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Comparing Invasive Plants from Their Native and Exotic Range: What Can We Learn forBiological Control?Author(s): Hariet L. Hinz and Mark SchwarzlaenderSource: Weed Technology, Vol. 18, Invasive Weed Symposium (2004), pp. 1533-1541Published by: Weed Science Society of America and Allen PressStable URL: http://www.jstor.org/stable/3989688Accessed: 19/02/2010 18:33

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Weed Technology. 2004. Volume 18:1533-1541

Symposium

Comparing Invasive Plants from Their Native and Exotic Range: What Can We Learn for Biological Control?1

HARIET L. HINZ and MARK SCHWARZLAENDER2

Abstract: This article reviews and summarizes data sets that attempt to compare performance, population dynamics, and herbivory of invasive plant species. Specifically, we review results from studies comparing (1) individual and population parameters of plant invaders in their native and exotic range, (2) herbivore pressure and natural enemy guilds associated with plant invaders in their native and exotic range, and (3) performance and defense levels of native and exotic populations of the invasive under standardized conditions and the performance of selected herbivores. We found a total of 39 published and 2 unpublished studies, investigating 40 plant species. The majority of studies within the first category showed that invaders form larger populations, grow denser, have higher reproductive output, larger seed banks, and higher regeneration rates in the exotic compared with the native range. In contrast, plant vigor was not always greater, presumably because of increased intraspecific competition. Nearly all studies within the second category showed that herbivory (percentage of attack, species number and load) was reduced and, if investigated, that the herbivore community shifted from specialists to generalists and from endophagous to exophagous species in the exotic compared with the native range. Under standardized conditions (category 3), an equal number of studies found increased vigor or no significant differences between plants from the exotic and native range. In a few cases, plants from the exotic range grew less vigorous. Specialist herbivores generally preferred or developed better on plants from exotic compared with native populations, which were also better defended. Studies comparing plant invaders at field sites in their native and exotic range or under standardized conditions can help identify factors facilitating successful invasions, which in turn can improve the selection and efficiency of biological control agents and the development of integrated management strategies. We therefore suggest to include comparative studies more frequently in biological control programs and to extend them to include manipulative experiments and comparisons of control agents. Additional index words: Biological weed control, comparative studies, enemy release hypothesis, evolution of increased competitive ability hypothesis. Abbreviations: EICA, evolution of increased competitive ability; ERH, enemy release hypothesis.

INTRODUCTION

Two mechanisms have been used to explain the success of invasive plants in their exotic range. First, more favorable abiotic conditions or release from biotic constraints in the exotic range, and second, postinvasion evolution of increased competitive ability (EICA) due to altered selection pressures in the exotic range (Blossey and Notzold 1995). Biotic constraints in the native range

I Received for publication February 23, 2004, and in revised form June 21, 2004.

2 Senior Research Scientist, CABI Bioscience Switzerland Centre, I rue des Grillons, 2800 Del6mont, Switzerland; Assistant Professor, Department of Plant, Soil, and Entomological Sciences, University of Idaho, Moscow, ID 83844-2339. Corresponding author's E-mail: [email protected].

include natural enemies (Keane and Crawley 2002; Shea and Chesson 2002) and competitively superior neighbors, such as plant competitors adapted to allelopathic effects (Bais et al. 2003; Callaway and Aschehoug 2000). More favorable abiotic conditions can, for example, include more favorable climatic conditions (Crawley 1987; Erfmeier and Bruelheide 2004) or an- thropogenically altered disturbance or nutrient regimes (or both) in the exotic range, to which plant invaders are often better suited to take advantage of than conspecific natives (Daehler 2003; Shea and Chesson 2002).

