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Keywords:

  • Centaurea maculosa;
  • community assembly;
  • diversity;
  • dominance;
  • evenness;
  • grassland;
  • niche complementarity;
  • seed addition

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  • 1
    Limiting similarity theory predicts that successful invaders should differ functionally from species already present in the community. This theory has been tested by manipulating the functional richness of communities, but not other aspects of functional diversity such as the identity of dominant species. Because dominant species are known to have strong effects on ecosystem functioning, I hypothesized that successful invaders should be functionally dissimilar from community dominants.
  • 2
    To test this hypothesis, I added seeds of 17 different species to two different experiments: one in a natural oldfield community that had patches dominated by different plant species, and one in grassland mesocosms that varied in the identity of the dominant species but not in species richness or evenness. I used indicator species analyses to test whether invaders had higher establishment success in plots with functionally different dominant species.
  • 3
    A large percentage of invader species (47–71%) in both experiments showed no difference in affinity across the different dominant treatments, although one-third of species did show some evidence for limiting similarity. Exotic invaders had much higher invasion success than native invaders, and seemed to be inhibited by dominant species that were functionally similar. However, even these invasion patterns were not consistent across the two experiments.
  • 4
    The results from this study show that there is some evidence that dominant species suppress invasion by functionally similar species, beyond the effect of simple presence or absence of species in communities, although it is not the sole factor affecting invasion success. Patterns of invasion success were inconsistent across species and experiments, indicating that other studies using only a single species of invader to make conclusions about community invasibility should be interpreted with caution.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Two questions are fundamental to understanding invasion: what makes a good invader and what makes a community invasible? (Richardson & Pysek 2006). Most studies investigating processes of invasion have focused on either characterizing traits of successful invaders (e.g. seed size or competitive ability; Rejmanek & Richardson 1996; Sharma et al. 2005) or characterizing attributes of a community that make it susceptible to invasion (e.g. species diversity or primary productivity; Shea & Chesson 2002; Levine et al. 2004). However, relatively few studies have considered aspects of both invaders and communities in unison in order to understand what types of invaders are most successful in different communities.

Despite the paucity of studies addressing both potential invaders and susceptible communities simultaneously, ideas concerning potential relationships between traits of successful invaders and invaded communities have a long history. In The Origin of Species, Darwin (1859) laid out his naturalization hypothesis, which predicted that invaders in the same genus as species already present in a system should be less likely to colonize successfully than species from different genera, owing to competition for similar habitats (Daehler 2001). Strauss et al. (2006) found this to be at least partially true for Californian grasses, where invasive species were less related to native species than were non-invasive exotic species. More recently, niche complementarity, ‘limiting similarity’ theory (Tilman 1982; Abrams 1983; Silvertown 2004) and rules of community assembly (Fox 1987) have been used as the bases of predictions that species already present in the community should suppress invasion by functionally similar species with similar resource requirements (Fargione et al. 2003; Tilman 2004; Von Holle & Simberloff 2004). In other words, communities may be invasible if they currently lack species that are ecologically similar to the invader (Lodge 1993; Stubbs & Wilson 2004).

Several recent studies have supported the hypothesis of limiting similarity for communities that are invaded by a single noxious invasive species (e.g. Naeem et al. 2000; Dukes 2001; Pokorny et al. 2005). Dukes (2002) found that the invasive species Centaurea solstitialis L. had the lowest establishment success in monocultures of a native forb that is functionally and phenologically similar to Centaurea. However, at least one study has found the opposite to be true, where the invasive species Conyza bonariensis (L.) Cronq. had greatest success in communities where the dominant resident species was functionally similar to the invader, possibly due to the dominant species ‘protecting’ seedlings from herbivory, with insects concentrated on the dominant species (Prieur-Richard et al. 2002). Other studies, using suites of native invaders instead of a single exotic invader, have also found mixed evidence for limiting similarity (Symstad 2000; Fargione et al. 2003; Von Holle & Simberloff 2004; Turnbull et al. 2005; Mwangi et al. 2007).

