In ecology, a priority effect is the impact
that a particular species can have on community
development due to prior arrival at a site.There
are two basic types: inhibitory priority effects
occur when a species that arrives first at
a site negatively impacts a species that arrives
later by reducing the availability of space
or resources.
Facilitative priority effects occur when a
species that arrives first at a site alters
abiotic or biotic conditions in ways that
positively impact a species arriving later.
Priority effects are a central and pervasive
element of ecological community development.
These effects have important implications
for natural systems as well as ecological
restoration efforts.
== Theoretical foundation ==
=== 
Community succession theory ===
Early in the 20th century, Frederic Clements
and other plant ecologists suggested that
ecological communities develop in a linear,
directional manner towards a final, stable
end-point: the climax community.
Clements indicated that a site's climax community
would reflect local climate.
He conceptualized the climax community as
a "superorganism" that followed a defined
developmental sequence.Early ecological succession
theory maintained that the directional shifts
from one stage of succession to the next were
induced by the plants themselves.
In this sense, succession theory implicitly
recognized priority effects; the prior arrival
of certain species had important impacts on
future community composition.
At the same time, the climax concept implied
that species shifts were predetermined.
A given species would always appear at the
same point during the development of the climax
community, and always have the same impact
on community development.This static view
of priority effects remained essentially unchanged
by the concept of patch dynamics, which was
introduced by Alex Watt in 1947.
Watt conceived of plant communities as dynamic
"mechanisms" that followed predetermined succession
cycles.
Although Watt questioned the idea of a stable
endpoint to community development, he seemed
to agree with Clements that each particular
species had a predetermined role to play in
community development.
They viewed succession as a process driven
by facilitation, in which each species made
local conditions more suitable for another
species.
=== Individualistic approach ===
In 1926, Henry Gleason presented an alternative
hypothesis in which plants were conceptualized
as individuals rather than components of a
superorganism.
Gleason suggested that the distribution of
various species across the landscape reflected
species-specific dispersal limitations and
environmental requirements rather than predetermined
associations among species.
Gleason set the stage for future research
on priority effects by explaining that initially
identical ponds colonized by different species
could develop through succession into very
different communities.
Thus, Gleason contested the idea of a predetermined
climax community and recognized that different
colonizing species could produce alternative
trajectories of community development.
Frank Egler (1954) built on Gleason's hypothesis
by developing the Initial Floristic Composition
model to describe community development in
abandoned agricultural fields.
According to this model, the set of species
present in a field immediately after abandonment
had strong influences on community development
and final community composition.
Although rooted in succession theory, this
approach foreshadowed the development of alternative
stable state and community assembly theory.
=== Alternative stable states ===
In the 1970s, it was suggested that natural
communities could be characterized by multiple
or alternative stable states.
In accordance with the conclusions of Gleason
and Egler, multiple stable state models indicated
that the same environment could support several
different combinations of species.
Theorists argued that historical context could
play a central role in determining which stable
state would be present at any given time.
Robert May explained, "If there is a unique
stable state, historical accidents are unimportant;
if there are many alternative locally stable
states, historical accidents can be of overriding
significance."
=== 
Community assembly theory ===
The development of assembly theory followed
from the emergence of alternative stable state
theory.
Assembly theory explains community development
processes in the context of multiple stable
states.
It asks why a particular type of community
developed when other stable community types
were possible.
In contrast to succession theory, assembly
theory was developed largely by animal ecologists
and explicitly incorporated historical context.In
one of the first models based on this theory,
Jared Diamond (1975) developed quantitative
"assembly rules" to predict avian community
composition on an archipelago.
Although the idea of deterministic community
assembly quickly drew criticism, the assembly
approach, which emphasized historical contingency
and multiple stable states, continued to gain
support.
Drake (1991) used an assembly model to demonstrate
that different community types would result
from different sequences of species invasions.
In Drake’s model, early invaders had major
impacts on the invasion success of species
that arrived later.
