Connell–Slatyer Succession Models

The Connell–Slatyer Succession Models constitute a foundational mechanistic framework in community ecology published in 1977 by the American marine and tropical ecologist Joseph H. Connell (1923–2020) and the Australian plant physiological ecologist Ralph O. Slatyer (1929–2012) in their American Naturalist paper "Mechanisms of Succession in Natural Communities and Their Role in Community Stability and Organization". For six decades prior to 1977, ecological succession—the directional change in species composition and community structure over time following a physical or biotic disturbance—was dominated by the holistic determinism of Frederic E. Clements (1916), who viewed succession as an orderly, embryonic development ("relay floristics") in which early pioneer species prepare the environment for a climatic climax superorganism, contrasted against the individualistic, stochastic Gleasonian perspective of Henry Allan Gleason (1926) and Frank E. Egler's (1954) "initial floristic composition" hypothesis.

Connell and Slatyer resolved this longstanding philosophical impasse by reformulating succession as a testable set of species-level demographic hypotheses. Focusing on how early-colonizing species modify the local abiotic and biotic environment—and how those modifications affect the subsequent establishment, growth, and survival of later-successional species—they delineated three distinct, falsifiable pathways: the Facilitation Model (where pioneers ameliorate harsh conditions to enable late-species recruitment while undermining their own regeneration), the Tolerance Model (where any arriving species can establish initially and directional replacement is driven by life-history longevity and shade/resource tolerance along a declining R* rule resource gradient), and the Inhibition Model (where early occupants monopolize space and resources, suppressing all later arrivals until disturbance or senescence kills the incumbent adults).

The Classical Polarity: Clements (1916) vs. Gleason (1926) and Egler (1954)

In Plant Succession: An Analysis of the Development of Vegetation (1916), Frederic E. Clements decomposed succession into six sequential processes: nudation (creation of a bare site by disturbance), migration (arrival of propagules), ecesis (establishment and growth), competition, reaction (autogenic modification of soil and microclimate by the resident biota), and stabilization (attainment of a self-perpetuating regional climatic climax). Clements assumed that autogenic "reaction" is obligatorily facilitative: only hardy pioneer species can survive in newly exposed bare sites, and by building organic humus and moderating temperature fluctuations, each seral stage renders the habitat unsuitable for itself while creating the cradle for the next wave of invaders—a pattern termed Relay Floristics.

Two major empirical challenges undermined universal relay floristics during the mid-twentieth century:

1. Gleason's (1917, 1926) Individualistic Concept: Gleason argued that plant associations are not integrated superorganisms, but rather coincidental assemblages of individual species sorting independently according to their seed-dispersal capacities and physiological tolerances.
2. Egler's (1954) Initial Floristic Composition (IFC) in Old-Field Succession: Studying abandoned agricultural fields in eastern North America, Frank E. Egler demonstrated that seeds, roots, and seedlings of annual weeds, perennial herbs, shrubs, and climax forest trees are often all present simultaneously in the first year after abandonment. The apparent "succession" of physiognomic stages over 100 years results not from wave-after-wave relay invasion, but from sequential differential growth rates and lifespans (r-selected annuals maturing in year 1, biennial/perennial forbs in years 2–10, fast-growing pines in years 15–50, and slow-growing oaks and hickories overtopping the canopy after 60+ years).

Connell and Slatyer (1977) recognized that Clements's relay floristics and Egler's initial floristic composition were not mutually exclusive dogmas, but rather subsets of a broader mechanistic logical tree defined by the net sign (+, 0, or −) of resident species' effects on invading colonist establishment.

Connell and Slatyer structured their model as a sequential dichotomous flow diagram beginning at Step 1 (a major disturbance opens a bare space) and Step 2 (propagules of various species arrive). Thereafter, the trajectory branches into three distinct models:

Model 1: The Facilitation Model

The Facilitation Model formalizes Clementsian relay floristics on an individualistic basis. Following a disturbance that strips away soil organic matter and biological legacies—most notably during primary succession on newly deglaciated till, volcanic lava flows, or mobile coastal foredunes—the abiotic microclimate is too harsh (high surface temperatures, low water-holding capacity, near-zero bioavailable nitrogen) for large-seeded, shade-tolerant late-successional species to germinate and survive.

