Shrub Invasion Decreases Diversity and Alters
Community Stability in Northern Chihuahuan Desert
Selene Ba ´ez*, Scott L. Collins
Department of Biology, University of New Mexico, Albuquerque, New Mexico, United States of America
Background: Global climate change is rapidly altering species range distributions and interactions within communities. As
ranges expand, invading species change interactions in communities which may reduce stability, a mechanism known to
affect biodiversity. In aridland ecosystems worldwide, the range of native shrubs is expanding as they invade and replace
native grassland vegetation with significant consequences for biodiversity and ecosystem functioning.
Methodology: We used two long-term data sets to determine the effects of shrub encroachment by Larrea tridentata on
subdominant community composition and stability in formerly native perennial grassland dominated by Bouteloua eriopoda
in New Mexico, USA.
Principal Findings: Our results indicated that Larrea invasion decreased species richness during the last 100 years. We also
found that over shorter temporal scales species-poor subdominant communities in areas invaded by Larrea were less stable
(more variable in time) compared to species rich communities in grass-dominated vegetation. Compositional stability
increased as cover of Bouteloua increased and decreased as cover of Larrea increased.
Significance: Changes in community stability due to altered interspecific interactions may be one mechanism by which
biodiversity declines in grasslands following shrub invasion. As global warming increases, shrub encroachment into native
grasslands worldwide will continue to alter species interactions and community stability both of which may lead to a
decline in biodiversity.
Citation: Ba ´ez S, Collins SL (2008) Shrub Invasion Decreases Diversity and Alters Community Stability in Northern Chihuahuan Desert Plant Communities. PLoS
ONE 3(6): e2332. doi:10.1371/journal.pone.0002332
Editor: Andy Hector, University of Zurich, Switzerland
Received March 21, 2008; Accepted April 25, 2008; Published June 4, 2008
Copyright: ? 2008 Ba ´ez, Collins. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: National Science Foundation
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: firstname.lastname@example.org
The measurement of ecological stability and the relationship
between community stability and diversity have been the subject of
much recent theoretical debate and empirical analysis . In the
past, empirical studies of stability were hindered by a lack of long-
term data, yet such datasets are beginning to accumulate in both
experimentally- and naturally-assembled communities. Both
theory and manipulative experiments have demonstrated a
positive relationship between community stability and species
diversity in ecological communities [1–3]. Critical evidence for this
relationship occurs primarily in competitive communities (e.g.,
producers) where species diversity and the structure of species
interactions affect species composition and turnover [4,5],
population stability (e.g., rates of variation in population densities)
[6,7], and community stability (e.g., temporal variation in net
primary production) [2,8]. However, empirical research exploring
how specific changes in the structure of species interactions alters
multiple aspects of community stability, including species diversity
and the temporal rates of population and community change, are
Theoretical and empirical research have shown that species
invasions and extinctions in competitive communities can affect
community compositional stability (e.g., temporal variability in
species diversity) because they can lead to further events of species
extinction or invasion [1,9,10]. Mathematical models attribute
these changes in community stability to shifts in the structure of
species interactions, with larger structural shifts leading to lower
community compositional stability [2,10–15]. Therefore, the
mode in which an invading species interacts is expected to be a
crucial factor affecting species diversity and community compo-
sitional stability in competitive communities [1,3,14,16–18]. Field
studies assessing the effects of species invasion on various aspect of
community stability are needed to validate predictions based on
theoretical models and to identify community and population
parameters that affect community compositional stability.
In arid and semiarid regions species interactions are important
factors structuring the diversity of plant communities [19–22].
Both competition and facilitation imposed by dominant species
affect temporal variation in the cover and species composition of
subdominant plant communities [6,23,24]. Therefore, in aridland
ecosystems, invasion or extinction of facilitating or competing
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dominant plant species may have a particularly strong effect on
various measures of the temporal stability of subdominant plant
species. Community instability or compositional community
instability can be defined as the gain or loss of species or changes
in species abundances that result in large directional changes in
community composition and diversity . Temporal variability
in population and community parameters, including rates of
change in population density and rates of species turnover, also
may occur following species invasions. Thus, population and
community instabilities linked to invasion and extinction events
can yield valuable insights into the mechanisms responsible for
maintaining species diversity and certain aspects of community
stability in time [1,2,4,26].
