Priority Effect

A priority effect occurs when the order and relative timing in which species arrive at a habitat patch during colonization, primary or secondary ecological succession, or microbiome assembly determine the outcome of interspecific interactions and long-term community structure (Sutherland, 1974; Fukami, 2015). Whereas classical deterministic niche models assume that a given set of abiotic conditions and a regional species pool will always converge onto a single predictable climax community, priority effects introduce historical contingency: two habitat patches with identical abiotic environments and identical species pools can diverge permanently onto alternative stable states (or alternative transient states) depending solely on whether Species A arrived hours, days, or years before Species B.

In the classical two-species Lotka–Volterra competition model, the per-capita population growth rates of Species 1 (N₁) and Species 2 (N₂) competing for shared resources with carrying capacities K₁, K₂ and interspecific competition coefficients α₁₂, α₂₁ are:

• dN₁ / dt = r₁ N₁ [ 1 − (N₁ + α₁₂ N₂) / K₁ ]
• dN₂ / dt = r₂ N₂ [ 1 − (N₂ + α₂₁ N₁) / K₂ ]

By Peter Chesson's modern coexistence theory, stable coexistence requires that intraspecific competition exceeds interspecific competition (α₁₂ < K₁/K₂ and α₂₁ < K₂/K₁, so that each species can invade when rare). Conversely, a deterministic priority effect (bistability / founder control) arises when interspecific inhibition exceeds intraspecific inhibition for both species simultaneously:

α₁₂ > K₁ / K₂ and α₂₁ > K₂ / K₁

Under this inequality (which corresponds to negative niche overlap or destabilizing positive frequency dependence), the interior coexistence equilibrium (N₁^*, N₂^*) becomes an unstable saddle point separated by a diagonal separatrix in phase space:

• Neither species can invade when rare once the other species is established near its carrying capacity.
• Whichever species arrives first (and expands above the separatrix threshold before the second species arrives) drives the latecomer to local extinction, locking the patch into a single-species monoculture (founder-controlled community, Yodzis, 1989; Ke & Letten, 2018).

In his 2015 synthesis, Tadashi Fukami classified all biological mechanisms generating priority effects into two distinct functional categories:

Because priority effects are easiest to manipulate in replicated microcosms and mesocosms where arrival order is controlled to the hour, several model systems have revealed the exact molecular and physiological drivers of historical contingency:

1. Floral Nectar Microbiomes (Metschnikowia Yeasts vs. Acinetobacter / Gluconobacter Bacteria)

Inside the ephemeral nectar of hummingbird-pollinated monkeyflowers (Mimulus aurantiacus), nectar-specialist yeasts (Metschnikowia reukaufii) and bacteria (Gluconobacter and Acinetobacter spp.) are dispersed from flower to flower on hummingbird bills (Peay et al., 2012; Vannette & Fukami, 2014):

• Niche Preemption via Amino Acid Depletion: If Metschnikowia yeast cells arrive in a newly opened flower just 24 hours before competing yeast species or bacteria, they consume virtually all dissolved amino acids and nitrogen in the nectar, preventing latecomers from growing.
• Niche Modification via pH Drop and Pollinator Behavior: Conversely, if acetic-acid bacteria (Gluconobacter) arrive first, they ferment nectar sucrose into gluconic and acetic acid, dropping nectar pH from ~5.5 to ~3.2 and reducing hummingbird visitation—an inhibitory niche modification that blocks yeast colonization and lowers plant seed set!

2. Wood-Decomposing Basidiomycete Fungi and Carbon Emissions

In fallen beech (Fagus sylvatica) and spruce logs on the forest floor, early-colonizing saprotrophic basidiomycetes establish territories delimited by melanized pseudosclerotial plates ("zone lines") and secrete combative extracellular laccase and peroxidase enzymes plus antifungal volatile organic compounds (Fukami et al., 2010):

• Inoculating sterilized wood blocks with a single pioneer species (Phlebiopsis gigantea vs. Trametes versicolor vs. Hypholoma fasciculare) just weeks before exposing the blocks to natural forest-floor colonization caused the fungal communities to diverge for years—resulting in a 3-fold difference in wood decomposition rate and CO₂ respiration flux purely as a function of which fungus arrived first!