Both mechanisms are expected to result in increased vigor, survival, seed output, and establishment rates in the exotic range. There are, however, surprisingly few

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HINZ AND SCHWARZLAENDER: COMPARING INVASIVE PLANTS

studies that quantitatively compared plant and population parameters for invasives between their native and exotic range to verify this assumption. A comparison of stem heights of 651 plant species occurring in Europe, Cali- fornia, and the Carolinas found that species do not grow taller in the exotic range (Thebaud and Simberloff 2001). The study did not, however, distinguish between invasive and noninvasive plant species, considered only one plant parameter, and was based on literature records rather than field data. Release from natural enemies, postulated in the enemy release hypothesis (ERH) (Keane and Crawley 2002), is often thought to be one key factor facilitating invasion success of exotics (Blossey and Not- zold 1995; Crawley 1987) and is used to justify the con- cept of classical biological control, the deliberate introduction of host-specific natural enemies from the native range of the invasive plant (DeBach and Rosen 1991). In addition to the direct benefit of reduced levels of herbivory, the release from natural enemies has also been hypothesized to cause a shift of resource allocation from defense toward increased growth in exotic populations of plant invaders, which in turn should lead to an improved performance of introduced biological control agents (Blossey and Notzold 1995).

In this article we review and summarize data sets that attempt to compare performance, population dynamics, and herbivory of invasive plant species. Specifically, we review results from studies comparing (1) individual and population parameters of plant invaders in their native and exotic range, (2) herbivore pressure and natural enemy guilds associated with plant invaders in their native and exotic range, and (3) performance and defense levels of native and exotic populations of the invasive under standardized conditions and the performance of selected herbivores. We first summarize the results of each type of study, then discuss their relevance for biological weed control, and finally suggest future research.

MATERIALS AND METHODS

We searched the CAB Abstracts (CAB International 2004) for studies published between 1976 and 2004 using various combinations of the following keywords and terms: evolution of increased competitive ability, enemy release hypothesis, comparative studies, comparative, comparison, exotic, invasive, introduced, native, range, populations, and invasive plants. Additional studies were found by cross-referencing of publications found during the initial search. We also contacted authors of recently published studies to ask for additional sources and unpublished data. Only those studies were considered that

investigated populations of an invasive plant at field sites in the native and exotic range. We excluded studies that compared field data from one range with literature data from the other range, as well as studies that compared invasive species with native congeners and literature reviews. We found a total of 38 published and 2 unpublished studies, investigating 40 plant species. All plant species investigated were described by the authors as invasive or at least "well established as naturalized aliens" (Fenner and Lee 2001). Approximately half of the plants (53%) are targeted for classical biological control (Julien and Griffiths 1998).

RESULTS AND DISCUSSION

Category 1: Studies Comparing Parameters of the In- vasive at Field Sites in the Native and Exotic Range. We found 17 published and 2 unpublished studies investigating 16 plant species that compared plant parameters in the native and exotic range (Table 1). We also included research by Sheppard et al. (2002) and Paynter et al. (1998, 2000) on scotch broom [Cytisus scoparius (L.) Link], which report data for similar manipulative experiments conducted in the exotic (Sheppard et al. 2002) and the native range (Paynter et al. 1998, 2000). The majority of studies found larger values for population size, plant density, vigor, reproductive output, and seed bank size in the exotic compared with the native range. For two shrub species, giant sensitive plant (Mi- mosa pigra L.) and rhododendron (Rhododendron ponticum L.), densities for mature plants were smaller or equal in the exotic range (Erfmeier and Bruelheide 2004; Lonsdale and Segura 1987). In both cases, results were explained by larger individual plant size in the exotic range, which resulted in fewer individuals per unit area. For common heliotrope (Heliotropium europaeum L.), only maximum plant densities were similar in the exotic and native range, whereas average density and infesta- tion size were greater and their occurrence more frequent in the exotic range (Sheppard et al. 1996). For studies in which no differences in plant vigor or reproductive output, or reduced values for the exotic range were found, authors assumed that larger densities per unit area in the exotic range increased intraspecific competition, which impaired individual plant performance (Paynter et al. 2003; Sheppard et al. 1996; Van et al. 2001; J. Payn- ter, unpublished data), or the outcome depended on the nutrient conditions of the sites compared (Edwards et al. 1998), the plant parameter investigated (Erfmeier and Bruelheide 2004), or whether values were given per plant or per unit area (Sheppard et al. 1996). A subset

1534 Volume 18, Invasive Weed Symposium 2004

Table 1. Summary of studies comparing traits of invasive plants in their native (N) and exotic (E) range. The references for each plant species listed are footnoted.

