SL and HL: Subtopic 2.1: Individuals, populations, communities and ecosystems. Populations interact through predation, mutualism, parasitism, and competition. These interactions affect survival, reproduction, population size, species distribution, and community structure.
How the interactions work
The symbols below show whether each population benefits , is harmed (-), or is largely unaffected .
| Interaction | Effect and mechanism | Named example |
|---|---|---|
| Predation | Predator , prey (-). One organism captures, kills, and consumes another, transferring matter and energy between trophic levels. Predation can regulate prey abundance through density-dependent negative feedback. | Reintroduced wolves prey on elk in Yellowstone National Park. |
| Mutualism | Population A , population B . Both populations gain a survival or reproductive advantage, although the benefits need not be equal. | Reef-building corals provide shelter and nutrients to photosynthetic algae, while the algae supply the corals with organic compounds. |
| Parasitism | Parasite , host (-). The parasite obtains resources from a host, reducing the host's fitness but usually not killing it immediately. | Varroa mites feed on honeybees and transmit viruses. |
| Competition | Population A (-), population B (-). Organisms use the same limited resource, so less is available to each competitor. Competition may be intraspecific or interspecific. | Chthamalus and Semibalanus barnacles compete for space on rocky shores. |
A common misconception is that parasitism is simply slow predation. Predators normally kill and consume prey, whereas parasites usually live on or in a host and depend on keeping it alive long enough to obtain resources or reproduce.
These interactions also have societal dimensions. In Yellowstone, conservationists may value wolves for restoring ecological relationships, while ranchers may oppose them because of livestock losses, showing how environmental perspectives influence management.
Exam technique
For an explain question, do more than name the interaction: identify which population benefits or is harmed, describe the mechanism, and link it to population size or ecosystem stability. Do not confuse mutualism with a relationship in which only one population benefits.