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Species Diversity, Invasion, and Alternative Community States in Sequentially Assembled
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Source: The American Naturalist, Vol. 178, No. 3 (September 2011), pp. 411-418
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vol. 178, no. 3
the american naturalist
september 2011
Species Diversity, Invasion, and Alternative Community States
in Sequentially Assembled Communities
Lin Jiang,1,* Lauren Brady,2 and Jiaqi Tan1
1. School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332; 2. Department of Biology, Kenyon College, Gambier,
Ohio 43022
Submitted April 12, 2011; Accepted May 13, 2011; Electronically published July 19, 2011
Online enhancement: appendix figure. Dryad data: http://dx.doi.org/10.5061/dryad.469kf.
abstract: The relationship between resident species diversity and
invasion is generally negative in experimental studies but takes various forms in observational studies of natural communities. We hypothesized that stochastic species colonization, which applies to natural communities but not to experimental communities generally
assembled through simultaneous species introduction, may lead to
nonnegative diversity-invasion relationships via incurring priority
effects. To test this hypothesis, we manipulated both resident species
diversity and colonization history in sequentially assembled communities of bacterivorous protist species. We found that, despite a
significant effect of assembly history on invader abundance, invader
abundance decreased with diversity. This result was largely driven
by positive selection effects associated with the dominant influence
of an invasion-resistant species, which shared the most similar resource use pattern with the invader, and by the overall weak priority
effects observed for the resident communities. Increasing species diversity, however, significantly strengthened priority effects, providing
the first experimental support for the idea that larger species pools
promote alternative community states. We suggest that elucidating
mechanisms regulating the strength of priority effects may help in
understanding variation in diversity-invasion relationships among
natural communities.
Keywords: alternative stable states, biological invasions, community
assembly, community invasibility, priority effects, species diversity.
Introduction
Many of the Earth’s ecosystems are undergoing major
changes in biodiversity, including both the loss of native
species and the addition of exotic species (Sax and Gaines
2003). One active area of research, linking native species
extinction with exotic species introduction, asks whether
species diversity of native (resident) communities influences their susceptibility to the invasion of exotic (nonresident) species. In agreement with the early proposition
* Corresponding author; e-mail: [email protected].
Am. Nat. 2011. Vol. 178, pp. 411–418. 䉷 2011 by The University of Chicago.
0003-0147/2011/17803-52971$15.00. All rights reserved.
DOI: 10.1086/661242
of Elton (1958), the majority of experiments that directly
constructed resident communities found that increasing
diversity tended to reduce invasion (Levine et al. 2002).
This pattern can often be attributed to one or both of the
following two processes—niche complementarity among
resident species, which allows more-diverse communities
to cover more of the niche space potentially utilized by
invaders, and the positive selection effect (also known as
the sampling effect), in which more-diverse resident communities are more likely to contain invasion-resistant species (e.g., Fargione and Tilman 2005). By contrast, studies
of invasions in natural communities have produced widely
different results, with positive, negative, and null relationships between resident species diversity and the success of
invading species all being common (Herben et al. 2004;
Fridley et al. 2007). Hypotheses explaining this discrepancy
have focused on associating positive diversity-invasion
relationships with broadscale processes, particularly the
role of spatial heterogeneity in abiotic and biotic conditions (summarized in Fridley et al. 2007). However,
although these hypotheses can potentially account for positive diversity-invasion patterns that dominate observational studies at large spatial scales, they do not provide
an explanation of the frequent nonnegative patterns found
in observational studies conducted at scales comparable
to those of experimental studies.
One factor that may potentially influence the diversityinvasion relationship but that remains unexplored in this
context is the history of community assembly. Resident
communities in experimental studies of diversity and invasion are generally assembled by introducing all member
species at the same time. However, this assumption of
simultaneous species arrival is unlikely to hold for natural
communities, which are typically colonized by species
from the regional pool sequentially (Fukami 2010). Frequently, in situations in which sequential community assembly leads to priority effects (e.g., Almany 2004), the
stochastic nature of species colonization can result in al-
412 The American Naturalist
ternative community states characterized by the dominance of different early-colonizing species in otherwise
similar habitats. Under this scenario, one may expect both
the positive selection effect and niche complementarity to
be less important. Priority effects reduce the importance
of the positive selection effect when invasion-resistant species, as late colonizers, attain small population sizes or
even fail to establish viable populations. Priority effects
also tend to reduce the importance of niche complementarity, because it results in communities with more-uneven
species abundance, in which the magnitude of complementary interactions is constrained by the limited contribution from the low-abundance organisms (Jiang et al.
