Zonation, succession and change in ecosystems

· Zonation = change in community composition across space along an environmental gradient.
· Succession = change in community composition over time.
· Exam must-know contrast: zonation is spatial; succession is temporal.
· Both are driven by changing abiotic factors and biotic interactions.

Zonation

· Zonation occurs when species are distributed in bands or zones because conditions change across a gradient.
· Common gradients: elevation, latitude, tidal level, soil horizons, distance from water source.
· Different species occupy different zones because they have different tolerances, competitive abilities and resource requirements.
· Typical exam idea: distribution changes because of changing abiotic conditions such as moisture, salinity, light, temperature or soil properties.
· A classic example is rocky shore zonation, where organisms are arranged by tidal exposure.

This diagram shows how species occupy different parts of a rocky shore according to tidal exposure. It is a clear example of zonation caused by changing abiotic conditions across a gradient. Use it to explain why different organisms dominate different shore levels. Source

Investigating zonation

· Use a transect across the environmental gradient to sample changes in species distribution.
· Measure both biotic factors (for example, species presence/abundance) and abiotic factors (for example, light, temperature, soil moisture, pH, salinity).
· Transects help identify which variables affect the distribution of species.
· Results may be shown in tables, graphs or kite diagrams.
· Kite diagrams show how abundance of different species changes along a transect.
· In a kite diagram, wider sections show greater abundance.

Succession: core idea

· Succession is the replacement of one community by another over time.
· It happens because each community changes the environment, making it more suitable for the next community.
· These changes may include increased soil depth, altered nutrient availability, more shade, more organic matter, and changed microclimate.
· Replacement happens through colonization and competition.
· A succession sequence is made of seral stages or seral communities (seres).
· Early colonizers are called pioneer species.
· The final relatively stable stage is the climax community.
· Evidence for long-term succession can come from pollen records in peat.

Seral stages and climax community

· Each seral community modifies the habitat.
· This allows the next seral stage to establish and outcompete the previous one.
· Example: mosses and lichens help break down rock and build soil, allowing grasses and then larger plants to grow.
· Over time, ecosystems usually show increases in complexity, biomass and species diversity.
· A climax community is the relatively stable end point of succession under those conditions.

This illustration shows the sequence from bare rock to a more mature plant community. It is a strong revision image for the idea that each stage changes the habitat and enables the next stage. Use it to explain pioneer species, seral stages and climax community. Source

Primary succession

· Primary succession begins on newly formed substratum with no soil and no pre-existing community.
· Examples: new volcanic rock, moraines exposed by retreating glaciers, wind-blown sand, waterborne silt.
· Earliest colonizers are pioneer species, often lichens and mosses.
· These begin soil formation and add organic matter.
· As soil develops, larger plants establish, followed by shrubs and trees if conditions allow.
· Primary succession is usually slow because soil must first form.

Secondary succession

· Secondary succession begins where there is bare soil but a pre-existing community existed before.
· Examples: abandoned farmland, forest after fire, land after another disturbance that leaves soil behind.
· It is usually faster than primary succession because soil, nutrients, seed banks or surviving organisms are already present.
· Communities still pass through a sequence of seral stages toward a more stable community.

This diagram shows how an ecosystem recovers after a disturbance while soil remains present. It helps distinguish secondary succession from primary succession, which starts without soil. Use it to compare the speed and starting conditions of the two processes. Source

How ecosystems change during succession

· During succession, energy flow, productivity, species diversity, soil depth and nutrient cycling all change.
· Soil depth generally increases over time.
· Nutrient cycling becomes more developed and efficient as biomass and decomposer activity increase.
· Species diversity usually increases through succession.
· Food webs usually become more complex.
· Biomass generally increases as the community develops.

This diagram summarizes how biomass, biodiversity and soil layer thickness increase through succession. It is especially useful for exam questions asking how ecosystem properties change from early to later seral stages. Use it to connect succession with increasing ecosystem complexity and stability. Source

Resilience, diversity and stability

· An ecosystem’s ability to tolerate disturbance and maintain equilibrium depends on its diversity and resilience.
· As succession proceeds, rising diversity often increases resilience.
· Greater resilience supports greater stability.
· Human interference can reduce diversity, resilience and stability.
· This is an important link between succession and human impacts on ecosystems.