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. 2012 Apr 6;110(7):1403–1410. doi: 10.1093/aob/mcs076

Response to enemies in the invasive plant Lythrum salicaria is genetically determined

Srijana Joshi 1, Katja Tielbörger 1,*
PMCID: PMC3489141  PMID: 22492331

Abstract

Background and Aims

The enemy release hypothesis assumes that invasive plants lose their co-evolved natural enemies during introduction into the new range. This study tested, as proposed by the evolution of increased competitive ability (EICA) hypothesis, whether escape from enemies results in a decrease in defence ability in plants from the invaded range. Two straightforward aspects of the EICA are examined: (1) if invasives have lost their enemies and their defence, they should be more negatively affected by their full natural pre-invasion herbivore spectrum than their native conspecifics; and (2) the genetic basis of evolutionary change in response to enemy release in the invasive range has not been taken sufficiently into account.

Methods

Lythrum salicaria (purple loosestrife) from several populations in its native (Europe) and invasive range (North America) was exposed to all above-ground herbivores in replicated natural populations in the native range. The experiment was performed both with plants raised from field-collected seeds as well as with offspring of these where maternal effects were removed.

Key Results

Absolute and relative leaf damage was higher for introduced than for native plants. Despite having smaller height growth rate, invasive plants attained a much larger final size than natives irrespective of damage, indicating large tolerance rather than effective defence. Origin effects on response to herbivory and growth were stronger in second-generation plants, suggesting that invasive potential through enemy release has a genetic basis.

Conclusions

The findings support two predictions of the EICA hypothesis – a genetically determined difference between native and invasive plants in plant vigour and response to enemies – and point to the importance of experiments that control for maternal effects and include the entire spectrum of native range enemies.

Keywords: Biological control, field experiment, herbivory, purple loosestrife, enemy release, EICA hypothesis, invasion, Lythrum salicaria

INTRODUCTION

Biological invasions have tremendous ecological impacts and economic costs, causing extinction of many native species and shifts in ecosystem function (Pimentel et al., 2000). Despite the large numbers of studies in invasion biology, no universal rules for predicting the success of invasive plants in novel environments have been found. The enemy release hypothesis (ERH) is a popular hypothesis that attempts to explain plant invasion success. It assumes that invaders are dislocated from their co-evolved natural enemies during introduction into their new range and thus have an advantage over native plants (Keane and Crawley, 2002; Colautti et al., 2004). However, invaders cannot completely escape all herbivores during introduction. In that case, they could either suffer from new generalist herbivores or may benefit when such herbivores have a greater impact on native competitors than on the invader (Keane and Crawley, 2002).

Many studies have tested the ERH and their results are quite contradictory. Some indirect studies provide experimental evidence for higher herbivore loads on native species than on introduced species (Wolfe, 2002; Mitchell and Power, 2003; Cincotta et al., 2009), while an opposite pattern – higher susceptibility of exotic species – has also been reported (Agrawal and Kotanen, 2003). Other studies have focused on one or few herbivore species, mostly for discovering biological control agents (Garcia-Rossi et al., 2003; Goolsby et al., 2004; Wang et al., 2011). However, in nature, plants are rarely attacked by a single enemy but have to deal with a diverse community of herbivores and pathogens. Several studies have also highlighted that specialist and generalist herbivores have differential effects on plant defence (Joshi and Vrieling, 2005; Hull-Sanders et al., 2007; Abhilasha and Joshi, 2009; Caño et al., 2009; Huang et al., 2010). This illustrates the need to study the response of invaders to a whole suite of herbivores.

An obvious prediction stemming from the ERH is that if we expose plants from the invasive range to the full range of enemies in their native habitat, they should suffer more from herbivory and pathogens than their native conspecifics. However, this prediction applies only if the consequences of enemy release (e.g. reduced defence) are genetically fixed and not only a passive response to reduced herbivore load in the new range. The ERH is not specific about whether enemy release results in microevolutionary changes or whether it is a passive response to reduced herbivore pressure in the new range. However, the evolution of increased competitive ability (EICA) hypothesis expands the ERH by assuming that enemy release drives an evolutionary change in introduced plants, in which resources are reallocated away from defence mechanisms to growth and reproduction (Blossey and Nötzold, 1995). Unfortunately, most tests of theories of invasion, including the EICA hypothesis, were not sufficiently controlled to distinguish between plastic and genetic responses to enemy release in the novel range. In particular, common garden tests of the EICA hypothesis that control for maternal effects are urgently needed (van Kleunen and Schmid, 2003) but virtually lacking, especially when studying the enemy release part of the EICA hypothesis (but see Oduor et al., 2011).

