Biology: Fundamental vs Realized - What Limits a Niche

Every organism has a fundamental niche — the full range of conditions where it could survive — but the niche it actually occupies, its realized niche, is often narrower because competitors push it out. On Scottish rocky shores, Chthamalus stellatus can physiologically tolerate heights from roughly 0.3 m to 3.5 m above MLWS. Yet Semibalanus balanoides, which grows faster and overgrows it, restricts Chthamalus to a realised band of about 1.5 m to 3.0 m. This distinction matters because it shows how biotic interactions, not just physical tolerance, shape where species live. Removal experiments reveal the mechanism directly: exclude S. balanoides and Chthamalus colonises lower, with densities rising at every height. Such shifts can be quantified as a percentage change, (new − original) ÷ original × 100. Comparing control and removal plots separates competition's effect from other limiting factors, such as wave action or predation, that may still cap the fundamental niche.