they simply deplete the shared resource. Think of it as a race to eat the food first.
Interference competition
Competition where individuals directly prevent others from accessing resources, through aggression, territoriality, or chemical inhibition. One organism actively blocks another.
Niche
The full range of resources, environmental conditions, and ecological roles that a species uses and occupies. It describes everything about where and how a species lives.
Fundamental niche
The full range of conditions and resources a species could use in the absence of competition. This is the species' potential ecological space.
Realized niche
The portion of the fundamental niche that a species actually occupies when competitors are present. Always smaller than or equal to the fundamental niche. Think of it as what the species is left with after competition squeezes it out of part of its range.
Competitive exclusion principle
Two species occupying the exact same niche cannot coexist indefinitely. The superior competitor will drive the other to local extinction. Also known as Gause's principle.
Niche differentiation
The evolutionary process by which competing species diverge in their use of resources, reducing overlap and allowing coexistence. Over time, species evolve to use different parts of their environment.
Character displacement
The evolutionary divergence of competing species' traits (body size, beak shape, feeding behaviour) in areas where they overlap. The traits become more different where the species coexist than where each lives alone.
Resource partitioning
The division of limited resources among species to reduce competition. Species use different food sources, forage at different times, or occupy different microhabitats.
Habitat heterogeneity
The variety of physical environments and microhabitats within an area. Greater heterogeneity provides more ways for species to differentiate their niches and coexist.
Limiting resources
Resources that are in short supply relative to demand and therefore constrain population growth. Competition centres on limiting resources.
Antagonistic interactions
Interactions with negative effects for at least one party. Competition (–/–) is antagonistic for both parties involved.
Competition is denoted (–/–) because both parties suffer reduced fitness.
It occurs whenever organisms use the same limiting resource.
Competition regulates population size (a density-dependent factor) and determines which species persist in a community.
Exploitative competition: indirect. Both species consume the same resource, depleting it for the other. Neither attacks the other directly.
Interference competition: direct. One species actively prevents the other from accessing a resource through aggression, territoriality, or chemical inhibition.
A niche encompasses everything about how a species makes its living: the resources it uses, the conditions it tolerates, its role in the food web, and when and where it is active.
The fundamental niche is the full set of conditions a species could exploit in the absence of competitors.
The realized niche is the subset it actually occupies once competition narrows its options.
Competition does not change the fundamental niche. The fundamental niche stays the same; the species simply cannot access all of it.
If two species have identical niches, they cannot coexist. The one with even a slight competitive advantage will eventually drive the other to extinction in that habitat.
Coexistence requires that niches differ in some way.
Habitat heterogeneity: a varied environment offers more distinct microhabitats, so weaker competitors can find refuges where they outperform the dominant species.
Niche differentiation: over evolutionary time, competing species diverge in their resource use, reducing overlap.
Character displacement: physical traits (beak size, body size) diverge in areas of overlap. Where species live apart, their traits may be similar; where they coexist, traits become more distinct.
Resource partitioning: species divide up resources by using different food items, foraging at different times of day, or occupying different vertical zones in a habitat.
When you observe two similar species coexisting with minimal apparent competition, it may be because competition in the past drove niche differentiation and character displacement.
The current low level of competition is the result of intense historical competition that selected for divergence. This concept is called the "ghost of competition past."
The barnacle example from this course is a classic demonstration of competitive exclusion and niche partitioning. Chthamalus stellatus and Balanus balanoides occupy different zones on rocky intertidal shores. Chthamalus can survive in both the upper and lower zones, but Balanus outcompetes it in the lower zone, restricting Chthamalus to the upper zone (its realized niche). The best way to test this is a removal experiment: take Balanus out of the lower zone and see whether Chthamalus recolonises.
The leopard frog and bullfrog scenario illustrates the same principle in freshwater. Before bullfrogs were introduced, leopard frogs used the entire pond (their fundamental niche). After introduction, bullfrogs dominated the centre, and leopard frogs retreated to the reeds along the shore (their realized niche). To test whether bullfrogs' fundamental niche extends beyond the pond centre, you would remove the leopard frogs and observe whether bullfrogs expand into the reeds.
Students often confuse fundamental and realized niches. The fundamental niche does not shrink when a competitor arrives. The species simply cannot access all of it. Its potential range stays the same; its actual range narrows.
