Consequences of invasion by the alien plant Mimulus guttatus on the species composition and soil properties of riparian plant communities in Scotland

ARTICLE IN PRESS Perspectives in Plant Ecology, Evolution and Systematics Perspectives in Plant Ecology, Evolution and Systematics 10 (2008) 231–240 www.elsevier.de/ppees

Consequences of invasion by the alien plant Mimulus guttatus on the species composition and soil properties of riparian plant communities in Scotland Anne-Marie Truscotta,b,, Steve C. Palmerc, Chris Soulsbyb, Sally Westawaya, Phil E. Hulmea,d a

Centre for Ecology and Hydrology, Banchory Research Station, Aberdeenshire, AB31 4BW, UK Department of Geography, University of Aberdeen, Aberdeen, AB24 3UF, UK c School of Biological Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK d National Centre for Advanced Bio-Protection Technologies, P.O. Box 84, Lincoln University, Canterbury, New Zealand b

Received 3 April 2007; received in revised form 26 February 2008; accepted 3 April 2008

Abstract Invasive plant species are widely recognised to have severe ecological impacts in a wide range of ecosystems throughout the world, yet there are few experimental studies measuring community-level effects of invasive plant species. Thus most evidence is from correlative studies, and as such often cannot easily disentangle cause and effect. Through a combination of an addition and removal experiment and a correlative approach (multi-site comparisons), this study aimed to quantify the effects of a widespread invasive species, Mimulus guttatus, on species richness and soil properties of riparian plant communities. The marked negative association between Mimulus cover and plant species richness identified through correlative multi-site comparisons was consistent with experimental removal studies which indicate Mimulus significantly alters the structure of riparian plant communities. Total C and N and soil moisture were marginally higher in invaded than in uninvaded disturbed sediment plots. Following Mimulus removal, there was an increase in the occurrence and abundance of another non-native species, Claytonia sibirica, as well as germination and establishment of Mimulus seedlings. This highlights that, although removal increased richness, bringing the plant community closer structurally to uninvaded vegetation, the application of removal as a management tool needs to be undertaken with caution, as it may create opportunities for other invaders. The impact of Mimulus appeared restricted to disturbed sediment communities, as addition experiments into herb–grass communities were relatively unsuccessful in establishing Mimulus. These patterns were consistent with the distribution of the species in riparian plant communities. The addition experiments highlight that, as well as competition from the resident vegetation community, mollusc herbivory further hinders the establishment of Mimulus. Many manipulation studies have removed invasive plant species from heavily invaded communities, and it is often

Corresponding author at: Centre for Ecology and Hydrology Edinburgh, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK. Tel.: +44 7780 681754. E-mail address: [email protected] (A.-M. Truscott).

1433-8319/$ - see front matter r 2008 Ru¨bel Foundation, ETH Zu¨rich. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.ppees.2008.04.001

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thought that invasive species usually affect plant community structure only where their cover is high. This study is unique in demonstrating impacts where cover of the invasive plant is relatively low. r 2008 Ru¨bel Foundation, ETH Zu¨rich. Published by Elsevier GmbH. All rights reserved. Keywords: Addition experiments; Biodiversity; Biological invasions; Ecosystem impacts; Removal experiments

