The concept of succession and its types

Succession is the natural process through which an ecosystem evolves over time, with changes in plant and animal life driven by shifts in environmental conditions such as water availability or soil presence. This transformation occurs in distinct steps known as seral stages, gradually altering the ecosystem's structure and composition.

Categories of succession

• Primary succession - This occurs on newly formed or exposed land where no soil exists initially, such as on bare rock after a volcanic eruption. Harsh conditions dominate at the start, requiring specialised species to begin the process.
• Secondary succession - This takes place on land that has been cleared of vegetation but retains soil, often following events like forest fires. It follows a similar pattern to primary succession but begins at a more advanced stage due to the presence of soil.

Primary succession and its progression

Primary succession starts in environments with no pre-existing soil, presenting challenging conditions for life. Over time, the actions of initial colonisers transform the area, paving the way for more complex ecosystems.

Stages of primary succession

• Arrival of pioneer species - Seeds and spores are carried by wind to barren land. Pioneer species, such as lichens, are adapted to survive in extreme conditions with minimal water and nutrients.
• Modification of environment - As pioneer species die and decompose, they contribute to the formation of a thin layer of basic soil, gradually making conditions less hostile.
• Soil development - Organic material builds up over time, enriching the soil with minerals and deepening it, which allows for the growth of small plants.
• Establishment of larger vegetation - With deeper and richer soil, larger plants can take root, further contributing to soil formation through decomposition.
• Increased complexity - Each stage sees new species out-competing earlier ones, leading to greater species diversity and abundance.

Secondary succession and its characteristics

Secondary succession occurs in areas where soil is already present, allowing the process to bypass the initial stages of soil formation. This often results in a quicker progression towards a mature ecosystem.

Features of secondary succession

• Starting point - Begins after disturbances like forest fires or deforestation, where soil remains intact, enabling a faster recovery compared to primary succession.
• Pioneer species - Unlike in primary succession, pioneer species here include larger plants such as grasses or small shrubs, which can establish more quickly due to existing soil.
• Similar progression - Follows the same pattern of increasing complexity and diversity as primary succession, with species replacement over time leading to a stable community.
• Speed of change - The presence of soil nutrients and organic matter accelerates the process, allowing ecosystems to recover more rapidly after disruption.

Seral stages and the development of climax communities

As succession advances through seral stages, ecosystems become more intricate, with each stage supporting different species better adapted to the changing conditions. The process culminates in a stable state known as the climax community.

Progression through seral stages

• Species replacement - At each seral stage, new species move in, out-competing existing ones to become dominant, driven by their adaptability to the evolving environment.
• Increase in diversity - Species diversity, or the number of different species, grows with each stage, alongside an increase in the abundance of individual species.
• Climax community - The final seral stage, often called the climatic climax, represents the most complex and largest community the environment can sustain. Here, biotic (living) and abiotic (non-living) factors reach a balance.
• Stability of climax - This community remains stable unless significant changes in abiotic conditions, such as climate shifts, disrupt the balance.

Alternative climax outcomes

• Sub-climax community - Occurs when succession is interrupted by local changes like fires or the introduction of new species, preventing the ecosystem from reaching its climatic climax and holding it at an earlier stage.
• Plagioclimax community - Results from human activity, such as farming or deforestation, which permanently alters the final community, preventing the natural climatic climax from forming.

Specific successions: lithosere and hydrosere

The type of succession and the resulting community depend on the initial abiotic conditions of the environment. Two distinct examples are lithosere, which occurs on bare rock, and hydrosere, which develops in water bodies.

Lithosere: succession on exposed rock

• Initial colonisation - Lichens, capable of surviving with minimal water and nutrients, are the first to colonise bare rock surfaces, beginning the breakdown of rock to release minerals.
• Moss establishment - As rocks become damper due to lichen activity, mosses grow, further weathering the rock and aiding in the formation of a thin soil layer.
• Soil deepening - Decomposition of mosses and lichens adds organic material, creating deeper soil that supports small plants and grasses.
• Growth of larger vegetation - Bacteria decompose dead plants, forming humus, which enriches the soil, allowing shrubs and small trees to establish.
• Climax development - Over time, the soil becomes deep and nutrient-rich, supporting slow-growing large trees that dominate the final climax community.

Hydrosere: succession in water bodies

• Early aquatic colonisation - Algae and floating plants are the first to colonise open water, starting the process of ecosystem development.
• Sediment accumulation - Dead plants sink and mix with sediments, gradually making the water shallower over time.
• Rooted plant growth - As the water becomes shallower, rooted aquatic plants establish, trapping more sediment and further reducing water depth.
• Transition to marsh - In very shallow water, marsh plants take hold, and plant litter accumulates, forming wet soil.
• Soil drying and deepening - Larger plants reduce soil moisture through transpiration, while decomposition adds depth and nutrients to the soil, eventually supporting large climax tree species as the dominant vegetation.