2009). We hypothesize that the diminished role of both
processes, which are associated with stochastic assembly
history, could potentially bring about nonnegative diversity-invasion relationships in natural communities.
To examine this hypothesis, we assembled aquatic bacterivorous protist communities that differed in species
richness and in the order of sequential species colonization, and we subjected these communities to the invasion
of a nonresident species. Protist microcosms are particularly suitable for answering community assembly questions (e.g., Fukami and Morin 2003; Jiang and Patel 2008),
mainly due to their amenability to experimental manipulations and the short generation times of protists that
permit communities to quickly attain steady states. Because invasion resistance is often considered as a type of
ecosystem function (Levine et al. 2002), our work is among
the few to explore potential functional consequences of
community assembly (also see Zhang and Zhang 2007;
Fukami et al. 2010), but is the first, to our knowledge, to
directly manipulate both species diversity and assembly
history. By doing so, we explored whether the assembly
history of a community affects its susceptibility to invasion
and whether communities assembled from different histories still give rise to negative diversity-invasion relationships, as seen in experiments with simultaneous species
introduction. Our experiment also allowed us to address
the question of how the size of the species pool affects the
likelihood of alternative community states. Theory suggests that differences in assembly history are more likely
to result in alternative community states as the species
pool size increases (Law and Morton 1993; Chase 2003;
Fukami 2004b), but an experimental test of this prediction
is lacking.
Material and Methods
Study Organisms
We assembled resident protist communities from a pool
of five bacterivorous ciliate species, including Colpidium
kleini, Loxocephalus sp., Paramecium aurelia, Spirostomum
teres, and Tetrahymena pyriformis, and subjected these
communities to the invasion of a sixth bacterivorous ciliate, Paramecium multimicronucleatum. Prior to the experiments, each of these species was grown separately in
its stock culture on a multispecies bacterial assemblage,
including Bacillus cereus, Bacillus subtilis, Serratia marcescens, and a number of unidentified species, which were
introduced to experimental microcosms along with their
protist consumers (see Microcosms).
Microcosms
Each microcosm was a 250-mL Pyrex glass bottle filled
with 100 mL of protozoan pellet medium (concentration,
0.55 g of protozoan pellet [Caroline Biological Supply,
Burlington, NC] per 1 L of deionized water). Bottles and
medium were autoclave-sterilized, and the medium was
inoculated with bacteria from the protist stock cultures.
To prepare for bacterial inoculation, we mixed samples
from the stock culture of each protist species and removed
the protists from the mixed sample by running it through
a 1.0-mm sterile filter. The filtrate, containing only bacteria,
was used to inoculate the medium. Each microcosm also
received two autoclave-sterilized wheat seeds as an additional carbon source.
Invasion Experiment
Resident protist communities differed in three aspects:
species richness (one, two, or five species), composition,
and colonization sequence. Single- and two-species communities each had five different compositions, with each
resident species represented once in single-species communities and twice in two-species communities, whereas
five-species communities included all five resident species.
We varied species colonization history for two- and fivespecies communities, allowing each species to be the first
colonizer once within each composition; this resulted in
two assembly sequences for each two-species composition
and five assembly sequences for the five-species composition (see fig. 1B for the list of treatments, in which resident species were represented by the first letter of their
genus names; e.g., CP stands for the two-species cultures
where Colpidium colonized before P. aurelia). We replicated each assembly sequence (20 total) three times, resulting in 60 microcosms. During resident community assembly, resident species were added at 4-day intervals.
Resident communities were allowed to equilibrate for another 2 weeks after the introduction of the last colonizers
before each was challenged with P. multimicronucleatum.
The inoculum of each protist species (including the invader) consisted of approximately 200 individuals, with
Community Assembly and Invasion 413
day-old cultures for inoculation. Microcosms were colonized by the first protist species 24 h after bacterial inoculation and were run as semicontinuous cultures for the
duration of the experiment (52 days), with 7% content of
each microcosm replaced with fresh medium each week.
All microcosms were maintained in an unlighted incubator
at 22⬚C.