Introduction Invasive plant species are commonly recognised to have severe ecological impacts in a wide range of ecosystems throughout the world (Vitousek et al., 1996; Levine et al., 2003). Direct and indirect ecosystem impacts of invasive plant species include competitive exclusion of native species, altered disturbance or fire regimes, nutrient enrichment, increased water loss and competition for pollinators (Bunn et al., 1998; D’Antonio et al., 2001; Chittka and Schu¨rkens, 2001; Ehrenfeld and Scott, 2001; Hulme and Bremner, 2006; Vila` et al., 2006). However, there are rather few experimental studies measuring community-level effects of invasive plant species (Tickner et al., 2001; Hulme, 2003; Levine et al., 2003; Richardson, 2004), as impacts are difficult to quantify. Studies have been principally correlative, and as such often cannot disentangle cause and effect (Vitousek, 1990; Vila` et al., 2006). Experimental approaches are also subject to constraints (Hulme, 2006). The experimental addition of an alien species to established plant communities has the advantage that pre-invasion states can be assessed prior to impact (Gentle and Duggin, 1998; Levine, 2000; Seabloom et al., 2003; Sans et al., 2004), and can be used to test the resilience to invasion. However, addition experiments are often not appropriate in ecologically sensitive areas unless accompanied by guaranteed removal to ensure that there is no subsequent risk of invasion. The experimental removal of an alien species from established plant communities allows the direct and indirect effects of the invasive species to be controlled for, as the original community composition and any species interactions are preserved (Ogle et al., 2003; MacDougall and Turkington, 2005; Hedja and Pysˇ ek, 2006; Hulme and Bremner, 2006), but may not be appropriate where invasion is so widespread that the presence of the invasive species has irreversibly altered ecosystem function (Zavaleta et al., 2001). The desired effect of removal of an invasive species is the restoration of the community to pre-invasion state. However, the compositional species change following removal has been rarely examined in detail. One of the consequences of invasive species removal may be to facilitate the proliferation of other invasive species (Alvarez and Cushman, 2002; Ogden and Rejma´nek, 2005; Hulme and Bremner, 2006) and cause soil and vegetation disturbance (D’Antonio et al., 1998; Zavaleta et al., 2001).

In riparian ecosystems, commonly documented impacts of plant invasions include declines in native species richness, modification of wildlife habitats, bank destabilisation, lowering of water tables, salinisation of soil and decreased channel capacity for flood flow (Willis and Hulme, 2002; Dawson and Holland, 1999; Dodd et al., 1994; Zavaleta et al., 2001; Hulme and Bremner, 2006). However, current understanding of the impacts of invasive plants on European ecosystems lags significantly behind North America and Oceania (Hulme, 2007). A widespread riparian invasive species, Mimulus guttatus L. Scrophulariaceae (hereafter Mimulus), has been assumed to have an adverse impact of moderate significance in Scotland (Welch et al., 2001; UKTAG, 2004), but no detailed assessment has been undertaken to support this claim. Through the combination of an addition and removal experiment and a correlative approach (multi-site comparisons), this study aimed to quantify the effects of Mimulus on native plant composition and soil properties. The magnitude and direction of the community responses following removal will indicate the potential success of control strategies and whether restoration of the vegetation to a pre-invasion state is achievable.

Materials and methods Correlative study We conducted an initial correlative assessment to evaluate the relationship between Mimulus cover and native species richness. Thirty-two 50 cm  50 cm plots containing Mimulus were established on disturbed sediment banks (see definition below) along the Cloak Burn, a tributary of the River Dee in north-east Scotland (571070 N, 021420 W, mean channel width 1.9 m) in July 2003. Vascular plants, mosses, bare ground and litter present within each plot were recorded to species level, and their cover estimated by eye to the nearest 5% or to the nearest 1% where species occupied o5% cover. The relationship between Mimulus cover and species number was assessed by a generalised linear model (GLIMMIX macro), with a Poisson error term and a logarithmic link function.

Manipulation experiments To assess the direct impact of Mimulus on riparian plant assemblages, 15 sites were established along the

