Ecological Succession: Types, Stages and Process

Ecological succession refers to a sequential change of organisms as an area progresses from a condition in which it is relatively poor in species composition and organic content, until it becomes a diverse an rich biological community. The first plants or organism to colonize an area are known as the pioneer community, and the final stage is the climax community. Succession is marked by increased productivity, nutrient shifts, rising diversity of organisms, niche development, and a gradual increase in the complexity of food webs.

Causes of Ecological Succession

Ecological succession is usually initiated by major disturbances that remove or destroy existing biological communities in an area. These disturbances open up space for new organisms to colonize and establish themselves. Natural disturbances like fires, floods, volcanic eruptions can cause the initial destruction. However, human activities like deforestation, agriculture, urbanization and pollution also disrupt ecosystems and trigger succession.

Types of Ecological Succession

Key types of ecological succession include primary succession, secondary succession, autogenic succession, and allogenic succession.

Primary Succession

Primary succession begins on rock, sand, or mud where little or no organic material or biological communities are present. Some examples include succession that happen after lava flows, glacial retreats, or the formation of sand dunes or salt flats.

The initial colonizers are hardy pioneer species like lichens, algae and mosses which require little soil. Over long periods, soils gradually develop, allowing more complex communities of grasses, herbs, shrubs and finally trees to establish. Primary succession can take centuries or millennia to reach a climax community.

Secondary Succession

Secondary succession occurs where existing biological communities have been disrupted but soil and nutrients remain intact. This occurs when events like fires, floods, windstorms, human land clearance or abandonment disturb ecosystems but do not destroy them completely. Since the soil remains, secondary succession tends to progress more rapidly through the sequential series of seral community stages.

Driving Factors: Autogenic vs. Allogenic Succession

The factors driving ecological succession can be either internal or external in nature.

Autogenic succession

Autogenic succession occurs through changes inherent within the ecological community itself. Some examples to understand autogenic succession are as follows:

• A pond gradually becomes filled with sediment, organic debris and plant matter over many years. This changes the pond depth, nutrient levels, light availability and soil accumulation along the edges – allowing the pond habitat to transition to a marshy wetland environment.
• Tall dense trees and shrubs establish themselves over an old field site initially colonized mainly by grasses and small plants. As the trees grow larger, they block increasing amounts of sunlight from reaching the ground, changing habitat conditions to favor shade-tolerant plants species in place of sun-loving initial colonizers.
• On a sand dune, early successional plants provide organic matter as they grow and die. This helps change the loose dry sandy conditions to allow woody shrubs and trees with greater water and soil demands to propagate themselves, transforming the landscape over time through the vegetation changes.
• Accumulation of animal droppings and gradual decomposition of plant remains in an area enriches and modifies soil chemistry and texture to be more hospitable for nitrogen-loving and organic-matter dependent organisms over many seasons. The animal/plant waste products change site suitability.

Allogenic Succession

Allogenic factors refer to external environmental influences like weather extremes, fires, climate changes which also affect ecosystem trajectories. Some examples are as follows:

• A major hurricane strips away trees and causes extensive wind damage and flooding in a forest ecosystem. This allogenic disruption resets the successional stages as pioneer species re-colonize.
• A severe multi-year drought changes environmental conditions in a wetland habitat to be drier. Many of the moisture-dependent plant and animal species die off as the community shifts toward more drought-resistant organisms.
• An intense, extensive wildfire burns up vegetation and alters soil chemistry across a mountain landscape over thousands of acres. This allogenic event causes substantial reordering of the ecosystem progression as new flora and fauna establish post-fire.
• Heavy monsoon rains cause massive landslides that remove soil, debris flows, and reshape valley terrain and drainage patterns. This completely modifies the physical land surface and available habitat niches, forcing ecological succession to divert or restart.
• A new viral disease is introduced into an aquatic ecosystem through runoff contamination, causing widespread death and die-off of resident fish populations that other species depended on. The cascade of allogenic changes redirects community dynamics.

We note here that in most cases, both autogenic and allogenic drivers impact succession and not necessarily work in isolation.

Stages of Succession

Ecological succession unfolds as a sequence of structural changes within the ecological community. Several distinct stages in this process of sequential development have been identified as follows:

• Nudation: Nudation refers to the opening of a site for potential colonization. It involves a disturbance which essentially resets the succession cycle by removing existing biological communities. This clearing of space sets the scene for pioneer species to establish themselves.
• Migration: The migration stage involves the arrival of propagules and dispersal agents which bring seeds, spores, eggs or organisms into the nudated site. Birds, wind, water and other animals transport the biological material which will form the basis of the next successional stage. The timing and availability of certain flora and fauna influences progression.
• Ecesis: Ecesis refers to the initial establishment and growth of those pioneer colonizers best able to thrive in the site’s conditions. How readily they can germinate, root, anchor themselves, draw nutrients and reproduce defines their colonizing success. Hardy, rapidly growing lichens, mosses or grasses often pioneer.
• Competition: Once pioneer species have established stable populations, competition plays a stronger role in driving succession. Fast-growing and spreading plants like shrubs and thickets often overtake the initial colonizers. As vegetation becomes denser, competition for space, sunlight and nutrients intensifies. Some present species thrive while others decline.
• Reaction: Existing community growth and development induces changes in the physical, chemical and biological habitat – organic matter accumulation, changing nutrient profiles, water retention abilities. These autogenic factors influence which species can persist. The habitat reacts back on the inhabitants by differential selection pressures.
• Stabilization: In late successional stages, community changes occur more gradually as species compositions stabilize. Climax communities represent highly diverse, complex ecosystems forged through species coevolution which are resilient to external disturbance. Stable associations and symbioses between species in quasi-equilibrium allow climax stability.