Previous studies have investigated isolated aspects of enemy release. For example, response to enemies was only investigated in the invasive range (Siemann and Rogers, 2003), and only for a small subset of enemies (e.g. Joshi and Vrieling, 2005; Stastny et al., 2005) or in a garden and not in natural populations (van Kleunen and Schmidt, 2003; Stastny et al., 2005; Huang et al., 2010; Wang et al., 2011), i.e. the herbivore spectrum may not have been relevant to the study goal. Other experiments were conducted with plants from both origins in the native range (Wolfe et al., 2004; Meyer et al., 2005; Zou et al., 2008a, b; Oduor et al., 2011) but these were done in gardens, albeit in the vicinity (metres to few kilometres) of natural populations. However, the above studies have not tested whether patterns of enemy release are maintained when conducted within established natural populations in the native range, and when maternal effects are removed (but see Oduor et al., 2011).

Here, we exposed plants from the invaded range and their native conspecifics to a full natural spectrum of above-ground herbivores in native habitats. We did this with both plants raised from field-collected seeds as well as with their offspring where maternal effects were removed, and we quantified herbivory effects both as direct damage as well as in plant performance. Our model species was purple loosestrife (Lythrum salicaria) because this species has been fundamental for developing the EICA hypothesis. Furthermore, biological control has also been applied for this species, indicating that enemy release may have occurred (Blossey et al., 2001; Landis et al., 2003). To examine the genetic basis of differences between native and invasive plants, we tested the following hypotheses:

  1. Introduced plants exhibit greater leaf damage than native plants when exposed to the natural herbivore spectrum in populations in the native range.

  2. Introduced plants exhibit increased growth compared with native plants.

  3. Response to enemies and growth are genetically fixed, and differences among origins are not the same in plants grown from seeds where maternal environment effects are removed and plants are raised from field-collected seeds.

MATERIAL AND METHODS

Study species

Lythrum salicaria L. (purple loosestrife, Lythraceae) is native to Europe and it is a well-known aggressive perennial invader in North America. The species was introduced to eastern North America accidentally by ship ballast and purposely for horticulture, as food source, and for ornamental and medicinal use in the early 19th century (Thompson et al., 1987). Since then it has expanded into a large variety of wetland ecosystems and produced large monospecific stands, often at the expense of native plants (Thompson et al., 1987). Genetic studies have indicated that multiple introductions occurred, with the older populations occurring in the east (Houghton-Thompson et al., 2005; Chun et al., 2009). Biological control programmes exist for this species. For example, two Chrysomelid species (Galerucella calmariensis and G. pusilla) have been identified as effective biological control agents for L. salicaria (Malecki et al., 1993; Blossey et al., 1994, 2001; Dech and Nosko, 2002). In Europe, 120 species of phytophagous insects and 64 species of floral visitors were found associated with purple loosestrife (Batra et al., 1986).

Study sites

The experiment was carried out in 2007 and 2008. Plant and seed material was raised in a common garden of the Botanical Gardens of the Tübingen University, Germany (48°32′N, 9°02′E). Two natural populations of L. salicaria were used for exposing the plants to native herbivores. The two populations are 9 and 14 km away from the common garden and are located near the villages of Reusten (48°55′N, 8°91′E) and Unterjesingen (48°52′N, 8°98′E). L. salicaria occurs in natural densities (approx. 1 plant m−2) at these sites which are characterized by naturally wet conditions, i.e. the root zone is saturated or even flooded throughout the year. Climatic conditions in the garden and field sites were identical and other natural L. salicaria populations grew within 2 km of the garden. This ensured that plants were exposed to near-natural conditions even during the phases of cultivation in the garden.