During the last century, C4-dominated grasslands worldwide
have experienced dramatic and rapid ecological changes due to
encroachment by native C3 shrubs [27–29]. In southwestern
North America more than 19 million hectares of arid and semi-
arid C4-dominated grassland have been invaded by Larrea tridentata
(creosote bush), a common C3shrub native to North American
desert ecosystems . Shrub invasion promotes loss of biodiver-
sity and impacts various aspects of ecosystem functioning [28–33].
Larrea invasion is thought to result from multiple drivers including
overgrazing, altered disturbance regimes, elevated atmospheric
CO2, and especially altered precipitation regimes and increased
temperatures [28,34]. Indeed, the northern distribution of Larrea is
known to be limited, in part, by winter night time low
temperatures which have increased by as much as 2uC over the
last century in parts of the US southwest [35,36]. Because climate
change is potentially an important driver of shrub encroachment,
this phenomenon is likely to continue across arid and semi-arid
Here, we explore the effects of Larrea invasion on the local
extinction of the native C4 grass Bouteloua eriopoda, and the
population and community stability of subdominant species in a
northern Chihuahuan Desert ecosystem. We hypothesize that
changes in species interactions due to the replacement of the
competitive grassBouteloua,withthepredominantlyfacilitative shrub
Larrea [19,37,38] result in altered patterns of population and
community stability. To test this hypothesis we used long-term
(1995–2004) vegetation data from permanent plots to quantify
population, compositional, and community cover stability [2,39] in
pre-invasion Bouteloua-dominated plant communities and post-
invasion Larrea-dominated communities at the Sevilleta National
Wildlife Refuge (SNWR), central New Mexico, USA. We also used
data from 1915 through 2001 to examine the effects of Larrea
invasion on species diversity in Bouteloua-dominated communities at
the Jornada LTER in southern New Mexico, USA. Understanding
the effects of invasion on community stability is particularly
important given the current global trends of biodiversity loss and
changes in species distribution and abundance .
We used two long-term datasets to evaluate the effects of shrub
invasion on richness and dynamics of subordinate species in
Chihuahuan desert plant communities. Historical photographs
from the SNWR show that Larrea has invaded formerly C4
dominated grasslands during the last 100 years, primarily
displacing the native C4grass Bouteloua eriopoda (black grama). In
this area, vegetation invaded by Larrea has consistently 20–30%
fewer species of vascular plants per m2than adjacent non-invaded
grasslands dominated by Bouteloua (14.05 SE 0.52 vs. 21.4 SE 0.59
1-m2, respectively; Repeated Measures MANOVA, F1,14=26.27,
P,0.001; ). Thus, to test for differences in community and
population stability following shrub invasion, we used 10-years
(1995–2004) of vegetation measurements in permanent plots
located in native grassland and nearby areas currently dominated
by Larrea but formerly dominated by C4grasses at the SNWR. Our
study sites were located 4 km apart, with the permanent vegetation
plots in grass vegetation located 1 km north of the current grass-
shrub transition zone, and the shrub invaded plots were 3 km
south of the transition zone. The SNWR is a 100,000 ha area
located along the Rio Grande in Central New Mexico. Mean
annual temperature at the study site (1989–2006) is 13.2uC and
the average annual precipitation is 255 mm, of which 60% occurs
during the monsoon season from July through September. In
addition, to assess the effects of Larrea invasion on plant species
diversity in Bouteloua-dominated grassland we used long-term
(1915–2001) vegetation measurements taken in permanent
quadrats in C4-dominated grassland that underwent shrub
invasion around 1960 at the Jornada Experimental Range (JER)
in south-central New Mexico [41,42] approximately 270 km south
of the SNWR. Mean annual temperature and average annual
precipitation at the JER are similar to values at the SNWR .