We sampled microcosms to estimate population density
of each protist species three times a week, starting from
2 days after the introduction of the first species (the first
sampling day designated as day 0). This procedure consisted of three steps. First, the microcosm was swirled to
ensure an even distribution of protist individuals in the
medium, and a small sample (0.3–0.4 mL) was withdrawn
using a sterile Pasteur pipette. Second, the sample was
weighed to determine its volume and distributed onto a
petri dish in small drops. Third, the petri dish was placed
under a stereoscopic microscope, and the number of individuals of each species in the sample was counted. We
diluted samples for high-abundance species before counting. All protist density data were recorded as the number
of individuals per milliliter.
Bacterial Experiment
Figure 1: Final population density of the invader at each resident
species richness level (A) and in each assembly history treatment
(B). In A, the Poisson generalized estimating equation line is shown
along with the data. In B, values are arithmetic means ⫹ SE. Horizontal bars in B indicate nonsignificant differences in the leastsquares means between two assembly sequences within the same
species composition treatment (e.g., PT vs. TP). C, Colpidium species;
L, Loxocephalus species; P, Paramecium species; S, Spirostomum species; T, Tetrahymena species.
the exception of S. teres, which consisted of 20 individuals
because of its lower stock culture abundance. To minimize
variation in physiological conditions of initial protist populations introduced at different times, we set up fresh stock
cultures of each species periodically and always used 14-
To gain a more mechanistic understanding of resourcebased competitive interactions between protists (see Fox
2002), we performed an experiment in which we estimated
the impacts of individual protist populations on bacterial
resources. This experiment had seven treatments, including a control with bacteria only and six protist monoculture treatments corresponding to each of the six protist
species used in the invasion experiment. Each treatment
was replicated five times, for a total of 35 microcosms.
This experiment proceeded as in the invasion experiment,
except that only a single protist species was introduced
into the microcosms. We ran the experiment for 3 weeks,
at the end of which period we sampled each microcosm
for bacterial abundance. Samples were serially diluted and
spread on nutrient agar plates, and the number of colonyforming units (CFUs) for each bacterial morphospecies
was counted after 10 days’ incubation at room temperature. Bacterial density data were recorded as the number
of CFUs per milliliter.
Data Analysis
Because protist communities appeared to approach their
steady states toward the end of the invasion experiment
(fig. A1 in the online edition of the American Naturalist),
data from the last sampling day were used to quantify
treatment effects. We used Poisson generalized estimating
equations (GEEs) to model invasion success, measured as
414 The American Naturalist
final invader population density, as a function of species
richness, composition, and colonization history. GEEs
were chosen over ordinary Poisson regression models because they can account for possible nonindependence
among data and tend to provide more-robust estimates of
standard errors of model parameters. For our data, GEE
yielded essentially the same parameter values as the Poisson regression model, but generated larger confidence intervals for the same parameters, compared with the latter
models; results based on GEEs are thus more conservative.
We ran three separate GEEs: the first to test for the general
relationship between resident species richness and invader
density, the second for the effect of resident species composition (nested within richness) on invader density, and
the third for the effect of assembly history (nested within
composition) on invader density. Given the overall significant effect of history (see “Results”), we also used
planned contrasts to assess the difference in invader density among communities experiencing different histories
within each species composition.
We used the overyielding index Dmax (Loreau 1998),
calculated for each multispecies culture, to assess the relative importance of complementarity and selection effects
on community invasion resistance. The additive partitioning model of Loreau and Hector (2001) was not used
because it was not possible to determine the contribution
of each resident species in multispecies cultures to invasion
resistance. The original Dmax was developed for biomass
production and attains positive values when the performance (biomass) of a polyculture exceeds the monoculture
performance of its most productive member species when
grown alone. In the context of invasion resistance, however, better performance corresponds to lower invader
abundance. To facilitate comparison with previous work,
we calculated Dmax on invader abundance according to its
original formula but reversed its sign; positive Dmax thus
corresponds to invader abundance of a polyculture being
lower than that of the monoculture of its most invasionresistant member species when grown alone. A one-sample
t-test tested whether Dmax value in each polyculture significantly differed from 0. Significant positive Dmax values,
indicative of the ability of multispecies assemblages to
lower invader abundance beyond that of the most invasion-resistant species, would suggest niche complementarity, whereas Dmax not different from 0, which is indicative of the dominant role of the most invasion-resistant
species, would suggest the positive selection effect.
