The impact on plant communities of an invasive alien herb, Oenothera drummondii, varies along the beach-coastal dune gradient

1 The impact on plant communities of an invasive alien herb, Oenothera drummondii, varies along the beach-coastal dune gradient Juan B. Gallego-Fernández1, M. Luisa Martínez2*, José G. García-Franco1,2, María Zunzunegui1 1 Universidad de Sevilla, Sevilla, España 2 Instituto de Ecología, A.C., Xalapa, Veracruz, México (permanent address) * Author for correspondence, [email protected] Manuscript File This is the peer-reviewed version of the article accepted for publication in FLORA Volume 260:151466. 2019, which has been published in final form at https://doi.org/10.1016/j.flora.2019.151466. 2 Abstract 1 One of the major threats to the diversity of coastal dunes is the expansion of invasive 2 species, such as Oenothera drummondii subsp. drummondii (Onagraceae). In 3 southwestern Spain, we studied the impact of this American invasive on community 4 structure and composition along a beach-dune gradient (beach, foredune, and inland 5 dunes). Differences in density, biomass, and the cover of O. drummondii, the cover of 6 perennial and annuals/biennials species, and Shannon diversity index H’ and dominance λ 7 were compared between invaded and uninvaded sectors. We observed that the intensity 8 and impact of the invasion by O. drummondii varies along the beach-dune gradient. The 9 abundance of the invasive plant increased inland and in consequence, its impact on 10 species richness and composition was highest in inland dunes. Here, plant cover of O. 11 drummondii represented 57.9% of total plant cover; species richness was reduced (with 12 3.3 fewer species per 2 x 2 plot), diversity H’ was lower while dominance λ was higher. At 13 a broader scale, species richness in the invaded sector was 25% larger than in the 14 uninvaded sector, because of the presence of ruderal species. Species composition also 15 was modified after the invasion. The abundance of a keystone native species was largely 16 reduced (Ammophila arenaria), and some natives became locally extinct (Otanthus 17 maritimus, Eryngium maritimum, Medicago marina and Elymus farctus). We conclude that 18 the high environmental severity of the beach and foredunes results in a reduced invasion 19 and impact of O. drummondii, whereas the milder conditions of inland dunes promote its 20 expansion. The shift in community structure and composition can have an increasing 21 domino effect and thus monitoring, and mitigation actions are necessary. When doing so, 22 3 the environmental heterogeneity of the beach-dune gradient should be considered, given 23 its relevance in the invasion process. 24 Keywords: Plant biomass; coastal dunes; gradient analysis; plant invasion; Species richness Highlights  We studied the impact of the invasive O. drummondii on Spanish coastal dunes.  The intensity and impact of the invasion increased along the beach-dune gradient.  The abundance of native species decreased, and some became locally extinct.  High environmental severity of the beach and foredunes attenuates invasion.  Milder conditions of inland dunes facilitate invasion and promote ruderals. Nomenclature and taxonomic reference: Valdés, B., S. Talavera and E. Fernández Galiano (eds.) (1987). Flora Vascular de Andalucía Occidental 1-3. Ketres Editora S.A., Barcelona. 4 1. Introduction 25 The negative impact of exotic invasive species on the integrity of plant 26 communities and ecosystems, as well as on the existence of rare and endangered species, 27 has been observed and demonstrated worldwide (Vilá et al., 2011; Pysek et al., 2012). 