Students sometimes think competition always leads to extinction. It does not. Competitive exclusion occurs only when niches are identical. If niches differ even slightly, coexistence is possible.
A common mistake is assuming that the stronger competitor always has the larger realized niche. The realized niche of any species depends on the specific conditions and resources present, not just competitive ability.
Students occasionally confuse niche differentiation (an evolutionary process occurring over generations) with resource partitioning (an ecological pattern observable at one point in time). They are related but operate on different timescales.
⚠️ Be able to define fundamental vs. realized niche and illustrate the difference with a specific example (the frog scenario is likely). This is a near-certain exam question.
⚠️ Know the competitive exclusion principle and be able to state what environmental feature promotes coexistence (habitat heterogeneity).
⚠️ Understand how to design a removal experiment to test competitive exclusion. The barnacle question tests this directly.
⚠️ Be able to identify niche differentiation from a diagram showing two species' resource-use curves shifting apart over time.
⚠️ Know the difference between exploitative and interference competition with examples.
True or false: Competition is a +/– interaction. (False. Competition is –/–; both parties are harmed.)
Fill in the blank: The niche a species occupies in the absence of competitors is its ____ niche. (fundamental)
True or false: Two species with identical niches can coexist indefinitely. (False. The competitive exclusion principle says the stronger competitor will drive the other to extinction.)
Fill in the blank: The evolutionary divergence of traits in areas where two species overlap is called ____. (character displacement)
True or false: Habitat heterogeneity reduces the chances of coexistence. (False. Greater habitat heterogeneity promotes coexistence.)
Q: What is the difference between population ecology and community ecology?
A: Population ecology studies the dynamics of a single species (size, growth, structure). Community ecology studies interactions among multiple species in the same area, including competition, predation, and mutualism.
Q: Factors that are usually biotic in nature and change in intensity as a function of population size are termed what?
A: Density-dependent factors.
Q: What is the competitive exclusion principle? Name one feature of the environment that promotes coexistence and explain how.
A: The competitive exclusion principle states that two species with identical niches cannot coexist; the superior competitor will eliminate the other. Habitat heterogeneity promotes coexistence because a varied environment provides distinct microhabitats, allowing the weaker competitor to find areas where it has an advantage.
Q: Which would be the best way to test whether C. stellatus is competitively excluded from the lower intertidal zone by B. balanoides?
A: Remove all B. balanoides from the lower intertidal zone and observe whether C. stellatus colonises it. (Answer B from the review guide.) This is a classic removal experiment: if the excluded species expands into the vacated area, competitive exclusion is confirmed.
Q: Define fundamental and realized niches using the leopard frog example.
A: The fundamental niche of the leopard frog is the entire pond (centre and reeds), the full range of habitat it can use in the absence of competitors. The realized niche is the reeds along the shore only, the restricted range it actually occupies after bullfrogs outcompeted it in the centre.
Q: Describe an experiment to distinguish the fundamental and realized niches of the bullfrogs.
A: Remove the leopard frogs from the pond entirely and observe whether bullfrogs expand from the centre into the reeds. If they do, the reeds were part of their fundamental niche but excluded from their realized niche by the leopard frogs. If they do not expand, the centre alone may be their fundamental niche.
Q: What term describes the process shown in a diagram where two species' resource-use curves shift apart over time?
A: Niche differentiation (also called character displacement when referring to the trait changes that drive the shift).
Q: When competition causes a species to lose some of its habitat, which niche has decreased?
A: Its realized niche. The fundamental niche remains the same; only the portion the species actually occupies (the realized niche) shrinks.
Competition is a density-dependent factor, which links directly to the logistic growth model from the Population Growth unit. As N increases, intraspecific competition intensifies, driving r down toward zero as N approaches K.
The niche concept also connects to broader community ecology topics you may encounter later in the course, including predation, mutualism, and food webs. Each species' niche is shaped not only by competition but by every interaction it has.
competition, interspecific competition, intraspecific competition, exploitative competition, interference competition, ecological niche, fundamental niche, realized niche, competitive exclusion principle, Gause's principle, niche differentiation, character displacement, resource partitioning, habitat heterogeneity, ghost of competition past, limiting resources, coexistence, antagonistic interactions, barnacle zonation, Chthamalus, Balanus, leopard frog, bullfrog, removal experiment, BBIO 180, intro biology, University of Washington