AP Syllabus focus:
‘Within ecosystems, species compete for territory, food, mates, and habitat. This competition can contribute to endangerment and extinction.’
Competition is a normal ecological interaction, but it becomes a conservation problem when it reduces survival or reproduction enough to drive population decline. This page explains how competition for key resources can push species toward endangerment.
Competition and limited resources
What competition is (and why it happens)
Competition: An interaction in which organisms use the same limited resource, reducing resource access, survival, growth, or reproduction for one or both.
Competition intensifies when resource supply is low or access is restricted, causing density-dependent declines in population performance (for example, lower birth rates as crowding increases).
Paired graphs showing each species’ zero-growth isocline (ZNGI) and the direction of population change on either side of the line. The intercepts (, , , ) clarify how carrying capacity and per-capita competitive effects translate into whether a population increases or declines. This links “limited resources” to predictable, density-dependent outcomes in two-species competition. Source
Types of competition that matter for endangerment
Intraspecific competition (within a species)
Often strongest because individuals share very similar needs.
Can sharply reduce reproduction when populations are already small or stressed.
Interspecific competition (between species)
Can limit a species’ realized habitat and diet, especially when needs overlap.
Exploitative competition
Indirect: individuals deplete shared resources (for example, grazing down forage).
Interference competition
Direct: individuals block access (for example, aggression, territorial defense).
The four resource arenas in the syllabus
Territory
Territory provides space to feed, breed, and avoid predators. When territory is scarce:
Dominant individuals or species secure the best areas, leaving others with low-quality space.
Reduced access to nesting sites, denning sites, or breeding displays lowers reproductive success.
Crowding in suboptimal areas can increase stress and injury, weakening individuals and increasing mortality.
Food
Food limitation is a common pathway from competition to decline:
Less food reduces body condition, slowing growth and decreasing survival (especially for juveniles).
Lower energy intake reduces fecundity (fewer offspring) and can delay breeding.
When prey or plant foods fluctuate seasonally, competition peaks during bottlenecks (for example, winter forage scarcity), producing repeated years of low recruitment.
Mates
Competition for mates can reduce population growth even when food and habitat seem adequate:
Skewed access to mates means fewer individuals contribute genes to the next generation.
Aggressive mate competition can increase injury and energy costs, lowering survival.
If only a small subset of males (or females) reproduce, the effective breeding population shrinks, increasing vulnerability to chance events and inbreeding.
Habitat (shelter and suitable living conditions)
Habitat: The set of environmental conditions and resources a species needs to survive and reproduce (including shelter, microclimate, and access to food and mates).
Habitat competition includes competition for:
Shelter (burrows, crevices, vegetation cover) that reduces predation and weather exposure
Microhabitats with suitable temperature, moisture, or light conditions
Breeding habitat such as shallow spawning grounds or specific host plants for larvae
How competition drives endangerment and extinction
Mechanisms linking competition to decline
Competition contributes to endangerment when it reduces key demographic rates:
Lower survival: insufficient food/shelter increases starvation, exposure, and predation risk
Lower reproduction: fewer mates, nest sites, or energy reserves reduces births
Lower juvenile recruitment: young individuals lose contests for food or safe habitat first
Constrained distribution: a species may be pushed into marginal habitat where survival is chronically low
Competitive exclusion and local extinction
When two species overlap strongly in resource use, persistent competitive pressure can eliminate one from a location.
Phase-plane (Lotka–Volterra) competition diagram showing two species’ zero-growth isoclines and population-change vectors. Because one species’ isocline lies entirely above the other (no stable intersection), the trajectories move toward an equilibrium where the inferior competitor is excluded (local extinction). This provides a mechanistic picture of how sustained resource overlap can convert competition into endangerment pressure. Source
Competitive exclusion: The principle that two species with very similar niches cannot stably coexist in the same place if a limiting resource is constant and one consistently outcompetes the other.
For endangered species, even local extinction (loss from part of the range) matters because:
Smaller range size reduces total population and increases isolation.
Fewer occupied sites increase vulnerability to fires, storms, disease, and other stochastic events.
Reduced numbers can create feedbacks: fewer breeders, weaker defense of territories, and lower ability to compete in subsequent seasons.
Why small populations are especially sensitive
Competition is most dangerous when populations are already low:
A small drop in births or survival can shift growth rate below replacement.
Loss of a few breeding territories can remove a large fraction of reproductive output.
Reduced genetic diversity can lower competitive ability (for example, poorer disease resistance), indirectly worsening competitive outcomes.
FAQ
They use manipulative designs (e.g., temporary exclusion barriers or removal experiments) and compare demographic rates (survival/reproduction) before and after, ideally with control sites.
Coexistence is more likely when species partition resources by time, space, or diet, when limiting resources fluctuate, or when predators/disease reduce dominance of the better competitor.
Interference involves direct prevention (aggression, territorial defence).
Exploitative competition is indirect depletion (one species consumes resources first).
If reproduction is concentrated in few individuals, the breeding population is effectively smaller, which can lower long-term growth and increase vulnerability to random events.
Time series linking competitor density to reduced survival, fledging success, or recruitment, plus evidence of reduced access to key resources (nest sites, prey abundance, or territory occupancy).
Practice Questions
State two resources listed in the syllabus that organisms compete for within ecosystems, and explain how competition for one of them can contribute to endangerment. (2 marks)
1 mark: Any two of: territory / food / mates / habitat.
1 mark: Explains a valid pathway to endangerment (e.g., reduced food lowers survival or reproduction; fewer territories reduces breeding success; limited mates reduces births).
A native bird species is declining in a woodland where another bird species uses similar nesting sites and feeding areas. Explain how competition could contribute to the native species becoming endangered, referring to territory, food, mates, and/or habitat. (5 marks)
(Any five points, 1 mark each):
Competition reduces access to nesting sites/territory, lowering breeding success.
Competition reduces food intake, decreasing adult survival or fecundity.
Juveniles are outcompeted, lowering recruitment into the breeding population.
Competition for mates skews reproduction so fewer individuals breed, reducing population growth.
Displacement into poorer habitat increases predation risk or exposure, raising mortality.
Persistent competition can cause local exclusion, shrinking range and increasing extinction risk.