1. Pioneer Establishment and Amelioration: Only stress-tolerant or ruderal pioneer species with wind-dispersed propagules, high photosynthetic light-saturation points, or symbiotic diazotrophic (nitrogen-fixing) root nodules can colonize the raw mineral substrate. As these pioneers grow, their canopies shade the soil surface (reducing evaporative demand), their root networks stabilize loose sediment, and their leaf litter adds humic organic carbon and mineralized ammonium/nitrate to the upper soil horizon.
2. Self-Limitation and Facilitated Invasion: The environmental modifications that allow late-successional species to invade simultaneously doom the pioneers: shade-intolerant pioneer seedlings cannot regenerate beneath their own closed canopy, or soil-borne host-specific pathogens and nematodes accumulate in the pioneer rhizosphere (as demonstrated for the European marram grass Ammophila arenaria on coastal dunes by Van der Putten et al., 1993). Late-successional tree seedlings germinate in the enriched, shaded humus, overtop the pioneers, and drive them to local extinction.

Classic Empirical Case Study: Glacier Bay, Alaska

Beginning with William S. Cooper (1923) and Reinmar Crocker and B. A. Major (1955), and tested via experimental seed additions and soil transplants by F. Stuart Chapin III, Lawrence R. Walker, Christopher L. Fastie, and Terry L. Sharik (1994), the 250-year glacial recession chronosequence at Glacier Bay, Alaska provides the classic demonstration of facilitation mixed with inhibition:

• Black Crust & Pioneer Dryas Stage (0–25 years post-deglaciation): Bare calcareous glacial till (total soil N < 5 g m⁻², pH 8.0–8.2) is colonized by cyanobacterial black crusts, Epilobium latifolium, and the mat-forming actinorhizal (Frankia-nodulated) subshrub Dryas drummondii.
• Alder (Alnus viridis subsp. sinuata) Stage (25–75 years): Symbiotic nitrogen fixation by dense thickets of Sitka alder fixes 60 to 150 kg N ha⁻¹ yr⁻¹, multiplying total soil nitrogen pools tenfold while leaf-litter organic acids leach calcium carbonate and lower soil pH from 8.0 to <5.0 within three decades.
• Sitka Spruce (Picea sitchensis) and Western Hemlock (Tsuga heterophylla) Climax (75–250+ years): Experimental plantings by Chapin et al. (1994) demonstrated that late-successional Sitka spruce seedlings planted into enriched Alnus soil grew substantially faster (showing strong nutritional facilitation via elevated foliar N and P) than spruce planted on bare pioneer moraines—even though Alnus thickets simultaneously exerted partial competitive inhibition on spruce germination and survival through dense shading and root competition. Once individual Sitka spruce trees emerge above the 6-meter alder canopy after ~50 years, they shade out the obligate heliophyte Alnus and transition toward an acid-humus Picea–Tsuga rainforest.

Model 2: The Tolerance Model

In the Tolerance Model, early-colonizing species are not required to prepare the physical environment for later species. Following a disturbance that leaves intact soil behind (typical of secondary succession after agricultural abandonment, logging, or windthrow), juveniles of both early- and late-successional species can establish from the start if their seeds are present—incorporating Egler's (1954) initial floristic composition—or late species can invade continuously thereafter without needing microclimatic amelioration by residents.

