Community Dominance Patterns, Not Colonizer Genetic Diversity, Drive Colonization Success in a Test Using Grassland Species
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Plant Ecol (2012) 213:1365–1380 DOI 10.1007/s11258-012-0097-2 Community dominance patterns, not colonizer genetic diversity, drive colonization success in a test using grassland species Stephen M. Hovick • Esra D. Gu¨mu¨s¸er • Kenneth D. Whitney Received: 19 April 2012 / Accepted: 4 July 2012 / Published online: 19 July 2012 Ó Springer Science+Business Media B.V. 2012 Abstract Successful colonization and/or invasion communities were no better at suppressing coloniza- depend on characteristics of the invaded community tion than communities dominated by a single func- and of the colonizer itself. Although many studies have tional group. Forb-dominated communities suppressed documented a negative relationship between invasi- A. thaliana colonization better than grass-dominated bility and biodiversity, the importance of community communities and suppressed seedbank colonizers evenness is rarely examined and thus poorly under- better than legume-dominated communities. Equal- stood. However, colonizer characteristics, including abundance communities were similar to forb-domi- population genetic diversity, can also be important nated ones in their eventual composition and in their determinants of colonization success. We conducted a invasibility, suggesting that forbs drove colonizer greenhouse experiment to assess the relative impor- suppression in that treatment rather than high evenness tance of community evenness and colonizer popula- itself. Most of our forbs grew quickly, yielding tion genetic diversity using the weed Arabidopsis productive forb-dominated communities; this points thaliana. We added seeds of A. thaliana (varying to the importance of growth and colonization phenol- genetic diversity while keeping propagule pressure ogy in our system. A. thaliana genetic diversity did constant) to four types of constructed plant communi- not affect colonization success, perhaps because ties: those dominated by legumes, grasses or forbs, or strong interspecific competition substantially limited with equal abundances of all three functional groups. A. thaliana seedling emergence. We selected community members from a large pool of species to avoid the confounding effects of species Keywords Functional group Á Population genetic identity. We also assessed the success of multiple diversity Á Biological invasions Á Community seedbank colonizers to assess generality in the dominance/evenness Á Invasion resistance Á effects of our evenness treatments. Equal-abundance Colonization Introduction Stephen M. Hovick and Esra D. Gu¨mu¨s¸er contributed equally to this study. A better understanding ofplant colonization dynamicsis S. M. Hovick (&) Á E. D. Gu¨mu¨s¸er Á K. D. Whitney necessary to identify potential invaders and the com- Department of Ecology and Evolutionary Biology, munities most threatened by invasion. In part, coloni- Rice University, MS-170, 6100 Main St., Houston, TX 77005, USA zation or invasion success reflects characteristics of e-mail: [email protected] the recipient community with some communities 123 1366 Plant Ecol (2012) 213:1365–1380 suppressing colonizers better than others (e.g., Crawley empirical findings (Wilsey and Polley 2002; Tracy and et al. 1999; Hector et al. 2001;FargioneandTilman Sanderson 2004; Zavaleta and Hulvey 2007). On the 2005;EmeryandGross2007;DrenovskyandJames other hand, if some optimal trait value exists for a site, 2010). Yet, colonization success also reflects key low-evenness communities containing a dominant characteristics of colonizing species (e.g., growth rates species that matches this optimum may have higher and fecundity; Pysˇek and Richardson 2007; van Kleun- productivity and therefore decreased invasibility than en et al. 2010) and their populations (e.g., propagule high-evenness communities (Norberg et al. 2001); this pressure, hybridization history, and population genetic pattern has also been confirmed experimentally diversity; Lockwood et al. 2005; Crawford and Whitney (Emery and Gross 2007). Thus, previous attempts to 2010;Hovicketal.2012). Studies investigating the study the relationship between invasibility and com- relative importance of the invaded community charac- munity evenness have produced conflicting results. teristics versus colonizer characteristics (i.e., invasibil- These discrepancies might be explained via species ity versus invasiveness) may provide new insights identity effects (Crawley et al. 1999; Emery and Gross regarding the determinants of successful colonization 2007; Daneshgar and Jose 2009). Most experimental and invasion (Richardson et al. 2000). low-evenness communities confound dominant spe- Species diversity and invasibility are often nega- cies identity