28 Such impacts increase over time and are usually irreversible or extremely difficult to deal 29 with, and hence, are acknowledged as one of the major threats to biodiversity worldwide 30 (EEA, 2012; Joppa et al., 2016). Studies aimed at understanding the ecological processes 31 that lead to biological invasions are ample (van Kleunen et al., 2010; Vilá et al., 2011), and 32 have mostly focused on species invasiveness (functional attributes, increments in 33 abundance and their differences with native resident species) (Stanisci et al., 2010) and 34 habitat invasibility (vulnerability to invasion) (Carboni et al., 2010; Santoro et al., 2012a; 35 Ledger et al., 2015). Nevertheless, both approaches oftentimes do not consider 36 community dynamics and environmental heterogeneity. In consequence, an increasing 37 number of studies have recently demonstrated that the invasion process is affected by 38 temporal changes in the abiotic environment, the invaded community, and in the invasive 39 species themselves (del Vecchio et al., 2015). Furthermore, the spatial scale at which 40 biological invasions are monitored (large areas -countries vs. small areas -vegetation plots) 41 and the environmental heterogeneity in which they occur have proved to be relevant in 42 understanding and assessing the invasion phenomena (Santoro et al., 2012a; del Vecchio 43 et al., 2015). However, although there is a growing number of studies that report the 44 impacts of invasive species on different communities, the role of environmental 45 heterogeneity remains mostly unknown (but see Santoro et al., 2012a). 46 5 Among the ecosystems and habitats that have been most affected by invasive 47 species, coastal dunes are outstanding and have been regarded as an “extreme case of 48 species invasion” (Castillo and Moreno-Casasola, 1996). Indeed, they are threatened by 49 invasive species owing to habitat heterogeneity and the frequent occurrence of intense 50 disturbance events (Acosta et al., 2008; Santoro et al., 2012a), which provide the 51 opportunity for the colonization by invasive species. The problem of species invasions on 52 coastal dunes is relevant because of the potential loss of the high biodiversity of very 53 specialized flora and fauna (van der Maarel, 2003), which includes species that are 54 tolerant to the extreme abiotic conditions of coastal dunes (García-Mora et al., 1999; 55 Maun, 1998). Furthermore, coastal dunes provide a host of ecosystem services to society, 56 including water quality, scenic beauty, recreation, and shoreline protection from storms 57 and sea-level rise through flood and erosion control (Everard et al., 2010; Salgado and 58 Martínez, 2017; Feagin et al., 2019). Thus, the relevance of preserving these ecosystems is 59 heightened because these ecosystem services rely on the integrity of coastal dune 60 communities. 61 Previous studies show that invasive species on coastal dunes are diverse, with 62 multiple origins and destinations. These earlier findings have demonstrated that several 63 species (i.e., Carpobrotus acinaciformis, C. edulis, Ammophila arenaria, Acacia longifolia, 64 Rosa rugosa, among others) are aggressively invasive and result in species decline through 65 different invasive processes (Marcantonio et al., 2014; Sarmati et al., 2019). Frequently, 66 native coastal dune plants are replaced by exotic invasives, and this species turnover 67 becomes more intense with invasion time. This pattern holds for invasives such as Acacia 68 6 longifolia (Marchante et al., 2015), Ammophila arenaria (Pickart, 2013) and Carpobrotus 69 aff. acinaciformis (Santoro et al., 2012b). Some relevant findings in this regard highlight 70 the biogeographic origin of the invasive species, which affects their impact in native 71 vegetation (Ledger et al., 2015). Other studies show that the expansion of invasive species 72 leads to the artificial stabilization of sand movement (Wiedemann and Pickart, 2004) as 73 well as changes in shoreline dynamics (Masterman and Ellison, 2018). Furthermore, del 74 Vecchio et al. (2015) also observed that the severity of the impact of the invasive species 75 might be associated with human disturbances, which facilitate dispersal and colonization 76 of invasive species. 