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.
Practice Questions
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.