Cultivation of plant material

Ripe seeds of L. salicaria were collected in late summer 2006 from four native (German) and four invasive (North American) populations (Table 1). Although climates (and latitudes) in the native and invasive range differ and this may affect plant trait expression (Colautti et al., 2009), Europe has probably been an important source for most invasions of purple loosestrife. The selected populations stem from the core distribution of that species in both ranges. As we aimed at generalizing our results about potential selection through enemy release and because multiple invasions are likely to have occurred in the US, we deliberately chose invasive populations from two regions for which a different invasion history has been suggested (Chun et al., 2009), i.e. similarity in traits among the regions are unlikely to be the result of the same bottleneck. The seeds were collected from 20 randomly selected individuals (i.e. maternal sibships) in each population and bagged individually. Handling of seeds, germination, plant cultivation and hand pollination followed a protocol that has been used successfully in our own previous studies (Moloney et al., 2009a). Seeds were air dried and stored in paper bags during winter and stratified at 4 °C for 4 weeks in March to maximize germination success. Two experiments were performed. In 2007 (Generation 1), we used the maternal sibships from the field-collected seeds. In 2008 (Generation 2), we used seeds from eight plants per population that were raised from field-collected seeds in our common garden in 2007. During the growing season in 2007, at least three flowering stalks per plant were covered prior to flowering with a light fabric organza to prevent access by insect pollinators. Plants were then moved to a greenhouse to perform controlled hand pollination. Flowers were carefully observed and fresh flowers were selected for pollination when their petals were fully expanded and the stigma had a bright colour. Approximately 50 flowers per plant were then successively hand-pollinated. For the within-population crosses used in this experiment, each plant was pollinated with pollen from one other plant from the same native or invasive population that had the appropriate flower morph (see Barrett, 1993, for a description of the pollination system). The resulting seeds from hand pollination were collected for each seed family (i.e. mother plant) individually and treated and stored as explained above until the next growing season.

Table 1.

Sources of native and invasive populations of Lythrum salicaria used in the experiment

Origin Country Population (region) Latitude Longitude
Native Germany Unterjesingen (Tübingen) 48°52′N 8°98′E
Germany Hagelloch (Tübingen) 48°32′N 9°01′E
Germany Grube (Potsdam) 52°43′N 12°97′E
Germany Golm (Potsdam) 52°40′N 12°97′E
Introduced USA Beaver Run (New Jersey) 40°51′N 79°55′W
USA Hainesville county (New Jersey) 41°25′N 74°80′W
USA Boone Forks (Iowa) 42°17′N 93°56′W
USA Manly (Iowa) 43°16′N 93°07′W
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Seeds of both native and introduced L. salicaria were germinated in a greenhouse next to our common garden in May (2007 and 2008, respectively) in standard potting soil that shares common properties with the natural soil conditions of the native range. Pots were placed among plastic trays filled with water and their position was randomized twice. After 4 weeks, plants with similar size were selected and transplanted into larger pots (30 cm diameter × 26 cm depth). Twenty grams of slow-release fertilizer Osmocote (18 : 10 : 11 NPK) was applied to each pot 14 d after transplanting to ensure that growth was not limited by nutrients. Seedlings that died during the first 2 weeks of transplantation were replaced. The experimental design was similar in both generations, but differed in the number of replicates and seed material. In 2007 (field-collected seeds), we used three replicates (i.e. the same three randomly selected maternal sibships) per population in each of the two sites, resulting in a total of 48 pots (three plants × eight populations × two sites). In 2008 (second-generation seeds), we used four replicates (i.e. seed families from within-population crosses) per population and site, resulting in a total of 64 pots (four plants × eight populations × two sites).

Field experiments

Exposure of plants to enemies in a natural population was done with two considerations in mind: first, we selected the phase for which the maximum damage by above-ground herbivory (i.e. specialists and generalists) was observed, which is in July and August, when young larvae feed on shoot tips of purple loosestrife, and later when older larvae may cause heavy defoliation of plants (Blossey, 1995). Secondly, to avoid outcrossing of the natural populations with potentially invasive plant material, we returned all plants to the common gardens prior to flowering. Therefore, exposure in the natural sites was limited to 4 weeks, starting on 5 July 2007 and on 7 July 2008, respectively. Prior and after this time, plants were exposed to near-natural conditions (including native herbivores) in our common garden, yet little damage was observed at the seedling stage. Observations in a parallel study indicated a large (>90 %) overlap in the herbivory spectrum between field and common garden and altogether approx. 20 species of herbivores were observed (about 15 insect species including three specialists, and about five generalist mollusc species, R. Wegerer, University of Tübingen, Germany, pers. comm.).

Plants were transferred to the two field sites (Reusten and Unterjesingen) by placing potted plants randomly among the plants of the natural populations. Back in the common garden, pots were placed in flooded pools with six pots per pool. Due to dry conditions in the second year, two native plants died prior to seed-set, resulting in a slightly unbalanced design with respect to populations.

