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

Zonation is the change in community along an environmental gradient. Transects measure biotic and abiotic factors along the gradient to find the variables that affect where species live, for example distance from the sea across sand dunes.

Succession is the replacement of one community by another over time due to changes in biotic and abiotic variables. Each sere changes conditions so that the next replaces it through competition until a stable climax community is reached. Primary succession starts on new substratum (volcanic rock, moraine, wind-blown sand, silt); secondary succession starts on bare soil where a community previously lived (abandoned fields, forest after a firestorm).

Energy flow, productivity, species diversity, soil depth and nutrient cycling change over time. An ecosystem's capacity to tolerate disturbance depends on its diversity and resilience.

HL only. The community that develops is influenced by climate, the properties of the local bedrock and soil, geomorphology, and fire and weather events. There can also be top-down influences from primary consumers or higher trophic levels, for example grazers preventing tree seedlings from establishing.

HL only. Patterns of net productivity (NP) and gross productivity (GP) change during succession: GP rises as biomass and leaf area increase, but in a mature community respiration takes most of GP, so NP falls as a share of GP.

r-strategists (small, many offspring, rapid reproduction, short life) are adapted to pioneer communities with plentiful space and little competition. K-strategists (large, few well-provisioned offspring, long life) are adapted to climax communities near carrying capacity with strong competition.

HL only. The concept of a climax community has been challenged, and there is uncertainty over which ecosystems would develop naturally without human influence. Human activity (grazing, burning, cutting, drainage, clearance) can divert and change the course of succession, leading to a plagioclimax: a community held at a stage below the natural climax, for example lowland heathland.