Common greenhouse• Lepidium draba plants from nine native (European) and 10

introduced (U.S.) populations were grown under standardized conditions in a greenhouse at CABI Bioscience, Switzerland

• After approximately 3 mon. growth the following parameters were recorded for each plant:

• Number of developing shoots• Length of longest leaf• Diameter of largest leaf• Above ground biomass• Below ground biomass

Leaf disc bioassay• Psylliodes wrasei adults (Fig. 1) were placed individually in tight-

locking Petri dishes lined with moistened filter paper for a 24hr starvation period.

• A single leaf of similar age was cut from each plant, and a 1.5cm diameter disc removed from each leaf with a cork borer and then placed into each dish.

• After a 24hr exposure period, leaf discs were removed from the Petri dishes and a digital photograph was taken of each disc.

• Leaf area consumed was measured using Scion Image © following the methods of O’Neal et al. (2002)

Analyses• Plant data were analyzed using a mixed effects model with

range (Europe/U.S.) as a fixed factor and populations within range as a random factor. For % leaf area consumed, plant type (L. draba Europe, U.S., L. draba spp. chalapense, and S. alba) was used as fixed factor and pair-wise t-tests were conducted to compare means among plant types.

No evidence for an ‘evolution of increased competitive ability’ for Lepidium drabaMichael G. Cripps, Jessica L. McKenney, William J. Price , Hariet L. Hinz†, and Mark Schwarzländer

Department of Plant Soil and Entomological Sciences, University of Idaho, Moscow ID, 83844-2339 Statistical Programs, College of Agricultural and Life Sciences, University of Idaho, Moscow, ID 83844-2337

†CABI Bioscience, Switzerland Centre, 1 Rue des Grillons, Delémont CH-2800, Switzerland

The ‘evolution of increased competitive ablity’ (EICA) hypothesis predicts that under standardized growing conditions plants from an introduced range should show increased performance relative to conspecifics from a native range, and that herbivores should show increased performance on less defended, introduced genotypes (Blossey & Nötzold 1995). This is because selection may favor reduced investment into costly herbivore defense, and more into growth or reproduction in the introduced range where plants are released from natural enemies. A biogeographic comparison of field populations of the invasive weed Lepidium draba L. (Brassicaceae) revealed that plants grow denser and more vigorously in the introduced U.S. range, compared to the native European range (McKenney 2005). To investigate whether the observed differences in growth were due to an EICA process, we grew L. draba populations from both ranges under standardized conditions and tested the feeding performance of the specialist herbivore Psylliodes wrasei Leonardi & Arnold.

DO575

Response

Variable

EUROPE

Mean ± (SE)

UNITED STATES

Mean ± (SE)

F1,17 P

Number of shoots 12.42 (2.34) 8.92 (1.33) 1.96 0.179

Longest leaf (cm) 23.1 (0.982) 22.8 (0.926) 0.02 0.878

Largest leaf diameter (cm)

6.18 (0.354) 5.86 (0.334) 0.43 0.522

Root biomass (g) 1.43 (0.128) 1.27 (0.121) 0.82 0.377

Shoot biomass (g) 3.24 (0.192) 2.62 (0.181) 5.48 0.0317

Total biomass (g) 4.67 (0.298) 3.89 (0.280) 3.64 0.0733

Fig. 1. Psylliodes wrasei (Chrysomelidae: Alticinae)

• All traits measured tended to be higher for the native European compared to the introduced U.S. populations of L.. Draba (Table 1).

• Shoot biomass was significantly greater for native populations (Fig. 2; Table 1).

• Mean proportion of leaf area consumed did not differ between native and introduced populations (t = 0. 92 df = 17, P = 0.369; Fig. 4).

• Hardly any feeding occurred on Sinapis alba, indicating that P. wrasei distinguished between plant species (t = 4.0, df = 17, P < 0.001).

Fig. 3. Leaf discs after 24hr feeding period. Left: Lepidium draba disc (8.16% consumed). Right: Sinapis alba disc (0% consumed).

Introduction

Methods

We thank J. Gaskin (USDA ARS, Sidney, MT) and A. Gassmann (CABI Bioscience, Switzerland) for seed collections, and A.Wins-Purdy and P. Jäger (CABI Bioscience, Switzerland) for technical assistance. This work was funded by a grant to M.S. from the USDA NRI Agreement no. IDA00108-CG.

Blossey, B., & Nötzold, R. (1995). Evolution of increased competitive ability in invasive nonindigenous plants: a hypothesis. Journal of Ecology, 83, 887-889.

Cripps, M.G. (2005). Enemy release and the evolution of Increased competitive ability as potential invasion mechanims of Lepidium draba L. in the western United States. M.Sc. Thesis. Moscow: University of Idaho.

McKenney, J.L. (2005). An inter-continental comparison of vigor and herbivory for the invasive plant Lepidium draba. M.Sc. Thesis. Moscow: University of Idaho.

Müller, C. & Martens, N. (2005). Testing predictions of the ‘evolution of increased competitive ability‘ hypothesis for an invasive crucifer. Evolutionary Ecology, 19, 533-550.

Müller-Schärer, H., Schaffner, U., & Steinger, T. (2004). Evolution in invasive plants: implications for biological control. Trends in Ecology & Evolution, 19, 417-422.

O'Neal, M.E., Landis, D.A., & Isaacs, R. (2002). An inexpensive, accurate method for measuring leaf area and defoliation through digital image analysis. Journal of Economic Entomology, 95, 1190-1194.

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(%

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Table 1. Results of mixed model ANOVA for the effects of continent (Europe, U.S.) and population within continent on growth parameters of Lepidium draba grown under standardized conditions.

Fig. 2. Root and shoot biomass (mean SE) for Lepidium draba populations from the U.S. and Europe. Each population consists of 9 to 10 replicates. The two bars on the right (US and EU) are the continent means computed by taking the mean of the population means.

Fig. 4. Mean ( SE) proportion of leaf discs consumed for populations of U.S. (10 populations) and European (9 populations) Lepidium draba and two related plants (gray bars), L. draba spp. chalapense and Sinapis alba. The two bars to the right (US and EU) are the continent means computed by taking the mean of the population means. There are 9 to 10 replicates for each population.

Results Summary and conclusions

Acknowledgments

References

United States Europe US EU

United States Europe US EU

L. d

. ch

alap

ense

S.

alba

• Contrary to the EICA hypothesis, native European L. draba grew equal to, or larger than their conspecifics from the introduced U.S. range; and the specialist herbivore, P. wrasei, showed no difference in feeding between populations from either range.

• One possible explanation is increased investment into generalist herbivore defense for introduced populations.

• While EICA assumes reduced herbivore pressure in the introduced range, a comparative study of L. draba found greater abundance of generalist herbivores in the introduced range (Cripps 2005).

• EICA also predicts decreased investment in herbivore defense, however, higher concentration of the primary glucosinolate (p-hydroxybenzyl) in seedlings and greater myrosinase activity in rosettes of L. draba from the introduced range was documented by Müller & Martens (2005).

• Therefore, increased investment in qualitative defense (i.e. glucosinolates) against generalists in the introduced range may explain the decreased performance of introduced L. draba observed in this study (Müller-Schärer et al. 2004).

• We suggest that invasion mechanisms other than EICA, such as release from specialized herbivores, or novel allelopathic interactions, may explain the invasive nature of L. draba in its introduced range.