Response variables

Because many plants did not produce seeds, either due to drought (year 2) or due to herbivory of apical meristems (both years), we selected final above-ground biomass as a proxy for fitness. Parallel investigations have indicated that plant biomass is a good correlate for seed production (r2 = 0·346, P < 0·001 for natives; r2 = 0·515, P < 0·001 for introduced plants, based on measurements on 200 plants per origin). Biomass was measured during October by harvesting all above-ground parts, drying them at 60 °C for 24 h and weighing them. To evaluate the potential for compensatory growth (which may be related to tolerance) in a non-destructive manner, we measured height of the plants repeatedly and calculated the plant relative height growth rate (RhGR) as: RhGR = [ln(harvest plant height)–ln(initial plant height at transplanting)]/growth days. We assessed both the total leaf area consumed and the percentage leaf damage for each plant by randomly selecting ten leaves per plant prior to measuring biomass. The leaves were scanned with an STD 1600+ scanner (Regent Instruments, Canada) and the absolute area consumed per leaf and percentage leaf damaged was estimated with the software WinFolia. Percentage leaf damage was calculated by reconstructing the leaf area before damage and dividing the leaf area consumed by the original area.

Statistical analyses

All statistical analyses were done with SPSS version 15·0 (SPSS Inc., 2007). Differences between origins in the dependent variables were tested for each year separately with hierarchical ANOVA models with continent as fixed factor and population (nested within continent) and field location as random factors. All data were log transformed to meet the assumptions of ANOVA.

RESULTS

In both generations, above-ground biomass was significantly larger for introduced than for native populations, but differences between the origins were larger in the second generation (Table 2; Fig. 1A). RhGR was greater for the native populations, and this difference was much more pronounced in the second generation, too (Table 2; Fig 1B). Overall there was much less leaf damage in the second generation than in the first one, but differences between origins in damage were much more pronounced in the second generation (Table 2; Fig. 1C, D). US plants experienced greater absolute and relative (percent) leaf damage than European plants in both generations, but the differences between the origins in relative damage were not significant in the first generation (Fig. 1D).

Table 2.

Summary of hierarchical ANOVAs constructed to test for the fixed effects of origin (invaded vs. native range), and the random effects of population (nested within origin) and location (two locations in the native range field experiment) on above-ground biomass, leaf area consumed, percentage leaf damage and relative height growth rate (RhGR)

Above-ground biomass
 Leaf area consumed
 Percentage leaf damage
 RhGR
Source of variation d.f. MS F P MS F P MS F P MS F P
Year 1
Origin 1 0·364 20·345 < 0·001 0·209 6·196 0·017 0·203 2·180 0·148 1·20 × 10P−4 4·649 0·037
Population (Origin) 6 0·032 1·875 0·110 0·019 0·570 0·752 0·110 1·182 0·336 1·44 × 10P−5 4·434 0·002
Location 1 0·001 0·083 0·775 <0·001 0·012 0·914 0·007 0·080 0·779 1·33 × 10P−5 0·517 0·477
Error 39 0·017 0·034 0·093 2·58 × 10P−5
Year 2
Origin 1 1·602 72·170 < 0·001 0·102 12·621 0·001 1·527 20·018 < 0·001 7·26 × 10P−4 25·582 < 0·001
Population (Origin) 6 0·009 0·385 0·885 0·003 0·370 0·895 0·018 0·239 0·962 9·26 × 10P−5 3·263 0·008
Location 1 0·046 2·078 0·155 <0·001 0·013 0·910 0·223 2·922 0·093 1·96 × 10P−5 000·691 0·410
Error 39 0·008 0·076 2·58 × 10P−5
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Two separate ANOVAs were performed for each of two experiments – with field-collected seeds (year 1) and with seeds raised from hand-pollination of first-year plants in a common garden (year 2). All data were log transformed prior to analysis. Significant (P < 0·005) effects are highlighted in bold type.

Fig. 1.

Fig. 1.

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(A) Above-ground biomass, (B) relative height growth rate, (C) leaf area consumed and (D) percentage leaf damage for native and introduced populations of Lythrum salicaria; data are means ± s.e. First-generation (2007) plants were grown from field-collected seeds, and second-generation (2008) plants from seeds produced in common conditions.

DISCUSSION

Our results provide evidence for the EICA hypothesis for L. salicaria because differences in plant performance and the release from natural enemies in invasive plants were accompanied by microevolutionary changes, indicated by apparent differences between origins in second-generation plants. In the following, we discuss our findings with respect to our initial hypotheses.