Although these mixed results could be due to an overly broad definition of plant functional groups (Petchey & Gaston 2006; Wright et al. 2006), it may be that the relative abundance, rather than simple presence or absence, of certain functional groups may be a useful predictor of invasibility. Very few studies have manipulated the relative abundance of resident species in studies of invasion (Wilsey & Polley 2002; Smith et al. 2004; Emery & Gross 2007), and even fewer have manipulated relative abundance of functional groups (Petchey & Gaston 2006). However, because dominant species in communities are known to have strong regulating effects on community structure and ecosystem function (McNaughton & Wolf 1970; Crawley et al. 1999), it is likely that dominant species in a community exert strong influences on the type of species that is a successful invader.

To examine relationships between traits of dominant species and successful invaders explicitly, it is necessary to design experiments that manipulate the identity of dominant species without altering species richness or disturbance intensity. Furthermore, because natural communities assemble differently than experimental communities (e.g. Ricklefs 2004), there is a need for parallel experiments in natural systems to document relationships between community composition and successful invader composition. In this study, I combined both approaches. The natural experiment used intact oldfield plant communities that had distinct vegetation patches differing in the identity of the dominant species. The second experiment involved constructed plant communities that varied in the identity of the dominant species, but not in species richness or evenness. To both systems, I added seeds of 17 plant species, spanning a range of origin and functional type, and tested the performance of these different invaders as a function of dominant species identity. Specifically, I asked the question: Are invaders that are functionally dissimilar to the dominant species more likely to succeed than other species? Importantly, this question can only be addressed when several species of invaders are added to communities simultaneously, an approach rarely taken in invasion ecology thus far. I also examined whether non-native species showed different patterns of invasion than native species, in order to frame results from this study in the context of other studies that have either added single exotic invader species or suites of native invaders. Results from this study provide some evidence for limiting similarity, but I conclude that other factors may be more important in predicting invasion success.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

experimental design: natural oldfields

In March 2004, I established 64 1 m × 1 m plots in an oldfield site at the Lux Arbor Reserve of Michigan State University's W.K. Kellogg Biological Station (KBS) in south-west Michigan, USA (Barry County; 42.2833°N, 85.2768°W), as part of a larger experiment (Emery & Gross 2006). This site was abandoned from agriculture 25 years prior, and had been mowed occasionally following abandonment. Within this site there were vegetation patches dominated by different species. I defined dominance as ≥ 40% relative cover of the dominant species in the plot based on visual estimates in August 2003 using a cardboard square that represented 2% cover for reference. I selected the four most dominant species – Andropogon virginicus L. (native C4 grass), Bromus inermis Leyss. (non-native C3 grass), Centaurea maculosa Lam. (non-native forb) or Solidago canadensis L. (native forb) and laid out 16 1 m × 1 m plots within each patch type (hereafter referred to by genus name). Patches ranged in size from 5 m2 to 50 m2, and were separated from other patch types by at least 5 m. I determined, among other measures, the species composition of each plot using visual estimates of percentage cover of each species (see Emery & Gross 2006 for more details). Due to haphazard animal disturbances and shifts in community composition from 2003 to the spring of 2004, nine of the 64 plots were excluded from all analyses.

experimental design: constructed community mesocosms

I conducted an outdoor mesocosm experiment at the KBS in south-west Michigan (Kalamazoo County; 42.2460°N, 85.2359°W) from July 2003 to September 2005. For both this experiment and the one described above, 2004 was slightly wetter (612 mm rain from April to August), while 2005 was slightly drier (362 mm rain from April to August), than the 30-year average for the area (465 mm, April–August). Both years had similar high average temperatures (24.9 °C and 26.7 °C) compared with the long-term average high (24.4 °C). Mesocosms consisted of buried pots (0.055 m3; 42 cm diameter × 40 cm deep) filled with a 3 : 2 mixture of local field soil (Kalamazoo sandy loam, Austin 1979) and sand in the ground inside a fenced area in July 2003. I placed a 2-cm layer of gravel in the bottom of each pot to aid drainage and buried the pots with the rim flush against the ground surface. All pots had 0.5-m walkways on all sides. I allowed the pots to settle, and then transplanted plugs to establish the treatments in September 2003.