with low-evenness treatments by not tively correlated (Hector et al. 2001; Herben et al. manipulating dominant species identity or not ran- 2004; Tilman 2004; Fargione and Tilman 2005; domly selecting replicates of dominant species from a Scherber et al. 2010). One common interpretation is larger pool, clouding inferences regarding the role of that species-rich communities use a greater proportion evenness in suppressing colonization (but see Tracy of available resources, therefore reducing the niche and Sanderson 2004). Although experiments with space available to colonizers; however, Wardle (2001) species that commonly occur as community domi- argued that species richness effects may instead result nants do provide valuable system-specific inferences, from differences in species composition. Species-rich a different approach is needed to identify more general communities are more likely to contain a dominant patterns. We suggest that testing for general patterns species that exploits resources effectively, thus suc- requires assessing colonizer suppression in experi- cessful colonization may depend on species identity mental communities that are assembled via random and not species richness per se. Nonetheless, most species selection and where evenness is manipulated invasibility studies use richness as the sole metric of at the level of the functional group to remove the diversity. An information-rich alternative to species confounding effects of species identity. richness is evenness, which reflects species’ relative Functional groupings aggregate species that use abundances within a community. In plant communi- and compete for resources similarly. Since resource ties, evenness can account for much more variation in availability is often central to understanding coloni- proportional diversity (H0) than does species richness zation and invasion dynamics, the number and identity (53 vs. 6 %; Stirling and Wilsey 2001), while also of functional groups in a community may significantly responding more rapidly to natural and anthropogenic affect invasibility (Roscher et al. 2009; Hooper and perturbations (Chapin et al. 2000; Walker et al. 2006). Dukes 2010). For instance, Arenas et al. (2006) found Because communities with equal species richness can that functional group identity was more important than differ significantly in species’ relative abundances, functional group richness in the ability of macroalgal evenness may provide additional valuable information communities to resist invasion. Similarly, Daneshgar about species diversity effects on invasibility. and Jose (2009) found that grasses reduced invasion by One challenge in making predictions regarding the grass Imperata cylindrica more than shrubs or evenness and invasibility is that, depending on the forbs, highlighting the importance of functional group context, invasibility may be expected to peak in either identity for community invasibility. In other cases, high- or low-evenness communities. On one hand, increasing functional group richness reduces invasion theory suggests that high-evenness communities can success (Symstad 2000; Xu et al. 2004), but to our result in greater resource drawdown, and thus greater knowledge no studies have assessed the relation- productivity, reducing the probability of colonization ship between invasibility and functional diversity by (Nijs and Roy 2000); this is consistent with several manipulating functional group evenness. We 123 Plant Ecol (2012) 213:1365–1380 1367 recognize that functional groupings have utility only This experiment jointly examines characteristics of to the extent that species within the group use plant communities and colonizers that can affect resources similarly, and that functional group classi- colonization and invasion success. We conducted a fications have their limitations (Eviner 2004). Never- greenhouse experiment using Arabidopsis thaliana as theless, we anticipate that broad generalizations a model invasive species, manipulating functional regarding community diversity and invasibility are group evenness in the invaded experimental commu- more likely to result from experiments manipulating nities and population genetic diversity in A. thaliana. community composition above the species level, For increased realism, we added field-collected soil to and functional groups provide a valuable tool for all treatments, which let us assess how functional doing so. group evenness affected colonization from a diverse Although community characteristics that affect seedbank in addition to colonization by A. thaliana. invasibility are important determinants of colonization We assembled initial plant communities using a robust success, many colonizer characteristics, such as pop- experimental design that avoids confounding species ulation genetic diversity, can also contribute. Intro- identity