Priority effect
Priority effects are the phenomenon in which the order of species arrival during community assembly determines the subsequent trajectory and final composition of the community. The early colonist — the species with priority — does not merely occupy space; it modifies the environment, establishes mutualisms, and creates competitive barriers that shape the niche space available to later arrivals. The same pool of species, arriving in different orders, can produce radically different communities. History is not a footnote in ecology. It is a causal force.
The concept is deeply connected to ecological succession and to the broader principle of path dependence in complex systems. Just as the QWERTY keyboard layout persists not because it is optimal but because it was first, ecological communities can lock into configurations determined by the identity and timing of their founding species. The priority effect is the ecological equivalent of a founder's advantage in economics or a first-mover advantage in technology: the early arrival gains a structural advantage that is self-reinforcing and difficult to dislodge.
Mechanisms of Priority
Priority effects operate through several mechanisms. Preemption occurs when early arrivals occupy limiting resources — space, light, nutrients — before later arrivals can access them. A grass that germinates quickly after fire may preempt the soil moisture and light that a slower-growing shrub would need, effectively eliminating the shrub from the community even though the shrub would dominate in the absence of the grass. Niche modification occurs when early arrivals change the environment in ways that favor themselves or their mutualists. A nitrogen-fixing shrub that colonizes a lava flow modifies the soil chemistry, making the site suitable for species that could not survive on bare rock. Enemy suppression occurs when early arrivals support predators or pathogens that suppress potential competitors. A mycorrhizal fungus that associates with an early tree species may form a network that preferentially supplies nutrients to that species, starving later arrivals.
These mechanisms are not mutually exclusive; they interact and amplify. A grass that preempts light may also modify soil chemistry through root exudates and suppress competitors through shared pathogens. The result is a community that is not just different from what would have assembled without the grass but is actively resistant to change. The priority effect has produced a lock-in.
Priority Effects and Restoration
The practical importance of priority effects is most visible in ecological restoration. A restorationist who seeds a site with one mix of species may produce a meadow; the same site, seeded with a different mix in a different order, may produce a shrubland. The difference is not in the site potential but in the assembly history. This is why restoration ecology is not merely applied ecology; it is historical ecology — the attempt to reconstruct not just a community but a trajectory.
The intermediate disturbance hypothesis has implications for priority effects. Intermediate disturbance creates a mosaic of patches at different successional stages, each with its own priority regime. The diversity of the landscape is not just a product of environmental heterogeneity; it is a product of historical heterogeneity, of the different assembly histories that each patch has experienced. A landscape managed to maintain intermediate disturbance is a landscape that maintains historical diversity, and historical diversity is the raw material of ecological resilience.
The Scale of Priority
Priority effects are not limited to local community assembly. They operate at the scale of biogeography. The Pleistocene extinctions of large herbivores in North America removed ecosystem engineers that had maintained grasslands and suppressed forest expansion. The priority effect of their absence is still visible: forests have expanded into former grasslands, and the communities that now occupy those sites are not the communities that would have existed if mammoths and ground sloths had persisted. The priority effect of extinction is a geological force.
At the global scale, human-mediated species introductions are massive priority effects. The introduction of European grasses to California, of kudzu to the southeastern United States, of cane toads to Australia — all are priority events that have restructured communities in ways that will persist for centuries. The introduced species gains priority not because it is superior but because it arrived first, and the self-reinforcing mechanisms of preemption, niche modification, and enemy suppression make its dominance difficult to reverse.
Priority effects are the mechanism by which history enters ecology. They are the proof that ecological communities are not timeless assemblages but historical products — the visible traces of invisible events. The species that arrived first did not just settle the land. They wrote its constitution.