77 Environmental sea-inland gradients are also relevant for the process and impact of 78 species invasions (Lortie and Cushman, 2007). The beach-dune gradients are characterized 79 by a variety of habitats, with pioneer communities on the beach, followed by embryo 80 dunes, foredunes, and inland dunes further away from the coastline. Different 81 environmental factors change along this gradient: wave impact, salinity (marine flooding 82 and salt spray), wind intensity, and sand burial decrease inland, while nutrient availability 83 and soil development increase. The above leads to a dynamic, but fragile, ecological and 84 vegetation gradients along this narrow strip that is determined by the tolerance to these 85 limiting factors. In this sense, Santoro et al. (2012a) observed that the increasing 86 population of an invasive alien species, Carpobortus aff. acinaciformis, resulted in the 87 collapse of the community structure. In this case, the invasive species became overly 88 abundant, while the natural vegetation zonation along the beach-dune gradient was 89 eliminated. Also, native plant assemblages are affected by invasive species in more subtle 90 7 ways through the disruption of species interactions which result in changes in community 91 structure (Trasevet and Richardson, 2014). 92 Oenothera drummondii subsp. drummondii Hook, native to Mexico and southern 93 USA, is an invasive species that has colonized many dune systems worldwide during the 94 last century. Studies on the impact of O. drummondii have focused on estimating the 95 degree of invasion (Campos et al., 2004; García-de-Lomas et al., 2015), its occurrence on 96 coastal plant communities of the Iberian Peninsula (Galán de Mera et al., 1997); its 97 physiological attributes that enable the species to be an effective invader (Zunzunegui et 98 al., 2014) and eradication actions (García-de-Lomas et al., 2016). Nevertheless, the 99 invasion patterns along the beach-dune gradient and the impact of O. drummondii in the 100 plant community has not been studied yet. 101 Within this framework, this paper reports the analysis of how the beach-dune 102 environmental gradient affects the invasion of O. drummondii on coastal dune plant 103 communities in Southern Spain, in terms of changes in community structure and the loss 104 of biodiversity. In detail, this paper addresses the following questions: 105 - Does the beach-dune gradient modify species invasion? 106 - Are there microsites along the beach-dune gradient (dune types or zones) that are 107 more prone to being invaded by alien plant species? 108 - How do community structure and composition change after the arrival and 109 expansion of the invasive alien species? 110 111 2. Materials and methods 112 8 2.1 Study species 113 The beach evening primrose, Oenothera drummondii subsp. drummondii 114 (Onagraceae) is, native to the coastal dunes in Eastern Mexico and South-eastern USA 115 (Dietrich and Wagner, 1988; Dietrich, 1997;). In its native range, O. drummondii is relatively 116 scarce (Moreno-Casasola, 1988; Gallego-Fernández and Martínez, 2011) but it is abundant 117 in invaded locations. It is considered invasive in Australia (ALA, 2014), China (Xu et al., 2012) 118 and Spain (García-de-Lomas et al., 2015) and naturalized in Egypt (Shaltout et al., 2016), 119 South Africa (Frean et al., 1997), Israel (Dufour-Dror, 2012), and New Zealand (Heenan et 120 al., 2002). In Spain, it was first recorded in the Gulf of Viscay (N Spain) in 1915 (Campos and 121 Herrera, 2009) and near the town Rota (Gulf of Cadiz, SW Spain) in 1957 (Silvestre, 1980) 122 and now it is considered invasive (García-de-Lomas et al., 2015). 