3. Amphibian Larvae in Temporary Vernal Ponds

In seasonal woodland ponds, the relative breeding phenology of early spring breeders (such as wood frogs, Lithobates sylvaticus, or spotted salamanders, Ambystoma maculatum) versus late spring breeders (such as American toads, Anaxyrus americanus, or treefrogs, Hyla versicolor) generates size-mediated intraguild priority effects (Wilbur & Alford, 1985):

• When winter warming advances salamander or predaceous beetle oviposition by 1 to 2 weeks, early-hatching larvae grow large enough to gape-limit and consume newly hatched tadpoles of later-arriving species, shifting the entire predator–prey food web of the pond.

4. Neonatal Human Gut Microbiome Assembly

During the first days and months of human infancy, the order in which pioneer bacterial strains—such as human-milk-oligosaccharide (HMO)-fermenting Bifidobacterium infantis, Bacteroides fragilis, and facultative anaerobes (Escherichia, Streptococcus) acquired via vaginal delivery vs. Cesarean section—colonize the sterile neonatal colon governs long-term microbiome structure (Sprockett et al., 2018):

• Early arrival of B. infantis consumes HMOs, lowers luminal pH via acetate and lactate production (niche modification), and trains mucosal regulatory T cells, whereas early colonization by hospital-acquired Enterococcus or Klebsiella following perinatal antibiotic exposure preempts spatial crypt niches and predisposes to dysbiosis.

When early colonizers arrive in an isolated habitat patch (such as a newly formed post-glacial lake or volcanic island) generations ahead of competing species, ecological priority effects can be reinforced by rapid local adaptation—a process termed the Monopolization Hypothesis by Luc De Meester and colleagues (2002):

1. Numerical Advantage (Ecological Niche Preemption): The first arriving genotype of a species (such as the water flea Daphnia magna or Daphnia pulex founding a resting-egg bank in lake sediment) expands rapidly to carrying capacity (N ≈ 10⁶–10⁸).
2. Genetic Advantage (Rapid Local Adaptation): Because the resident population is large, natural selection acts on standing genetic variation and new mutations to adapt the founder population to the lake's specific thermal, UV, and fish/invertebrate predation regime.
3. Evolutionary Exclusion of Latecomers: When genetically superior or conspecific immigrant propagules arrive decades later, they are both numerically outnumbered by millions of resting eggs and locally maladapted compared to the evolved resident population—creating a permanent eco-evolutionary priority effect that explains why neighboring lakes just meters apart frequently harbor genetically distinct clones or species despite high waterfowl-mediated dispersal.

1. Sutherland, J. P. (1974). "Multiple stable points in natural communities." The American Naturalist, 108(964), 859–873. https://doi.org/10.1086/282961
2. Wilbur, H. M., & Alford, R. A. (1985). "Priority effects in experimental pond communities: field responses of amphibians to timing of hatching." Ecology, 66(4), 1106–1114. https://doi.org/10.2307/1939162
3. De Meester, L., Gómez, A., Okamura, B., & Schwenk, K. (2002). "The Monopolization Hypothesis and the dispersal–gene flow paradox in aquatic organisms." Acta Oecologica, 23(3), 121–135. https://doi.org/10.1016/S1146-609X(02)01145-101145-1)
4. Fukami, T., Dickie, I. A., Paula Wilkie, J., Paulus, B. C., Park, D., Roberts, A., Buchanan, P. K., & Allen, R. B. (2010). "Assembly history dictates ecosystem functioning: evidence from wood decomposer communities." Ecology Letters, 13(6), 675–684. https://doi.org/10.1111/j.1461-0248.2010.01465.x
5. Fukami, T. (2015). "Historical contingency in community assembly: integrating niches, species pools, and priority effects." Annual Review of Ecology, Evolution, and Systematics, 46, 1–23. https://doi.org/10.1146/annurev-ecolsys-110411-160340