As part of a larger experiment (Emery & Gross 2007), I created six unique mesocosm communities with six different dominant species common to Midwest US grasslands: Bromus inermis (non-native C3 grass), Coreopsis lanceolata L. (native forb), Elymus canadensis L. (native C3 grass), Panicum virgatum L. (native C4 grass), Schizachyrium scoparium Michx. (Nash) (native C4 grass) and Solidago nemoralis Ait. (native forb). Each mesocosm also contained two native legume species: Desmodium canadense (L.) DC. and Lespedeza capitata Michx. The six treatments varied in identity of the dominant species, but not in species richness (eight per pot), evenness (34 : 2 : 2 : 2 : 2 : 2 : 2 : 2 planting ratio) or number of individuals (48 per pot). Each treatment was replicated eight times (48 pots total). All plugs were grown from seed in the greenhouse during June 2003, and transferred outside under shade cloth and regularly watered in July and August. After transplanting, pots were regularly watered for 2 weeks to help establishment, and then left alone through the autumn and winter. In early May 2004, I weeded the pots to remove winter annual species that had germinated from the seedbank or from dispersal into the pots. Individuals were removed from pots by clipping stems with small scissors just below the soil surface to minimize soil disturbance.

seed addition

In March 2004, I added 50 seeds each of 17 different species (850 seeds total), all common to Michigan grasslands and oldfields (Rabeler 2001) and representing a variety of functional types and origin (Table 1) to a 0.25 m × 0.5 m subsection in each 1-m2 plot in the field experiment (the rest of the plot was undisturbed or used for other experiments), and 50 seeds of each species to each pot in the mesocosm treatment. I classified the invader species into three functional groups: C3 (cool season) grasses, C4 (warm season) grasses and forbs, based on phenology and growth form. Species were also classified as native or non-native to North America based on information from the USDA PLANTS Database (http://plants.usda.gov). Seeds were added in early spring (March) to take advantage of possible freeze/thaw requirements needed for germination. Seeds of native species were purchased from a native-plant grower in Minnesota (Prairie Moon Nursery, Winona, MN, USA), non-native grass seeds were purchased from a forage crop seed supplier (Michigan State Seed Solutions, Grand Ledge, MI, USA) and Centaurea maculosa seeds were collected from nearby fields. All species were also sown into outdoor common gardens to confirm seed viability. In August 2004, I determined which species had colonized each experiment by counting seedlings. For the natural experiment, I also censused seedlings in adjacent control subplots of the same size to quantify any natural invasion of the 17 species from the local community. I used the difference in seedling number between seed-added and control plots to quantify invasion of each added species in each patch type. Although there was variability in edaphic characteristics among patches in the natural experiment, these were not significant predictors of invasibility (Emery & Gross 2006). To examine second-year survival of invaders, the mesocosm experiment was extended through 2005 (second-year survival in the natural experiment was not monitored owing to comparatively low initial establishment). Seedlings censused in pots in 2004 were marked with toothpicks, and censused again in August 2005.

Table 1.  Species added as seeds to experiments (field and mesocosm)
SpeciesLife formOrigin
  1. Origin refers to North American presence. Species in bold are the eight species transplanted to create initial communities in the mesocosm experiment.

Achillea millefolium L.ForbNative
Andropogon gerardii VitmanC4 bunchgrassNative
Bromus inermis Leyss.C3 clonal grassNon-native
Bromus kalmii GrayC3 bunchgrassNative
Calamagrostis canadensis (Michx.) Beauv.C3 clonal grassNative
Centaurea maculosa Lam.ForbNon-native
Coreopsis lanceolata L.ForbNative
Dactylis glomerata L.C3 bunchgrassNon-native
Echinacea pallida (Nutt.) Nutt.ForbNative
Elymus canadensis L.C3 clonal grassNative
Liatris aspera Michx.ForbNative
Muhlenbergia racemosa (Michx.) B.S.P.C4 clonal grassNative
Panicum virgatum L.C4 clonal grassNative
Poa pratensis L.C3 clonal grassNon-native
Ratibida pinnata (Vent.) Barnh.ForbNative
Schizachyrium scoparium (Michx.) NashC4 bunchgrassNative
Solidago nemoralis Ait.ForbNative

data analyses

I used indicator species analysis (Dufrene & Legendre 1997; McCune et al. 2002) to examine whether individual invading species showed differences in their colonization success across plots dominated by different dominant species. Indicator species analysis involves calculating a metric (indicator value, IV) that summarizes both the relative abundance and the frequency of each species in each treatment such that:

  • IVkj = RAkj × RFkj × 100

where RA is the relative abundance of a given species j (i.e. a species added as seed) in a given group k (i.e. a dominant species treatment) and RFkj is the proportional frequency of species j in group k (i.e. the proportion of plots in each treatment that contain species j). Values range from 0 (no presence of a species in a given patch type) to 100 (perfect indication). A perfect indication score means that a given invader occurred only in a given treatment, and always in that treatment. The observed IV is compared with an expected IV calculated using Monte-Carlo randomizations of the data, where species frequency and abundance data from each plot are randomly assigned to a group/treatment 1000 times. The null hypothesis is that the observed IV is no larger than would be expected by chance (as calculated by the randomization procedure). I also used seedling failure data (i.e. 50 seeds added – the number of surviving seedlings) for each species to test in which treatments each invader species did significantly ‘worst’. Similarly, I used 2005 invader survival data from the mesocosm experiment to calculate in which treatments each invader species survived the best or worst. I also summarized invader species and dominant species into functional groups, to see in which dominant functional group treatment each invader functional group did best and worst. To summarize evidence broadly for limiting similarity in both experiments, I also compared IV numbers for each invader-dominant functional group pair in a simple t-test. The indicator species analyses were performed in PC-ORD (McCune & Mefford 1999).

I also compared relative establishment success of the different functional groups of invaders (C3 grasses, C4 grasses and forbs), and of native and non-native invaders in the two experiments using one-way analysis of variance (anova) with dominant species or dominant functional group as the main factor for both the field and the mesocosm experiments (separate analyses). Establishment and survival percentages of seedlings were arcsine square root transformed for analyses to meet normality assumptions better. All anova analyses were performed using Systat v. 11 (Systat Software Inc. 2004).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

indicator species analysis

In the natural experiment, 29% of invader species (5/17 species) showed differential responses to patch type and the identity of the dominant species (Table 2). Echinacea pallida, a forb, showed significant affinity with Bromus (C3 grass) patches, and was least successful in Solidago (forb) patches. Schizachyrium scoparium, a C4 grass, showed significant affinity with Centaurea (forb) patches, but did not do significantly worse in any one patch type. Three C3 grasses (Bromus kalmii, Dactylis glomerata and Poa pratensis) all showed highest failure in Bromus (C3 grass) patches. When invaders were grouped into functional groups, ignoring species-level classification, there were no significant associations between invaders and community dominants.

Table 2.  Results from the indicator species analysis for the field experiment
Species2004 establishment
High groupLow group
  1. Species are grouped by functional group. Significant relationships between each invader species (or functional group) and the patch dominant species are shown for both greatest establishment (‘high group’) and lowest establishment (‘low group’). Numbers in parentheses are the actual indicator values (IV). Significance values are calculated based on 1000 randomizations in a Monte Carlo simulation, with *P < 0.10 and **P < 0.05.

B. inermisNSNS
B. kalmiiNSBromus (25.4**)
C. canadensisNSNS
D. glomerataNSBromus (25.4**)
E. canadensisNSNS
P. pratensisNSBromus (25.2**)
C3 grassesNSNS
A. gerardiiNSNS
M. racemosaNSNS
P. virgatumNSNS
S. scopariumCentaurea (30.6**)NS
C4 grassesNSNS
A. millefolium NSNS
C. maculosaNSNS
C. lanceolataNSNS
E. pallidaBromus (29.5**)Solidago (25.1*)
L. asperaNSNS
R. pinnataNSNS
S. nemoralisNSNS
ForbsNSNS

For the mesocosm experiment, 9 of 17 species (53%) showed some significant differentiation with respect to the identity of the dominant resident species (Table 3). Consistent with the hypothesis of limiting similarity, establishment and survival of forbs in general showed affinity for Bromus/C3 grasses and was lowest in Coreopsis/forb treatments, although individual species of forbs did not always match this pattern. For example, Achillea millefolium had highest establishment success, but lowest survival success in Panicum/C4 grass treatments. C4 grass invaders in general had significantly lower establishment in Schizachyrium (C4 grass) pots, but showed no difference among treatments in second-year survival. Again, however, this was not consistent for all C4 invader species. Muhlenbergia racemosa, for example, had lowest establishment in C3 grass treatments. Finally, C3 invaders in general had highest establishment success in Panicum treatments, and highest survival in Bromus/C3 grass treatments, and did not do significantly worse in any one treatment. Only two of six C3 grass invaders showed any differentiation among treatments (Bromus kalmii and Dactylis glomerata), and these were not similar. Bromus kalmii had lowest establishment success in C3 grass-dominated treatments, while D. glomerata had highest survival in the same C3 treatments.