123 The main stems of this short-lived perennial herb are erect to procumbent, and the 124 basal side-stems are prostrate or ascending to about 50 cm tall with a strong taproot. The 125 plant is self-compatible, outcrossing and pollinated by hawkmoths in its native habitats 126 (Wagner et al., 2007). The species reproduces and spreads by small and numerous seeds 127 which have a high germination rate. 128 129 2.2 Study site 130 The study was carried out in a coastal dune system of Huelva, Spain (37° 9’ 20’’ N, 6° 131 54’ 40’’ W). These dunes are very recent, formed after the construction of the jetty Juan 132 Carlos I in 1981 (Rodríguez-Ramírez et al., 2008). The dune system stretches for 7 km 133 running parallel to the jetty and is between 200 to 400 m width. The area is included in the 134 9 protected natural area “Marismas del Odiel” since 1989 and integrated into the European 135 NATURA 2000 network (ES0000025). 136 Ever since this dune system was formed, the mobile dunes became colonized by 137 native coastal dune species, which slowly increased plant cover and increased biodiversity. 138 Four habitat types included in Annex I of the European Habitat Directive 92/43/EEC (2013) 139 occur in the study site: (1) Embryo dune, characterized by Cakile maritima and Polygonum 140 maritimum (habitat type 1210 and 2010; Royo and Traveset, 2009; Gracia et al., 2009), (2) 141 foredune, characterized by the dominance of Ammophila arenaria (habitat type 2120; 142 Gracia, 2009), (3) Fixed coastal dunes with herbaceous vegetation dominated by Crucianella 143 maritima (habitat type 2130 and 2230; Gracia and Muñoz, 2009; Gómez-Serrano and 144 Sanjaume, 2009). 145 The climate is Mediterranean with winter rains and summer droughts, a mean 146 annual temperature of 17.8 °C and a mean annual rainfall of 467 mm. The dune system and 147 beach cover 110 ha, of which ca. 70 ha are invaded by O. drummondii, mostly along the 4 148 km of the western margin. The eastbound dunes have not been invaded yet (Fig. 1). The 149 first records of O. drummondii in the southwest of the Iberian Peninsula date from 1957 150 (Silvestre, 1980), and in the study area it was registered for the first time in 1996 (M.R. 151 García-Mora personal communication). 152 153 16 3.2 Species composition 259 A total of 55 vascular plant species from 21 families were recorded (Appendix 1). 260 From these, 7.3% of the species are exotics: O. drummondii, O. laciniata, Arctotheca 261 calendula, and Conyza canadensis. Furthermore, 41.8% are considered typical dune 262 species. Invaded and uninvaded sectors had a total of 49 and 35 species, respectively, 263 and this trend was also observed in the three zones, with a higher number of species in 264 the invaded sites (Table 1). From the total number of species, 29 were observed in both 265 sectors, invaded and uninvaded, 20 were only found in the invaded sector, and 6 were 266 exclusive to the uninvaded site (Appendix 1). The local distribution of species revealed 267 that a relatively large number 37 (65%) of species only grew on inland stabilized dunes. 268 The only species that were observed growing in all zones considered were Pancratium 269 maritimum, Ammophila arenaria, Polygonum maritimum, Euphorbia paralias, and 270 Otanthus maritimus. 271 Most of the species (92.7%) were herbaceous, and only 7.3 % were woody. Life 272 cycles of the species found were diverse: el 34.5% are perennials, 58.2 % annuals, and 273 7.3 % biennials. In both sectors, species richness of annual and biennial species 274 increased inland. However, differences between sectors only were significant on inland 275 dunes. In contrast, species richness of perennials decreased inland. Again, the number 276 of perennial species (excluding O. drummondii) was significantly larger in inland dunes 277 in the uninvaded sector (Figure 2). 