Number of studies in which

x ~~~~~~~~~~~~~~~~~~~~~~E > N E =N E <N Total no. of

Trait studies No. Plant speciesa No. Plant species,, No. Plant species

Population size 3 3 Lythruin salicaria3 0 0 Senecio inaequidens'`

0 Solidago gigantea9 ,, Density 12 12 Acroptilon repens' 2 Heliotropium europaeum 1 Mimosa pigra

Carduus nutans9 Rhododendron ponticum5 0 Cytisus scoparius'2

Genista monspessulana'3 Heliotropium europaeum'5 Lepidium draba7,8 Lythrum salicarial4 Melaleuca quinquenervia'7 Rhododendron ponticum5 Solidago gigantea9

Vigor 10 6 Acroptilon repens'? 4 Cytisus scoparius'2 2 Heliotropiuni europaeum'9 Lythrum salicarial Heliotropium europaeum'5 Rhododendron ponticumt Mimosa pigra" Lythrum salicaria4 Rhododendron ponticum5 Melaleuca quinquenervia'7 Senecio inaequidens"4 Solidago gigantea9 m

Reproductive output 11 8 Acacia longifolia'8 4 Echium plantagineum6 I Heliotropium eurotaeum'5 Carduus nutans'9 Genista monspessulana'3 Chrysanthemoides monilifiral5 Lythrumn salicaria4 Heliotropium europaeum'5 Rhododendron ponticum5

Lythrum salicaria4 Melaleuca quinquenervia'7 Mimosa pigra 8 Senecio inaequidens'4 Solidago gigantea9

Seed bank 5 5 Acacia longifolial' 0 0 Carduus nutans'9 Chrvsanthemoides moniliferalx Genista monspessulana'3 Heliotropium europaeum'5 Mimosa pigra"

Seed size 1 1 Cvtisus scoparius2 I Ulex europaeus2 0 Seedling establishment rates 2 2 Echium plantagineum6 0 0

Rhododendron ponticum5 Seedling survival 2 0 0 2 Cytisus scoparius'w

Echium plantagineumf Germination or reproductive period 3 2 Acroptilon repens"'' I Acacia longifjlia'5 0

Carduus nutansI9 Chrysanthemoides moniliferall Proportion of young stages 3 3 Cvtisus scoparius'2 0 0

Genista monspessulana'3 Melaleuca quinquenervia"7

Average age I 0 1 Rhododendron ponticum5 I Rhododendron ponticum5 Maximum age 2 1 Genista monspessulana'l I Cytisus scoparius'2 0

t References for plant species are ' Bastlova-Hanzelyova (2001); 2 Buckley et al. (2003); ' Eckert et al. (1996); Edwards et al. (1998); 5Erfmeier and Bruelheide (2004); ' Grigulis et al. (2001); 7H

L. Hinz, M. Cripps, W. Fu, K. Medina, and H. Reicher (unpublished data); 8H. L. Hinz, M. Cripps, J. L. Renteria Bustamante, and A. Wins-Purdy (unpublished data); 9 Jakobs et al. (2004a); (" J. e Littlefield and U. Schaffner (unpublished data); " Lonsdale and Segura (1987); 2 Paynter et al. (2003); 11 J. Paynter, A. Sheppard, C. Preston, and R. Roush (unpublished data); '4 Prati and Bossdorf (,, (2002); 15 Sheppard et al. (1996); "t Sheppard et al. (2002); 17 Van et al. (2001); "8 Weiss and Milton (1984); " Woodburn and Sheppard (1996).