Table 3.  Results from the indicator species analysis for the mesocosm experiment
SpeciesBy dominant speciesBy dominant functional group
2004 establishment2005 survival2004 establishment2005 survival
High groupLow groupHigh groupLow groupHigh groupLow groupHigh groupLow group
  1. Species are grouped by functional group. Significant relationships between each invader species (or functional group) and the community dominant (species or functional group) are shown for both greatest establishment or survival (‘high groups’) and lowest establishment or survival (‘low groups’). Numbers in parentheses are the actual indicator values (IV). Significance values are calculated based on 1000 randomizations in a Monte Carlo simulations, with *P < 0.10, **P < 0.05, and ***P < 0.01. Dashes (–) indicate lack of data to test for relationship.

B. inermisNSNSNSNSNSNSNSNS
B. kalmiiPanicum (30.9**)Bromus (17.7*)NSNSC4 grass (43.3*)C3 grass (35.2***)NSNS
C. canadensisNSNSNSNS
D. glomerataNSNSNSNSNSNSC3 grass (44.3**)NS
E. canadensisNSNSNSNSNSNSNSNS
P. pratensisNSNSNSNS
C3 grassesPanicum (24.2**)NSBromus (26.1**) NSNSNSC3 grass (44.8**)NS
A. gerardiiNSNSNSNS
M. racemosaNSNSNSC3 grass (33.8**)
P. virgatumPanicum (30.5**)Schizy (17.9**)NSNS
S. scopariumBromus (29.7*)Schizy (17.6**)NSNSNSNSNSNS
C4 grassesNSSchizy (17.5***) NSPanicum (19.3*)NSNSNSNS
A. millefoliumNSNSNSPanicum (27.8**)C4 grass (34.1**)Forb (33.6**)C3 grass (18.7*)C4 grass (44.7***)
C. maculosaNSCoreopsis (18.7***)Bromus (36.8**)Coreopsis (20.1**)NSForb (36.8***)C3 grass (56.9***)Forb (39.7***)
C. lanceolataNSNSNSNSNSNSNSNS
E. pallidaNSNSNSNS
L. asperaNSNSNSPanicum (24.2**)NSNSC3 grass (20.7*)NS
R. pinnataNSNSNSNS
S. nemoralisNSCoreopsis (17.0***)NSNSNSNSNSNS
ForbsNSCoreopsis (17.0***)Bromus (38.0***)Coreposis (19.8***)C3 grass (41.9*) Forb (33.9***)C3 grass (61.0***)Forb (38.5***)

In general, invaders had higher IV indices in plots where the dominant species was functionally different from the invader (Fig. 1, natural experiment: F1,33 = 1.0, P = 0.33; mesocosm experiment: F1,102 = 4.79, P = 0.031).

image

Figure 1. Summary of evidence for limiting similarity between dominant species and invaders in the natural and mesocosm experiments. Higher indicator values represent higher colonization success.

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When relative success of invaders was compared using just average establishment data (as opposed to the IV index combining frequency and abundance data), there were still differences among some groups, although the patterns were not as strong. As with the indicator species analysis, there were no significant differences among patch types for the different functional groups of invaders in the natural experiment (Fig. 2, C3: F3,55 = 0.44, P = 0.72; C4: F3,55 = 2.45, P = 0.06; Forb: F3,55 = 0.22, P = 0.88). In the mesocosm experiment, only forbs showed differences in 2004 establishment success, with lower germination in pots dominated by forbs (Fig. 3a, C3: F2,48 = 0.73, P = 0.49; C4: F2,48 = 0.42, P = 0.66; Forb: F2,48 = 8.12, P = 0.001). Through 2005, all three groups showed significantly higher survival in those pots dominated by C3 grasses in comparison with C4 grasses or forbs (Fig. 3b, C3: F2,48 = 4.32, P = 0.19; C4: F2,48 = 4.48, P = 0.017; Forb: F2,48 = 14.73, P < 0.001).

image

Figure 2. Establishment success of three functional groups of invaders in each of the four community patch types of the natural experiment. Bars represent one standard error.