278 279 280 281 17 282 Fig. 2. Species richness and relative plant cover (mean ± se) of annual and biannual and 283 perennial (not O. drummondii) species of coastal dune communities surveyed at three zones 284 (beach, foredune, and inland dunes) in sectors that were invaded (black) and uninvaded 285 (white) by the exotic herb Oenothera drummondii. Comparisons between zones (invaded vs. 286 uninvaded) are indicated by the lines above each pair of bars (Mann-Whitney tests; ns = not 287 significant; *** = P < 0.001). Comparison between zones of the invaded sector is indicated by 288 capital letters. Comparisons between zones of the uninvaded sector are shown in small letters. 289 Both analyses with ANOVA and Tukey comparisons; same letters indicate non-significant 290 differences. 291 292 3.3 Changes in plant cover 293 To determine the impact of O. drummondii on species abundance according to 294 their life cycles, we compared relative plant cover of annuals, biennials, and perennials 295 but did not consider O. drummondii. The plant cover of annual and biennial species 296 increased inland in both sectors. However, the only significant difference was observed 297 in inland dunes, where plant cover was significantly larger in the uninvaded sector. 298 Unlike annuals and biennials, plant cover of perennials decreased inland and, in all cases, 299 it was significantly larger in the uninvaded sector (Figure 2). 300 301 18 3.4 Diversity 302 In both sectors, species richness and Shannon´s diversity were significantly larger 303 on inland dunes than on the beach and foredunes (Figure 3). Furthermore, species 304 richness and Shannon´s diversity were significantly smaller in the invaded sector, but 305 this was not so on the beach and foredunes (t = 5.259, P < 0.001 and t = 7.544, P < 0.001, 306 respectively). In contrast with the above, dominance (Simpson Index) was significantly 307 smaller in inland dunes, but it was in inland dunes where we found significant 308 differences between sectors (Figure 3). In this case, dominance was significantly larger 309 in the invaded sector than in the uninvaded sector. The comparisons of the Simpson 310 Index between invaded and uninvaded sectors showed that differences were only 311 significant in inland dunes, where the invaded sector had larger values (t = 4.353, P < 312 0.001). 313 314 19 315 316 Fig. 3. Diversity indices (mean ± se) of coastal dune communities surveyed at three zones 317 (beach, foredune, and inland dunes) in sectors that were invaded (black) and uninvaded 318 (white) by the exotic herb Oenothera drummondii. Comparisons between zones (invaded vs. 319 uninvaded) are indicated by the lines above each pair of bars (ns = not significant; *** = P < 320 0.001). Comparison between zones of the invaded sector is indicated by capital letters. 321 Comparison between zones of the uninvaded sector is shown in small letters. Both analyses 322 with ANOVA and Tukey comparisons; same letters indicate non-significant differences. Cover 323 of O. drummondii was omitted when the indices were calculated. 324 325 Rank abundance curves show that O. drummondii was the dominant species, 326 especially on inland dunes in the invaded sector (Figure 4). In turn, plant communities 327 in the uninvaded sector were more equitable, with a larger number of species with 328 intermediate values, and the absence of extremely dominant species. This trend was 329 more marked on inland dunes. The dominant species in the uninvaded sectors varied 330 20 between zones: Polygonum maritimum was the dominant species on the beach, 331 Ammophila arenaria in the foredune, and Malcolmia littorea in inland dunes. 332 333 Fig. 4. Rank-abundance curves of plant communities on the beach, foredunes, and 334 inland dunes in sectors that were invaded and uninvaded by the exotic herb Oenothera 335 drummondii. 