of the studies compared aspects of the population biology of the invasive, such as transition rates, phenology, or age structure. Seedling establishment rates and the proportion of young life stages were higher, and germination, flowering, or reproductive periods were generally longer in the exotic compared with the native range (Ta- ble 1). These results were explained with greater resource allocation to reproduction and more suitable environmental conditions for seedling establishment (Erf- meier and Bruelheide 2004; Van et al. 2001) or lower seed predation rates and interspecific competition (Gri- gulis et al. 2001; Weiss and Milton 1984). Consequently, regeneration rates were higher in the exotic range, en- abling invasive plants to persist once established instead of being replaced by other plant species (Paynter et al. 2003; Van et al. 2001; Paynter, unpublished data). The authors concluded that introduction of biological control agents that reduce the reproductive output in combina- tion with management practices, which increase interspecific plant competition, thereby reducing seedling establishment rates, might be the most effective control approach (Grigulis et al. 2001; Paynter et al. 2003; Van et al. 2001). Using a simple population model, Noble (1989) reached a similar conclusion. Reduced seedling survival to mature plant in the exotic compared with the native range was linked either to increased intraspecific competition (Grigulis et al. 2001) or to different patterns of seedling germination leading to higher drought-in- duced mortality (Sheppard et al. 2002). The hypothesis that invasive plants grow older in the exotic range was not confirmed (Table 1). Another interesting trait, which to our knowledge, has so far not been compared quantitatively, is the life history type. There are observations that some invasive plants appear to have changed from a predominantly semelparous to an iteroparous life history in their exotic range (Miiller-Scharer et al. 2004). Plants of the facultative biennial houndstongue (Cynog- lossum officinale L.), for example, are strictly semelparous in their native Eurasian range, whereas in the exotic range in North America, a proportion of plants behave iteroparously (M. Schwarzlinder and J. Williams, personal observations). There is at least circumstantial evidence that reduced survival to reproduction caused by herbivory in the native range can lead to a monocarpic habit, whereas a polycarpic life cycle might increase in the absence of herbivory in the exotic range (Muller- Scharer et al. 2004).

Considering the magnitude of problems caused by invasive plant species, the number of studies quantifying plant and population parameter differences between the

native and exotic range is surprisingly small. There might be two reasons for the lack of such studies: (1) plant growth differences between the exotic and native range seem so obvious that studies actually testing the assumption were not perceived necessary, and (2) comparative studies on different continents are logistically difficult and costly. The summarized studies show for most plant traits that invasive plants tend to perform better in their exotic compared with their native range, thereby supporting assumptions of the ERH and EICA hypothesis. However, there were also cases in which performance was poorer in the exotic range or where no differences were found, and the outcome often depended on the parameters investigated or environmental conditions. Therefore, results cannot necessarily be extrapo- lated between exotic ranges (K. Shea, unpublished data) or between habitats within one exotic range (Sheppard et al. 2002). It should also be noted that the studies listed differ greatly in quality, for example, number of populations investigated, rigor in data collection and statistical analysis. Despite these limitations, our review showed that comparing plant and especially population parameters of invaders in their native and exotic range can help identify potential factors facilitating invasion success. This in turn can aid more effective control ef- forts, including biocontrol agent selection (Sheppard 2000, 2003). In addition, simulation models that aim to predict the outcome of different management strategies could be rendered more realistic and robust by using quantitative data on population parameters from both ranges (Paynter et al. 2003; Rees and Paynter 1997). Data on population sizes and densities are also useful to quantify the magnitude of invasive plant species problems in the exotic range and provide arguments for or against the need of a biological control program. Finally, such studies contribute important baseline data for pre- and postrelease comparisons to quantify the effective- ness of biological control agent introductions.