Introduced plants exhibit larger leaf damage than native plants

In both generations, absolute and relative leaf damage was markedly larger for introduced plants than for native ones when exposed to the natural herbivore spectrum in the native range. This indicates that release from enemies occurred during introduction of L. salicaria, and it was accompanied by evolutionary changes in resistance to herbivory, although differences in plant quality for herbivores cannot be ruled out entirely. Empirical studies attempting to test the ERH are plentiful, but they have yielded equivocal results. One reason may be that the approaches to test the ERH are rather diverse. For example, previous tests were conducted either only in the novel range for the invasive plants and not for their native conspecifics (e.g. Daehler and Strong, 1997; Agrawal and Kotanen, 2003; Mitchell and Power, 2003; Agrawal et al., 2005; Schutzenhofer et al., 2009; Hawkes et al., 2010). Other studies focused on few enemies (e.g. van Kleunen and Schmidt, 2003; Siemann and Rogers, 2003; Stastny et al., 2005; Zou et al., 2008b; Huang et al., 2010; Wang et al., 2011) and have produced results with either increased (Siemann and Rogers, 2003; Stastny et al., 2005; Zou et al., 2008b) or decreased (Oduor et al., 2011) damage of invasive provenances, or no difference (van Kleunen and Schmid, 2003).

Specialist and generalist herbivores may have different effects on invasive plant defence (Keane and Crawley, 2002; Bossdorf et al., 2004b; Joshi and Vrieling, 2005; Huang et al., 2010). Therefore, our approach of exposing invasive plants to the entire herbivore spectrum in the native range may be powerful because the probability of detecting reduced resistance is larger than with single enemies. Therefore, there is an urgent need for studies that expose both native and invasive conspecifics to the full enemy spectrum in the native range (Bossdorf et al., 2005). Although there are yet no field experiments conducted in natural populations, there is evidence from studies that shared some aspects with our design: they were conducted in the native range, plants were subjected to more than a single enemy, and plants from both the native and novel range were included (Wolfe et al., 2004; Meyer et al., 2005; Zou et al., 2008a, b; Oduor et al., 2011). In contrast to the studies from the native range, the results of these and our study were consistent and yielded strong support for reduced resistance (but see Oduor et al., 2011) in invasive plants against native enemies.

A key difference between our study and most of the above is that we used plants where maternal effects were removed. Furthermore, our field sites were replicated, outcrossing of invasive genotypes with native plants was avoided and plants were exposed to natural conditions in field populations. We believe that all these features are desirable for studies of the EICA hypothesis in the native range. The lack of, and need for, control of maternal environment effects has been highlighted previously (Bossdorf et al., 2005), but only few studies have included this consideration in their design (van Kleunen and Schmid, 2003; Meyer et al., 2005; Oduor et al., 2011). Furthermore, it has been shown that experiments conducted in a single garden in either or both ranges may yield spurious results (Maron et al., 2004; Williams et al., 2008; Moloney et al., 2009a). We therefore believe that our approach is useful for future studies of the EICA hypothesis, but it could be further complemented by a systematic appraisal of several possible enemy groups (e.g. Wolfe et al., 2004; Meyer et al., 2005; Zou et al., 2008a, b), including below-ground enemies (Rogers and Siemann, 2004). However, caution should be applied when transplanting invasive plants into native populations, in order to not to initiate a ‘back-invasion’ of novel genotypes.

Interestingly, decreased resistance did not translate into detectable fitness loss for the introduced plants, i.e. US plants grew consistently larger than European plants. Because biomass is positively correlated with seed number in L. salicaria, release from enemies seems to be associated with reallocation of resources from defence to growth and reproduction. This corroborates the observation made for L. salicaria by Blossey and Nötzold (1995), which initially inspired the EICA hypothesis. Although ideally competitive ability should be tested directly rather than inferred from plant size (e.g. Bossdorf et al., 2004a), our results provide some support for the EICA hypothesis. Namely, enemy release did occur in L. salicaria during transport and there were evolutionary changes in plant size associated with the invasion. Whether the large size of the US plants is a direct consequence of enemy release may be debated, but our differences in plant size in both generations and for populations from different source regions (Europe) and with different invasion history (US) indicate that the difference in vigour and response to herbivory are probably a result of evolutionary processes in the novel range. The observed larger biomass of US plants despite being attacked more indicates that invasives may be more tolerant to herbivory. Such findings have been reported before (Müller-Schärer et al., 2004; Rogers and Siemann, 2004), suggesting that tolerance could be more important than resistance in the novel range where most herbivores are generalists. However, while additional analyses indicated a very large tolerance of invasive plants (i.e. no effect of damage on performance), tolerance was not larger than for plants in the native range. Therefore, tolerance may be a trait that increases invasiveness of Lythrum in general, but this trait is characteristic for the species and did not evolve in the novel range.

