OIKOS 106: 253 /262, 2004
Dominance not richness determines invasibility of tallgrass prairie
Melinda D. Smith, Julia C. Wilcox, Theresa Kelly and Alan K. Knapp
Smith, M. D., Wilcox, J. C., Kelly, T. and Knapp, A. K. 2004. Dominance not richness
determines invasibility of tallgrass prairie. / Oikos 106: 253 /262.
Many recent studies suggest that more diverse communities are more resistant to
invasion. Community characteristics that most strongly influence invasion are
uncertain, however, due to covariation of diversity with competition and crowding.
We examined separately the effects of species richness and dominance on invasion by
an exotic legume, Melilotus officinalis, in intact, native Kansas grassland. We
manipulated dominance of C4 grasses by reducing their abundance (i.e. ramet
densities) by /25 and 50%. In addition, richness was reduced by removing species
that were mainly rare and uncommon as might be expected with environmental changes
such as drought and fragmentation. In both years of the study (2001 /2002),
invasibility, measured as peak establishment of Melilotus, was not affected by a 3fold reduction in species richness, nor was there an interaction between loss of species
and reduced dominance on invasion. In contrast, reductions in abundance of the
dominants significantly reduced invasibility of the grassland plots in both years.
Because the abundance of dominants was highly correlated with measures of
competition (i.e. ratio of dominant biomass to total biomass) and crowding (total
stem densities), this pattern was opposite to that expected if competition were indeed
limiting invasion. Rather, invasion appeared to be facilitated by the dominant species,
most likely because reduced dominance increased environmental stress. Our results
suggest that dominance is the key community characteristic determining invasibility,
because highly competitive and space-filling species can either enhance or reduce
susceptibility to invasion depending on whether dominants create a more competitive
environment or alleviate stressful conditions.
M. D. Smith, National Center for Ecological Analysis and Synthesis, Santa Barbara, CA
93101, USA. Present address: Dept of Ecology and Evolutionary Biology, Yale Univ., PO
Box 208105, New Haven, CT 06511, USA (melinda.smith@yale.edu). / J. Wilcox, T.
Kelly and A. K. Knapp, Division of Biology, Kansas State Univ., Manhattan, KS 66506,
USA.
Few ecosystems remain free from the threat of invasion
by exotic (non-native, alien) biota (Vitousek et al. 1997),
but there is ample evidence that ecosystems and communities differ considerably in their susceptibility to
invaders (i.e. invasibility; Crawley 1987, Rejmanek 1989,
Levine and D’Antonio 1999). Identifying the underlying
mechanisms that explain this variation has emerged as a
focal area of ecological research (Naeem et al. 2000).
Recent research suggests that invasibility of plant communities can be influenced by a number of factors,
including traits of invading species (Grotkopp et al.
2002), abundance of natural enemies (Maron and
Montserrat 2001, Keane and Crawley 2002), disturbance
and resource availability (Hobbs and Huenneke 1992,
Davis et al. 2000), and propagule supply (Tilman 1997,
Smith and Knapp 2001). However, the relative importance of these factors in determining invasion can vary
considerably at different stages of the invasion process
(Prieur-Richard et al. 2000) and at different spatial scales
(Stohlgren et al. 2003) making generalizations difficult.
An alternative approach to understanding invasibility
has focused on characteristics of the invaded plant
Accepted 4 December 2003
Copyright # OIKOS 2004
ISSN 0030-1299
OIKOS 106:2 (2004)
253
community, particularly the role of diversity in influencing invasion. Diverse plant communities are hypothesized to be more resistant to invasion due to more
intense competition and fewer resource opportunities
that result from more complete resource use (Elton 1958,
Shea and Chesson 2002). Both theory and experiments
provide support for this hypothesis by demonstrating
greater resistance to invasion with increasing diversity
(Knops et al. 1999, Stachowicz et al. 1999, Tilman 1999,
Levine 2000, Naeem et al. 2000, Prieur-Richard et al.
2000, Symstad 2000, Lyons and Schwartz 2001, Moore
et al. 2001, Kennedy et al. 2002, Troumbis et al. 2002)
and evenness (Wilsey and Polley 2002) at small spatial
scales and in relatively homogeneous systems. However,
an opposite pattern or a lack of relationship also has
been found in other experiments (Robinson et al. 1995,
Burke and Grime 1996, Palmer and Maurer 1997,
Crawley et al. 1999, Lavorel et al. 1999, Foster et al.
