Difficulty: Introductory to Intermediate | Prerequisites: Basic understanding of species interactions and habitats.
This topic sits at the intersection of ecology and evolutionary biology. Species do not live in isolation; they compete for resources, and that competition shapes where each species can survive. Understanding niches and competition is foundational for making sense of community ecology, biodiversity, and how ecosystems are structured. You should already be comfortable with the idea that organisms interact with their environment and with each other.
The classic barnacle zonation experiment (often called Connell's experiment) is the go-to example for this material. It appears repeatedly in introductory biology courses because it cleanly demonstrates how interspecific competition restricts where a species lives, even when it could survive elsewhere.
A species' fundamental niche is everywhere it could survive; its realized niche is where it does survive once competitors are in the picture. The barnacle experiment proves this by removing a competitor and watching the weaker species expand into territory it was previously excluded from. Long-term competition can drive niche differentiation and character displacement, leaving a "ghost of competition past."
Fundamental niche
The full range of environmental conditions and resources in which a species can survive and reproduce, absent any competition or predation. Think of it as the theoretical maximum habitat a species could occupy if nothing else got in its way.
Realized niche
The portion of the fundamental niche that a species actually occupies once biotic interactions (competition, predation) are taken into account. In simple terms, it is where the species ends up living in the real world.
Interspecific competition
Competition between individuals of different species for shared resources such as food, space, or light. Think of it as a turf war between species, not within a species.
Competitive exclusion principle
The ecological rule stating that two species competing for the exact same limiting resource cannot coexist indefinitely; one will outcompete and exclude the other. In simple terms, if two species need the same thing and one is better at getting it, the weaker one gets pushed out.
Niche differentiation (resource partitioning)
The evolutionary or behavioural process by which competing species diverge in their use of resources, reducing direct competition. Think of it as species learning to "share" by specialising in slightly different things.
Character displacement
The phenomenon where two similar species that overlap geographically evolve to become more different from each other in traits related to resource use, compared to populations where only one species is present. In simple terms, competition pushes species to look or behave less alike over time.
Ghost of competition past
The idea that current niche differences between species are the evolutionary legacy of past competitive interactions, even if those species no longer directly compete. Think of it as the historical fingerprint of competition, still visible in species' traits long after they stopped fighting.
Two barnacle species share the intertidal zone: Chthamalus stellatus lives in the upper zone, Balanus balanoides lives in the lower zone.
Connell removed B. balanoides from the lower zone and observed what happened to C. stellatus.
Result 1: With B. balanoides present, C. stellatus stays confined to the upper zone.
Result 2: With B. balanoides removed, C. stellatus colonises the lower zone and thrives.
Conclusion: C. stellatus can survive in the lower zone but is excluded by competition from B. balanoides.
C. stellatus's fundamental niche includes both the upper and lower intertidal zones, because it can survive in both when competition is absent.
C. stellatus's realized niche is only the upper intertidal zone, because B. balanoides outcompetes it in the lower zone.
The difference between the two is driven entirely by interspecific competition.
B. balanoides is the stronger competitor in the lower intertidal. We know this because removing B. balanoides allows C. stellatus to move in, but C. stellatus cannot displace B. balanoides when it is present.
The experiment demonstrates the competitive exclusion principle: B. balanoides excludes C. stellatus from the lower zone because both compete for the same space.
Complete exclusion would occur if C. stellatus had no refuge; the upper zone acts as a habitat where B. balanoides is less competitive, allowing coexistence.
The independent variable (what the researcher manipulates) is the presence or absence of B. balanoides.
The dependent variable (what is measured) is the distribution and survival of C. stellatus.
Over evolutionary time, sustained competition between C. stellatus and B. balanoides could select for individuals that specialise in different microhabitats or resources.
Niche differentiation: the two species evolve to reduce overlap in resource use, each becoming better adapted to its own zone.
Character displacement: traits related to survival in each zone (shell shape, desiccation tolerance) diverge more where the two species overlap than where only one is present.
These evolutionary shifts are driven by natural selection: individuals that avoid direct competition with the other species have higher fitness, so traits reducing competition spread through the population.
Once niche differentiation and character displacement have occurred, the two species may appear to occupy non-overlapping niches naturally.
An observer arriving after the fact might not realise that competition drove the separation. The current niche differences are the "ghost" of historical competition.
This concept is important because it explains why species that seem perfectly suited to their separate niches were actually shaped by past competitive pressure.
No formulas for this topic. The key diagram to visualise is a niche overlap model:
Draw two overlapping circles (or bell curves along an environmental gradient such as "tidal height").
Circle 1 = C. stellatus fundamental niche (upper + lower intertidal).
Circle 2 = B. balanoides (lower intertidal).
