Practice question

When species with overlapping fundamental niches coexist under competition, each often uses only a subset of the conditions or resources it could exploit alone. Competition removes or discourages individuals from shared niche space, causing realized niches to contract or shift. This ecological restriction reduces direct overlap and can stabilize coexistence if each species has conditions where it performs relatively better. Fundamental niches are defined by potential performance in the absence of restrictive competitors and therefore do not automatically expand when competitors meet. Realized niches would expand after competitor removal, not during coexistence with competition. The claim is a common generalization rather than a universal rule: one species may be excluded entirely, positive interactions can expand realized occupancy, and weak competition may cause little contraction. Short-term narrowing is ecological compression, while heritable divergence accumulated over generations is character displacement. The key mechanism is that actual resource use reflects both physiological capability and the biotic environment. Coexistence becomes possible when differential use prevents either competitor from having an overwhelming advantage across all limiting conditions.