We quantified the degree of similarity in the final structure of protist communities that shared the same species
composition, but with different assembly histories, using
the Morisita-Horn similarity index. This index ranges from
0 (no species shared by the two communities being compared) to 1 (two communities sharing the same species
and the same relative abundance of these species) and is
known to be robust to changes in species richness (Wolda
1981), which facilitates the comparison between species
richness levels. For comparison, we also calculated the
widely used Bray-Curtis similarity index. Results based on
the two similarity measures were similar, and we report
only those based on the Morisita-Horn measure here. To
test the idea that species pool size may influence the likelihood of alternative community states, we compared similarity values in the two- and five-species cultures using a
permutation test that randomly shuffled data between the
two richness levels 1,000 times. Analyses included the invader as a component of the protist community, but results
were qualitatively similar when considering resident protist
species only.
We performed principal component analysis (PCA) of
bacterial density data from the bacterial experiment to
discern the difference in bacterial community structure
among protist monocultures. Principal components were
calculated based on the correlation matrix of bacterial densities, which were log transformed (log10[x ⫹ 1]) prior to
the analysis.
Results
Invasion success, measured as invader population density
at the end of the experiment, decreased significantly as the
number of resident species increased (fig. 1A; GEE:
x 2 p 8.58, df p 1, P p .0034 for richness). The species
richness effect remained significant (x 2 p 9.18, df p 2,
P p .0101) in a GEE model that treated species composition as a nesting factor within species richness, which
also revealed a significant effect of species composition
(x 2 p 20.12, df p 8, P p .0099). A third GEE model that
treated species assembly history as a factor nested within
species composition indicated significant effects of both
factors on invasion success (composition: x 2 p 26.81,
df p 10, P p .0028; history: x 2 p 18.09, df p 9, P p
.0342). The effect of assembly history, however, was not
uniform across species composition treatments. Planned
contrasts revealed significant differences in the invader
density of communities sharing the same composition but
with different assembly histories in three of five cases for
two-species communities and three of 10 cases for fivespecies communities (fig. 1B).
Negative values were attained for Dmax in most multispecies cultures (fig. 2); only in one treatment (LS) was it
significantly different from 0 (t 2 p 4.38, P p .0484).
None remained significant after Bonferroni adjustment.
The dominance of early-colonizing species over later
ones was observed only in the five-species communities,
not two-species communities (fig. A1). For example, reversing the order of the introduction of the two most
Community Assembly and Invasion 415
Figure 2: Dmax value (mean ⫹ SE) for each assembly history treatment. In all but one treatment (LS), Dmax is not significantly different
from 0 in one-sample t-tests. None of the values are significantly
different from 0 after Bonferroni adjustment. C, Colpidium species;
L, Loxocephalus species; P, Paramecium species; S, Spirostomum species; T, Tetrahymena species.
ident communities, such that species dominance patterns
were unaffected by history, the abundance of the subordinate species did change with history in these communities (fig. A1). In particular, Spirostomum attained larger
populations as a subordinate species when it was the first
colonizer rather than when it was the second colonizer,
and this increased abundance of Spirostomum corresponded to the reduced invader abundance in the twospecies communities in which Spirostomum was present
(fig. A1). Additional evidence for the linkage between Spirostomum and invasion success came from the five-species
communities, where stronger priority effects led to changes
in the identity of the dominant species as well as to changes
in Spirostomum abundance. A GEE model relating invader
abundance to all resident species in these communities
identified Spirostomum abundance as the only significant
predictor of invader abundance (x 2 p 4.93, P p .0264).
Note that Spirostomum was the least abundant resident
species in these communities (fig. A1), and some of the
other resident species, when alone, posed greater resistance
to invasion (fig. 1B). Therefore, the negative association
between Spirostomum and invasion may not be solely attributable to the inhibitive effect of Spirostomum on the
invader via the reduction of their shared bacterial re-
abundant species (Colpidium and Loxocephalus) in the
five-species communities caused the reversal of their relative abundance (fig. A1). As a result, similarity in the
final structure of communities that shared identical species
composition but underwent different assembly histories,
measured as the Morisita-Horn index, was significantly
greater for two-species communities than for five-species
communities (fig. 3; randomization test: P ! .001).
PCA revealed both similarities and differences in the
structure of bacterial communities among protist monocultures. In particular, bacterial communities in the invader (Paramecium multimicronucleatum) cultures were
most similar to those in the cultures of Paramecium aurelia, which shares the same genus as the invader; Spirostomum cultures possessed distinct bacterial communities
from other protist cultures (fig. 4).
Discussion
Our experiment clearly showed that the assembly history
of a community can affect its susceptibility to invasion.
This result was closely linked to changes in the structure
of resident communities with different assembly histories.