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of Mimulus while plants were small, thereby minimising disturbance. In the herb–grass community, there were two randomly assigned treatments: uninvaded (as above) and addition – an uninvaded plot to which Mimulus was added. There were no naturally invaded areas in the herb–grass community. There were, therefore, 30 herb– grass community plots (2 plot treatments at 15 sites). For the addition plot, 13 plants were introduced to alternate 10 cm  10 cm sub-plots, and a further 9 added to a 10 cm buffer zone surrounding each plot (Fig. 1). Mimulus plants were obtained from neighbouring disturbed sediment areas that were not part of the experimental set-up and were therefore of a similar developmental stage as if they were invading the plots naturally at that time. Plots were not cleared prior to addition. If plants failed to establish, this was recorded and individual plants were replaced at monthly intervals. The cover of all plant species in each plot was estimated by eye to the nearest 5% or to the nearest 1% when species occupied o5% cover prior to treatment (May) and in July and September. In addition, all plant species were recorded as present/absent within twentyfive 10 cm  10 cm sub-plots within each plot (Fig. 1). This method gives a less subjective measure of abundance with which to monitor change, and allows species associations to be examined at a finer spatial scale. Where present, the height of the tallest Mimulus stem in each plot was measured. Damage to leaf tissue by caterpillars, other insects and molluscs was assessed where percentage of leaf area lost was greater than 10%. Nomenclature and species status follow Preston et al. (2002).

Beltie Burn, a tributary of the River Dee in north-east Scotland (571080 N, 021380 W, mean channel width 1.7 m), in May 2005. Each of the 15 sites comprised three 50 cm  50 cm plots located in disturbed sediment communities and two 50 cm  50 cm plots located in herb–grass communities. The disturbed sediment banks comprised vegetated gravel, sand and silt banks that would be inundated during high flow events, whereas the herb–grass communities represented more established and stable plant communities further up the river bank. The 15 sites were chosen to be as similar as possible in respect to substrate, plant composition and habitat characteristics. Within each site, the plots were no more than 15 m apart within each plant community, and the two plant communities at each site were no more than 50 m apart. In the disturbed sediment community, there were three plot treatments: (1) uninvaded – in an area having no naturally occurring Mimulus; (2) invaded – in an area naturally colonised by Mimulus (425% cover) and (3) removal – a colonised plot (425% cover) from which all Mimulus was removed. The plots initially having Mimulus were randomly assigned to invaded and removal treatments. There were, therefore, 45 disturbed sediment community plots (3 plot treatments at 15 sites). When removing Mimulus in May, care was taken to minimise soil disturbance and avoid damage to the shoot or root systems of other species present. A 10 cm buffer zone surrounding the plots was also weeded of Mimulus plants to minimise edge effects (Fig. 1). Weeding was undertaken at monthly intervals to remove any newly established Mimulus and to record the level of re-colonisation. This frequent weeding allowed removal

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Fig. 1. Experimental set-up showing the 10 cm  10 cm sub-plots within the 50 cm  50 cm plot, the buffer zone surrounding the plots which was weeded in Mimulus removal plots to minimise edge effects and the positions where Mimulus plants were added in the Mimulus addition plots.

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In May (prior to treatment) and September (four months after treatment), three soil samples were taken from the buffer zone around each plot with a 2.5 cm diameter corer to a depth of 5 cm, and combined. In the uninvaded plots, soil samples were taken at three randomly selected points in the buffer zone. In the removal plots, soil samples were taken prior to removal directly beneath or adjacent to Mimulus plants at 3 randomly selected points in the buffer zone, and in the addition plots soil samples were taken directly adjacent to 3 of the 12 randomly selected locations where Mimulus was planted in the buffer zone. The soil samples were oven dried at 30 1C for three to four days, coarsely sieved (2 mm mesh) to remove stones and roots, and milled to produce a fine powder. These were then analysed for total C (TC) and total N (TN) using an Elementar vario EL Elemental Analyser (Elementar Analysensysteme GmbH, Hanau, Germany). The difference between wet and dry weight provided a measure of soil moisture.