Category 2: Studies Comparing Herbivore Pressure and the Natural Enemy Complex of the Invasive Plant in the Native and Exotic Range. We found 15 field studies comparing levels of herbivory or the natural enemy complexes (or both) of 25 plant species in their native and exotic range (DeWalt 2003; Edwards et al. 1998; Fenner and Lee 2001; Goeden 1974; Grigulis et al. 2001; Hinz et al. 2003, 2004; Memmott et al. 2000; Paynter et al. 1996; Prati and Bossdorf 2002; Sheppard et al. 1996; Van et al. 2001; Weiss and Milton 1984; Wolfe 2002; Woodburn and Sheppard 1996). In all but one (Sheppard et al. 1996), herbivory was reduced in the

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WEED TECHNOLOGY

exotic range, indicated through a reduction in percentage of attack, lower phytophagous species numbers, or reduced herbivore loads, for example reduced herbivore biomass (Memmott et al. 2000). In addition, there was a shift from specialists to generalists and from endophagous to exophagous arthropods in the exotic compared with the native range (Goeden 1974; Memmott et al. 2000). These results are confirmed by studies not included in our review that compared field with literature data (Jobin et al. 1996; Strong et al. 1984; Syrett et al. 1999; Wilson et al. 1990). They are also in concordance with a recent literature review that found that the number of pathogen species was smaller on plants in their naturalized compared with their native range, and that species that were more completely released from pathogens were also more widely reported as harmful invaders (Mitchell and Power 2003).

Maron and Vila (2001) stated that plant invaders are often rapidly discovered by native herbivores and that the phytophagous fauna of introduced species can be as diverse as that of native species. However, in one of the examples they used, only a small group of herbivore species, i.e., leaf-miners, was considered (Auerbach and Simberloff 1988). In addition, although similar numbers of herbivores might accumulate on plants in the exotic compared with the native range (Wilson et al. 1990), these might consist of many individuals of only a few generalist species. On hoary cress (white top) (Lepidium draba), for example, Lygus hesperus Knight (Hemiptera, Miridae) accounted for 54% of the total number of spec- imens collected in the exotic range (M. Schwarzlaender and J. McKenney, unpublished data). The number of herbivore species acquired depends on the time period the invasive plant is already present in the introduced range and the family the plant invader belongs to, i.e., the abundance of native congeners in the introduced range (Andow and Imura 1994; Strong et al. 1984). The weevil Euhrychiopsis lecontei (Dietz) (Coleoptera, Curculioni- dae), for example, is a herbivore of the native North American watermillfoil (Myriophyllum sibericum Ko- marov) that moved to the invasive Eurasian watermillfoil (M. spicatum) and is currently considered as a control agent for the aquatic invader (Sheldon and Creed 1995). In some instances, specialist herbivores have been acci- dentally introduced into the exotic range along with the invasive plant (Syrett et al. 1999; Tewksbury et al. 2002). Information on the natural enemy complex present, and the impact on the invasive, is important for the decision to proceed with new biological control introductions or not and for the selection of potential control agents.

In summary, there is substantial indication for a cor- relation between release from natural enemies and invasion success and a shift in the composition of natural enemies toward more exophagous generalists. However, to prove this hypothesis, parallel herbivore exclosure experiments conducted in the native and exotic range of a plant invader are necessary, as suggested by Maron and Vil'a (2001) and Keane and Crawley (2002).

