Succession is the gradual change in ecosystems over time. It's a key process in terrestrial biomes, shaping landscapes and communities. From barren rock to lush forests, succession drives ecosystem development through distinct stages.

Primary succession starts from scratch, while secondary succession occurs after disturbances. Both involve pioneer species paving the way for more complex communities. Understanding succession helps us grasp how ecosystems evolve and respond to change.

Types of Succession

Primary Succession

• Occurs on newly exposed or created surfaces that lack soil and have never supported vegetation before such as volcanic islands, glacial moraines, and lava flows
• Begins with the establishment of pioneer species that can survive in harsh conditions with little to no soil (lichens, mosses)
• Pioneer species gradually modify the environment by breaking down rocks and adding organic matter to the substrate, making it more suitable for other species to colonize
• Over time, the ecosystem develops through a series of successional stages until it reaches a stable climax community

Secondary Succession

• Occurs in areas where an existing ecosystem has been disturbed or destroyed, but the soil remains intact such as abandoned agricultural fields, clear-cut forests, and areas affected by natural disasters (wildfires, hurricanes)
• Begins with the recolonization of the disturbed area by species that can quickly establish themselves in the existing soil (grasses, herbaceous plants)
• Successional stages progress more rapidly than in primary succession because the soil is already developed and contains a seed bank and nutrients
• The ecosystem eventually reaches a climax community that may resemble the original ecosystem before the disturbance, but with some differences in species composition

Sere

• The entire sequence of successional stages from the initial colonization of a bare area to the establishment of a stable climax community
• Includes all the intermediate stages of succession, each characterized by a distinct set of species and environmental conditions
• The duration and specific stages of a sere can vary depending on factors such as climate, soil type, and the nature of the disturbance
• Examples of seres include the development of a forest ecosystem on an abandoned agricultural field (old-field succession) and the recolonization of a lava flow by vegetation (primary succession on volcanic substrates)

Successional Stages and Species

Pioneer Species

• The first species to colonize a newly exposed or disturbed area, initiating the process of succession
• Adapted to survive in harsh conditions with limited resources such as water, nutrients, and soil (lichens, mosses, grasses)
• Often have high reproductive rates, efficient dispersal mechanisms, and the ability to tolerate extreme environmental conditions (temperature fluctuations, drought)
• Modify the environment by breaking down rocks, fixing nitrogen, and adding organic matter to the substrate, making it more suitable for other species to establish

Climax Community

• The final, relatively stable stage of succession where the ecosystem reaches a state of equilibrium with the environment
• Characterized by a diverse assemblage of species that are well-adapted to the prevailing environmental conditions and can maintain their populations through self-replacement
• Examples of climax communities include mature forests (temperate deciduous forest, tropical rainforest), grasslands (tallgrass prairie), and tundra ecosystems
• The specific composition and structure of a climax community can vary depending on factors such as climate, soil type, and disturbance regime

Facilitation

• The process by which early successional species modify the environment in ways that make it more favorable for the establishment and growth of later successional species
• Pioneer species facilitate the colonization of other species by improving soil conditions, providing shade, and creating microhabitats
• Examples of facilitation include the role of nitrogen-fixing plants (legumes) in enriching the soil with nitrogen, making it more suitable for other plants, and the creation of nurse logs by fallen trees, which provide a substrate for the germination and growth of seedlings
• Facilitation is a key mechanism driving the progression of succession, as each stage creates conditions that enable the establishment of the next stage

Factors Influencing Succession

Disturbance

• Any event or process that disrupts the structure and function of an ecosystem, often setting back or altering the course of succession
• Can be caused by natural factors such as wildfires, hurricanes, floods, and volcanic eruptions, or by human activities such as deforestation, agriculture, and urbanization
• The type, intensity, frequency, and scale of disturbance can have significant impacts on the trajectory and outcomes of succession
• Examples of how disturbance influences succession include:
  • Periodic wildfires in fire-adapted ecosystems (chaparral, ponderosa pine forests) that maintain a mosaic of successional stages and promote the regeneration of fire-dependent species
  • Clear-cutting of forests for timber harvest, which resets succession to an earlier stage and can lead to the establishment of different species compared to the original forest
  • Agricultural abandonment, which allows secondary succession to occur on former croplands, often resulting in the development of old-field ecosystems and, eventually, forest regrowth
• Understanding the role of disturbance in shaping successional processes is crucial for predicting ecosystem responses to environmental change and informing management strategies for conservation and restoration