2002) and in observational studies at larger spatial scales
(Knops et al. 1995, Wiser et al. 1998, Lonsdale 1999,
Smith and Knapp 1999, Stohlgren et al. 1999, Levine
2000).
These conflicting patterns may result from uncontrolled (extrinsic) factors (e.g. disturbance, productivity,
resource availability, or size of the species pool), which
covary with diversity and confound observed patterns of
invasion in observational studies and in some experiments (Levine and D’Antonio 1999, Naeem et al. 2000,
Wardle 2001, Shea and Chesson 2002). An alternative
explanation may be that extrinsic factors are more
important than diversity in determining invasion at
larger spatial scales (Kennedy et al. 2002). At smaller
spatial scales where extrinsic factors are held constant
(i.e. plot or field), biotic factors appear to be most
important for invasion by defining the competitive space
that potential invaders encounter (Kennedy et al. 2002)
or by controlling resource opportunities that invaders
can exploit (Shea and Chesson 2002). Indeed, experiments in which diversity is directly manipulated and
extrinsic factors are controlled suggest that more diverse
communities are more resistant to invasion due to
increased neighborhood crowding and competition for
limiting resources (Stachowicz et al. 1999, Naeem et al.
2000, Kennedy et al. 2002).
Wardle (2001) argued that variation in dominant
species abundance could explain conflicting relationships observed between diversity and invasion in grasslands, a focal system of many recent invasion studies. In
highly productive sites where dominance is strong,
dominant species competitively exclude sub-ordinate
(uncommon and rare) species and suppress potential
invaders. This could produce a positive relationship
between diversity and invasion when sites that vary in
productivity and abundance of dominant species are
included in examinations of invasibility, a scenario
common to observational studies (Levine and D’Anto254
nio 1999, Smith and Knapp 1999). Naeem et al. (2000)
suggested that experimental studies in which diversity is
directly manipulated and that are conducted at smaller
spatial scales remove such covariation. However, these
experiments often employ synthetic communities comprised of random assemblages of species. Here the
chance of including the most competitive or space-filling
species increases as diversity increases (i.e. the sampling
effect, Wardle 2001). As a result, crowding and competition covary with diversity (Kennedy et al. 2002), making
it difficult to separate the effects of diversity per se from
such biotic factors as the composition or abundance of
resident species. Thus, despite evidence for diversity,
competition and crowding increasing resistance to invasion at smaller spatial scales, it is still unclear which of
these community characteristics most strongly influences
invasibility.
The goal of this study was to examine the effects of
species richness and dominance (abundance of competitive species) on invasibility while holding constant
extrinsic factors, such as propagule supply, productivity
and disturbance. We measured invasion by an exotic
legume species, Melilotus officinalis L., in response to
experimentally manipulated levels of plant species richness and dominance in intact, mesic grassland (tallgrass
prairie) plots over a two-year period. The capacity of
tallgrass prairie plant communities to respond rapidly to
a variety of perturbations (Tilman and El Haddi 1992,
Steinauer and Collins 1995, Turner and Knapp 1996)
makes it an excellent system to examine the immediate
consequences of manipulations of richness and dominance. In addition, these communities are characterized
by steep rank-abundance relationships, where a few C4
grasses species dominate (control resources and competitively exclude other species) and a suite of sub-ordinate
C3 grass and forb species comprise a bulk of the diversity
but are generally found at lower abundance (Collins and
Steinauer 1998, Smith and Knapp 2003).
By directly manipulating the dominance, but not the
presence, of key C4 grasses, we controlled crowding and
competition effects that covary with diversity, and
examined these effects on invasion independent of other
community characteristics. We also manipulated species
richness by eliminating plant species from experimental
plots based inversely on their probability of occurrence
in the community, so that rare and uncommon species
were removed before more abundant species. Drought,
fragmentation or altered disturbance regimes are known
to cause this type of nonrandom loss of species from
grasslands (Smith and Knapp 2003). Thus, our intent
was to simulate a realistic pattern of species loss in the
field. This experimental design also allowed us to
decouple diversity effects from the sampling effect
(Wardle 2001), because the most common and dominant
species were never lost from the communities and were
present at all richness levels (Smith and Knapp 2003).