The overlap region is where competition occurs.
The realized niche of C. stellatus is the non-overlapping portion (upper zone only).
Competitive exclusion and niche partitioning explain why similar species often occupy slightly different habitats or food sources. This principle is used in conservation biology to predict what happens when invasive species arrive: the invader may competitively exclude native species from their realized niche, potentially driving population declines. It also underpins agricultural pest management, where understanding which species competes for what resource informs biocontrol strategies.
Students often confuse the fundamental niche with the realized niche. The fundamental niche is the theoretical full range; the realized niche is the smaller, actual range after competition. If the question says "restricted" or "actual habitat," it means realized niche.
Students sometimes think the upper intertidal zone is the fundamental niche of C. stellatus. It is not. The upper zone is its realized niche. Its fundamental niche includes both zones.
Some students assume competitive exclusion means one species always goes extinct. It does not. Coexistence is possible when each species has a refuge or uses resources differently.
Students may confuse character displacement with niche differentiation. Character displacement is a specific evolutionary change in traits (morphology, behaviour); niche differentiation is the broader outcome of reduced resource overlap. Character displacement is one mechanism that produces niche differentiation.
⚠️ The barnacle experiment is a favourite exam scenario. Expect questions that give you the removal experiment results and ask you to identify which niche concept applies.
⚠️ "Restricted to the upper zone" = realized niche. "Survives in the lower zone when competitor is removed" = fundamental niche includes the lower zone. Get this distinction locked in.
⚠️ Be ready to identify the independent variable (presence/absence of B. balanoides) and dependent variable (distribution/survival of C. stellatus) in experimental design questions.
⚠️ Long-answer questions may ask you to connect competition to evolutionary outcomes (niche differentiation, character displacement, ghost of competition past). Know the chain: competition → natural selection → trait divergence → reduced niche overlap.
True or False: The fundamental niche of C. stellatus is only the upper intertidal zone. (False, it includes both zones.)
Fill in the blank: When B. balanoides is present, C. stellatus occupies its ______ niche. (realized)
True or False: The competitive exclusion principle states that two species can never coexist. (False, they can coexist if they partition resources.)
Fill in the blank: The variable the researcher controls in an experiment is called the ______ variable. (independent)
True or False: Character displacement causes two species to become more similar over time. (False, they become more different.)
Q: Using the barnacle experiment results, describe the difference between a fundamental niche and a realized niche. Which species is the stronger competitor in the lower intertidal, and how do you know?
A: The fundamental niche is the full range of conditions where a species can survive (for C. stellatus, both upper and lower zones). The realized niche is the subset actually occupied when competitors are present (upper zone only for C. stellatus). B. balanoides is the stronger competitor in the lower intertidal because its presence prevents C. stellatus from colonising that zone, but removing it allows C. stellatus to thrive there.
Q: Explain how long-term competition between C. stellatus and B. balanoides could lead to niche differentiation and character displacement through natural selection.
A: Over generations, individuals of each species that are best adapted to their own zone (and thus avoid direct competition) have higher fitness and produce more offspring. Natural selection favours traits that reduce niche overlap, such as improved desiccation tolerance in C. stellatus for the upper zone or faster growth in B. balanoides for the lower zone. This trait divergence is character displacement. The resulting reduction in resource overlap is niche differentiation.
Q: What is the "ghost of competition past" and how does it relate to the barnacle system?
A: The ghost of competition past refers to present-day niche separation between species that arose from historical competitive interactions. If C. stellatus and B. balanoides have already undergone character displacement, an observer today would see two species neatly occupying separate zones and might not realise competition drove that separation. The current pattern is the evolutionary legacy of past competition.
Q: In the barnacle removal experiment, what is the independent variable being manipulated by the researcher?
A: The presence or absence of B. balanoides in the lower intertidal zone.
Q: C. stellatus is excluded from the lower intertidal when B. balanoides is present. This phenomenon is best explained by which ecological principle?
A: The competitive exclusion principle.
This material connects directly to evolutionary mechanisms (natural selection, speciation). The competitive pressures described here are the same selective forces that drive adaptive radiation when organisms colonise new environments with empty niches. It also links to population ecology: competition is a density-dependent factor that can limit population growth and influence carrying capacity. Understanding niches is essential for grasping biodiversity and conservation biology, where habitat fragmentation and invasive species alter competitive dynamics.
interspecific competition, intraspecific competition, fundamental niche, realized niche, competitive exclusion, Gause's principle, niche partitioning, resource partitioning, character displacement, niche differentiation, ghost of competition past, Connell barnacle experiment, Chthamalus stellatus, Balanus balanoides, intertidal zonation, removal experiment, independent variable, dependent variable, coexistence, ecological niche, habitat competition