336 337 338 21 3.5 Community changes in invaded and uninvaded dunes. 339 We observed that the cover of native species was negatively correlated with the cover 340 of O. drummondii. Also, the cover of native plants was larger in the uninvaded sector. 341 Nevertheless, these trends were not statistically different. Additionally, PERMANOVA 342 tests performed with species frequency and plant cover showed that differences 343 between sectors (invaded and uninvaded plant communities) and between zones 344 (beach, foredunes and inland dunes) were highly significant (Table 2). Pair-wise a 345 posteriori comparisons indicated that such differences were significant for the beach 346 and inland dunes but not so in the foredunes (Table 2). PERMDISP tests showed that 347 multivariate dispersion was significantly greater within uninvaded communities in 348 inland dunes (Table 2). 349 Table 2. Results of two way fixed factor PERMANOVA tests on vegetation community data for 350 invaded and uninvaded sectors of three zones: beach, foredune, and inland dunes. Results of 351 PERMDISP tests on vegetation community data for invaded and uninvaded sectors in three 352 zones: beach, foredune, and inland dunes, and analysis testing the variability between zones. 353 PERMANOVA test PERMDISP test Source of variation df SS MS F P F P Sector 1 12337 12337 4.3612 0.0003 6.7208 0.0001 Zone 2 118500 59252 20.945 0.0001 53.535 0.001 Sector x Zone 2 6148.9 3074.5 1.0868 0.3478 Residual 182 514850 2828.9 Total 187 662520 PERMDISP tests Invaded vs. Uninvaded t P Pair-wise test t P Beach 1.7737 0.0212 Beach – Foredune 3.9923 0.002 Foredune 1.4463 0.0671 Beach – Inland dune 11.317 0.003 Inland dunes 2.5093 0.0001 Foredune – Inland dune 4.0724 0.001 354 SIMPER analyses showed that the average dissimilarity between invaded and 355 uninvaded communities was lowest at the beach (46.92) and highest (81.43) in inland 356 dunes (Table 3). The species that mostly discriminated between invaded and uninvaded 357 plant communities were Ammophila arenaria and Othantus maritimus, both perennials 358 and with a reduced cover on invaded dunes. Polygonum maritimum grows exclusively 359 22 on the beach, and the beach was not largely colonized by the invasive plant. 360 Consequently, this species contributed to the dissimilarity between invaded and 361 uninvaded communities, because colonization by the invasive on the beach was 362 reduced. Also, in inland dunes, annual species such as Polycarpon alsinifolium, 363 Hedypnois arenaria, and Erodium cicutarium contributed significantly to the dissimilarity 364 between invaded and uninvaded areas. 365 Table 3. Discriminating species for dissimilarity between zones invaded and non-invaded zones 366 by Oenothera drummondii identified by the dissimilarity percentage procedure analyses 367 (SIMPER). The list includes the cumulative species contribution, up to 90%, to the average 368 dissimilarity. Data are untransformed. 369 Species Average Abundance Invaded sector Average Abundance Uninvaded sector Average dissimilarity Dissimilarity /SD Contribution (%) Cumulative contribution (%) Beach. Average dissimilarity between Invaded and Uninvaded sectors: 46.92 Polygonum maritimum 73.14 46.47 16.30 1.45 34.74 34.74 Ammophila arenaria 7.78 16.13 8.26 0.74 17.60 52.34 Otanthus maritimus 5.12 14.17 6.98 1.04 14.87 67.21 Euphorbia paralias 4.76 11.75 6.58 0.87 14.03 81.24 Elymus farctus 6.08 4.29 3.97 0.86 8.46 89.70 Eryngium maritimum 0.99 6.95 3.64 0.66 7.77 97.47 Foredune. Average dissimilarity between Invaded and Uninvaded sectors: 66.83 Ammophila arenaria 42.02 51.55 20.51 1.42 30.69 30.69 Otanthus maritimus 8.91 20.97 11.89 0.92 17.79 48.48 Euphorbia paralias 11.96 10.14 8.57 0.87 12.83 61.32 Polygonum maritimum 12.75 0.41 6.30 0.75 9.43 70.75 Elymus farctus 0.49 6.77 3.35 0.76 5.02 75.77 Pancratium maritimum 