OIKOS 106:2 (2004)
We hypothesized that both decreased richness and
reduced dominance would increase invasion. We expected that the loss of less common and rare plant
species from grassland plots would increase susceptibility to invasion by creating open niches and making
available resources previously unavailable (MacArthur
1972, Davis et al. 2000), whereas communities with lower
abundance of dominant species would be less competitive and crowded and thus less resistant to invasion by
non-resident species. However, we expected that reduced
dominance, by more strongly influencing resources
(light, space) important for establishment at local scales
(Foster et al. 2002), would more strongly impact invasion
than richness. We used seedling establishment as our
measure of invasibility of the experimental plots.
Although seedling germination and establishment are
only the first steps in the invasion process (Crawley
1987), there is strong evidence for this phase of life
history, rather than survivorship, being of critical
importance in regulating species distributions and invasion in grasslands (Gross and Werner 1982, Foster 1999,
2002).
Methods
In May 2000, we established 72 1.0/0.5 m plots on a
relatively homogeneous upland site in an infrequently
burned (/every 4 years) watershed at the Konza Prairie
Biological Station (Manhattan, KS, USA). The site had
been burned several weeks prior. Plots were laid out in a
grid with rows of plots separated by 1 m, and plots
within rows separated by 0.5 m. Vegetation of the site
was dominated by a few warm-season (C4) grass species,
mainly Andropogon gerardii Vitman and Sorghastrum
nutans (L.) Nash and to a lesser extent Bouteloua
curtipendula (Michx.) Torr., A. scoparius Michx. and
Panicum virgatum L. Numerous less common and rare
warm- and cool-season (C3) grass and forb species made
up the bulk of the diversity. Exotic species are relatively
uncommon at Konza Prairie, comprising only /9% of
the species in grassland sites (Smith and Knapp 1999),
and were absent from the study site.
Dominance and richness treatments
We determined levels of the dominance and richness
treatments by first censusing the number of ramets
(individual tillers) of each species in each plot at the
beginning of the 2000 growing season. The range in
dominance, i.e. total ramet densities of the three most
abundant C4 grasses (A. gerardii , S. nutans and B.
curtipendula ), for the plots was 173 to 519 ramets
(average: 368.99/87.5 SD). Average richness for the plots
was 14.1 (9/2.0 SD) with a minimum of 10 species and a
OIKOS 106:2 (2004)
maximum of 18 species per plot. Based on these
community characteristics, we selected dominance treatments of high (ambient), medium (/25% reduction in
dominance) and low ( /50% reduction). Richness treatments selected were 13 /16 species (ambient), 10 /12, 7 /
9, and 4 /6 species for the most depauperate community.
Only six plots had less than 12 species, thus richness was
reduced to some extent in all the plots.
Our assignment of the dominance and richness
treatments to the plots (12 treatment combinations
each with 6 replicates) was stratified and semi-random
due to constraints of natural richness and dominance.
We first divided the plots into the three dominance
levels, so that high (ambient) dominance plots had
at minimum 307 stems of the dominant C4 grasses
(average: 429.29/53.8 SD), medium dominance plots had
at minimum 243 stems (average: 378.59/69.2 SD),
and low density plots had at minimum 173 stems
(average: 298.99/83.3 SD). Each had similar initial
average richness: high / 14.8 (range: 10 /18 species),
medium / 13.6 (10 /18), and low / 13.9 (12 /17). For
each dominance treatment, six plots (of /15 plots per
dominance level) that had 13 or greater species were first
randomly assigned to the high (13 /16 species) richness
treatment. The remaining plots were then assigned
randomly to the other richness treatments. This resulted
in an average initial richness of 15.4 (1.4 SD; range:
13 /18) for the 13 /16 richness treatment, 13.6 (1.7 SD;
range: 10 /17) for the 10 /12 richness treatment, 14.3 (2.4
SD; range: 10 /18) for the 7 /9 richness treatment, and
13.1 (1.7 SD; range: 11 /18) for the 4 /6 richness
treatment. Thus, for all but the ambient (high) richness
treatment, average initial richness did not differ significantly among the richness reduction treatments. For
each richness treatment, plots were then randomly
assigned to a particular target richness value.