Data analysis All analyses were carried out in SAS v.9.01 (SAS, 1999). Mimulus itself, whether naturally occurring or introduced, was excluded from calculations of species richness and evenness. Only two other invasive species were recorded (Claytonia sibirica and Lamium album), one of which, an archeophyte (L. album, Preston et al., 2002), was only found in two plots, and these were analysed together with native species. If several invasive species were present and frequent, we would have contrasted the changes in the abundance of invasive species following treatment with those of native species (see Hulme and Bremner, 2006). Plant species were assigned an Ellenberg indicator value for fertility, light, moisture and pH taken from Hill et al. (1999). For each plot, we calculated indices of fertility, light, moisture and pH as the mean of individual species’ Ellenberg indicator values, weighted by the square root of the species’ frequency (Truscott et al., 2005). Similarly, we calculated indices based on Grime et al. (1988) plant growth strategies, viz. competitor, stress-tolerator and ruderal. For each of these seven traits, an average score across all species was derived for each plot. This enabled plots to be compared in relation to the moisture or fertility requirements of the vegetation or the extent to which the plant community was dominated by competitive or ruderal species. Species frequency data were used to calculate Pielou’s evenness (J0 ) scores at the plot scale (Pielou, 1975). Plot characteristics based on species number, Ellenberg, Grime and evenness scores were analysed by fitting linear mixed models (MIXED procedure), with

treatment, month and a treatment by month interaction term included as fixed effects and site and a plot by month repeated measures term as random effects. Where a significant effect of treatment was detected, Tukey’s HSD test was used to contrast mean values for significant differences. Degrees of freedom were calculated using the approximation of Satterthwaite (1946). Differences in individual species frequency between treatments and changes over time were also analysed as above. Significance levels for multiple tests were adjusted by sequential Bonferroni correction (Rice, 1987). Differences in Ellenberg and Grime indices between uninvaded disturbed sediment and herb–grass communities were analysed by fitting linear mixed models (MIXED procedure), with community, month and a community by month interaction term included as fixed effects and site and a plot by month repeated measures term as random effects. Detrended Correspondence Analysis (DCA: ter Braak and Smilauer, 2002) was used to analyse differences between treatments in the disturbed sediment community based on species frequency measures. Site was included as a covariable, Mimulus was excluded, rare species were downweighted in importance and the frequency data were logN(y+1) transformed. Scores of the uninvaded, removal and invaded plots along the first axis were analysed by fitting a linear mixed model (MIXED procedure). Treatment, month and a treatment by month interaction term were fixed effects and site was included as a random effect.

Results Correlative assessments revealed species richness decreased significantly with increasing Mimulus cover (F1,30 ¼ 5.6, po0.05; Fig. 2), such that a doubling of Mimulus cover from 25% to 50% reduced species richness by 3.4 species (23%) based on the modelled regression of plant species number against Mimulus cover (Fig. 2).

Disturbed sediment community Species richness differed significantly between treatments (F2,29 ¼ 10.9, po0.001; Fig. 3a), and there was a highly significant treatment by month interaction (F2,81 ¼ 10.3, po0.001). In May, there were 24% more species in uninvaded plots than invaded plots (Tukey HSD test, po0.05), but no difference between invaded and removal plots. Uninvaded plots also had significantly more species than invaded plots in July (Tukey HSD test, po0.01) and September (Tukey HSD test, po0.001). Removal of Mimulus led to a highly significant increase in species richness, resulting in

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Fig. 2. Number of plant species in relation to the cover of Mimulus in 50 cm  50 cm plots in July 2003. The line shows the predicted relationship from a generalised linear model.

a mean of 2.6 (21%) and 4.0 (34%) additional species per plot in July and September compared to May (Tukey HSD test, po0.001). Species richness was 28% and 48% higher in removal compared to invaded plots in July and September (Tukey HSD test, po0.05, po0.001, respectively). Evenness declined significantly during the season (F2,84 ¼ 6.3, po0.01; Fig. 3b) but there was no treatment or treatment by month interaction effect. A total of 74 species was recorded across all plots, of which 57 occurred in uninvaded plots compared to 45 in invaded plots. Uninvaded plots had a greater abundance of C. sibirica (Tukey HSD test, po0.01) compared to invaded plots. In addition, Urtica dioica (Tukey HSD test, po0.001) and Lathyrus pratensis (Tukey HSD test, po0.001) occurred in more uninvaded plots than invaded plots. In the removal treatment, the total number of species increased from 48 prior to removal to 57 four months after removal. Several species colonised plots following Mimulus removal, including the graminoids Holcus lanatus, Elymus repens, Phleum