Although priority effects were weak in the two-species res-
Figure 3: Morisita-Horn similarity between communities with the
same species composition but different assembly sequences, averaged
across all compositions for each species richness level. A permutation
test that randomly assigned values to the two richness treatments
showed that the similarity among the five-species communities was
significantly smaller than that among the two-species communities
(P ! .001). Values are mean ⫹ SE.
416 The American Naturalist
Figure 4: Plot of the first and second principal component scores
(PC1 and PC2, respectively) from the principal component analysis
of bacterial community structure in the bacterial experiment. PC1
and PC2 accounted for 19.5% and 12.5% of variation in the data,
respectively. Each treatment had five replicates. B, bacteria only; C,
Colpidium monocultures; I, invader (Paramecium multimicronucleatum) monocultures; L, Loxocephalus monocultures; P, Paramecium
aurelia monocultures; S, Spirostomum monocultures; T, Tetrahymena
monocultures.
sources, independent of other species in resident communities. More likely, it may have reflected its complementary use of bacterial resources with other resident
species. Consistent with this idea, we found that Spirostomum imposed different influences on bacterial communities than did other protist consumers (fig. 4), which
suggests its distinct niche. Overall, our findings agree with
those of several recent studies that found significant effects
of species colonization history on ecosystem functioning,
including biomass production in micro-algal communities
(Zhang and Zhang 2007) and grassland plant communities
(Korner et al. 2008) and wood decomposition in fungal
communities (Fukami et al. 2010). Together, these results
suggest that history-dependent alternative community
states that differ in species composition and/or abundance
may exhibit different levels of ecosystem function, which
underscores the need for future assembly studies to consider the functional consequences of assembly history.
As in many experimental studies that directly manipulate resident species diversity (Levine et al. 2002), a negative diversity-invasion relationship emerged in our experiment. This negative effect of resident species diversity
on invasion can be primarily attributed to the positive
selection effect, as indicated by the Dmax values, which
rarely differed from 0. The positive selection effect was
largely driven by the fact that more-diverse communities
have an increased likelihood of including Paramecium aurelia, whose monoculture showed the greatest resistance
to invasion; resident communities containing this species
exhibited a substantially lower level of invasion than did
those communities that lacked this species (fig. 1B;
planned contrast: P ! .0001). Note that the invader (Paramecium multimicronucleatum) belongs to the same genus
as P. aurelia but does not belong to the same genus as
other resident species. Darwin (1859) hypothesized the
difficulty of a nonnative species becoming naturalized in
habitats where its congeners are present, reasoning that
the substantial ecological similarity of congeners results in
strong competition between them. The lower abundance
of P. multimicronucleatum in the presence of P. aurelia,
coupled with the similarity in their use of bacterial resources, provided strong support for this hypothesis (see
Jiang et al. 2010 for a formal experimental test of the
hypothesis). Notably, changing assembly history did not
eliminate this positive selection effect associated with interactions of closely related species. In both two-species
communities with weak priority effects and five-species
communities with stronger priority effects, P. aurelia, as
the most invasion-resistant species, reached appreciable
abundance, which rendered the positive selection effect
robust to variation in assembly history. The positive selection effect also dwarfed any complementarity effects
among resident species (undetected using Dmax) and any
possible change in the strength of complementary interactions as a result of history-dependent species abundances. In all, the strong positive selection effect led to a
decrease in invasion success with increasing species diversity, despite the influence of assembly history on
invasion.
As species diversity increased, the structure of communities that shared the same species composition but
underwent different assembly histories became less similar.
One could argue that the lower similarity in the five-species
communities may simply be a transient phenomenon that
may go away if the experiment runs longer, because these
communities experienced a longer initial assembly period
(with the last resident species introduced on day 20) than
did the two-species communities (in which the last resident species introduced on day 8). Our data, however,
indicate that most communities attained relatively steady
states by the end of the experiment (fig. A1). Moreover,
even if the five-species communities were not at their
steady states (see Fukami 2004a for an example of community assembly with long transient dynamics), they demonstrated longer-lasting priority effects than did the two-
Community Assembly and Invasion 417
species communities. For instance, P. aurelia was more
abundant than was Tetrahymena for only 5 days in the PT
treatment after both species were introduced, before yielding its dominance to the latter; Tetrahymena dominated
P. aurelia in the TP treatment throughout the experiment
(data not shown). By contrast, when introduced earlier in
the five-species communities, each of the two species held
its dominance over the other for the duration of the experiment (fig. A1); this is especially evident when comparing treatments CSLPT and LTPCS, in which the two
species were introduced 4 days apart, as in the two-species
communities. Considering the short generation times (less
than 2 days) of these organisms, these results suggest that
species pool size can influence priority effects over ecologically meaningful timescales. Indeed, these results are
consistent with the theoretical prediction that increasing
species pool size tends to increase the likelihood of alternative community states (Law and Morton 1993; Chase
2003; Fukami 2004b). It has been suggested that communities with larger species pools tend to contain more
species with similar traits, the interaction of which promotes priority effects (Chase 2003). This is certainly the
case for the invader, for which the priority effects associated with its resident congener P. aurelia, which was more
frequently present in more-diverse communities, were primarily responsible for its decrease with an increase in resident species diversity (i.e., the positive selection effect).
