Understanding Succession
Definition
Ecological succession is the orderly, directional, and somewhat predictable process of community development that involves changes in species composition and community processes over time. It is a cumulative process driven by the interactions between organisms and their environment, leading to a more or less stable, self-perpetuating community called the climax community.
Historical Context & Timeline
Henry Cowles (1899)
One of the first to formally study succession, observing vegetation changes on sand dunes along Lake Michigan. Recognized the developmental nature of plant communities.
Frederic E. Clements (early 20th Century)
Developed the "superorganism" concept of plant communities, progressing through predictable stages (seres) towards a climatically determined climax.
Henry A. Gleason (1920s)
Challenged Clements with his "individualistic concept," arguing succession resulted from individual species' responses and dispersal.
Modern View
While Clements' deterministic view is now seen as too rigid, the general concept of directional change remains central. Modern ecology acknowledges both deterministic (biotic interactions, site modification) and stochastic (chance events, dispersal) factors. The idea of a single, stable climax has been replaced by a more dynamic view of multiple possible stable states or a shifting mosaic of patches.
Key Terminology
Sere (or Seral Stage)
An individual stage or community in the sequence of succession. The entire sequence of communities is called a sere.
Pioneer Species/Community
The first species or community to colonize a barren or disturbed area. Typically hardy, fast-growing, and good dispersers (e.g., lichens, annual weeds).
Climax Community
The relatively stable, mature, self-perpetuating community that is the final stage of succession, in equilibrium with prevailing environmental conditions.
Types of Succession
Primary Succession
Starting from Scratch
Occurs in an area where no community existed before, or where previous life has been completely obliterated, leaving a barren substrate devoid of soil and organic matter.
Examples:
- Bare rock (after volcanic eruption, glacier retreat)
- Newly formed sand dunes
- Newly created ponds
Secondary Succession
Rebuilding After Disturbance
Occurs in an area where a pre-existing community has been removed or disturbed, but the soil and some organic matter (and often seeds/propagules) remain.
Examples:
- Abandoned agricultural fields
- Clear-cut forests
- Areas after a forest fire (if soil remains)
Generally much faster than primary succession.
Autogenic Succession
Driven by biotic processes within the community itself. Organisms modify the environment, making it more suitable for some species and less for others.
Allogenic Succession
Driven by external abiotic factors or disturbances (e.g., climate change, volcanic eruptions, geological changes in water levels).
Mechanisms of Succession (Connell and Slatyer, 1977)
Early species modify the environment in ways that make it more suitable for later successional species to establish and grow, and less suitable for themselves. This is the classic Clementsian view.
Example:
Lichens and mosses on bare rock break it down, contributing to soil formation, which allows grasses and herbs to colonize. Nitrogen-fixing plants (like alders) can enrich the soil.
Early species modify the environment in ways that make it less suitable for the establishment of later species (or even themselves). Later species can only colonize if the early colonizers are removed or die, or if they can overcome the inhibition.
Example:
Some plants release allelopathic chemicals that inhibit the growth of competitors. Dense stands of early colonizers might shade out seedlings of later species.
Later successional species are able to establish and grow independently of the presence or absence of early species. They are simply better competitors for resources or more tolerant of conditions that develop. They can invade and grow because they are better adapted to the late-successional environment.
Example:
Shade-tolerant tree species can grow under the canopy of earlier, less shade-tolerant species and eventually replace them.
General Trends During Succession
These are general patterns and can vary depending on the specific ecosystem and type of succession.
Key Ecological Metrics
Life History Strategies
Early Successional (r-selected): High reproduction, rapid growth, good dispersal, short lifespans (e.g., weeds).
Late Successional (K-selected): Lower reproduction, slower growth, better competitors, longer lifespans (e.g., large trees).
Stability
Resilience (recovery) might be high in early stages. Resistance (withstanding disturbance) increases in later stages. Overall ecosystem stability generally increases.
