AP Syllabus focus:
‘Competition, predation, and symbioses—parasitism, mutualism, and commensalism—drive population dynamics within communities.’
Organisms in a community constantly interact as they seek energy, space, and mates. Competition, predation, and symbioses shape which populations grow, decline, or coexist by altering survival and reproduction.
Core community interactions
Competition
Competition occurs when individuals or species use the same limiting resource (e.g., food, nesting sites, light), reducing access for at least one participant.
Competition: a (−/−) interaction in which two individuals or species each experience reduced fitness because they rely on the same limited resource.
Competition can be:
Intraspecific (within a species): often strongest because niches overlap heavily; can regulate population size by lowering birth rates and/or raising death rates.
Interspecific (between species): can reduce population growth of one or both species and reshape community composition.
Key outcomes ecologists emphasise:
Time-series data (from Gause’s Paramecium competition experiments) illustrating competitive exclusion: when two species share strongly overlapping niches in the same environment, one competitor increases while the other declines to local extinction. This provides a concrete example of a (−/−) interaction producing a clear shift in community composition over time. Source
Competitive exclusion: one species drives the other locally extinct when niches overlap too much.
Resource partitioning: coexistence via niche differentiation (different microhabitats, times of activity, or resource types), reducing direct competition.
Predation
Predation transfers energy and matter from prey to predator and can strongly influence prey abundance and trait frequencies.
Predation: a (+/−) interaction in which a predator increases fitness by killing and consuming a prey organism, decreasing prey fitness.
Predator–prey dynamics commonly produce population fluctuations because:
Phase-plane diagram of Lotka–Volterra predator–prey dynamics, where each closed orbit represents cyclical changes in prey abundance (x-axis) and predator abundance (y-axis). It visualizes how predator increases typically lag behind prey increases, producing repeating oscillations rather than convergence to a fixed point in the simplest model. Source
Rising prey density can increase predator survival and reproduction (more food).
Higher predator density increases prey mortality, which can later reduce predator numbers due to food scarcity.
Common prey adaptations include:
Behavioral: grouping, vigilance, fleeing, alarm calls.
Morphological/chemical: camouflage, spines, toxins. Common predator adaptations include stealth, speed, cooperative hunting, and sensory specialisations.
Symbiosis: long-term close relationships
Symbiosis: a persistent, close ecological relationship between organisms of different species in which at least one benefits.
Symbioses alter population dynamics by changing survival, reproduction, and competitive ability.
Parasitism (+/−)
Parasites gain resources from a host, reducing host fitness (often without immediate death).
Effects on hosts can include reduced growth, lowered fecundity, increased vulnerability to predation, and altered behaviour that increases parasite transmission.
Selection may favour host defences (immune responses, avoidance behaviours) and parasite strategies (immune evasion, high transmission).
Mutualism (+/+)
Both species benefit; interactions can increase population growth rates by improving access to resources or protection.
Mutualisms can be obligate (required for survival/reproduction) or facultative (helpful but not essential).
Mutualistic benefits often depend on environmental context; if costs exceed benefits, the interaction may weaken.
Commensalism (+/0)
One species benefits while the other experiences no measurable fitness effect under current conditions.
Apparent neutrality can shift with changing conditions (e.g., resource limitation), so commensal relationships may become mutualistic or parasitic over time.
How these interactions drive population dynamics
Birth and death rates change when interactions alter energy intake, disease burden, or exposure to predators.
Carrying capacity for a species can effectively drop if competitors monopolise limiting resources.
Community composition shifts as strong predators suppress some prey, allowing other species to increase; parasites can prevent competitive exclusion by keeping dominant hosts in check.
Coevolution can occur: reciprocal selection between interacting species (e.g., predator–prey arms races; host–parasite cycles), changing trait distributions and, indirectly, population trajectories.
FAQ
By suppressing potential dominant competitors, predators can reduce competitive exclusion.
This can maintain multiple prey populations at moderate densities, leaving resources available for additional species.
Obligate mutualisms are required for survival or reproduction of at least one partner.
Facultative mutualisms improve fitness but partners can persist without each other, especially when conditions provide alternative resources.
If a parasite depends on the host for prolonged feeding or transmission, rapid host death can reduce parasite fitness.
Selection can favour intermediate virulence that balances resource extraction with host survival and opportunities to spread.
Where two species overlap, selection can favour individuals that use different resources or habitats, reducing competition.
Over generations, trait differences (e.g., beak size, jaw morphology) can become more pronounced in sympatry than in allopatry.
If environmental changes make the shared resource limiting, the “neutral” partner may start experiencing reduced fitness.
For example, increased density or reduced food can turn a harmless hitchhiker into a competitor for space or nutrients.
Practice Questions
State whether each interaction is positive, negative, or neutral for each species: (i) mutualism, (ii) commensalism, (iii) parasitism. (2 marks)
Correct signs for mutualism: +/+ (1)
Correct signs for commensalism: +/0 and parasitism: +/− (1)
In a pond community, two fish species consume the same insect larvae, and a larger fish preys on both. Explain how competition and predation together can affect the population sizes of the two smaller fish species over time. (5 marks)
Identifies interspecific competition for a limiting resource reducing fitness of one or both fish species (1)
Explains reduced larvae availability can lower birth rate and/or raise death rate in competing fish (1)
Describes predation increasing mortality of the smaller fish (1)
Links prey abundance to predator abundance (e.g., more prey supports more predators; fewer prey later reduces predator numbers) (1)
Integrates both interactions: predation can prevent competitive exclusion or shift competitive outcomes by suppressing the better competitor (1)