In early June 2000, we reduced dominance and
richness in the plots by clipping individuals and precisely
applying herbicide (glyphosate) to the cut stem. This
minimized non-target effects to neighbors. Dominance
was reduced by permanently removing stems of mainly
A. gerardii , S. nutans and B. curtipendula . In general,
A . gerardii comprised 80% of the stems removed and
S. nutans and B. curtipendula comprised 10 /20% of
stems removed. These proportions reflected natural
abundance patterns observed in the plots (data not
shown). For the richness treatments, plant species were
selected for removal based on their frequency of
occurrence at the study site; those that occurred least
frequently were removed first and species that were more
frequent were removed until a target richness level was
achieved. Thus, the dominant and most frequent species
were never removed completely from the study plots
(Smith and Knapp 2003).
After all treatments were applied, ramet densities were
recensused to confirm treatment levels. We reduced C4
255
grass ramet densities in low dominance plots by 47%
(average 137.79/76.9 SD ramets per plot) and in medium
dominance plots by 23% (average 86.39/15.4 SD ramets
per plot). In 2001, the plots required only minimal
adjustments of the richness levels, and although total
densities increased, the dominance treatments remained
in place (Fig. 1A). At the end of the growing season (late
August to mid September) each year, we harvested, dried
and weighed all aboveground biomass of each species
separately. Relative dominance, an estimate of competition in each plot, was calculated as ratio of the sum endof-season biomass of the three dominant C4 grasses, A.
gerardii , S. nutans and B. curtipendula to total aboveground biomass.
Invasibility
We added seeds of Melilotus officinalis (Fabaceae), an
annual/biennial legume native to Eurasia, to plots to
assess invasibility. Only one target invader was used to
avoid confounding effects of competition that could
occur between multiple target invaders. Melilotus invades grasslands throughout the Great Plains (Great
Plains Flora Association 1986) and is considered a
noxious weed in some areas. We chose Melilotus
because, although found often at high local abundance
in scattered populations on Konza Prairie (Smith and
Knapp 1999, 2001), it was absent from the study site.
Commercially grown seeds (Sharp Bros. Seed Co., Healy,
KS) of uniform quality were used each year.
In June 2000, seeds of Melilotus were sown into each
plot at a density of 300 seeds plot 1 (equivalent to
average 0.58 g plot 1). In preliminary laboratory tests,
we obtained germination rates in excess of 60% in
microcosms (4/50 /75 cm) consisting of blocks of
intact, burned grassland sod obtained from the field (M.
D. Smith unpubl.). Water (equivalent to 1 cm of rain)
was gently added to all plots immediately after seed
additions and three additional times during the first
three weeks to stimulate germination. To determine if
variation in propagule supply and spring environmental
conditions altered patterns of invasion, seeds of Melilotus were added earlier (26 April) and at a higher density
(600 seeds plot 1, equivalent to average 1.26 g plot 1)
in 2001. Again, all plots were watered immediately after
seed additions and five more times over the next two
weeks to enhance germination. The number of seedlings
that established was measured on 22 June, 2 July, 13 July,
23 August 2000 and on 24 May, 24 June, 7 July and 3
September 2001. Seedlings were not allowed to persist
beyond establishment to avoid unwanted introductions
to the site. Peak establishment (2000: 22 June, 2001: 24
May) was used as the measure of invasibility in
subsequent analyses.
256
Since light is an important aboveground resource
limiting growth and establishment in tallgrass prairie
(Knapp and Seastedt 1986, Foster and Gross 1998,
Foster 2001), light penetration (photon flux density;
PFD) through the canopy was measured each time
seedlings were counted. Using a Sunfleck ceptometer
(Decagon, Pullman, WA), PFD was quantified above the
canopy and at the soil surface in two locations in each
plot on cloudless days. Light penetration at peak invader
establishment was used as a measure of resource
availability in the plots in subsequent analyses.
Statistical analyses
Effects of dominance and richness treatments on light
availability, dominance (ramet densities of three dominant C4 grasses), and establishment of Melilotus seedlings were evaluated using two-way ANOVA for each
year separately, and LSD tests detected differences in
treatment means (Ott 1993). Simple linear regressions
also were used to assess invasibility across the continuum
of richness and C4 grass ramet densities across the plots.