Growth is larger in introduced populations

Interestingly, there were apparently contradicting results for biomass and relative height growth rate. As expected, invasive plants attained a much higher biomass than native ones, even when exposed to herbivory. However, although differences in relative growth rate and biomass allocation patterns may explain differences among plants in tolerance (Strauss and Agrawal, 1999; Stowe et al., 2000), we would expect plants that grow faster (i.e. native plants) to attain a larger final biomass. In addition, differences in total biomass were probably even more pronounced, because US plants also exhibit a massive root crown compared with European plants (Moloney et al., 2009b). One possible explanation for the contradictory pattern is that increased herbivory at the apical meristems may have stimulated lateral rather than vertical growth, or investment in below-ground biomass and that plant height, which we used as a non-destructive response variable, may be inappropriate for evaluating compensatory growth. However, Chun et al. (2010) detected similar differences among origins in a common garden in the invaded range. In their experiment, growth measurements were taken more frequently and the authors showed that whereas height growth rate was larger for native origins, they terminated growth earlier and a lower asymptote was reached for height and other growth parameters than for invasive plants. The authors explained this pattern with local adaptation of the two origins to the length of the growing season. Namely, plants of the native range flower earlier than those in the invaded range (see also Bastlová & Kvĕt, 2002, for large range of populations), and cessation of growth is correlated with flowering time (Chun et al., 2010).

Variation between generations

Although our findings were qualitatively similar in both generations, there were large differences in susceptibility to herbivory. All plants exhibited a greater average damage by herbivores in the first than in the second generation but relative differences between origins were more pronounced in the second year. Some caution is warranted in interpreting the findings for the two generations as such comparisons are necessarily confounded with differences between years. However, genetic effects are probably a main factor explaining differential response to herbivory between generations because absolute differences in performance between generations were similar across origins. Also, herbivore pressure was lower rather than higher in the second than in the first year, which makes it unlikely that the larger difference between continents in herbivore effects was generated by greater herbivore loads (i.e. larger effect size).

Few studies have been conducted for longer than a single season, but all found large differences among years. For example, Funk and Throop (2010) detected differences in herbivore damage between years, but these were in the opposite direction with larger effects in the drier than in the wetter year. Changes in herbivore fauna composition across years was observed, too, and could be another reason for annual variations in leaf damage (Agrawal et al., 2005).

The more interesting pattern in our study is the relative difference between origins in the two generations. Differences among origins were much greater in the generation where maternal effects were removed. This has important consequences for future studies of the EICA hypothesis because maternal effects seem to influence the outcome of tests of enemy release and competitive ability. Bossdorf et al. (2005) have highlighted this problem and suggested that inaccurate estimates of population differentiation are obtained if genetic control is lacking. Two studies grew second-generation plants (van Kleunen and Schmid, 2003; Meyer et al., 2005), but these plants were raised from rhizomes rather than from seeds, which may have caused unwanted effects. For example, Meyer et al. (2005) obtained a smaller origin effect on response to herbivory in Solidago gigantea in their second generation and attributed this finding to induced defence in the first year. Our results highlight the importance of conducting studies with appropriate seed material (Oduor et al., 2011), especially when testing hypotheses assuming evolutionary changes during and after introduction.

Conclusions

The lack of comprehensive tests of enemy release and the EICA hypothesis not only limits our understanding of invasion success but may also deprive us of the ability to discover effective means of biological control. For example, our findings of larger biomass in invasive plants despite being attacked more indicate that biological control with leaf-eating beetles that is applied for L. salicaria may not be very effective. Our results also support previous suggestions that replicated common garden experiments are needed for testing theories of invasion success.

ACKNOWLEDGEMENTS

We thank M. Seifan for support with statistics and K. Moloney for generously sharing his ideas about the design, and K. Moloney and R. Leimu for helpful comments on an earlier draft of the manuscript. F. Schurr, K. Moloney and C. Holzapfel provided seed material. This work was supported by the German Research Foundation – DFG [TI-338/8-1].

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