Examples of Succession
Primary Succession on Bare Rock (Xerosere)
Pioneer Stage
Lichens, Mosses
Herbaceous Stage
Grasses, Herbs
Shrub Stage
Shrubs, Small Trees
Forest Stage (Climax)
Mature Forest
Primary Succession in an Aquatic Environment (Hydrosere - e.g., Pond)
Phytoplankton
Microscopic Algae
Submerged Stage
Rooted Plants
Floating Stage
Water Lilies, Duckweed
Reed-Swamp
Reeds, Cattails
Sedge-Meadow
Sedges, Grasses
Woodland Stage
Willows, Alders
Climax Forest
Terrestrial Forest
Secondary Succession in an Abandoned Field (Old-Field)
Pioneer Stage
Fast-growing annual weeds (e.g., crabgrass, ragweed).
Perennial Weeds & Grasses
Perennials invade and outcompete annuals.
Shrub Stage
Shrubs (e.g., sumac) and sun-loving trees (e.g., pines) establish.
Young Forest Stage
Faster-growing deciduous trees (e.g., aspen, birch) or pines dominate.
Mature Forest Stage (Climax)
Slower-growing, shade-tolerant hardwoods (e.g., oak, hickory) form a stable climax forest.
Climax Concept – Nuances & Modern Understanding
Monoclimax Theory (Clements)
Proposed one true climax per region, determined by climate. Others (edaphic, disclimax) seen as deviations.
Polyclimax Theory (Tansley)
Argued for multiple climax communities in a region, controlled by climate, soil, topography, fire, etc.
Climax Pattern Hypothesis (Whittaker)
Suggested climax communities grade into one another along environmental gradients, forming a continuum.
Cyclic Climax
Some communities exist in a cycle of stages rather than a single stable state (e.g., due to periodic fire).
Disclimax (Disturbance Climax)
A community maintained in a successional stage by recurrent disturbance (e.g., grasslands maintained by grazing/fire).
Current View
Ecosystems are seen as dynamic and less deterministic. The exact path and final state can be influenced by chance, disturbance regimes, and species availability. A "shifting mosaic" of successional patches is often more realistic than a uniform climax.
Significance of Ecological Succession
Ecosystem Development
Process by which ecosystems develop and mature from simple to complex.
Biodiversity
Generally leads to increased species diversity and habitat complexity.
Soil Development
Contributes to soil formation and enrichment, especially in primary succession.
Nutrient Conservation
Mature ecosystems (climax) are more efficient at conserving nutrients.
Ecosystem Restoration
Crucial for ecological restoration projects (reforesting, wetland restoration).
Predicting Changes
Helps predict community changes in response to disturbances.
Case Study: Succession on Krakatoa Island
Background
In 1883, the volcanic island of Krakatoa, Indonesia, erupted catastrophically, sterilizing the island. This provided a natural laboratory for studying primary succession.
Colonization Process
- Within months: Airborne spores of blue-green algae and ferns.
- Few years: Grasses and herbaceous plants (wind/sea dispersed seeds).
- Later: Shrubs and trees (e.g., Casuarina, figs - bird/bat dispersed seeds).
- Over decades: A tropical forest gradually developed.
Observations & Significance
- Relatively rapid colonization due to tropical climate and proximity to propagule sources.
- Animal colonization followed plant establishment.
- Influenced by dispersal, environmental modification by pioneers, and species interactions.
- Classic example of large-scale primary succession, highlighting resilience and dispersal importance.
UPSC Relevance
Prelims Focus
- Definitions: Succession, pioneer, climax, primary/secondary.
- Differences: Primary vs. Secondary succession.
- General trends: Diversity, biomass.
- Mechanisms: Facilitation, inhibition, tolerance.
- Examples: Xerosere, hydrosere.
- Pioneer species knowledge (e.g., lichens).
Mains (GS Paper III) Application
- Direct conceptual questions on succession processes.
- Link to ecosystem stability and biodiversity.
- Application in ecosystem restoration, afforestation.
- Understanding impacts of deforestation (sets back succession).
- Explaining ecosystems like grasslands as disclimax.
Key Insight for Answers
Understanding ecological succession provides a framework for analyzing environmental changes, conservation strategies, and the long-term consequences of human activities on natural ecosystems.