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Fig. 3. (a) Number of plant species and (b) evenness (J0 ) based on species frequency data in invaded, removal and uninvaded plots in disturbed sediment communities in May (prior to treatment), July and September.

pratense and Juncus acutiflorus/articulatus and the herbs Alchemilla vulgaris, Cardamine amara, Senecio aquaticus and Veronica officinalis, as well as seedlings of Fraxinus excelsior. The species showing the most marked response to removal were U. dioica and Agrostis

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stolonifera, the former being found in one additional plot and increasing in mean cover from 13% to 25%, while the latter was recorded in an additional five plots and its mean cover increased from 8% to 15%. Moss and litter occurrence also increased (4 plots in May, 10 plots in September and 12 plots in May, 15 plots in September, respectively) but not significantly. The only significant effect on Ellenberg indices was for the moisture index (F2,30 ¼ 8.7, po0.01), which was higher for invaded plots than for uninvaded plots (Tukey HSD test, po0.001). Ellenberg fertility, light and pH indices did not differ significantly between treatments. There were no significant differences between the three months. Invaded plots had a higher Grime ruderal index than uninvaded plots, but this was only marginally significant (p ¼ 0.056). There were no significant differences in Grime’s stress-tolerator or competitor indices between treatments. Grime’s stresstolerator index differed between months (F2,82 ¼ 4.3, po0.05), being significantly higher in September than May or July (Tukey HSD test, po0.05). The reverse pattern was found for Grime’s ruderal index (F2,83 ¼ 4.6, po0.05), having significantly higher values in May than in September (Tukey HSD test, po0.05). DCA demonstrated that the species composition of invaded plots was distinct from uninvaded plots (Fig. 4). The first axis accounted for 9% of the explained variance in the species data and the first two axes together accounted for 16%. Treatment had a significant effect on vegetation composition (F2,126 ¼ 89.7, po0.001), as measured by the scores of invaded, removal and uninvaded plots on the first axis. Invaded plots were significantly different from univaded plots in May, July and September (Tukey HSD test, po0.001). Removal plots were not significantly different from invaded plots in May prior to removal. However, two 0.9

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and four months after removal, plots were significantly distinct from invaded plots (Tukey HSD test, po0.01) and were becoming more similar to uninvaded plots (May Tukey HSD test, po0.001; July and September Tukey HSD test, po0.05).

Herb–grass community There were no significant differences between addition and uninvaded plots in species richness (mean 14.770.9 S.E. and 13.270.8 S.E., respectively) or evenness (mean 0.8670.01 S.E. and 0.8670.01 S.E.). There was a significant time effect (F2,53 ¼ 6.1, po0.01), evenness scores being significantly lower in July than either May or September (Tukey HSD test, po0.05; po0.01, respectively). Mimulus plants initially added to the herb–grass plots suffered intense herbivory by slugs (especially Arion ater), with 61% of plants showing damage (410% leaf area lost) attributed to slugs, and only 71% survived after one month. Despite replacement of plants each month, ca. 50% of plants suffered herbivory and survivorship each month ranged from 54% to 89%. Ellenberg fertility, light, moisture and pH indices and Grime’s competitor, stress-tolerator and ruderal indices did not differ significantly between addition and uninvaded plots.

Soil properties Treatment and month of sampling had no significant effect on total C, total N or soil moisture in either disturbed sediment or herb–grass communities. Soil total C, total N and soil moisture levels were higher in invaded disturbed sediment plots compared to uninvaded disturbed sediment plots, but not significantly so (Fig. 5). Removal plots were more similar to uninvaded plots than invaded plots in terms of total C and N in September. In all treatments, soil C and N levels were higher in September than in May but not significantly so.

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Fig. 4. Simplified DCA ordination diagram showing the mean and S.E. on the first two axes of invaded, removal and uninvaded plots in disturbed sediment communities in May (prior to treatment), July and September based on species frequency measures.