Other modeling studies suggested that priority
effects may be especially important when invasion
frequency is low enough to allow species to
become established before replacement, or
when other factors that could drive assembly
(e.g., competition, abiotic stress) are relatively
unimportant.
In a 1999 review, Belyea and Lancaster described
three basic determinants of community assembly:
dispersal constraints, environmental constraints,
and internal dynamics.
They identified priority effects as a manifestation
of the interaction between dispersal constraints
and internal dynamics.
== Empirical evidence ==
On January 25, 2007, a search of the ISI Web
of Science citation index using the key word
"priority effect*" returned 65 ecology-related
studies.
The first study to explicitly mention priority
effects was published in 1983.
Although early research focused on animals
and aquatic systems, more recent studies have
begun to examine terrestrial and plant-based
priority effects.
Inhibitory priority effects have been documented
more frequently than facilitative priority
effects.
Priority effects among species within the
same trophic level and functional group, or
guild, have been documented more frequently
than effects across trophic levels or guilds.
Studies indicate that both abiotic (e.g. resource
availability) and biotic (e.g. predation)
factors can affect the strength of priority
effects.
=== Marine ===
Most of the earliest empirical evidence for
priority effects came from studies on aquatic
animals.
Sutherland (1974) found that final community
composition varied depending on the initial
order of larval recruitment in a community
of small marine organisms (sponges, tunicates,
hydroids, and other species).
Shulman (1983) and Almany (2003) found strong
priority effects among coral reef fish.
The former study found that prior establishment
by a territorial damselfish reduced establishment
rates of other fish.
The authors also identified cross-trophic
priority effects; prior establishment by a
predator fish reduced establishment rates
of prey fishes.
In the late 1980s, several studies examined
priority effects in marine microcosms.
Robinson and Dickerson (1987) found that priority
effects were important in some cases, but
suggested, "Being the first to invade a habitat
does not guarantee success; there must be
sufficient time for the early colonist to
increase its population size for it to pre-empt
further colonization."
Robinson and Edgemon (1988) later developed
54 communities of phytoplankton species by
varying invasion order, rate, and timing.
They found that although invasion order (priority
effects) could explain a small fraction of
the resulting variation in community composition,
most of the variation was explained by changes
in invasion rate and invasion timing.
These studies indicate that priority effects
may not be the only or the most important
historical factor affecting the trajectory
of community development.
In a striking example of cross-trophic priority
effects, Hart (1992) found that priority effects
explain the maintenance of two alternate stable
states in stream ecosystems.
While a macroalga is dominant in some patches,
sessile grazers maintain a "lawn" of small
microalgae in others.
If the sessile grazers colonize a patch first,
they exclude the macroalga, and vice versa.
=== Amphibian ===
In two of the most commonly cited empirical
studies on priority effects, Alford and Wilbur
documented inhibitory and facilitative priority
effects among and toad larvae in experimental
ponds.
They found that hatchlings of a toad species
(Bufo americanus) exhibited higher growth
and survivorship when introduced to a pond
before those of a frog species (Rana sphenocephala).
The frog larvae, however, did best when introduced
after the toad larvae.
Thus, prior establishment by the toad species
facilitated the frog species, while prior
establishment by the frog species inhibited
the toad species.
Studies on tree frogs have also documented
both types of priority effect.
Morin (1987) also observed that priority effects
became less important in the presence of a
predatory salamander.
He hypothesized that predation mediated priority
effects by reducing competition between frog
species.
Studies on larval insects and frogs in water-filled
tree holes and stumps found that abiotic factors
such as space, resource availability, and
toxin levels can also be important in mediating
priority effects.
=== Terrestrial ===
Terrestrial studies on priority effects are
rare.
The aforementioned ISI Web of Science search
retrieved only 19 studies on fully terrestrial
organisms, and all of these studies were published
during or after 1993.
Most studies have focused on arthropods or
grassland plant species.
