Biology: Potential vs Actual - What Limits a Species

Every organism has a theoretical range of conditions where it could survive — its fundamental niche — but the reality of sharing a habitat with competitors, predators, and limited resources usually shrinks that range into a realized niche. This distinction sits at the heart of Form and Function in Biology SL, where the intertidal zone offers one of ecology's clearest illustrations. The classic barnacle study shows how biotic interactions reshape where a species actually lives. Chthamalus stellatus can survive and reproduce across the entire intertidal zone when grown in isolation, revealing its full fundamental niche. Yet when Semibalanus balanoides is present, competition pushes Chthamalus into the high intertidal zone alone — its realized niche. Understanding this gap between potential and actual range explains why species distributions depend as much on ecological relationships as on physical tolerance, and why removing a competitor can dramatically expand where a population persists.