For the community stability analyses we used annual data from
1995 to 2004 collected in 193 permanently located 1-m2quadrats
in a native grassland community dominated by Bouteloua eriopoda
(black grama), and in a former grassland area (prior to 1950) that is
now dominated by Larrea tridentata (creosote bush). At the grassland
and shrubland sites, quadrats were located in four replicate
36636 m plots. In each plot vegetation was measured in 36
permanently located 1-m2quadrats placed five meters apart in an
evenly spaced 666 grid. The cover of all plant species was
measured annually using a 1-m2frame divided into 10 cm units to
facilitate cover estimates (for further details on the experimental
layout see ). Within each site we restricted our analyses to plots
where the dominants, Larrea or Bouteloua, had a minimum mean
cover of 0.1% over 10 years. A total of 193 1-m2quadrats met this
criterion, 100 of which were located in Bouteloua-dominated
grassland, and 93 in Larrea-invaded shrubland. In this system, as
in other aridland ecosystems, grass- and shrub-dominated areas
resulted in spatially structured communities in which soil moisture
and nutrients are concentrated in ‘‘islands of fertility’’ beneath
patches of vegetation [43–45]. In both vegetation types annual
forbs and grasses are common. Shrub-dominated vegetation, in
particular, has a relatively high cover of large seeded winter annual
forbs. In contrast, grass-dominated vegetation has high richness
and cover of small-seeded annual forbs, which are primarily
present during summer . The annual cover of all plant species
in each quadrat was estimated in May when winter annuals peak
in abundance and biomass, and again in September when
perennial and summer annual species peak in abundance and
biomass in response to the summer monsoon. Cover for each
species was expressed as a percentage of the summed total of
species maximum cover values in each quadrat in each year.
In addition, we used data from the JER to determine if plant
species diversity declines as Larrea invasion occurs or if Larrea
invades areas that are already low in species diversity. To do so, we
evaluated changes in species diversity in nine permanently located
1-m2quadrats where species composition was recorded at
irregular time intervals between 1915 and 2001 . All quadrats
were originally dominated by Bouteloua eriopoda in 1915 but are now
completely dominated by Larrea tridentata.
All analyses of data from the SNWR were conducted by pooling
quadrats across the four blocks within each vegetation type and
considering the quadrats as independent samples . Pooling
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samples was deemed appropriate because cover of dominant
species was randomly distributed within plots and blocks, quadrats
were separated by distances of 5 to 272 m (i.e., 5 to 272 times the
area of a sampling unit), and there were no significant block effects
(R2=0.022, F3,88=0.073, P=0.54) cover.
Ecological stability has many different definitions and metrics
[1,25]. In this study, we quantified four measures of ecological
stability: cover stability, compositional stability, population stability,
and species turnover as defined below. Following Tilman et al.
(2006), cover stability of the subdominant community was measured as
the mean cover divided by the temporal standard deviation of cover.
Compositional stability was measured as the mean of the Euclidian
distances calculated for all the possible temporal pair comparisons of
the subdominant community in each quadrat (i.e., yr 1 vs. yr 2, yr 1
vs. yr 3, yr 1 vs. yr 4… and yr 9 vs. yr 10 ) and multiplied by 21
to make the variable more intuitive in terms of stability. Because
indices of stability may be affected by differences in species richness,
we used the log-transformed residuals of a linear regression of log-
transformed compositional stability vs. mean species richness in our
ANCOVA models to eliminate possible effects of species richness on
this measure of stability. Turnover was the sum of the probabilities of
colonization and extinction for each species that occurred in a
quadrat. Turnover was calculated using a Markovian chain
probabilistic function (colonization l+extinction d, where l=k/k+l
and d=m/m+n, where k number of colonization events, l=number
of stage persistence events from absent to absent, m=number of
extinction events, n=number of presence persistence events . In
addition, we evaluated the population stability of subdominants by
dividing the mean cover of each species over 10 years by the
temporal standard deviation of cover for that species and taking the
mean of these values .