For 2000 and 2001, separate multiple regression (backward elimination) analyses were used to evaluate the
dependence of invasibility on richness, total ramet
density (a measure of available space at the time of
invasion), dominance, relative dominance, and light
availability. The extent of collinearity was evaluated
since there were inter-correlations among some of the
predictor variables. Acceptable levels were judged using
the following criteria: condition indexB/30, variance
inflation B/1/(1/r2), and variance proportions not close
to one. All analyses were performed using SAS statistical
software (SAS version 8.1, SAS Institute, Inc. 1999).
Significance levels were set at P5/0.05.
Results
Dominance and light availability
Experimental manipulations of dominance effectively
reduced total ramet densities of the three most abundant
C4 grasses in 2000, with these reductions persisting in
2001 (Table 1, Fig. 1A). The dominance reductions were
independent of the richness reduction treatments (Table
1). Dominance was similar across all richness levels,
except in 2001 where total C4 ramet densities differed
with richness only for the low dominance treatment.
Dominance was significantly lower in plots with 10 /12
or 13 /16 species when compared to those with only 4 /6
species (data not shown). The dominance reduction
treatments significantly altered light availability at the
soil surface (Table 1). Light penetration increased by
/25% and 50% over that for the high dominance plots
with equivalent reductions in ramet densities of the C4
OIKOS 106:2 (2004)
Table 1. Results from ANOVAs of effects of dominance (D) and richness (R) treatments on dominance (C4 grass ramet density) and
light penetration (%) in tallgrass prairie for (A) 2000 and (B) 2001.
Source
A) 2000
Model
D
R
D/R
Error
B) 2001
Model
D
R
D/R
Error
df
Dominance
Light penetration (%)
MS
F
P
F
MS
P
11
2
3
6
60
98067.58
522063.04
1720.27
11576.08
5938.31
16.51
84.55
0.29
1.95
B/0.001
B/0.001
0.83
0.09
500.87
2259.47
33.78
148.52
97.45
5.14
23.19
0.34
1.52
B/0.001
B/0.001
0.80
0.19
11
2
3
6
60
102873.38
475060.50
4468.68
28013.32
10282.38
10.00
46.20
0.43
2.72
B/0.001
B/0.001
0.73
0.02
254.44
842.83
93.84
138.61
112.02
2.27
7.52
0.84
1.24
0.02
0.001
0.48
0.30
grasses (Fig. 1B). In 2001, light availability was still
higher with reduced dominance, however differences
between the dominance treatments were not as
pronounced (Fig. 1B). Light availability was not affected
by the richness treatments in either year of the study
(Table 1).
Invasibility
Maximum establishment of Melilotus seedlings, our
measure of community invasibility, was not affected by
reduced richness in either year of the study regardless of
whether data were analyzed with richness as a categorical
variable or with realized (measured) levels of richness
(Table 2, Fig. 2). These results indicate that invasion was
independent of the number of resident species in these
communities. In contrast, community invasibility was
significantly affected by reduced dominance (Table 2). In
2000, the low density plots had /30% fewer Melilotus
seedlings establishing than the moderate or high dominance plots (Fig. 3A). A similar pattern of invasion was
observed in 2001 (Fig. 3B), although the reduction in
Melilotus establishment in low dominance plots was not
significant (Table 2). Because there was a continuum of
total dominant density for the plots the data were also
analyzed with dominance as an independent regression
variable (Fig. 3C, D). Results were consistent with the
ANOVA analyses. The number of Melilotus seedlings
establishing declined significantly with decreasing abundance of the dominant C4 grasses in both years (2000:
F1,70 /26.13, PB/0.001; 2001: F1,70 /6.26, P /0.01).
Invasibility also was correlated with other biotic and
abiotic factors that we measured. Establishment of
Melilotus increased with increasing total ramet densities
for the plots (2000: r /0.45, PB/0.001; 2001: r/0.40,
PB/0.001), and with increasing competition, as measured as the ratio of end-of-season C4 grass biomass to
total biomass (2000 only: r /0.32, P/0.006), but
decreased with increasing light availability (2000: r /
/0.46, PB/0.001; r/ /0.32, P/0.006). All of these
measures also were significantly correlated with dominance, but only competition was correlated with richness (2000, r / /0.38, PB/0.001; 2001, r/ /0.31, P/
0.007). Light availability decreased (2000, r / /0.62,
PB/0.001; 2001, r / /0.51, PB/0.001), whereas competition (2000, r /0.65, PB/0.001; 2001, r/0.64, PB/
0.001) and crowding, i.e. total ramet densities (2000:
r /0.85, P B/0.001; 2001: r/0.74, PB/0.001), increased
with dominant density.
