# Ecological Succession Ecological succession is the process by which the species composition of a community changes over time. This occurs as species colonize a habitat and modify it, making it more suitable for other species to invade and replace them, until a stable community is formed. --- ## Key Concepts - **Ecological Succession:** The gradual and orderly process of change in an ecosystem brought about by the progressive replacement of one community by another until a stable climax is established. - **Sere:** An individual succession pathway. - **Seral Stages:** The developmental phases or intermediate communities within a sere. - **Pioneer Species:** The first species to colonize a new or disturbed habitat. They are typically hardy and can withstand harsh conditions. - **Climax Community:** The final, relatively stable community that is the end product of succession. It is in equilibrium with the environment. --- ## Kinds of Succession Ecological succession can be broadly categorized into two main types: primary and secondary. | Feature | Primary Succession | Secondary Succession | | :--- | :--- | :--- | | **Starting Condition** | Occurs in a habitat that was not previously inhabited by organisms. No soil is present. | Occurs in a habitat already modified by a pre-existing community that has been disturbed. Soil is already present. | | **Examples** | Bare rock surfaces such as volcanic lava, land exposed by retreating glaciers. | Abandoned farm fields, forests after a fire or flood, areas affected by a hurricane. | | **Pioneer Species** | Lichens, mosses. | Weeds, grasses, fast-growing plants. | | **Rate of Succession** | Very slow, as soil must be formed first. | Relatively fast, as soil and some life such as seeds already exist. | ### Succession Based on Water Availability - **Hydrarch (Hydrosere):** Succession that begins in aquatic environments such as ponds, lakes, and marshes. - **Xerarch (Xerosere):** Succession that begins in dry habitats with a deficiency of water, such as bare rock or sand dunes. --- ## Stages of Xerarch Succession (Primary Succession on Bare Rock) Xerarch succession is a classic example of primary succession, detailing the development of a forest from bare rock. ### 1. Crustose-Lichen Stage (Pioneer Stage) - Pioneer species such as crustose lichens (*Licanora*, *Rhinodina*) colonize the bare rock. - They are highly resistant to desiccation (drying out). - They initiate soil formation by secreting acids that decompose the rock and by trapping wind-blown particles. ### 2. Foliose-Lichen Stage - Leaf-like (foliose) lichens (*Permelia*, *Dermatocarpon*) grow on the thin layer of soil created by the pioneers. - They grow faster and overshadow the crustose lichens, accelerating soil accumulation. ### 3. Moss Stage - Xerophytic (drought-tolerant) mosses colonize the soil accumulated in crevices. - They form a mat that holds more water and adds a significant amount of organic matter (humus) upon death and decay. ### 4. Herb Stage - Herbaceous plants, initially annuals, invade the deeper soil. - Their roots penetrate the rock, enhancing weathering. - Leaf litter adds more humus, and the plants provide shade, which reduces evaporation. - Biennial and perennial herbs and grasses follow. ### 5. Shrub Stage - Xeric shrubs invade, outcompeting the herbs for light and resources. - Birds are attracted to the shrubs and help disperse seeds, leading to a denser growth. - The environment becomes less suitable for herbs, which begin to migrate out. ### 6. Tree Stage - The improved soil conditions allow tree saplings to grow among the shrubs. - The trees eventually form a canopy, shading out the shrubs. - Deeper root systems and increased leaf litter continue to build and enrich the soil. ### 7. Climax Stage - The initial xeric trees are eventually replaced by more mesophytic (moderate moisture) tree species as soil depth and water retention improve. - This results in a stable, self-perpetuating climax community, often a forest, which is in equilibrium with the local climate. --- ## Summary - Ecological succession describes the predictable and orderly changes in the composition of an ecosystem. - The process moves through a series of intermediate communities called seral stages, starting with pioneer species and ending with a climax community. - The main types of succession are primary and secondary, distinguished by their starting conditions. | Type | Starting Point | Soil | Speed | Example | | :--- | :--- | :--- | :--- | :--- | | **Primary** | New, uninhabited land | Absent | Slow | Bare rock (Xerarch) | | **Secondary** | Disturbed land | Present | Fast | Abandoned field | - A xerosere (succession on bare rock) progresses from lichens to mosses to herbs to shrubs to trees to climax forest, as each stage modifies the environment (builds soil) to make it suitable for the next. - The biological significance of succession is that it is a fundamental process for ecosystem development, recovery from disturbance, and the establishment of biodiversity and stability.