Uninvaded herb–grass community plots had plants with lower fertility (F1,14 ¼ 3.5, p ¼ 0.08), light (F1,14 ¼ 16.4, po0.01), moisture (F1,14 ¼ 15.3, po0.001) and pH (F1,83 ¼ 4.7, po0.05) indices than uninvaded disturbed sediment community plots. In terms of Grime indices, there was no significant difference between the two uninvaded communities in respect to the competitor index, but herb–grass communities had a higher stress-tolerator index (F1,14 ¼ 15.7, po0.05) and lower ruderal index (F1,14 ¼ 10.1, po0.01) than the disturbed sediment

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Fig. 5. (a) Mean (71 S.E.) total C, (b) total N and (c) soil moisture content (%) in May (prior to treatment), and September for experimental plots in herb–grass and disturbed sediment communities.

communities. Uninvaded herb–grass community plots also had significantly higher soil total C and total N (F1,14 ¼ 5.3, po0.05; F1,14 ¼ 4.5, p ¼ 0.05; Fig. 5).

Discussion The marked negative association between Mimulus cover and plant species richness identified through

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correlative multi-site comparisons was consistent with experimental removal studies indicating that Mimulus significantly alters the structure of riparian plant communities. Similarly, removal of Impatiens glandulifera (Hulme and Bremner, 2006) provides support for the widespread finding of reduced species richness following invasion of riparian vegetation (Maskell et al., 2006). Plots invaded by Mimulus contained 24% fewer plant species than invaded disturbed sediment plots. Comparable levels of species reduction following invasion have been found for other alien weeds (Pysˇ ek and Pysˇ ek, 1995; Alvarez and Cushman, 2002; Yurkonis et al., 2005). However, this study is unique in demonstrating impacts where cover of the invasive plant is relatively low (mean cover of 30%). Previous manipulation studies have worked in heavily invaded communities where cover of the invasive species ranged from 60% to 95% (Frappier et al., 2004; MacDougall and Turkington, 2005; Ogden and Rejma´nek, 2005; Hedja and Pysˇ ek, 2006; Hulme and Bremner, 2006; Vila` et al., 2006). Therefore, it has often been thought that invasive species need to attain high cover in order to affect plant community structure (Richardson et al., 1989; Pysˇ ek and Pysˇ ek, 1995; Bı´ mova´ et al., 2004). Mimulus possesses a suite of life history characteristics that we hypothesise are responsible for the effects on plant assemblages in disturbed sediment communities. The species produces erect stems (50–100 cm, although sometimes exceeding 150 cm tall) and exhibits rapid shoot growth (Waser et al., 1982; Truscott et al., 2006), which undoubtedly leads to shading of neighbouring plants and alters the structural diversity of vegetation. High propagule pressure and rapid germination of seeds, in conjunction with the high survival, regeneration and colonisation of fragments, leads to effective long- and short-distance dispersal (Truscott et al., 2006). Furthermore, the species can tolerate a range of environmental conditions including shade and temperature (Truscott et al., 2008). Competition for space, water, nutrients and light, or some combination of these resources, has been shown to be the main negative effect of invasive plants on native species (Woods, 1993; Almasi, 2000; Case and Crawley, 2000; Shea and Chesson, 2002; Levine et al., 2003). Changes in community composition following Mimulus removal occurred rapidly in disturbed sediment communities. Species already dominant in the plots increased, but there was also colonisation by new species from outside the plots, e.g. C. amara and V. officinalis. The consequence of removing Mimulus was a proliferation of nitrophilous competitive perennials such as U. dioica and A. stolonifera, as has also been found with I. glandulifera removal (Hulme and Bremner, 2006; Hedja and Pysˇ ek, 2006). Many of the newly colonised species are fast growing and spread by fragments, rhizomes or stolons and therefore could have colonised