Fig. 1. Effects of dominance
treatments on (A) dominance,
i.e., ramet densities of three
most abundant C4 grasses
(Andropogon gerardii ,
Sorghastrum nutans and
Bouteloua curtipendula ), and
(B) light penetration (%) at the
soil surface in grassland plots in
2000 (closed bars) and 2001
(open bars). Each year was
analyzed separately. Different
letters indicate significant
(P B/0.05) differences among
treatments within a year.
OIKOS 106:2 (2004)
257
Table 2. Results from ANOVAs of effects of dominance (D) and richness (R) treatments on establishment of an invading legume,
Melilotus officinalis, in tallgrass prairie for 2000 and 2001.
Source
Model
D
R
D /R
Error
df
11
2
3
6
60
2000
2001
F
MS
P
F
MS
P
516.86
1876.68
173.07
235.48
156.65
3.30
11.98
1.10
1.50
0.001
B/0.001
0.35
0.19
352.37
1083.58
43.15
263.25
321.28
1.10
3.37
0.13
0.82
0.38
0.04
0.94
0.56
In multiple regression (backward elimination) analyses, we included the following as predictors of invasibility: realized richness, total ramet density as an index
of community crowding, dominance (C4 grass ramet
density), relative dominance (ratio of end-of-season C4
biomass to total biomass) as an estimate of competition,
and light availability. Light availability was not included
in the final model due to collinearity (a variance inflation
index/1/(1/r2) and variance proportion close to one).
For 2000, total ramet density and relative dominance
were retained as significant predictors of invasion by
Melilotus (Table 3). The parameter estimates for both
were positive, which suggests that community crowding
and competition by the dominant C4 grasses each had
independent and direct positive effects on community
invasibility. In 2001, only total ramet density was
retained as a significant predictor of Melilotus establishment. As in 2000, the parameter estimate for total ramet
density was positive, indicating that crowding was the
best predictor of invasibility.
Discussion
We examined the effects of species richness and dominance (abundance of highly competitive species) on
invasibility of tallgrass prairie by separately manipulating these community characteristics in intact grassland
plots. Counter to our expectations, establishment by the
invading legume Melilotus officinalis, our measure of
Fig. 2. Number of Melilotus
officinalis invaders in response to
richness reduction treatments (A /
B) and as a function of realized
richness in the plots (C /D) in 2000
(left) and 2001 (right). Note that
twice as many seeds were added to
plots in 2001 vs 2000. Closed
circles/4 /6 richness treatment,
open triangles/7 /9 richness
treatment, closed squares /10 /12
richness treatment, open
diamonds/13 /16 richness
treatment.
258
OIKOS 106:2 (2004)
Fig. 3. Number of Melilotus officinalis invaders in response to dominance reduction treatments (A /B) and as a function of total C4
grass (dominant) density in the plots (C /D) in 2000 (left) and 2001 (right). Note that twice as many seeds were added to plots in
2001 vs 2000. Closed circles/low dominance treatment, open triangles/medium dominance treatment, closed squares/high
invasibility in this system, was not influenced by nonrandom species loss in either year of the study. We found
Table 3. Parameter estimates from backward elimination multiple regression of the dependence of Melilotus establishment
(invasibility) on realized richness, total ramet density as an
index of crowding, dominance (total C4 ramet density), relative
dominance as a measure of competition (ratio of end-of-season
biomass of dominant C4 grasses to total biomass), and % light
penetration in 2000 and 2001. Final models: 2000/r2 /0.28,
F2,70 /13.27, P B/0.001; 2001/r2 /0.16, F1,70 /13.00,
P B/0.001.
