Ecological Succession

Historical Context & Motivation

Long before ecology became a formal science, naturalists noticed that landscapes do not remain static. A field left unplowed gradually fills with grasses, then shrubs, then trees. A volcanic island, once lifeless, accumulates life over decades. These observations prompted one of ecology's most enduring questions: how and why do biological communities change over time in a directional, often predictable way?

The concept of ecological succession — the sequential replacement of species and communities in an area following a disturbance or the creation of new habitat — was among the first unifying ideas in ecology. Understanding its history helps reveal how our modern framework was built from competing ideas, field observations, and vigorous debate.

The debate between Clements' deterministic view and Gleason's stochastic view persists in modern ecology, though most ecologists now favor a nuanced position: succession often shows general trends, but outcomes are influenced by chance events, species traits, disturbance history, and spatial context. This chapter will build your understanding from foundational definitions through the mechanisms that drive successional change.

Core Principles & Definitions

Ecological succession is the process by which the structure and species composition of a community change over time following a disturbance or the colonization of new, previously lifeless substrate. It is driven by interactions among species, between species and their physical environment, and by the availability and arrival of propagules (seeds, spores, larvae). Several foundational ideas anchor our understanding of this process.

Visual Explanation — Stages of Primary Succession

The following diagram illustrates the classic sequence of primary succession from bare rock to a mature forest community. Notice how soil depth, species diversity, and structural complexity increase at each stage, while light availability at the ground level decreases as taller vegetation casts more shade.

In Stage 1, lichens — symbiotic partnerships between fungi and photosynthetic algae or cyanobacteria — colonize bare rock. Their acids slowly weather the rock surface, and dead lichen tissue combines with mineral particles to form a thin proto-soil. In Stage 2, mosses establish in this nascent soil, trapping moisture and accelerating soil formation through their own decomposition. By Stage 3, enough soil has accumulated to support grasses and herbaceous plants whose root networks further stabilize and enrich the soil. Stage 4 sees the arrival of shrubs and shade-intolerant pioneer trees such as birches or aspens. Finally, in Stage 5, tall shade-tolerant canopy species — oaks, maples, beeches — dominate, forming a relatively stable climax community that can persist for centuries barring major disturbance.

Mechanisms of Species Replacement

Why do early species give way to later ones? In 1977, Connell and Slatyer proposed three mechanistic models that explain how one seral stage transitions to the next. Understanding these models is essential because real successional sequences often involve elements of all three operating simultaneously or at different stages.

The facilitation model is most evident in primary succession, where the physical environment is so harsh that only pioneer organisms can survive initially, and their presence is required to create conditions for subsequent colonizers. The tolerance model is common in secondary succession in forests: shade-tolerant seedlings establish beneath a canopy of shade-intolerant species and gradually replace them as the pioneers die. The inhibition model is frequently observed in marine intertidal communities, where mussels or seaweeds monopolize space and prevent recruitment of other species until they are removed by waves, predators, or disease.

While succession is not inherently a mathematical process like population growth, ecologists use quantitative indices to track how communities change. The Shannon diversity index (H') is one such tool: it captures both the number of species (richness) and the evenness of their abundances. During early succession, H' is typically low because only a few pioneer species are present. As succession proceeds, more species colonize and establish, increasing H'. In very late succession, competitive dominance by a few canopy species can reduce evenness, sometimes causing H' to level off or slightly decline.

Detailed Breakdown — Primary vs. Secondary Succession

The distinction between primary and secondary succession is one of the most important classifications in ecology. It determines the starting conditions, the rate of community development, and the types of organisms that initiate the process. The following diagram and table provide a side-by-side comparison.

Worked Example — Calculating Shannon Diversity Across Seral Stages

Let us track how species diversity changes through three stages of old-field succession (secondary succession on an abandoned farm) using the Shannon diversity index. We will compare an early stage (Year 2), an intermediate stage (Year 20), and a late stage (Year 80).

Ecosystem Properties Across Succession

Eugene Odum's seminal 1969 paper described a suite of ecosystem-level properties that tend to change predictably during succession. This framework, while somewhat idealized, remains a valuable heuristic for understanding how ecosystems mature. The following table summarizes key trends.

A critical insight from Odum's framework is the relationship between production (P) and respiration (R). In early succession, production vastly exceeds community respiration because fast-growing pioneer plants photosynthesize rapidly while total community respiration is low. This means the ecosystem is accruing biomass. As the community matures, respiration from the large standing biomass (maintenance costs of big trees, decomposers processing dead material) approaches total production, so net community productivity approaches zero — the ecosystem is essentially in a "steady state" of carbon balance.

Connection to Advanced Theory

The classical model of succession as a unidirectional march toward a single climax has given way to more sophisticated frameworks. Modern ecology recognizes that real landscapes are dynamic mosaics of patches in different successional stages, and that disturbance is not an aberration but a fundamental driver of community structure.

The Intermediate Disturbance Hypothesis (IDH), proposed by Joseph Connell in 1978, posits that species diversity is maximized at intermediate levels of disturbance. Too little disturbance allows competitive exclusion (a few dominant late-successional species monopolize), while too much disturbance eliminates all but the hardiest pioneers. At intermediate disturbance frequency and intensity, a mosaic of early and late-successional patches coexists, maximizing landscape-level diversity.

Another important modern framework is the concept of alternative stable states. Rather than converging on a single climax, some ecosystems can settle into different persistent configurations depending on the history and intensity of disturbance, the order of species arrival (priority effects), and feedback mechanisms. For example, a tropical forest cleared and burned repeatedly may shift to a grassland state that resists reforestation because grass fires prevent tree seedling establishment — a positive feedback loop that maintains the alternative state.

The patch dynamics model, developed in part by Steward Pickett and P. S. White, views landscapes as collections of patches in different stages of development. A windthrow here, a fire there, a landslide elsewhere — each creates a gap that resets the local successional clock. At any given time, the landscape is a mosaic, and the overall diversity of the landscape depends on the variety and frequency of these patches. This model is directly applicable to forest management, where controlled burns or selective logging are used to mimic natural disturbance regimes and maintain biodiversity.

Understanding succession at this advanced level also connects to pressing global issues like ecological restoration (how do we guide succession to rebuild damaged ecosystems?), climate change (will warming alter which species arrive and which succeed?), and invasive species (can non-native species derail successional pathways and create novel ecosystems with no historical analog?).

Practice Problems

Lesson Summary