Analysis of covariance (ANCOVA) and linear regressions were
used to assess the effects of the dominant species, Bouteloua and
Larrea, on subdominant population and community stability at the
quadrat scale. ANCOVA models were used to evaluate the
dynamics, structure, and species richness of subdominant
communities as a function of vegetation type (i.e., site), cover of
the dominant species, Bouteloua or Larrea, and the interaction
between site and cover of dominants. The temporal mean cover of
Bouteloua and Larrea was used to evaluate the effect of the dominant
species on the subdominant communities of a given quadrat
because cover fluctuated from one year to the next but there was
no overall net change in cover of the dominant species over the
study period . We used ANCOVA analyses to evaluate
whether Bouteloua and Larrea had similar effects on subdominant
community structure and stability. Thus, we assessed how the
cover of the dominant species affected the cover and species
richness (10-year mean of species present in a given quadrat) of
subdominants as a whole, as well as by life form (forbs, grasses and
shrubs), life history, (summer annuals, winter annuals, and
perennials), and distribution (shared between grass and shrub
vegetation, or restricted to either vegetation type). Most of the
dependent variables were square root or log-transformed to
At the SNWR, fifty-six percent of subdominant species were
found in both grass- and shrub-dominated vegetation (Jaccard
index of dissimilarity=0.44). In each community, the cover of the
dominant species affected the stability, structure, and species
richness of the subdominant communities (Table 1, Figure 1). A
significant site by dominant species cover interaction indicated
that Bouteloua and Larrea had opposite effects on the directionality
of compositional stability (Table 1, Figure 1). As cover of Bouteloua
increased, compositional stability of subordinate species increased
due to decreased turnover rates. In contrast, as cover of Larrea
increased, compositional stability of subordinate species decreased
and turnover rates increased. However, both Bouteloua and Larrea
had a negative effect on cover stability of the subdominant
communities as a whole (F1,99=5.7, P=0.02, Tables 1, 2). That is,
variation in the total cover of subdominant species increased as
cover of the dominants increased in both grass- and shrub-
dominated vegetation. In addition, high cover values of Bouteloua
significantly decreased the population stability of subdominant
species (i.e., variations in the cover of subdominant species
increased), whereas the cover of Larrea had no effect on population
stability of subdominants (Tables 1, 2).
Bouteloua and Larrea also had opposite effects on the structure
and richness of subdominant communities (Appendices 1, 2). The
cover of virtually all subdominants grouped according to life forms
and life histories decreased as cover of Bouteloua increased, whereas
cover of subdominants increased as cover of Larrea increased. This
was true for species of subdominants found only in the Bouteloua or
Larrea communities as well as for those species that occurred in
both community types (Appendix S1). Consistent with these
trends, species richness of the subdominant communities was
negatively related to cover of Bouteloua, and positively related to
cover of Larrea, although this relationship was marginally non-
significant for the latter (Appendix S2).
Despite the negative relationship between Bouteloua cover and
cover of subdominants, after controlling for cover of the dominant
species at each vegetation type, cover of forbs, shrubs, summer
annuals, and shared species of subdominants was higher in
Bouteloua- compared to Larrea-dominated sites (Appendix S1). In
contrast, the cover of winter annuals, perennials, and species
unique to each community type did not differ between Bouteloua-
and Larrea-dominated areas. Overall, cover of Bouteloua in
grassland vegetation was not significantly different than cover of
Larrea in shrubland areas (ANOVA, F1,191=0.091, P=0.863), but
cover of subdominants was slightly higher in Bouteloua than in
Larrea dominated vegetation (ANOVA, F1,190=3.78, P=0.053;
Table 2). Thus, despite the apparent facilitation of subdominant
species by Larrea, total cover of subdominants was lower in Larrea-
dominated areas (Table 2).
It is conceivable that differences in compositional stability and
species richness between grass- and shrub-dominated areas are a
function of different environmental conditions . Our study sites
at the Sevilleta are only a few kilometers apart so they experience
similar seasonal and annual climate variability. Soil conditions
(carbon and nitrogen) and annual net primary productivity do not
differ between our Larrea and Bouteloua-dominated sites [44,49].
In addition, data from the long-term permanent quadrats at
JER show that Larrea invaded species-rich Bouteloua-dominated
sites, and that as Larrea invasion progressed through time, total
species diversity (Linear regression: F1,65=17.83, R2=0.22,
P,0.001) and the cover of Bouteloua decreased (Quadratic
regression: F1,65=6.43, R2=0.17, P=0.002; Linear regression:
y=8336.524.0x, R2=0.06, P=0.001, Figure 2). For these
analyses, time was used as a proxy for increasing cover of Larrea,
as woody species were not sampled in the permanent vegetation
quadrats but the areas in which these quadrats were located were
invaded by Larrea during the sampling period. Therefore, this
evidence supports the idea that at the SNWR, Larrea invasion, and
not initial differences in species diversity or environmental factors,
was the main cause of species loss in areas that were formerly
dominated by C4grasses.
