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- Materials and Methods
- Supporting Information
Invasions represent excellent opportunities to synthesize evolution, ecology, and genetics over contemporary time-scales, and especially to address how human-induced changes in the environment and species distributions influence ecological and evolutionary processes. While extensive research focuses on the ecological processes that play a role in plant invasions, an understanding of the evolutionary processes involved is still maturing (Hendry et al., 2008; Catford et al., 2009; Buswell et al., 2011). It is clear, however, that the success of an invasion may depend on adaptation to novel conditions, potentially many times, during the course of range expansion (Hufbauer et al., 2012).
Such adaptation should be observable as ecologically important differentiation between populations from the native and invaded ranges. Increased growth rate or reproductive capacity is frequently reported from field observations in the invaded range (Elton, 1958; Crawley, 1987; Thébaud & Simberloff, 2001; Parker et al., 2013), and increasingly from common garden experiments (reviewed in Felker-Quinn et al., 2013). This improved vigor could contribute to rapid spread and population growth in the invaded range. Multiple hypotheses have attempted to explain increased performance of invasive populations, including the evolution of increased competitive ability hypothesis (EICA; Blossey & Notzold, 1995). EICA posits that selection will favor genotypes with reduced allocation to herbivore defense and increased allocation to growth, reproductive output, or competitive ability in the absence of herbivores characteristic of the native range (Blossey & Notzold, 1995). EICA thus predicts that, as a consequence of trade-offs, individuals from the native range will produce less biomass than individuals from the introduced range, and specialist herbivores will show improved performance on individuals from the introduced range in a common environment (Blossey & Notzold, 1995; Joshi & Vrieling, 2005). EICA is only supported if increases in growth are linked to decreases in defense. Though increased performance in invasive individuals relative to native is often observed in common garden experiments, shifts in defenses are less common (Kumschick et al., 2013), and thus evidence for EICA is equivocal (Felker-Quinn et al., 2013). Some authors have expanded ideas about trade-offs to include tolerance to other types of stress, not just herbivore pressure (Bossdorf et al., 2005; He et al., 2010). If novel habitats are less stressful than native ones, selection would favor individuals that shift resources from stress tolerance to increased vigor and fecundity. Invoking trade-offs assumes that plants are unable both to be highly tolerant to stressful environments and to be highly competitive or to have high reproductive output (Grime, 1977). Several species demonstrate such trade-offs. For example, tolerance to an abiotic stress, such as serpentine soils or drought, comes at the expense of competitive ability or growth rate (Sambatti & Rice, 2007). EICA and other trade-off hypotheses suggest that invasive individuals evolve a lower tolerance to biotic or abiotic stress, and therefore will perform relatively poorly under stressful conditions, as assessed in several studies (e.g. Hodgins & Rieseberg, 2011; Lachmuth et al., 2011; Kumschick et al., 2013). However, trade-offs can occur in multiple directions (e.g. Burton et al., 2010). Additionally, strategies favored by selection may change over time, between different phases of an invasion (Dietz & Edwards, 2006) or depending on the habitats invaded (Lachmuth et al., 2011). A shift in resource allocation under benign conditions is not the only explanation for increased growth and reproduction in a species' invaded range. Other genetically based changes between the native and invaded ranges that could lead to increased performance include inter- or intra-specific hybridization (Rieseberg et al., 2007; Schierenbeck & Ellstrand, 2009; Lai et al., 2012).
Observing the phenotypes of a single generation in a common environment may be insufficient to demonstrate adaptation to a novel habitat. Latitudinal clines can impede our ability to infer evolutionary change from common gardens (Colautti et al., 2009). Even within the invaded range of a species, local adaptation can vary along latitudinal and environmental clines as the invading populations adjust to local environments, shifting phenology, biomass, and other trait means (Colautti et al., 2010; Lachmuth et al., 2011). It is also necessary to rule out differences between the native and invaded ranges caused by maternal environmental effects. Maternal effects can have strong, even adaptive, impacts on offspring phenotype in some systems (Galloway, 2005). Yet their influence in shaping performance differences between ranges is only rarely experimentally controlled (except see Monty et al., 2009; Hodgins & Rieseberg, 2011).
To test trade-off hypotheses, and to look for evidence of rapid adaptation to novel habitats, we ask: (1) Do native and invasive populations show consistent phenotypic differences in growth and reproduction, and do such differences remain even after controlling for latitude and maternal effects? and (2) Is there evidence of a trade-off between the ability to grow quickly under benign conditions and the ability to tolerate stressful conditions? To address these questions, we conducted two large glasshouse experiments with Centaurea diffusa (diffuse knapweed), one of North America's worst weedy invaders (Lejeune & Seastedt, 2001). The first experiment, hereafter the broad CG, included 28 native European and 18 invasive North American populations of C. diffusa (Fig. 1), using one of the highest levels of population replication in studies of this type (see also Kumschick et al., 2013). The second experiment, hereafter the maternal CG, assessed whether patterns observed in the broad common garden were maintained after controlling the maternal environment by using seeds produced from glasshouse crosses of four populations from each range. Both gardens included a benign control treatment and biotic and abiotic stress treatments (simulated herbivory, nutrient deficiency, drought, and flooding). Differences between the two experiments would suggest that the maternal environment (as well as experimental variation) exerts a significant influence on phenotypic response. Similarity between them would suggest that genetic polymorphisms control the phenotypic divergence between native and invasive individuals (Moloney et al., 2009). Furthermore, if plants from the invaded range outperform plants from the native range in the control treatment, but not in the stress treatments, hypotheses invoking trade-offs would be supported. However, a lack of significant differences between the two ranges may indicate that C. diffusa success in North America does not rely upon evolutionary differences between ranges.
Figure 1. Range and collection map for Centaurea diffusa in the Northern Hemisphere, by country. Populations used in the various experiments are indicated by point style; all populations were used in the broad common garden (open circles), and four from each origin in both broad and maternal common gardens (filled circles). Origin status in a particular country is indicated by color, and was determined from the classification in Greuter (2006), ISSG.org (2013), Kartesz (2013), Tropicos.org (2013), and USDA (2013). ‘Present, status unknown’ also includes countries where Centaurea diffusa is considered naturalized. Degrees of latitude are indicated on dotted lines, and degrees of longitude are indicated on solid lines. CG, common garden experiment.
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