A central challenge in community ecology is to understand the connection between biodiversity and the functioning of ecosystems. The traditional approach in community ecology is based on the premise that predictions can be made by treating a species as a homogenous entity. However, no population is truly homogenous and individuals within a species often vary considerably in their ecology. By far the largest source of this functional variation between individuals stems from differences in size and ontogenetic stage. The unresolved question is how this functional variation within species affects the structure and functioning of natural ecosystems. Using a combination of experiments and theoretical studies I demonstrate that these changes in the ecology of individuals during their development have important consequences on the dynamic, structure and functioning of entire ecosystems and how they respond to natural or anthropogenic disturbances.