What is ecological succession?

Ecological succession is the process by which the structure and composition of an ecosystem change over time. This natural progression occurs as species colonize an area, modify their environment, and are gradually replaced by other species better suited to the new conditions. Succession reflects the dynamic nature of ecosystems, showing how they evolve in response to both internal interactions and external disturbances.

Key characteristics of ecological succession

• Gradual change - Ecosystems transform slowly as species composition shifts, altering the community's structure.
• Directional process - Succession typically moves toward a more stable and diverse state, often called a climax community, unless interrupted by disturbances.
• Species replacement - Early species modify the environment, making it suitable for later species, which eventually replace them.

Types of ecological succession

There are two primary types of ecological succession, each starting under different conditions and following distinct pathways. Understanding these types helps explain how ecosystems develop or recover over time.

Primary succession

Primary succession occurs in areas where no previous ecosystem existed, such as on bare rock, newly formed volcanic islands, or glacial retreat areas. There is no soil or organic matter present at the start.

Characteristics of primary succession:

• Starting conditions - Begins with a barren, lifeless substrate where environmental conditions are harsh.
• Process - Pioneer species, often lichens or mosses, colonize the area first, breaking down rock into soil over long periods. This allows other plants and animals to establish.
• Timeframe - This process is very slow, often taking centuries to develop a mature ecosystem due to the need to build soil from scratch.
• Example - The formation of an ecosystem on a new volcanic island where lava has cooled into bare rock.

Secondary succession

Secondary succession happens in areas where an existing ecosystem has been disturbed or destroyed, but soil and some organic matter remain, such as after a forest fire, flood, or agricultural abandonment.

Characteristics of secondary succession:

• Starting conditions - Begins with pre-existing soil, seeds, or root systems, making recovery faster than in primary succession.
• Process - Pioneer species, often fast-growing plants like grasses or weeds, quickly recolonize the area, followed by shrubs and eventually trees as the ecosystem rebuilds.
• Timeframe - This process is relatively quicker, often taking decades rather than centuries, due to the presence of soil and remnants of the previous community.
• Example - Regrowth of a forest after a wildfire, where soil and seeds from the original ecosystem facilitate recovery.

Roles of keystone and indicator species in ecosystems

Certain species play critical roles in shaping and reflecting the state of an ecosystem during succession. These species influence community structure or indicate environmental conditions, providing insight into the health and progression of succession.

Keystone species

A keystone species is a species whose activities have a disproportionately large effect on the structure and function of an ecosystem relative to its abundance. These species can influence which other species thrive during succession by altering habitats or controlling populations of other organisms.

For example, sea otters in kelp forest ecosystems are keystone species because they control sea urchin populations, preventing overgrazing of kelp, which supports diverse marine life.

Indicator species

An indicator species is a plant or animal whose presence, abundance, or absence reflects specific environmental conditions or the overall health of an ecosystem. These species help scientists monitor the stages of succession by signaling changes in habitat quality or ecosystem characteristics.

For example, lichens are often indicator species for air quality; their presence or scarcity in an area undergoing succession can indicate levels of pollution or environmental recovery.

Effects of ecological succession on ecosystems

As succession progresses, it brings significant changes to the ecosystem's structure and function. These changes influence the total biomass, species richness, and productivity, shaping the long-term development of the community.

Changes in ecosystem characteristics

• Total biomass - Biomass, the total mass of living matter in an area, increases over time during succession. Early stages have low biomass due to sparse vegetation, while later stages support more complex life forms and greater biomass.
• Species richness - The number of different species in an ecosystem, or species richness, also increases as succession advances. Early stages are dominated by a few pioneer species, while later stages host a diverse array of plants, animals, and microorganisms.
• Net productivity - Net productivity, the rate at which energy is stored as biomass after accounting for energy used in respiration, typically rises during early and middle stages of succession. It may stabilize or decrease slightly in mature ecosystems as energy is directed toward maintenance rather than growth.

Succession often moves ecosystems toward a more stable state, where species composition and environmental conditions reach a balance. This stability, however, can be disrupted by natural or human-induced disturbances, potentially restarting the successional process.

Adaptation and evolution during succession

Succession not only changes the ecosystem but also drives adaptations and potentially the origin of new species. Early colonizers, known as pioneer species, play a crucial role in this transformative process.

Pioneer species

• Initial colonization - Pioneer species are the first to move into unoccupied or disturbed habitats, often possessing traits like rapid growth, high dispersal ability, and tolerance for harsh conditions.
• Environmental modification - These species adapt to the specific conditions of their habitat, such as extreme temperatures or nutrient scarcity, and modify the environment by creating soil, adding organic matter, or altering light and water availability.
• Facilitation of later species - By changing the habitat, pioneer species make it suitable for other species to establish, leading to shifts in community composition over time.

Potential for new species

Over long periods, pioneer species and their successors may undergo genetic changes as they adapt to the specific challenges of their evolving habitat. These adaptations can result in the origin of new species, a process known as speciation, where populations become reproductively isolated and develop distinct traits suited to their niche within the successional ecosystem.

On isolated volcanic islands undergoing primary succession, unique environmental pressures can lead to the evolution of endemic species, such as the diverse finches of the Galápagos Islands, which adapted to different food sources over time.

Understanding ecological succession provides a window into the resilience and adaptability of ecosystems, showing how life persists and evolves even in the face of dramatic change.