Predictor variable
A) 2000
Realized richness
Total ramet density
Dominance
Relative dominance
Light penetration (%)
B) 2001
Realized richness
Total ramet density
Dominance
Relative dominance
Light penetration (%)
OIKOS 106:2 (2004)
Invasibility
Parameter
estimate
Partial
correlation
P
eliminated
0.03 (0.008)
eliminated
15.03 (7.86)
eliminated
/
0.49
/
0.20
/
/
B/0.001
/
0.06
eliminated
0.03 (0.009)
eliminated
eliminated
eliminated
/
0.40
/
/
/
/
B/0.001
/
/
/
that establishment remained constant despite a 3-fold
loss in the number of uncommon and rare species. This
result contrasts with other studies that also have directly
manipulated richness (Knops et al. 1999, Stachowicz et
al. 1999, Levine 2000, Naeem et al. 2000, Prieur-Richard
et al. 2000, Symstad 2000, Kennedy et al. 2002,
Troumbis et al. 2002; but see Palmer and Mauer 1997,
Crawley et al. 1999, Lavorel et al. 1999). The negative
relationship between richness and invasion observed in
these studies may be, in part, due to increased resource
use, density and crowding with increasing richness
(Stachowicz et al. 1999, Naeem et al. 2000, Dukes
2001, Kennedy et al. 2002), but also may be due to the
sampling effect (Wardle 2001), in which the chance of
including a highly productive, space-filling, or more
competitive species increases as richness increases.
Although the sampling effect is thought to be a likely
process in communities shaped by stochastic assembly
processes (Tilman et al. 1997), it is also viewed as an
artifact of experiments that manipulate community
richness by constructing synthetic communities comprised of random assemblages of species (Huston 1997,
Wardle 1999, 2001).
Our study differed from these studies because we
removed species based on their abundance in the
259
community. This allowed us to maintain the most
dominant and productive species in all communities,
irrespective of richness. In addition, the chance of
including or excluding a more or less productive, rare
and uncommon species was not related to community
richness. Thus, the invasion patterns we observed were
not influenced by the sampling effect (Smith and Knapp
2003). The lack of change in invasion resistance across
the gradient of community richness we created, as well as
the absence of an interaction between the richness and
dominance reduction treatments, suggests that resource
use, density and crowding were not related to richness in
our experimental communities. Indeed, we found that
light availability at peak Melilotus establishment, a key
measure of aboveground resource availability, and total
densities of all species were not affected by species loss.
Thus, richness does not appear to be an important factor
influencing invasion of intact, tallgrass prairie communities when factors known to covary with diversity are
held constant.
In a study comparable to ours, Lyons and Schwartz
(2001) found that loss of uncommon species decreased
invasion resistance of an alpine meadow. In these shortstatured, herbaceous communities, they hypothesized
that invasion resistance decreased with species loss due
to increased availability of limiting resources, particularly water, allowing for greater establishment by the
target invader, Lolium multiflorum Lam. In contrast,
uncommon and rare species in the present study did not
appear to have disproportionate effects on key limiting
resources for establishment, i.e. light or space availability, and thus their loss did not influence invasion
resistance in this mesic grassland, at least in the shortterm.
Although non-random loss of species had no impact
on invasibility, reduced dominance of the C4 grasses
significantly decreased invasion by Melilotus. This is
counter to the pattern we expected. We hypothesized
that reduced dominance would increase aboveground
resource availability and decrease competition and
crowding, which it did by increasing light availability
and decreasing relative biomass of C4 grasses and total
stem densities. However, these alterations did not result
in increased establishment of Melilotus. Other studies
have shown that susceptibility to invasion is strongly
influenced by abundance of dominant species and
regulated by light availability. For example, Burke and
Grime (1996) found that invasion was highest in grassland sites that were disturbed. These sites not only
contained the highest availability of bare ground (indicative of more available space and light) but also were
those with the greatest damage to the dominants. They
found that decreased abundance of the dominant species
as a result of their disturbance treatments was key in
determining invasion in their grassland plots. Similarly,
Robinson and colleagues (1995) found that annual
260
grassland communities with lower dominance (percent
cover) of Bromus diandrus, the primary dominant in
their study plots, as well as those subjected to greater
disturbance, were more susceptible to invasion. In a
successional Kansas grassland, invasion was found to be
strongly influenced by variation in light availability
across a natural gradient of plant diversity, with invasion
increasing as light availability increased (Foster et al.
2002). Invasion also was lowest in sites with low
diversity, which coincided with high dominance
by a few grass species. In contrast, Wilsey and Polley
(2002) found that invasion of synthetic grassland communities was enhanced with decreasing evenness
(i.e. increasing dominance), but could find no change
in resources among the different levels of evenness
manipulated. Instead, they suggested that invasion may
have been enhanced by within-plot variability in resources.
