Species Interactions

Objective: Distinguish niches from habitats and classify resource competition.

A habitat is the physical setting where an organism lives; an ecological niche includes the conditions, resources, and interactions associated with its role and persistence. Competition occurs when organisms use a shared limiting resource, potentially reducing performance of one or both. Competition may be within a species or between species; its strength depends on resource supply and overlap. Resource partitioning can reduce direct overlap, but it need not eliminate competition. Similar niches can lead to competitive exclusion under strong simplifying conditions, yet coexistence is possible through spatial, temporal, or other differences. Experiments and field observations must distinguish competition from correlated responses to an environment.

Objective: Trace direct and indirect effects of consumer–resource interactions.

Predation broadly describes one organism consuming another; herbivory is consumption of plant or algal tissues and may not kill the whole producer. Predators and herbivores can influence prey or producer abundance, distribution, and traits; resources, refuges, and other interactions can modify effects. A population graph showing oscillations is consistent with several mechanisms and does not prove a simple predator–prey cycle. Defensive traits and consumer responses may evolve under selection, but not every observed trait is necessarily an adaptation. Food-web consequences can spread indirectly, so cause should be tested with suitable comparisons.

Objective: Interpret interaction signs without assuming fixed outcomes.

Mutualism benefits both interacting species under defined conditions; parasitism benefits one and harms the host; commensalism benefits one while the other shows no detectable effect under the measured conditions. These labels summarize net effects and can vary with environment, stage, or availability of alternatives. A pollinator–flower interaction may provide resources and pollination, but an actual benefit must be measured. Symbiosis refers to close, often lasting associations and is not synonymous with mutualism. The plus, minus, and zero notation is a model that depends on which outcome and time scale are measured.