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as a result of vegetative expansion from the surrounding vegetation. Removal and uninvaded plots were clearly distinguished in multivariate space, and there was evidence that following removal, plots became increasingly similar in species composition to uninvaded plots. This indicates that the disturbed sediment communities may be resilient and have the potential to recover and return to a pre-invasion state (Mitchell et al., 2000). However, as riparian systems are so dynamic, resilience is difficult to conceptualise in these systems due to the frequency of disturbance (Richardson et al., 2007), which favours regeneration via a seedbank or frequent colonisation via seeds or vegetative fragments. In addition, the response following removal may change over time and vary as a function of abiotic factors (Berlow et al., 2003; Ogden and Rejma´nek, 2005). Total C and N and soil moisture were higher, albeit not significantly, in invaded compared to uninvaded disturbed sediment plots, as has been previously demonstrated for other invasive species (Blank and Young, 1997; Saggar et al., 1999; Scott et al., 2001; Vila` et al., 2006). The trend suggests that Mimulus may influence soil chemistry though the consequences for the vegetation community are unclear. Invasive plants species have been shown to affect nutrient cycling and soil chemistry, which may subsequently affect native vegetation (Christian and Wilson, 1999; Stohlgren et al., 1999; Ehrenfeld and Scott, 2001; Vila` et al., 2006). The impact of Mimulus appeared restricted to disturbed sediment communities, as experimental addition into herb–grass communities was relatively unsuccessful in establishing Mimulus, despite repeated replacement of plants. This supports evidence from field surveys in that Mimulus is found within the most frequently disturbed area of the river bank (principally restricted to within 1 m of the river edge) and rarely further up the river bank spreading into adjacent habitats (Truscott et al., 2008). Herb–grass communities had higher stress-tolerator indices and lower ruderal indices than disturbed sediment communities, highlighting that relatively closed vegetation comprising stress-tolerant rather than ruderal species is relatively resistant to invasion by Mimulus. This species appears to be another example of invasion being dependent on disturbance regimes (Davis et al., 2000; Levine et al., 2003; Seabloom et al., 2003). Also, the addition experiments in herb–grass communities highlight that, as well as competition from the resident vegetation community, mollusc herbivory further hinders the establishment of Mimulus, as has been shown for other invasive plant species growth and competition (Scherber et al., 2003). How much of a threat is Mimulus? The species is capable of successfully colonising disturbed sediment communities along small streams with a resultant loss of native species. As in previous studies in riparian

habitats, most of the plant species threatened are widespread ruderal plants or other non-native weeds, few if any of direct conservation value. Potentially, Mimulus may impact upon the invertebrate community, particularly since its flowers are a poor nectar source (Robertson et al., 1999). Where vigorous growth occurs, there are concerns regarding the obstruction of streamflow in the smaller tributaries (Truscott, 2007). Taken together, the evidence does not support a strong case for managing the species. A further constraint on management is that although Mimulus removal increased species richness bringing the plant community closer in structure to uninvaded vegetation, there was also an increase in the occurrence and abundance of another non-native species. C. sibirica, is more widespread in the UK than Mimulus and can itself become locally dominant, suppressing native vegetation under its lush mass of spring foliage (Preston et al., 2002). It therefore appears that the disturbed sediment communities are prone to colonisation by fast growing, clonal species. The evidence that C. sibirica was more abundant on uninvaded plots is suggestive that in the absence of Mimulus, C. sibirica may simply occupy similar sites with likely comparable consequences for native vegetation. Thus it seems that in highly disturbed communities, removal of the target invasive species may simply result in the spread of other invasive species (Alvarez and Cushman, 2002; Ogden and Rejma´nek, 2005; Hulme and Bremner, 2006; Hulme, 2006).

Acknowledgements We thank the Findrack Estate and local landowners for access to land, Laura Flegg for field work assistance, Ruth Mitchell for advice on multivariate techniques and David Elston of BioSS for statistical support. We also thank the Environmental Analysis Group at CEH Lancaster, particularly Clive Woods, for conducting the soil analysis. This study was supported by the European Union within the FP 6 Integrated Project ALARM (Settele et al., 2005; GOCE-CT-2003-506675). Two anonymous referees provided valuable comments.

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