Category 3: Comparing Populations from the Native and Exotic Range under Standardized Conditions. The EICA hypothesis predicts that in the exotic range, invasives should shift their resource allocation from defense to growth or reproduction (or both) because they were introduced without their specific natural enemies (Blossey and Notzold 1995). When growing native and exotic populations under standardized conditions, exotics should therefore grow more vigorously, exhibit reduced defense, and herbivores should show preference for or improved performance on (or both) exotic populations of the invasive plant. We found 17 studies, covering 14 plant species, testing the EICA hypothesis in respect to plant vigor, seven studies that tested the response of herbivores, and two that compared levels of defense (Table 2). In respect to plant vigor, equal numbers of studies supported EICA (exotic range [E] > native range [N]) as ones that did not (E = N). However, when considering the number of plant species investigated, results of only six species supported EICA (E > N), whereas results were similar (E N) for 12 species tested (Table 2). In four cases, effects were reversed, i.e., plants from exotic populations showed in at least one parameter measured a reduced performance compared with plants from native populations (E < N). The outcome often depended on the parameter investigated, i.e., for the same plant species, some parameters were greater, some smaller, and some did not differ significantly between plants from the native and exotic range (Jakobs et al. 2004b; Van Kleu- nen and Schmid 2003). Similarly, different studies yield- ed contrasting results for the same plant, for example, St. Johnswort (Hypericum perforatum) (Maron et al. 2004; Pritchard 1960; Vil'a et al. 2003). We can only speculate about the underlying reasons for this variability. Plants might have changed their resource allocation from, for instance, vegetative growth toward higher sex- ual reproduction (DeWalt 2003), leading to contrasting results between plant parameters, or the outcomes might be due to specific genotypic characteristics of the populations selected from each range for comparison. It has been shown for saltcedar (Tamarix spp.), for instance, that the most common genotype in the exotic range is a

Volume 18, Invasive Weed Symposium 2004 1537

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Table 2. Summary of studies comparing invasive plant species vigor (including reproductive output), defense levels, and herbivore preference and performance between native and exotic populations under standardized conditions. The references for each plant species listed are footnoted.

Number of cases in which

Total no. of E> N E N E<N Trait studies No. Plant species6 No. Plant speciesb No. Plant speciesi N

Plant vigor 17 10 Eschscholzia californica"' 10 Alliaria petiolata4 4 Alliaria petiolata4 Z Hypericum perforatutl ' Carduus nutans'8 Lepidium draba7 Lythrum salicaria',23c,'6 Clidemia hirta6 Solidago canadensis'4 Sapium sebiferum'2 '3 Digitalis purpureal6 Spartina alrerniflora5d Solidago canadensis'4 Echium vulgare'l6 Solidago gigantea8 Eschscholzia californica'? N

Hypericum pefoioratuml- Lepidium draba7 Lythrum salicaria'6 7 Senecio jacobaea8II Solidago canadensis'4 Solidago gigantea8 0

Herbivores Specialists 5 4 Alliaria petiolata4 2 Lythrum salicaria' 27 0

Lythrum saiicaria23 c

Spartina alterniflora5l Generalists 4 1 Sapium sebiferum'3 4 Alliaria periolata. 0

Lythru.n salicaria'7 Sapium sebiferum9S3 s

Defense 2 O (0 2 Lythra j iAil Sa,piumsbfeul

D~~~~~~~~~~~~~~~~~~~~~~~~~~~ Abbreviations: E, exotic range; N, native range. References for plant species are I Bastlova and Kvet (2002); 2 Blossey and Notzold (1995); Blossey and Kamil (1996); 4 Bossdorf et al. (2004); Daehler and Strong (1997); 6 DeWalt et al. (2004);

H. L. Hinz, M. Cripps, J. L. Renteria Bustamante, and A. Wins-Purdy (unpublished data); I Jakobs et al. (2004b); 9 Lankau et al. (2004); '0 Leger and Rice (2003); 11 Pritchard (1960); 2 Siemann and

cko Rogers (2001); '3 Siemann and Rogers (2003); 14 Van Kleunen and Schmid (2003); i5 Vila et al. (2003); 6 Willis and Blossey (1999); 17 Willis et al. (1999); 18 Willis et al. (2000). c No statistical test provided. dComparison within North America.