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Overall, the dominant species, Bouteloua and Larrea, had radically
different effects on the compositional stability of subdominant
communities at the Sevilleta. Although relationships are relatively
weak in some cases, the key is how they differ significantly in
direction. In particular, compositional stability increased as cover
of Bouteloua increased and decreased as cover of Larrea increased.
Our results also suggested that the dominant species had opposite
interspecific interactions with subdominant species. That is, total
cover of subdominants decreased as cover of Bouteloua increased
suggesting that Bouteloua competes with virtually all groups of
subdominants regardless of their life form, life history, and
distribution. In contrast, cover of subdominants increased as
Larrea cover increased, suggesting that Larrea may facilitate
subdominant species in this aridland ecosystem.
It has been proposed that invasive species disrupt the
community properties that maintain species diversity at small
spatial scales [50,51]. However, field studies exploring the
mechanisms that cause biodiversity loss during biological invasions
are scarce. Our long-term study shows that the invasion of Larrea
and the local extinction of Bouteloua affected community compo-
sitional stability, and species diversity. We found that subdominant
communities in areas invaded by Larrea over 50 years ago at the
SNWR differed in their current patterns of community stability, as
Larrea invaded areas had lower compositional stability compared
to communities in Bouteloua-dominated vegetation. Thus, our
findings confirm predictions from theoretical studies that species
invasion can lead to changes in community structure, species
interactions, and loss of species diversity. In addition, our study
expands on previous studies by demonstrating that species
invasion can alter compositional stability by increasing the rate
of species turnover.
Recent analyses indicate that species diversity is rarely a
primary driver of community stability, when stability is defined as
changes in species richness [1,14]. Although communities with
high species diversity may be more resistant to invasion and have
higher compensation potential following local extinction events,
species invasion and extinction are more likely to cause larger
species losses in species rich than in species poor communities
[1,10,18]. In contrast, a growing body of evidence suggests that the
structure of species interactions is a critical determinant of stability
in natural communities [3,16]. Thus, in our aridland ecosystem,
lower community stability is likely due to changes in the structure
of species interactions due to the replacement of a competitive
grass with a predominantly facilitative shrub, rather than to initial
differences in species diversity in invaded vs. non-invaded areas,
and to species loss after Larrea invasion.
Our long-term field observations demonstrated that the
compositional stability of subdominant communities depended
Table 1. ANCOVA results for the effects of vegetation type (site) and cover of Larrea and Bouteloua on the population and
community stability of subdominants.
Estimates Whole model
Termd.f. Estimate SEt -ratio
Population stabilityIntercept0.4540.009 50.29
site x cover1 0.001 0.00023.03 0.002
Cover stabilityIntercept1.283 0.06121.1
site1 0.0590.028 2.050.041
site x cover1 0.001 0.0010.85 0.394
Compositional stability (log)Intercept0.04 0.0950.42 0.672 0.14
site x cover1
Turnover Intercept0.63 0.013 46.05
site x cover1 0.0010.0003 2.740.006
Colonization Intercept0.331 0.0136.72
site x cover1 0.0007 0.00022.85 0.004
Extinction Intercept0.297 0.00740.62
site x cover1 0.00020.0002 1.40.163
Compositional stability was logarithmically (log) transformed.
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on the interactions exerted by the dominant species. Thus our
results confirm predictions from theoretical and empirical research
suggesting that community stability is often determined by one or
a few species that have strong effects on the structure of
interspecific interactions within a community [12,52,53]. Indeed,
prior research in grasslands demonstrates that patterns of species
dominance may be better predictors of compositional stability
than total cover . Furthermore, theoretical work indicates that
incompetitive communities (e.g.,primary producers) the structure of
interspecific interactions is the most relevant factor determining
community stability . In particular, community models reveal
that the mean and variance of interspecific interactions are decisive
parameters that dictate community stability . Specifically, these
models show that competitive communities are destabilized when an
invading species has a large effect on the mean and variance of
interspecific interactions within a community [14,17]. For instance,
in communities where weak interactions predominate, the invasion
of a strongly interacting species causes large deviations in the mean
and variance of overall interspecific interactions and leads to species
loss. Although we do not evaluate interspecific interactions in
Bouteloua- and Larrea-dominated communities in detail, other work at
SNWR has shown that Bouteloua competes with subordinate species
whereas Larrea facilitates subordinate species . Thus, our
empirical results are consistent with theoretical models that show
that lower community stability resulted from altered interspecific
interactions as a result of the replacement of a competitive by a
As consequence, we argue that in this aridland ecosystem, Larrea
invasion destabilized subdominant communities because Larrea
altered the structure of interspecific interactions. That is, the mean
and variance of the intensity of interspecific interactions of invaded
communities changed because Larrea facilitates subdominants
instead of competing with them as does Bouteloua  (Table 2,
Appendix S2). These interactions are themselves unstable (or
variable), however, because facilitation by shrubs has been shown
to have both positive and negative effects on subdominant
communities depending on environmental conditions [24,54].