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image

Figure 3. Establishment (a) and survival (b) success of three functional groups of invaders in each of three dominant species functional groups in the mesocosm experiment. Bars represent one standard error.

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native vs. non-native invaders

Non-native species had significantly higher establishment than native species in all patch types of the natural experiment (F1,110 = 53.45, P < 0.001; Fig. 4a). The high success of non-native species was mostly driven by the high establishment success of Centaurea maculosa (Fig. 4b). With C. maculosa removed from the analysis, there was no difference between native and non-native invasion success (F1,110 = 2.77, P = 0.099). In the mesocosms, non-native species again had higher establishment (F1,96 = 134.82, P < 0.001) and survival (F1,95 = 32.86, P < 0.001) than native species (Fig. 3a). This higher success of non-native species was mostly due to very high establishment rates of C. maculosa and Dactylis glomerata (Fig. 4b). When C. maculosa and D. glomerata were removed from this analysis, the remaining non-native species actually had lower establishment success than the native species (F1,96 = 12.34, P = 0.001).

image

Figure 4. Relative establishment success (2004) of (a) native and non-native invaders, and (b) individual non-native species, in both the natural and the mesocosm experiments. Bars indicate one standard error.

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The indicator species analysis shows some evidence for limiting similarity regulating invasion success of the four exotic species. In the natural experiment, two of the three C3 exotic species (D. glomerata and P. pratensis) had low establishment in Bromus (C3) patches, while the other two exotic species (B. inermis and C. maculosa) showed no variable success across patch types (Table 2). In the mesocosm experiment, B. inermis and P. pratensis showed no variable success across treatments, while C. maculosa (a forb) had lowest establishment in Coreopsis/forb-dominated treatments (Table 3). In contrast to the predictions of the limiting similarity model, D. glomerata (C3) actually had the highest survival in C3-dominated treatments.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

The results from this study show only weak evidence for a predictable relationship between successful invaders and dominant species in communities. Based on mechanisms of limiting similarity, I expected that successful invaders should be functionally different to dominant species in plant communities. However, invaders did not consistently have less success in those communities that contained a dominant species in the same functional group as the invader. For example, C4 grass invaders in the mesocosm experiment had the lowest establishment success in pots dominated by Schizachyrium, a C4 species, as expected with limiting similarity. However, C4 invaders showed no performance difference across treatments when dominants were categorized by functional group. The indicator species analysis further indicated that the different functional groups of invaders in the field experiment showed no association or disassociation with any one dominant patch type. In fact, only forb invaders in the mesocosm experiment consistently met the predictions of limiting similarity, where forb invaders had lowest establishment and survival success in forb-dominated communities.

Furthermore, I found little evidence for limiting similarity when examining individual species responses across patch types. In the field experiment, only five species (29%) showed differentiation among patch types as measured by the indicator species analysis, although these five species all supported patterns of limiting similarity (e.g. Schizachyrium scoparium, a C4 grass, was most closely associated with the Centaurea, a forb, patch type, while Echinacea pallida, a forb, had lowest establishment in the Solidago, another forb, patch type). In the mesocosm experiment, only 9 of 17 species (53%) showed any differentiation among dominant treatments, and these differences were even less predictable in terms of limiting similarity. Even establishment and survival of a given invader varied. For example, Achillea millefolium, a forb, had the highest establishment in C4-dominated communities and lowest in forbs, but persistence of A. millefolium was highest in C3-dominated and lowest in C4/Panicum-dominated treatments. A large percentage of species (47–71%) in both experiments showed no difference in affinity across the different dominant treatments.

Although the functional group categories used were broad (C3 grasses, C4 grasses and forbs), distinct differences in phenology and rooting structure between these groups promote coexistence of species in different groups. For example, many forbs have taproots that extend below grass root systems, allowing them to access resources not available to more competitive grass species. These classifications have also proven useful for predicting ecological dynamics in other studies (e.g. Tilman et al. 1997). The coexistence of C3 and C4 grasses in Midwestern US grasslands has been attributed, at least in part, to differences in phenology (Fargione & Tilman 2005 and references therein). Furthermore, the few studies that have examined the relationship between community and invader composition have found some evidence supporting limiting similarity as a mechanism inhibiting invasion of these functional groups (C3/C4/forb). For example, Fargione et al. (2003) found that resident functional groups (C3 grasses, C4 grasses, legumes and forbs) inhibited invasion from members of their own functional group in experimental communities at Cedar Creek, MN.

