7.1 Competition and Competitive Exclusion

Interspecific vs. Intraspecific Competition

Competition is one of the main forces shaping ecosystems. It happens whenever two or more organisms need the same limited resource, and there isn't enough to go around. The outcome of competition affects which species live where, how populations grow, and how communities are structured over time.

There are two broad types:

• Interspecific competition occurs between individuals of different species going after the same limited resource (food, water, space, light).
• Intraspecific competition occurs between individuals of the same species fighting over resources like mates, nesting sites, or territory.

Intraspecific competition is actually the more intense of the two, because members of the same species have nearly identical resource needs. But interspecific competition is what drives many of the big ecological patterns you'll study in this unit.

What Determines How Intense Competition Gets?

Three main factors control how strong competition is between species:

• Resource availability — When resources are abundant, competition is weak. As resources become scarce, competition intensifies.
• Population density — More individuals in an area means more mouths competing for the same pool of resources.
• Niche overlap — The more similar two species' resource needs are, the more directly they compete.

Over evolutionary time, competition pushes species to adapt. Two important outcomes are character displacement, where competing species evolve to become more different from each other (reducing overlap), and resource specialization, where a species narrows its diet or habitat use to avoid direct competition.

Measuring Competition's Effects

Ecologists track competition's impact by measuring changes in birth rates, death rates, and overall population size over time. Two classic experimental approaches are used:

1. Removal experiments — Remove one species from an area and observe whether the remaining species' population grows, shifts its behavior, or expands its range.
2. Addition experiments — Introduce a new competitor and watch for declines in the resident species.

On the modeling side, the Lotka-Volterra competition equations are the standard mathematical tool for predicting what happens when two species compete. These equations extend the logistic growth model by adding a term for the competitive effect of each species on the other.

Competitive Exclusion Principle

Core Concept and Assumptions

The competitive exclusion principle (also called Gause's principle, after Russian ecologist Georgy Gause) states that two species competing for the exact same limiting resource cannot coexist indefinitely in the same habitat. One species will always have a slight edge in resource use, and over time it will drive the other to local extinction.

Gause demonstrated this in the 1930s using two species of Paramecium grown in lab cultures. When P. aurelia and P. caudatum were raised together on the same bacterial food source, P. aurelia consistently outcompeted P. caudatum, which declined to extinction.

The principle rests on key assumptions:

• The two species occupy identical ecological niches (same resource, same way of obtaining it).
• Environmental conditions remain constant over time.
• No other interactions (like predation or mutualism) intervene.

These assumptions are strict, which is why the principle works cleanly in the lab but gets more complicated in nature.

Ecological Implications and Exceptions

Competitive exclusion helps explain several real-world patterns:

• Species distributions — Why certain species are found in some areas but absent from others, even when the habitat looks suitable.
• Community assembly — How the mix of species in a community changes as new species arrive or conditions shift.
• Niche evolution — Why species under competitive pressure tend to evolve differences over time rather than staying identical.

In natural ecosystems, complete competitive exclusion is less common than you might expect. Several factors can prevent it:

• Environmental heterogeneity — Varied habitats create pockets where different species have the advantage, so neither wins everywhere.
• Predation — Predators can keep the dominant competitor's population low enough that the weaker competitor survives.
• Disturbance — Storms, fires, or floods periodically reset competitive hierarchies.
• Mutualistic interactions — Relationships that benefit both species can offset competitive disadvantages.

Resource Partitioning for Coexistence

If competitive exclusion says identical competitors can't coexist, then resource partitioning is how species get around that rule. Species divide up shared resources so they aren't in direct competition anymore, which allows them to live in the same habitat.

Partitioning happens along several dimensions:

• Spatial — Species use different parts of the same habitat. Caribbean Anolis lizards are a textbook example: multiple species live on the same island but occupy different microhabitats (tree trunks, branches, leaf litter), each with body types adapted to their specific perch.
• Temporal — Species use the same resource but at different times. Some pollinators visit flowers in the morning while others are active in the afternoon.
• Dietary — Species consume different food items from the same environment. On African savannas, zebras eat tall, tough grass stems; wildebeest graze on the mid-level leafy layer left behind; and gazelles crop the short new growth near the ground.

Robert MacArthur's classic 1958 study of five warbler species in New England spruce forests is another key example. All five species ate insects in the same trees, but each foraged at different heights and positions within the canopy, reducing direct competition enough to coexist.

Why Resource Partitioning Matters

• It explains how high species diversity persists in environments that seem uniform, like coral reefs or tropical rainforests. What looks like one habitat actually contains many distinct micro-niches.
• It helps ecologists predict how communities will respond to environmental change. If a disturbance eliminates one micro-niche, the species that depended on it may decline even if the overall habitat looks intact.
• It informs conservation strategies by highlighting the importance of habitat complexity for maintaining biodiversity.

Over evolutionary time, resource partitioning leads to niche differentiation: species become increasingly specialized for their particular slice of the resource, reinforcing coexistence.

Competition's Impact on Diversity

Competition and Community Structure

Competition shapes community structure in ways that aren't always straightforward. Intense, sustained competition in stable environments tends to reduce diversity through competitive exclusion. But moderate levels of competition can actually promote diversity by preventing any single species from dominating completely.

Several factors moderate how competition affects diversity:

• Environmental heterogeneity — More varied habitats support more species because different species win in different microhabitats.
• Disturbance regimes — Periodic disturbances (fires, floods, storms) create openings for less competitive species that would otherwise be excluded. This connects to the intermediate disturbance hypothesis, which you may encounter later.
• Trophic interactions — Predators that preferentially eat dominant competitors can free up resources for weaker species, a process called predator-mediated coexistence.

Evolutionary Consequences

Competition is one of the strongest drivers of evolutionary change in ecological communities:

• Character displacement — When two similar species overlap geographically, they tend to evolve greater differences in the overlap zone than where each lives alone. A classic example is Darwin's finches on the Galápagos, where beak sizes diverge on islands where competing species coexist.
• Adaptive radiation — When a lineage colonizes a new environment with many open niches, competition among early arrivals can drive rapid diversification as different populations specialize on different resources.

Conservation Implications

Understanding competitive interactions is directly useful for conservation:

• Invasive species management — Invasive species often outcompete natives for resources. Knowing which resources are contested helps target management efforts.
• Habitat restoration — Restoring habitat complexity (not just habitat area) supports resource partitioning and higher diversity.
• Climate change predictions — As species ranges shift with changing temperatures, new competitive matchups will form. Predicting outcomes depends on understanding competitive dynamics.