1. Life-History Trade-Off Between Colonization/Growth Rate and Resource Tolerance: Under the Tolerance Model, early-successional dominance is achieved by fast dispersal and high maximum relative growth rate (RGR_max) (corresponding to Ruderals and Competitors in CSR Plant Strategy Theory and fast colonizers in the competition–colonization trade-off).
2. Directional Replacement via Declining Light (I*) and Soil Resources (R*): As the pioneer stand closes its canopy, light intensity at the forest floor drops from 100% full sunlight to <2%–5% full sunlight. Early-successional trees (such as loblolly pine Pinus taeda, trembling aspen Populus tremuloides, or birch Betula pendula) have high whole-plant light compensation points (high I*) and cannot regenerate in their own shade. Conversely, late-successional trees (such as sugar maple Acer saccharum, American beech Fagus grandifolia, and eastern hemlock Tsuga canadensis) have low dark respiration rates, horizontal leaf display, and low equilibrium light requirements (low I*; Horn, 1971; Kobe et al., 1995). Their seedlings persist in the shaded understory for decades, growing upward to replace the shorter-lived pioneer canopy without having depended on the pioneers for soil preparation.

Model 3: The Inhibition Model

The Inhibition Model reverses Clementsian determinism: rather than facilitating their successors, early colonists prevent the establishment or maturation of all subsequent arrivals—both early- and late-successional species alike. Under pure inhibition, priority effects govern early community assembly: whichever species arrives first and captures the open patch holds it against challengers:

1. Mechanisms of Incumbent Suppression: Resident species pre-empt light via dense, low-hanging evergreen or clonal canopies, deplete soil moisture and nitrate via fibrous root mats, release allelopathic phytotoxins (e.g., juglone, phenolic acids, or monoterpenes in Kalmia angustifolia, Empetrum hermaphroditum, and Pteridium aquilinum), mechanically abrade invading seedlings, or harbor refuge populations of seed-eating rodents and herbivorous mollusks.
2. Disturbance- or Senescence-Driven Replacement: Because late-successional invaders cannot penetrate an intact inhibitory stand, succession stalls—forming a recalcitrant understory layer or arrested succession (Royo & Carson, 2006)—until external agents such as selective herbivory, insect outbreaks, fungal pathogens, wave/wind damage, or natural adult senescence kill the early occupants and open micro-gaps. Crucially, if early colonists have short lifespans and high susceptibility to herbivores whereas occasional invading late-species individuals are long-lived and well defended against grazers, a directional shift from short-lived to long-lived species still occurs over time—not because late species outcompeted healthy early adults, but because late species outlive and out-resist the mortality agents that eliminate the early colonists.

Classic Empirical Case Study: Sousa's (1979) Rocky Intertidal Boulder Experiments

The first experimental test designed to discriminate among all three Connell–Slatyer models was conducted by Wayne P. Sousa (1979, Ecological Monographs) in an intertidal boulder field at Ellwood Beach near Santa Barbara, California:

• Successional Sequence on Overturned Boulders: When winter storm waves overturn a cobble or boulder, scouring it bare, the rock is colonized within 1 month by a green mat of the opportunistic ephemeral green alga Ulva (sea lettuce). Over the next 12 to 24 months, Ulva is replaced by middle-successional perennial red algae (Gigartina leptorhynchos, Gelidium coulteri, Rhodoglossum affine), and within 2 to 3 years by a dense turf of the late-successional perennial red alga Gigartina canaliculata (now Chondracanthus canaliculatus).
• Selective Removal and Grazer Manipulation Experiments: When Sousa experimentally scraped away the early Ulva carpet on replicate boulders and concrete blocks, middle- and late-successional red algae colonized and grew markedly faster than on untouched control boulders covered by Ulva—refuting the Facilitation and Tolerance models and proving strong Inhibition. In unmanipulated plots, succession progressed from Ulva to Gigartina only because the intertidal herbivorous shore crab Pachygrapsus crassipes preferentially grazed the tender, palatable Ulva thalli (and seasonal desiccation stressed Ulva), opening bare microsites for the tougher, unpalatable perennial red algae to establish.