'0

0

c

WEED TECHNOLOGY

novel hybrid that is not found in the plants' native range (Ellstrand and Schierenbeck 2000; Gaskin and Schaal 2002). In some studies, trends were observed, but they were not statistically significant, often because of an in- adequate number of populations tested per origin or high variability between populations of the same origin (or both) (Willis and Blossey 1999; Willis et al. 1999). In addition, Maron et al. (2004) found strong latitudinally based differences in parameters and argued that at least reciprocal common gardens in the native and exotic range are necessary to unambiguously demonstrate ge- netically based changes. Studies comparing the response of herbivores and plant defense mostly confirmed the EICA hypothesis, i.e., specialist herbivores preferred or performed better on populations from the exotic range of the invasive, whereas plant defense was reduced (Ta- ble 2). In contrast, most generalist herbivores did not distinguish between exotic and native populations. This result was expected because generalist herbivores are also present in the exotic range, and a reduction in generalist defense therefore provides no fitness gain for the invasive. The overall outcome of studies testing the EICA hypothesis probably depends on the defense sys- tem of the plant investigated, whether a trade-off between growth and defense actually exists, and on the type and intensity of herbivory in the exotic range (Muill- er-Scharer et al. 2004).

The results summarized indicate that introduced biological control agents should perform better or at least equally well on exotic as compared with native populations of an invasive plant. At the same time they show that postinvasion genetic changes can occur and that these have the potential to influence the probability for successful biological control (Gaskin and Schaal 2002).

Therefore, studies quantifying potential performance differences of biocontrol agents on populations from the exotic and native range of the invasive before introduction would be valuable. It has been shown for example that in particular very specific agents may prefer or de- velop better (or both) on certain genotypes (Lym and Carlson 2002; Lym et al. 1996) or only control one particular form of the invasive, which obviously reduces their efficiency to sustainably control the plant (Burdon et al. 1981).

Few studies compared both plants at field sites in the exotic and native range and exotic and native populations under standardized conditions. Jakobs et al. (2004a, 2004b) found similar results in both comparisons, indi- cating that the increased vigor observed at field sites likely has a genetic basis. In contrast, Hinz et al. (2003,

2004) found that plants performed better at field sites in the exotic range, but no evidence for improved growth of exotic populations under standardized conditions, in- dicating that factors other than genetic changes are re- sponsible for the invasion success of the plant studied. Ideally, both types of studies should therefore be conducted.

CONCLUSIONS

The few studies comparing plant invaders at field sites in their native and exotic range or under standardized conditions that we summarized here revealed many useful applications. The most interesting, in our opinion, is that they can help identify factors facilitating invasion success (Grigulis et al. 2001; Paynter et al. 2003). Apart from contributing to a better understanding of general invasion mechanisms, this knowledge can also improve the selection and efficiency of biological control agents and the development of integrated management strategies to control invasives. At the same time such studies can provide arguments for or against a biological control program, contribute baseline data for pre- and postrelease comparisons, and data to feed into more realistic simulation models that aim to predict the outcome of different management strategies.

We therefore suggest including comparative studies more frequently in biological control programs, ideally combining comparisons at field sites and under standardized conditions. In particular, detailed investigations on the demography of the invasive are best suited to determine potential invasion mechanisms. Ideally, studies should also be extended to include manipulative treatments to investigate the differential effects of, for example, interspecific competition, herbivory, or nutrient and water levels on the invasive in its native and exotic range, and should be conducted over a range of environmental gradients with adequate replication. Such comparisons would allow to causally link specific factors with invasion success, which in turn can be used to de- velop specific management strategies. Finally, we suggest to conduct similar comparisons on the demography of released biological control agents in their native and introduced range. We believe that such comparisons could help determine why some biological control agents reach outbreak densities in their introduced range, whereas others stay rare or fail to establish. This could further improve the predictability and therefore credibil- ity of biological control.



ACKNOWLEDGMENTS

We thank A. Erfmeier, G. Jakobs, J. Littlefield, U. Schaffner, A. Sheppard, T. Van, and E. Weber, who gen- erously shared their unpublished data or submitted man- uscripts with us, and A. Sheppard and Q. Paynter for inspiring discussions. We also thank H. Muiller-Scharer and two anonymous reviewers for improving earlier ver- sions of the manuscript.

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