Ecological Succession and Disturbance Regimes

Ecological succession is the directional, non-seasonal transformation of biological community structure, taxonomic composition, and ecosystem biogeochemistry over time following the creation of an open substrate or the removal of pre-existing biomass by an environmental disturbance. Early plant ecologists, including Henry Chandler Cowles (1899) studying Lake Michigan sand-dune chronosequences and Frederic E. Clements (1916), formalized the process into discrete developmental stages: nudation (exposure of bare substrate by glacial retreat, volcanism, or wildfire), migration (arrival of reproductive propagules such as spores and seeds), ecesis (germination, physiological establishment, and initial growth), competition (biotic interactions for light, water, and mineral nutrients), reaction (autogenic modification of soil chemistry and microclimate by resident organisms), and stabilization (convergence toward a mature community state).

Ecologists distinguish two principal trajectories based on initial substrate conditions. Primary succession occurs on newly exposed mineral landforms devoid of organic soil, seed banks, or mycorrhizal networks—exemplified by basaltic lava flows, newly emerged volcanic islands such as Surtsey, sand dunes, and deglaciated moraines. Secondary succession unfolds after a disturbance—such as windthrow, wildfire, insect defoliation, or agricultural abandonment—partially or wholly removes standing vegetation while leaving intact humic soil horizons, microbial communities, vegetative rootstocks, and buried seed banks, enabling recovery over decades rather than centuries. Together with the spatial and temporal characteristics of ecological niches and regional food webs, disturbance regimes govern biodiversity maintenance across landscapes.

Clementsian Climax versus the Gleasonian Continuum

For much of the twentieth century, theoretical debate centered on whether successional communities assemble as tightly integrated biological units or as shifting assortments of independent species. In Plant Succession: An Analysis of the Development of Vegetation (1916), Frederic E. Clements proposed the organismic (monoclimax) hypothesis, viewing the plant community as a supra-organism whose developmental seres proceed deterministically toward a single regional climatic climax determined by macroclimate. Under Clements's framework, pioneer guilds modify the physical environment in an obligate sequence that prepares the site for subsequent seral stages until a self-replacing climax forest or grassland stabilizes.

In 1926, Henry Allan Gleason challenged this holistic orthodoxy with the individualistic concept of the plant association, later supported by Robert H. Whittaker's gradient analyses in the Great Smoky Mountains and John T. Curtis's ordination studies in Wisconsin. Gleason argued that every plant species distributes independently along continuous environmental gradients according to its physiological tolerances, dispersal stoichiometry, and stochastic colonization history. Consequently, communities are not discrete bounded entities; rather, succession represents a population-level shift in species abundances driven by differential dispersal, growth rates, and longevity along changing resource axes.

Connell and Slatyer's Three-Pathway Mechanistic Model

To replace descriptive debates with falsifiable experimental hypotheses, Joseph H. Connell and Ralph O. Slatyer (1977) formulated three mechanistic pathways explaining how early-arriving species influence later colonists:

1. Facilitation: Early pioneer colonists are the sole species capable of establishing on harsh, nutrient-poor substrates and modify the abiotic environment so that it becomes less suitable for their own recruitment but favorable for shade-tolerant, late-successional species. At Glacier Bay, Alaska, where glaciers have retreated over 100 km since 1794, Lawrence R. Walker and F. Stuart Chapin III documented how pioneer Dryas drummondii (actinorhizal rosaceous shrubs) and Alnus sinuata (Sitka alder) host symbiotic Frankia bacteria that conduct nitrogen fixation, elevating soil nitrogen and organic matter and enabling Picea sitchensis (Sitka spruce) to establish and eventually shade out the alder canopy.
2. Tolerance: Any species arriving via propagule dispersal (including late-successional taxa) can establish after disturbance. Early seral pioneers dominate initially only because of high vagility and rapid juvenile growth, whereas late-successional species persist through resource depletion, tolerate deep shade and lower soil nutrient availability, and gradually outgrow pioneers without requiring abiotic facilitation.
3. Inhibition: Early colonists preempt space, light, or soil moisture—or release allelopathic biochemicals—thereby suppressing the invasion of both early- and late-successional competitors. Succession proceeds only when localized mortality (such as herbivory, disease, or windthrow) kills inhibiting individuals and releases resources for long-lived replacement species, as observed when dense algal mats (Ulva) inhibit perennial red algae (Chondrus crispus) on rocky intertidal shores.

Odum's Strategy of Ecosystem Development

In his 1969 Science synthesis "The Strategy of Ecosystem Development," Eugene P. Odum integrated community succession with system-level thermodynamics and the carbon cycle. During early succession, gross primary production (P) exceeds total community respiration (R), yielding a ratio P / R > 1 and positive net community production (NCP = P − R > 0). Because standing structural biomass (B) is initially sparse, the ratio of biomass supported per unit energy flow (B / P) is low, and plant assimilates are allocated primarily to foliage and abundant small seeds.

As succession progresses toward an old-growth forest, woody structural tissues (heartwood, coarse roots, and soil humus) accumulate, increasing total maintenance respiration R until P / R approaches 1.0. Simultaneously, F. Herbert Bormann and Gene E. Likens's watershed experiments at Hubbard Brook Experimental Forest demonstrated that streamwater export of dissolved nitrate (NO₃⁻), calcium (Ca²⁺), and potassium (K⁺) spikes immediately after forest clear-cutting due to halted plant uptake and accelerated microbial nitrification, then declines during the aggradation phase as vegetative uptake immobilizes minerals, before stabilizing in mature forests where internal nitrogen cycle mineralization balances uptake.