In a lab experiment, Shorrocks and Bingley
(1994) showed that prior arrival increased
survivorship for two species of fruit flies;
each fly species had inhibitory impacts on
the other.
A 1996 field study on desert spiders by Ehmann
and MacMahon showed that the presence of species
from one spider guild reduced establishment
of spiders from a different guild.
More recently, Palmer (2003) demonstrated
that priority effects allowed a competitively
subordinate ant species to avoid exclusion
by a competitively dominant species.
If the competitively subordinate ants were
able to colonize first, they altered their
host tree’s morphology in ways that made
it less suitable for other ant species.
This study was especially important because
it was able to identify a mechanism driving
observed priority effects.
With the exception of an early study exploring
the facilitative effects of litter deposition,
studies that explicitly addressed terrestrial
plant priority effects began to appear in
the literature around the year 2000.
A study on two species of introduced grasses
in Hawaiian woodlands found that the species
with inferior competitive abilities may be
able to persist through priority effects.
At least three studies have come to similar
conclusions about the coexistence of native
and exotic grasses in California grassland
ecosystems.
If given time to establish, native species
can successfully inhibit the establishment
of exotics.
Authors of the various studies attributed
the prevalence of exotic grasses in California
to the low seed production and relatively
poor dispersal ability of native species.
== Emerging concepts ==
=== Long-term implications: convergence and
divergence ===
Although many studies have documented priority
effects, the persistence of these effects
over time often remains unclear.
Young(2001) indicated that both convergence
(in which "communities proceed towards a predisturbance
state regardless of historical conditions")
and divergence (in which historical factors
continue to affect the long-term trajectory
of community development) are present in nature.
Among studies of priority effects, both trends
seem to have been observed.
Fukami (2005) argued that a community could
be both convergent and divergent at different
levels of community organization.
The authors studied experimentally-assembled
plant communities and found that while the
identities of individual species remained
unique across different community replicates,
species traits generally became more similar.
=== Trophic ecology ===
Some studies indicate that priority effects
can occur across guilds or trophic levels.
Such priority effects could have dramatic
impacts on community composition and food
web structure.
Even intra-guild priority effects could have
important consequences at multiple trophic
levels if the affected species are associated
with unique predator or prey species.
Consider, for example, a plant species that
is eaten by a host-specific herbivore.
Priority effects that influence the ability
of the plant species to establish would indirectly
affect the establishment success of the associated
herbivore.
Theoretical models have described cyclical
assembly dynamics in which species associated
with different suites of predators are able
to repeatedly replace one another.
=== Intraspecific aggregation ===
In situations where two species are introduced
at the same time, spatial aggregation of a
species' propagules could cause priority effects
by initially reducing interspecific competition.
Aggregation during recruitment and establishment
could allow inferior competitors to coexist
with or even displace competitive dominants
over the long-term.
Several modeling efforts have begun to examine
the implications of spatial priority effects
for species coexistence.
Rejmanek (2002) suggested that only 10 empirical
studies examining intraspecific aggregation
had been published by 2002.
=== Mechanisms and new organisms ===
The literature on priority effects is currently
growing in both depth and breadth.
A few studies have begun to explore the mechanisms
driving observed priority effects.
Moreover, although past studies focused on
a small subset of species, recent papers indicate
that priority effects may be important for
a wide range of organisms, including fungi,
birds, lizards, and salamanders.
=== Ecological restoration ===
Priority effects have important implications
for ecological restoration.
In many systems, information about priority
effects can help practitioners identify cost-effective
strategies for improving the survival and
persistence of certain species, especially
species of inferior competitive ability.
For example, in a study on the restoration
of native Californian grasses and forbs, Lulow
(2004) found that forbs could not establish
in plots where she had previously planted
bunchgrasses.
When bunchgrasses were added to plots where
forbs had already been growing for a year,
forbs were able to coexist with grasses for
at least 3–4 years.
Lulow’s results suggested that planting
forbs before grasses might improve forb persistence
in this system