4.62 1.21 2.85 0.31 4.26 80.03 Malcolmia littorea 2.54 3.35 2.53 0.61 3.79 83.81 Lotus creticus 4.86 0.00 2.43 0.25 3.64 87.45 Erodium cicutarium 1.68 2.56 2.00 0.38 2.99 90.44 Inland dune. Average dissimilarity between Invaded and Uninvaded sectors: 81.43 Ammophila arenaria 14.48 22.55 13.49 1.05 16.56 16.56 Malcolmia littorea 14.25 12.93 8.68 0.84 10.67 27.23 Otanthus maritimus 4.62 15.05 8.53 0.80 10.48 37.71 Polycarpon alsinifolium 11.93 6.73 7.37 0.71 9.05 46.76 Hedypnois arenaria 7.20 7.96 5.32 0.76 6.54 53.30 Erodium cicutarium 4.23 6.59 4.40 0.72 5.41 58.71 Pancratium maritimum 4.80 4.24 4.20 0.41 5.16 63.87 Cyperus capitatus 4.97 1.04 2.90 0.34 3.57 67.44 Silene nicaeensis 3.17 2.55 2.53 0.44 3.10 70.54 23 Hypochaeris glabra 4.67 0.70 2.52 0.50 3.09 73.63 Pseudorlaya pumila 1.61 2.87 1.86 0.65 2.28 75.91 Anthemis maritima 3.49 0.00 1.74 0.22 2.14 78.05 Reichardia gaditana 3.12 0.56 1.70 0.26 2.09 80.14 Medicago littoralis 0.21 2.95 1.55 0.31 1.91 82.05 Eryngium maritimum 0.00 3.06 1.53 0.44 1.88 83.93 Linaria pedunculata 2.56 0.44 1.40 0.39 1.72 85.64 Lotus creticus 1.96 0.35 1.14 0.23 1.40 87.04 Conyza canadensis 1.15 1.53 1.14 0.55 1.40 88.44 Echium gaditanum 0.23 1.71 0.94 0.26 1.16 89.60 Andryala arenaria 0.97 1.05 0.88 0.50 1.09 90.69 370 As expected, the species that contributed to similarities within zones of invaded 371 and uninvaded sectors were unlike those that contributed to dissimilarities. Polygonum 372 maritimum contributed the most to the similarity between invaded and uninvaded 373 beaches. In the foredunes, Ammophila arenaria was the species that most contributed 374 to the similarity between invaded and uninvaded sectors (more than 67%). In inland 375 dunes, Ammophila arenaria and Malcolmia littorea explained over the 47% within 376 uninvaded communities (Table 4). In invaded inland dunes, Malcolmia littorea and 377 Polycarpon alsinifolium explained over the 55% of the overall similarity. 378 379 Table 4. Species contributing to similarities within invaded and uninvaded zones by Oenothera 380 drummondii, identified by similarity percentage analyses (SIMPER). The list includes the 381 cumulative species contributing up to 90% of the average similarity. The average abundance 382 (cover percentage per plot) is calculated across three sites. Data are untransformed. 383 Species Average abundance Average similarity Similarity /SD Contribution (%) Cumulative contribution (%) Beach Group Invaded. Average similarity: 67.85 Polygonum maritimum 73.14 61.77 3.36 91.03 91.03 Group Uninvaded Average similarity: 47.89 Polygonum maritimum 46.47 33.06 1.78 69.04 69.04 Otanthus maritimus 14.17 5.07 0.58 10.59 79.64 Ammophila arenaria 16.13 4.22 0.57 8.81 88.44 Euphorbia paralias 11.75 3.11 0.37 6.50 94.94 Foredune Group Invaded. Average similarity: 21.41 Ammophila arenaria 35.61 14.52 0.57 67.80 67.80 Polygonum maritimum 9.88 1.96 0.30 9.16 76.96 Euphorbia paralias 7.26 0.98 0.16 4.57 81.53 24 Hedypnois arenaria 7.89 0.83 0.18 3.89 85.42 Otanthus maritimus 8.65 0.81 0.15 3.76 89.18 Silene nicaeensis 6.67 0.71 0.20 3.31 92.49 Group Uninvaded. Average similarity: 42.92 Ammophila arenaria 51.55 32.35 1.21 75.37 75.37 Otanthus maritimus 20.97 6.45 0.45 15.03 90.40 Inland dune Group Invaded. Average similarity: 17.02 Malcolmia littorea 16.07 5.82 0.66 34.17 34.17 Polycarpon alsinifolium 14.36 3.55 0.38 20.85 55.01 Ammophila arenaria 12.20 1.99 0.22 11.70 66.72 Hedypnois arenaria 4.89 1.33 0.48 7.80 74.51 Hypochaeris glabra 5.12 0.95 0.31 5.57 80.08 Erodium cicutarium 5.10 0.63 0.21 3.69 83.77 Linaria pedunculata 3.02 0.43 0.27 2.52 86.29 Pancratium maritimum 5.78 0.42 0.09 2.49 88.78 Cyperus capitatus 5.98 0.40 0.10 2.36 91.14 Group Uninvaded. Average similarity: 29.50 Ammophila arenaria 22.55 8.99 0.59 30.47 30.47 Malcolmia littorea 12.93 4.98 0.76 16.87 47.34 Otanthus maritimus 15.05 4.67 0.45 15.84 63.18 Hedypnois arenaria 7.96 3.11 0.72 10.54 73.72 Erodium cicutarium 6.59 2.34 0.60 7.93 81.65 Polycarpon alsinifolium 6.73 1.82 0.49 6.16 87.81 Pseudorlaya pumila 2.87 0.79 0.48 2.68 90.49 Silene nicaeensis 2.55 0.64 0.42 2.18 92.66 384 385 386 25 4. Discussion 387 4.1 Does the beach-dune gradient modify species invasion? Are there microsites along 388 the beach-dune gradient (dune types) that are more prone to being invaded by alien 389 plant species? 