During periods of extreme heat and drought shrubs facilitate
subdominants through water uplift and shade, whereas they
compete with subdominants under more favorable environmental
conditions [20,21,55]. In contrast, Bouteloua consistently competes
with subdominant species [19,38,56]. Hence, the broad range of
interactions exerted by Larrea on subdominant species may cause
drastic changes in the structure of interspecific interactions within
shrub-dominated areas which can decrease compositional and
community stability , while the steady competitive effects of
Figure 1. Linear regressions (solid lines) and means (dotted lines) of stability and species turnover of subdominant communities as
a function of the cover of Larrea and Bouteloua, at the Sevilleta National Wildlife Refuge, New Mexico, USA.
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Bouteloua on subdominants is likely to reduce species turnover rates
and enhance compositional stability. These divergent interactions
are further enhanced at our study site because nutrient
concentrations and soil moisture although comparable beneath
grass and shrub canopies have higher temporal variation beneath
the canopy of Larrea compared to Bouteloua , indicating that
subdominants beneath Larrea are subjected to higher temporal
heterogeneity of resources.
Processes responsible for community stability can be linked to
the temporal variation (i.e., stability) of different parameters at
both the community and population levels. Therefore, examining
population and community variation can yield insights into the
processes responsible for compositional stability in natural
communities. In particular, field and theoretical studies indicate
that in plant communities, high species diversity stabilizes
community productivity or cover due to population level instability
[2,7,13]. In our study, however, population and community
parameters for compositional stability were counter to those
predicted from these studies. In fact, Larrea and Bouteloua had
opposite effects on compositional and population stability, but
equally negative effects on the cover stability of subdominants as a
whole (Figure 1, Table 1). For instance, as cover of Bouteloua
increased, subdominants experienced increasing variability in total
community cover and population size, while the composition of
subordinate species remained stable through time. In contrast, as
cover of Larrea increased, total community cover and species
composition of subdominants were less stable, while population
stability was unaffected by cover of Larrea. In our system, only the
rates of species turnover (colonization and extinction) explained
the overall patterns of compositional stability of subdominant
communities (Table 1, Figure 1).
It is possible that ten years is not long enough to capture
ecologically meaningful links among species invasion/extinction,
compositional stability, and patterns of variation of population size
in this system. It is also possible that increased rates of species
turnover caused population instabilities that led to species
extinctions some time in the past. In such case, the present
patterns of compositional stability would be relicts of past
community dynamics, and do not currently influence other aspects
of community stability. Due to the lack of information on
community and population dynamics during the time when Larrea
invasion occurred at SNWR, our results add to the continuing
conundrum regarding how population variability contributes to
different aspects of community stability . However, these
findings support the idea that species interactions and composition
play a critical role in defining the stability-diversity relationship
 and the role of biotic interactions in plant invasions .
Therefore, interspecific interactions imposed by dominant species
may play a more critical role in defining community stability than
previously realized .
Although long-term observational data necessarily limits the
identification of causes and effects due to the correlative nature of
many processes in nature, such data help to identify factors that
may be drivers of key ecological processes. In our study, we can
not reject the hypothesis that changes in compositional stability
result from loss of species diversity at the SNWR. However,
diversity per se appears to be a secondary driver of community
stability in this system and diversity is itself subject to variations in
Table 2. Means, Standard Errors, and estimates of linear regression of community stability, structure, and species richness of
subdominant species as a function of the cover of the dominant species, Larrea tridentata and Bouteloua eriopoda over a 10 year
period at the Sevilleta NWR.
