2.5 Zonation, succession and change in ecosystemsIB Environmental Systems and Societies SL: Revision notes

Zonation is the change in a community along an environmental gradient, such as up a mountain, down a rocky shore or inland from a sand dune. Transects measure biotic and abiotic factors along the gradient to determine the variables that affect the distribution of species, for example how long each level of a shore is covered by the tide.

Succession is the replacement of one community by another over time as biotic and abiotic variables change. Each seral community (sere) changes conditions so that the next can replace it through competition, until a stable climax community is reached.

Primary succession starts on newly formed substratum with no soil or community (new volcanic rock, moraines revealed by retreating glaciers, wind-blown sand, silt). Secondary succession starts on bare soil where a community previously existed (an abandoned field, forest after a firestorm) and is faster.

As a succession proceeds, energy flow, productivity, species diversity, soil depth and nutrient cycling change. Early on, productivity and biomass are low; they rise as larger plants establish, soil deepens and nutrient cycling becomes more efficient. Species diversity typically rises through the mid stages and may fall in the climax when a few large species dominate.

An ecosystem's capacity to tolerate disturbance and maintain equilibrium depends on its diversity and resilience. High diversity gives functional redundancy and a bank of surviving species. Very large disturbances can still push an ecosystem past a tipping point.