However, this hypothesis cannot account for the fact that
priority effects involving the same pair of resident species
(e.g., P. aurelia and Tetrahymena) were stronger with increasing diversity in our experiment, which instead may
be better explained by complementary resource use (fig.
4) among earlier-colonizing resident species leaving fewer
resources for later colonizers in more-diverse communities
(i.e., niche complementarity). Future studies should further explore the idea that niche complementarity and positive selection effects may both contribute to the increase
of priority effects with diversity. We note that both mechanisms are often found to play important roles in causing
the increase of ecosystem biomass production with diversity (Cardinale et al. 2007).
Two aspects of our invasion experiment warrant some
clarification. First, for convenience, we confined the term
“community assembly” to resident communities and the
term “invasion” to the nonresident species, which gives
the impression that community assembly and invasion are
two independent concepts. These two concepts, however,
are in fact closely linked, because community assembly
involves the invasion and subsequent interactions of species, regardless of whether the species are resident (native)
or nonresident (nonnative). Both terms have often been
used together in the same context (e.g., Fargione et al.
2003; Tilman 2004). As such, our experiment (and many
other invasion experiments) may also be considered as an
assembly experiment with the nonresident species introduced last. Second, our experiment had only three replicates. We chose to replicate treatments three times because
our previous assembly experiments (Jiang and Patel 2008;
Jiang et al. 2010, 2011) showed that this level of replication
was adequate at detecting treatment differences, which is
also the case for the present experiment. There was indeed
some unexpectedly appreciable variation among replicates
in a few treatments, which nevertheless did not prevent
us from detecting significant diversity and assembly history
effects. One may also ask whether increasing replication
may increase the chance of finding stronger priority effects
in some of the treatments, based on the premise that replicates may randomly diverge along different trajectories
(e.g., Jiang et al. 2011). However, such random divergence
has not been found for communities of bacterivorous protists (Jiang et al. 2011), probably because stable bacteriaprotist interactions minimize the effects of demographic
stochasticity that trigger divergence. Consistent with this,
there was little sign of random divergence in our experimental communities of bacterivorous protists. Nevertheless, our experiment could certainly benefit from more
replication, which would have increased statistical power.
Our study provides the first experimental evidence for
the increased likelihood of alternative community states
in more-diverse communities and demonstrates the importance of resident community assembly history for
invasion. However, we still found a negative diversityinvasion relationship in our sequentially assembled communities, largely driven by an invasion-resistant resident
species that remained fairly abundant with changes in
assembly history. Nevertheless, we caution against jumping
to the conclusion that assembly history contributes little
to the nonnegative diversity-invasion patterns that are observed in many natural communities. Priority effects involving all resident community members (including the
invasion-resistant species) in our experiment were generally weak, which limited the effect of assembly history
on invasion. In contrast, priority effects of various
strength, including strong effects that result in the extinction or severe reductions in the density of later colonizing
species (e.g., Zhang and Zhang 2007), have been reported
(Chase 2003). Our understanding of the role of assembly
history for community resistance to biological invasions
and other ecosystem functions will certainly benefit from
a better knowledge of the mechanisms regulating the
strength of priority effects (e.g., Chase 2003, 2007, 2010).
Acknowledgments
We thank Z. Pu, W. Ryberg, and two anonymous reviewers
for constructive comments that significantly improved this
418 The American Naturalist
manuscript. This project was supported by the National
Science Foundation (grants OCE-0851606 and DEB064041), and L. Brady was supported by OCE-0851606 as
a National Science Foundation Research Experience for
Undergraduates student.
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Associate Editor: Oswald J. Schmitz
Editor: Judith L. Bronstein