Stability Mean (SE)Equationr2
Pn Mean (SE)Equationr2
Population stability0.43(0.01)y=0.42+0.0003x 0.020.22892 0.44(0.004) y=0.4820.001x0.13
Community stability 1.16(0.05)y=1.3020.01x 0.080.00890 1.04(0.03)y=1.2720.01x0.09 0.002 100
Compositional stability (log)3.95216(0.09)y=0.3520.011x0.17
,0.001 91 1.10215(0.04)y=20.27+0.01x 0.060.013 100
Probabilites of turnover0.57(0.01)y=0.54+0.001x
0.06 0.018 910.68(0.007)y=0.7220.001x 0.040.037100
Probabilities of colonization 0.29(0.01) 0.050.03 930.36(0.004) y=0.3920.001x0.07 0.008100
Probabilities of extinction 0.27(0.01)0.01 0.257930.32(0.004)y=0.3320.0004x 0.01 0.243 100
Structure and richness
Cover according to life forms
,0.001 921.34(0.04) y=1.6620.01x0.12
Grasses (sqrt)1.58(0.12) 0.0010.8375 1.64(0.06)y=1.8720.01x 0.020.145 97
Shrubs (sqrt)1.43(0.095) 0.060.058 571.15(0.05) y=1.5520.01x0.090.002 95
Cover according to life history
Summer annuals (sqrt)1.03(0.06) y=0.86+0.01x
0.080.007 881.51(0.04) y=1.7620.01x0.060.018 100
Winter annuals (sqrt) 1.08(0.06) 0.39
,0.001 91 1.03(0.04)y=1.1920.01x 0.03 0.11399
Perennials (sqrt)1.5(0.09) 0.010.319831.45(0.04) y=1.8820.01x 0.16
Cover according to distribution
Restricted (log) 0.51(0.07)y=0.25+0.01x
0.270.002 33 0.34(0.04)y=0.4720.004x 0.030.218 52
Shared (log) 0.43(0.03)0.070.01 92 0.5(0.01)y=0.6520.01x0.22
Species richness 2.38(0.15) 0.06 0.089934.73(0.11)y=6.3320.05x 0.33
Cover dominant species 30.08(3.15)93 31.07(1.26)100
Some of the variables were logarithmically (log) or square root (sqrt) transformed.
Shrub Invasion and Stability
PLoS ONE | www.plosone.org6 June 2008 | Volume 3 | Issue 6 | e2332
community stability and environmental factors . In conclusion,
our long-term analysis of population and community dynamics
demonstrated that Larrea invasion into Bouteloua-dominated
grassland reduced cover of the dominant C4 grass and total
species richness, as well as total cover and community stability of
subordinate species. These changes, in concert with other factors,
such as changes in plant chemical compounds , altered niche
differentiation at the plant scale [19,23,59] and decreased niche
dimensionality at the community scale due to shifts in soil resource
availability , may contribute to biodiversity loss as native C3
shrubs invade areas formerly dominated by long-lived perennial
C4grasses . To the extent that cover reflects abundance, lower
cover leads to lower biodiversity by increasing the probability of
local extinction through both stochastic (drift) and deterministic
(competition) mechanisms [2,61]. As global change continues to
create conditions that favor C3shrubs over C4grasses in arid and
semiarid ecosystems worldwide , compositional stability will
likely decline further through altered species interactions ,
increased environmental variability, and altered disturbance
cycles. Therefore, loss of compositional stability may be an
increasingly important cause of species loss during biological
invasions. In addition, long-term studies of the consequences of
invasion by native species can serve as valuable surrogates to
understand the impacts of invasion by non-indigenous species on
community structure and function under future climate scenarios.
(site) and cover of Larrea and Bouteloua on the cover of functional
groups, and species richness of subdominant plant communities.
Found at: doi:10.1371/journal.pone.0002332.s001 (0.09 MB
ANCOVA results for the effects of vegetation type
lines) of the 10-year mean cover and species richness of
subdominants as a function of the cover of Larrea and Bouteloua
at the Sevilleta NWR.
Found at: doi:10.1371/journal.pone.0002332.s002 (0.07 MB TIF)
Linear regressions (solid lines) and means (dotted
We thank James H. Brown, William Pockman, Katie N. Suding, David
Tilman and Emilio Bruna for comments on earlier versions of the
Conceived and designed the experiments: SC SB. Performed the
experiments: SC SB. Analyzed the data: SB. Contributed reagents/
materials/analysis tools: SC SB. Wrote the paper: SC SB.
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