Pioneer Communities & Climax Communities

Pioneer communities

Pioneer communities refer to the hardy species which can readily colonize severely disturbed, exposed environments with few initial resources. Lichens, mosses, grasses often play this role. Over time, pioneers modify habitats by accumulating organic matter, trapping sediments etc, facilitating later colonizers. Their adaptations allow resilience. Examples of Pioneer communities are:

• Lichens: Lichens, which are symbiotic association of algae and fungi are one of the hardiest pioneer species to colonize bare rock, they help initiate soil formation that aids later plant life establishment. Different lichen species can inhabit diverse newly exposed surfaces.
• Algae: Similar to lichens in colonizing capability, algal crusts containing Cyanobacteria and Green algae species can coat harsh ground like volcanic lava flows and help fix nitrogen to enrich the site.
• Mosses and Liverworts: Often the first small plants to propagate on rocks/boulders, tree trunk bases, and other exposed areas after sufficient time, generating organic matter.
• Grasses: Tough pioneering grass varieties adapted to grow in poor soils are quick colonizers of drier newly disturbed sites from fields to sand dunes. Their dense networks stabilize surfaces.
• Annual and Perennial Herbs: Quick spreading low vegetation like dandelions, goldenrod, mullein or plantains capable of colonizing open ground in a wide range of habitats through wind dispersed seeds.

Climax Communities

Climax communities represent the finale, stable ecosystems of late ecological succession. Complex forests or grasslands illustrate such structurally and functionally diverse systems.

Stability through Coevolution

The resilience of climax communities emerges partly from multi-species coevolution over successive generations reinforcing one another’s niche roles. For example, long-term adaptation between dependent butterflies and host plants or trees and fungal partners. This biological accommodation helps maintain ecosystem integrity despite disruptions.

Types of Seral Succession

Based on the substrate where succession starts, several types of seral successions have been recognized. Each type of seral succession is characterized by the initial conditions of the environment and the dominant type of vegetation that colonizes it.

Xerosere

Succession in dry, xeric conditions, often in areas lacking significant soil moisture. It typically begins with lichens and mosses and progresses to grasses and shrubs, eventually leading to a forest if moisture conditions improve. It is of further two types:

• Psammosere: Succession on sandy environments, such as sand dunes. It begins with pioneer species that are tolerant to high levels of sand and salt, progressing to grasses, shrubs, and finally to woodland or forest.
• Lithosere: Succession on a bare rock surface. This type of succession starts with organisms like lichens and mosses that can grow on rock surfaces, gradually leading to the formation of soil as these organisms decompose, allowing higher plants to establish.

Hydrosere

Succession in aquatic environments, such as ponds, lakes, or bogs. It starts with phytoplankton and progresses through various stages including submerged and floating plants, followed by reed-swamp vegetation, sedge meadow, and ultimately to a climax forest community.

Halosere

Succession in saline environments like salt marshes. It begins with salt-tolerant plants (halophytes) and progresses through various stages, potentially leading to a non-saline climax community if salinity decreases over time.

Glaciosere

Succession that occurs on recently deglaciated land. Pioneer species that can tolerate the harsh, nutrient-poor conditions colonize first, followed by a gradual progression to more complex plant communities.

Mire succession

Succession in peat-forming wetland areas. It starts with aquatic or semi-aquatic plants and progresses through various stages of marsh and bog development, potentially leading to forested peatland.

Ruderosere

Ruderosere is a term used to describe a type of ecological succession that occurs in habitats heavily disturbed by human activities, particularly those where the soil has been disrupted or is covered with rubble and waste materials. The term “ruderal” refers to plant species that are the first to colonize such disturbed lands.

Trends in Biomass in Ecological Succession

As ecological succession progresses from pioneer species to the climax community, biomass tends to increase continuously over time. Pioneer species first colonize an area after a major disturbance removes any existing ecological community. They increase total biomass from near zero levels. As succession proceeds through various seral intermediate stages, additional vegetation layers, larger plants, and more complex organisms establish themselves. This leads to steadily increasing accumulation of living and decaying organic biomass in the ecosystem. In the climax community with multiple canopy levels, abundant vegetation, and diverse animal populations, the system-level biomass reaches its highest levels, stabilized through complex symbioses and coevolved mechanisms. Hence biomass follows a pattern of continual increase during succession, with the climax showing maximum accumulation.