The decline in invasibility with decreasing dominance
that we observed may have occurred for several reasons.
The study site was burned at the beginning of the
growing season (mid- to late-March) in both years of the
study, and thus all plots lacked a litter layer. Light levels
in the high (ambient) dominance plots also were
relatively high (40%). Reduced abundance of the
dominants further increased light availability by as
much as 35%, perhaps to levels that negatively affected
establishment. The expected positive effects of increased
light and space availability resulting from reduced
dominance were likely offset by increased stress at the
time of establishment via increased canopy temperatures
and evapotranspiration, and decreased soil moisture
(Knapp and Seastedt 1986). Indeed, average light availability in the experimental plots (average: 46.3%,
range: 21.3 to 80.0%) was double that observed in
nearby, intact plots that were burned and varied
naturally in dominance (average: 24.5%, n/30 plots,
range: 7.5% to 45.5%; Smith 2002). Decreased density of
the dominant species also may have indirectly influenced
invasion by increasing the apparancy of Melilotus
seedlings to herbivores (Prieur-Richard et al. 2002).
We observed mortality of seedlings as a result of
herbivory in the study plots, but this was not directly
quantified.
Regardless of which of these mechanisms were operating, invasion was not affected by richness but was
facilitated by increased abundance of the dominant
species in the grassland plots. Recently, Bruno and
coauthors (2003) suggested that facilitation by dominant
species might be an important, yet unrecognized, process
influencing invasibility of communities. Under stressful
conditions, dominant species may facilitate rather
than compete with invading or colonizing species,
because they provide specific environmental conditions
or ameliorate harsh conditions. We hypothesize that
our manipulations of the dominant species made
OIKOS 106:2 (2004)
microclimatic conditions much harsher in low and
moderate density plots than the high (ambient) density
plots, and thus seedling establishment was facilitated by
increased abundance of the dominants. Moreover, we
observed similar patterns for natural recruitment of
native species into the plots as observed for invasion
by Melilotus ; richness of colonizing species increased
with dominance, but was unrelated to community
richness (Smith 2002). This suggests that increased
abundance of dominants also can facilitate establishment of a suite of other colonizing species in this
grassland.
Bruno et al. (2003) also point out that facilitation by
dominant species may change the shape of the relationship between richness and invasion by increasing
richness of both native and exotic species under harsh
conditions but then decreasing their richness when
competition becomes intense. Our results indicate that
variation in abundance of dominant species can strongly
influence invasion independent of community richness.
This suggests that for studies that manipulate richness by
randomly constructing communities or by randomly
removing species from intact communities, variation in
the presence and/or abundance of dominant species
could be driving observed relationships between richness
and invasion.
Finally, because we focused on establishment of the
invading species, it is unclear how important facilitation
will be during other stages of the invasion process.
Indeed, there is evidence that invasion resistance
can vary depending on the life stage of the invading
species (Prieur-Richard et al. 2000, Franzen 2001). In
our grassland communities, dominant species may have
an opposite effect on growth, persistence and reproduction of invaders once they are established, because
the negative effects of competition may outweigh any
positive effects related to improved microclimatic
conditions for establishment. Patterns of invasion
with natural variation in dominant species observed in
intact, grassland plots (Smith and Knapp 1999)
suggest that dominant species negatively affect persistence in invaders in the long-term. Thus, we contend
that dominant species and their effects on other community characteristics and resources are key in determining resistance, as well as susceptibility, of
communities to invasions. This emphasizes the need to
examine the role of dominant species as competitors or
facilitators in influencing diversity-invasibility relationships.
Acknowledgements / This work was supported by an
EPA STAR fellowship to MDS and the NSF LTER program.
This work was conducted as part of the Knowledge Network for
Biocomplexity Working Group supported by the National Center for Ecological Analysis and Synthesis, a Center
OIKOS 106:2 (2004)
funded by NSF (Grant #DEB-0072909). M. Lett, J. Rivers and
J. Gasper assisted with fieldwork. J. Blair, S. Collins,
D. Hartnett, J. Pontius and B. Wilsey provided helpful
comments on the manuscript.
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