The Intermediate Disturbance Hypothesis (IDH)

Natural landscapes rarely reach a static climax everywhere; instead, they form shifting mosaics shaped by a disturbance regime quantified by spatial extent, intensity (physical energy released), severity (proportion of biomass killed), and return interval (frequency). Formulated by Joseph H. Connell (1978) for tropical rainforests and coral reefs, the Intermediate Disturbance Hypothesis (IDH) proposes that local biodiversity (species richness S) follows a unimodal humped curve across gradients of disturbance frequency or intensity.

When disturbances are rare and mild, competitive exclusion goes to completion, allowing a few shade-tolerant or space-monopolizing late-successional dominants (such as * Porites* corals or canopy dipterocarps) to eliminate inferior competitors. Conversely, when disturbances are frequent and intense, only a small subset of fast-maturing, vagile ruderal species can complete their life cycles between lethal events. At intermediate disturbance frequencies or times since disturbance, pioneer colonizers and late-successional competitors coexist within the same patch mosaic, maximizing local species richness. In fire-prone Mediterranean shrublands and boreal forests, pyrophytic plant lineages have evolved traits synchronized with historical fire return intervals, including serotinous cones (sealed by resin bonds that melt at 45–60 °C in Pinus contorta and Banksia species), thick insulating suberized bark, epicormic sprouting from dormant sub-bark buds (Eucalyptus), and smoke-triggered seed germination via karrikinolide butenolide compounds.

Holling's Adaptive Cycle and Alternative Stable States

Moving beyond linear equilibrium recovery, C. S. Holling (1973, 2001) introduced the concept of ecological resilience and the four-phase adaptive cycle governing ecosystem reorganization:

• Exploitation (r phase): Rapid colonization of disturbed patches by opportunistic pioneers with high reproductive rates.
• Conservation (K phase): Slow accumulation and tight sequestration of biomass and nutrients into connected canopy structures, gradually increasing structural rigidity and fuel loads while reducing system resilience to external shocks.
• Release (Ω phase): Triggered by wildfire, bark-beetle outbreak, or storm windthrow, rapidly liberating bound carbon and mineral nutrients back into labile pools.
• Reorganization (α phase): Soil microbial mobilization, seed germination, and stochastic assembly that either reset the previous successional loop or flip the ecosystem across a critical bifurcation threshold (hysteresis) into an alternative stable state, such as the shift of overgrazed semi-arid grasslands to woody Prosopis shrublands or the transition of coral reefs to macroalgal turfs following herbivore loss and eutrophication.

How do chronosequences allow ecologists to study multi-century succession?

Because primary succession on lava flows, sand dunes, or glacial moraines requires 200 to 1,000+ years—far exceeding the duration of direct longitudinal field monitoring—ecologists employ a space-for-time substitution called a chronosequence. By sampling spatially separated sites of known geological or historical age that share identical parent material, regional climate, and topography (such as the 250-year recessional moraine sequence at Glacier Bay, Alaska, or the 4-million-year Hawaiian substrate age gradient), researchers reconstruct temporal trajectories of soil development, nitrogen accumulation, and canopy turnover, while controlling for stochastic site-history differences through permanent plot validation.

Why do nitrogen-fixing plants dominate early primary succession but decline later?

Newly exposed mineral substrates from glaciers or volcanoes contain moderate rock-derived phosphorus (PO₄³⁻) and cations (Ca²⁺, Mg²⁺) but virtually zero biologically available nitrogen (NH₄⁺ or NO₃⁻). Pioneer plants bearing root nodules with N₂-fixing bacteria (Alnus, Dryas, Coriaria, or legumes) overcome nitrogen limitation despite the high energetic cost of the nitrogenase reaction (16 ATP per N₂ reduced). Once litterfall builds a rich soil organic nitrogen pool, non-fixing late-successional trees invest that saved metabolic energy directly into taller woody stems, overtopping and shading out the light-demanding nitrogen fixers.

What is the difference between autogenic and allogenic succession?

Autogenic succession is driven internally by the biological activities of the resident organisms themselves—such as canopy shading, litter accumulation, humic acid soil acidification, peat formation in hydrarch bogs, and root-mediated mineral weathering—which progressively alter the local micro-environment. Allogenic succession, by contrast, is driven by external abiotic geoforces independent of the biota, including river-meander sediment deposition on floodplains, chronic coastal salt-spray shifts, tectonic uplift, or regional climatic aridification. In natural ecosystems, community trajectories reflect the simultaneous interplay of autogenic biotic reaction and allogenic environmental forcing.

1. Clements, F. E. (1916). Plant Succession: An Analysis of the Development of Vegetation. Carnegie Institution of Washington Publication No. 242. doi:10.5962/bhl.title.56234
2. Odum, E. P. (1969). "The Strategy of Ecosystem Development." Science, 164(3877), 262–270. doi:10.1126/science.164.3877.262
3. Connell, J. H., & Slatyer, R. O. (1977). "Mechanisms of Succession in Natural Communities and Their Role in Community Stability and Organization." The American Naturalist, 111(982), 1119–1144. doi:10.1086/283241
4. Connell, J. H. (1978). "Diversity in Tropical Rain Forests and Coral Reefs." Science, 199(4335), 1302–1310. doi:10.1126/science.199.4335.1302