Although I only examined the initial establishment of invading perennial species, seedling recruitment is often a key life stage in population growth (e.g. Silvertown et al. 1993; Shea & Kelly 1998; Parker 2000). For future work, it may be more informative to test mechanisms of limiting similarity by examining more specific species traits (e.g. plant biomass, plant height, specific leaf area, leaf nitrogen content; Stubbs & Wilson 2004; Mason et al. 2005; Petchey & Gaston 2006; Wright et al. 2006).

invasion by non-native species

There has been some attempt to predict and control the spread of invasive species by understanding what types of communities are most susceptible or resistant to invasion. For example, non-native C3 grasses are rapidly invading C4-dominated tallgrass prairie in the US Great Plains (Cully et al. 2003). Management efforts that focus on reintroducing C3 native species (in the same functional group as the invaders) have shown some promise in controlling the spread of these invasive species. For instance, restorations that involved planting native C3 grasses in Canada successfully slowed invasion by Agropyron cristatum (L.) Gaertn., an introduced C3 grass (Bakker & Wilson 2004).

However, the results from my study suggest that attempts to control invasion of non-native species by manipulating functional groups may not always be successful. Centaurea maculosa, the most noxious invader of the four non-native species, showed some aversion for forb-dominated communities in the mesocosm experiment, but performed equally well (and 3–10 times greater than native species) across the different patches in the field experiment. Another non-native species, Dactylis glomerata, also had very high invasion success compared with native species. Paradoxically, it was not very successful in the Bromus patches of the field experiment, but had highest survival in C3 grass-dominated pots of the mesocosm experiment. Perhaps the bigger question coming from these data is why these two exotic species were so much more successful at invading than the native species? Managing community composition may slow invasions (Bakker & Wilson 2004), but other factors, such as enemy-release or below-ground interactions, may be more important for controlling invasive species (Callaway et al. 2004; Levine et al. 2004). In fact, these factors may in part explain the lower establishment rates of invaders in the natural experiment compared with the mesocosm experiment, which had more homogeneous soils and was protected from larger herbivores. Although it is possible that several years of competitive interactions will alter the relative success of these invasive species (Levine et al. 2004), this study shows community composition exerts very little effect on the initial establishment and survival of these species.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

The results from this study show that the identity of invaders and community dominants can be important factors to consider when trying to understand mechanisms of the invasibility of communities. I did find differences in colonization success among different functional groups of invaders in both experiments, as well as between native and non-native invader species. Overall, I found that invaders do better in communities dominated by a species that belongs to a different functional group, although this generalization broke down when looking at individual invading species. Many studies of invasibility summarize invasion based on the establishment success of a single, often non-native, species (e.g. Robinson et al. 1995; Smith et al. 2004). My results call into question the generality of results from such single-species addition studies. As the data presented here illustrate, a community that is susceptible to invasion by one species may be highly resistant to another species. Growing concern over the negative ecological and economic impacts of ‘exotic’ species makes understanding what factors drive their invasion critical (Lonsdale 1999), while increased insight into the mechanisms of ‘native’ species invasions is important for protecting and maintaining species diversity, successional dynamics and community stability (Crawley 1987; Tilman 1997; Mwangi et al. 2007). Studies that add seeds of a variety of species may create undesired seedling competition effects, but they also give a more complete picture of how a community may respond to any given novel invader (e.g. Symstad 2000; Foster et al. 2002). Furthermore, although the hypothesis of limiting similarity would make success of invaders easier to predict, these results show that such predictions are rarely upheld by experimental data, and are often inconsistent across species.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Thanks to Kay Gross, Jennifer Rudgers, Jeff Conner, Doug Schemske and Carolyn Malmstrom for the development of ideas and comments on earlier versions of this manuscript. Funding for this research was provided by a US Environmental Protection Agency STAR graduate fellowship to S.M.E. This is KBS contribution number 1423.

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  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
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