390 We detected a modest number of aliens, which represented 7.3% of a total of 55 391 vascular plant species from 21 families that were recorded. This finding was slightly 392 larger than reports from similar studies. For example, Carboni et al. (2010) found that 393 alien species represented 7% of total species richness. However, and like the findings by 394 Carboni et al. (2010), alien spread in our study area was correlated with changes in 395 community structure and composition. Our results showed that, indeed, the invasion by 396 O. drummondii increased inland and differed along the beach-dune gradient. This trend 397 was observed in terms of frequency, density, biomass and plant cover of the invasive 398 plant. The least invaded zones were the beach and foredunes, whereas inland dunes 399 were invaded the most. Similar studies have shown that the changing environmental 400 conditions along the beach-dune gradient largely affect the successful expansion and 401 impact of exotic invasive species, although the results are divergent. In coincidence with 402 our results, Kolb et al. (2002) and Acosta et al. (2008) found that in Californian coastal 403 ecosystems, and on Italian coastal dunes, respectively, greater invasion occurred in less 404 harsh sites. These authors concluded that the beach and embryo dunes had limited alien 405 invasions because the environmental stress reduces the ability of invasives to thrive and 406 use the available resources (Davis et al., 2000). On the contrary, Lortie and Cushman 407 (2007) observed higher levels of invasion when environmental conditions were extreme, 408 while Carboni et al. (2011) found that intermediate levels of disturbance and stress 409 offered the best conditions for colonization and spread of invasive species (Carpobrotus 410 32 2011; Santoro et al., 2012a). Finally, the differences observed between zones in terms 553 of the impact of invasion should be considered in management and conservation 554 targets. 555 556 5. Conclusions 557 This study showed that the invasion of O. drummondii in the coastal dunes of 558 southwestern Spain is associated with changes in community structure and 559 composition and, that these depend on local conditions. The differences in 560 invasiveness and impact of the invasive species along the beach-dune environmental 561 gradient can help predict where the impact will be more intense, and thus, where 562 mitigation actions are most necessary. In zones with high environmental severity such 563 as the beach and foredunes, the invasive species was less abundant, and thus, changes 564 in native vegetation were less drastic. The contrary was observed on inland dunes, 565 which are exposed to milder conditions. Additionally, the abundance of native dune 566 builder species such as Ammophila arenaria and Otanthus maritimus was significantly 567 reduced in the presence of O. drummondii, while other native species were locally 568 eliminated in the invaded sector. Thus, the shift in community structure and 569 composition could have a domino effect on coastal dune biodiversity and needs to be 570 carefully monitored and mitigated as much as possible. When doing so, the 571 environmental heterogeneity of the beach-dune gradient should also be considered, 572 given its relevance in the invasion process. 573 574 6. 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