Subsequent experimental reviews across terrestrial and marine seres (Pickett, Collins & Armesto, 1987; Walker & Chapin, 1987; Connell et al., 1987) demonstrated that Facilitation, Tolerance, and Inhibition are not mutually exclusive across an entire ecosystem, but frequently operate simultaneously on different demographic transitions (germination vs. survival vs. growth) or shift predictably along environmental stress gradients:

• Integration with the Stress-Gradient Hypothesis (Bertness & Callaway, 1994): Across both primary and secondary succession, the Facilitation Model dominates early seral stages under high abiotic stress (barren glacial till, arid nurse-plant deserts, hypersaline salt marshes, and upper rocky intertidal zones), where neighbor shade and nitrogen fixation ameliorate lethal physical conditions. Conversely, as soil organic matter accumulates and abiotic stress declines—or in benign, productive secondary successions—interactions shift toward Tolerance and Inhibition.
• Mycorrhizal and Soil-Microbiome Feedbacks: Contemporary succession ecology links the three Connell–Slatyer models to plant–soil feedbacks (PSFs) (Bever et al., 1997; Kardol et al., 2006): early-successional ruderals accumulate host-specific root pathogens in their rhizosphere (negative PSF, accelerating self-inhibition and species turnover), whereas late-successional species depend on the buildup of arbuscular or ectomycorrhizal fungal networks (positive PSF, driving microbial facilitation).

What is the main difference between the Facilitation, Tolerance, and Inhibition models of succession?

The three models differ in how early-colonizing pioneer species affect the establishment of later species: under Facilitation, pioneers improve harsh soil or microclimatic conditions so late species can invade; under Tolerance, late species can establish from the start without pioneer help and win dominance because they tolerate lower resource levels (especially deep shade); and under Inhibition, early colonists block later arrivals until disturbance, herbivory, or death removes the incumbent adults.

Why does succession still move from short-lived to long-lived species under the Inhibition Model?

Even when early-successional pioneers inhibit the recruitment of late-successional seedlings, early pioneers typically have shorter lifespans and weaker defenses against herbivores, pathogens, and physical stress than late-successional species. Whenever a pioneer individual dies from grazing, disease, or old age and is replaced by a long-lived, stress-resistant late-successional individual, the late species holds that spot for decades or centuries—causing a net directional ratchet toward long-lived climax species.

How did Wayne Sousa's (1979) algal boulder experiments test the Connell–Slatyer models?

On intertidal boulders in California, early green algae (Ulva) are normally replaced over 2–3 years by perennial red algae (Gigartina). By experimentally removing the early Ulva layer from replicate boulders, Sousa showed that late-successional red algae colonized right away and grew much faster in the absence of Ulva. This proved that Ulva inhibits rather than facilitates red algae, and that natural succession depends on shore crabs (Pachygrapsus) selectively eating the palatable Ulva.

Can Facilitation and Inhibition occur at the same time in a single successional community?

Yes. At Glacier Bay, Alaska, Chapin et al. (1994) showed that nitrogen-fixing alder (Alnus) thickets simultaneously facilitate the growth of late-successional Sitka spruce (Picea sitchensis) by adding 60–150 kg N ha⁻¹ yr⁻¹ to raw glacial till, while also inhibiting spruce seed germination and early survival through dense canopy shading, root competition, and heavy leaf-litter burial.

1. 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. https://doi.org/10.1086/283241
2. Sousa, W. P. (1979). "Experimental investigations of disturbance and ecological succession in a rocky intertidal algal community." Ecological Monographs, 49(3), 227–254. https://doi.org/10.2307/1942484
3. Chapin, F. S., III, Walker, L. R., Fastie, C. L., & Sharik, T. L. (1994). "Mechanisms of primary succession following deglaciation at Glacier Bay, Alaska." Ecological Monographs, 64(2), 149–175. https://doi.org/10.2307/2937039
4. Pickett, S. T. A., Collins, S. L., & Armesto, J. J. (1987). "Models, mechanisms and pathways of succession." The Botanical Review, 53(3), 335–371. https://doi.org/10.1007/BF02858321
5. Bertness, M. D., & Callaway, R. (1994). "Positive interactions in communities." Trends in Ecology & Evolution, 9(5), 191–193. https://doi.org/10.1016/0169-5347(94)90088-490088-4)