1Determinants of plant species invasions in an arid island:

2evidence from Socotra Island (Yemen)

3 4Ali S. Senan • Federico Tomasetto • Alessio Farcomeni • Rayasamuda K. Somashekar • Fabio At- 5torre 6 7A.S. Senan • R.K. Somashekar 8Department of Environmental Science, Bangalore University, India 9 10F. Tomasetto 11Bio-Protection Research Centre, Lincoln University, New Zealand 12 13A. Farcomeni 14Department of Public Health and Infectious Diseases, Sapienza University of Rome, Italy 15 16F. Attorre (✉) 17Environmental Biology Department, Sapienza University of Rome, 18P.le A. Moro 5, 00185 Rome, Italy 19e-mail: [email protected] 20 21 22Abstract Understanding the factors that affect the distribution of alien plants in arid islands is 23complicated by the complex and stochastic nature of the invasion process per se, the harsh 24environmental conditions and the low number of researchers and sampling effort. We present the 25results of the most comprehensive inventory to date of alien vascular plant species occurring in 26Socotra Island, a global biodiversity hotspot just beginning to be developed. A floristic survey was 27conducted between 2006 and 2008, in 36 grid cells of 10 × 10 km. We integrated this data from this 28survey with those from scientific literature. We recorded 88 alien plant species. Tree and 29herbaceous species were the most common growth forms. Species from Asia and edible species 30were prevalent. We identified 80 species considered weeds worldwide with >50% adapted to arid 31conditions. We used a two-part model to analyze the spatial distribution of naturalized and alien 32plant species in relation to environmental and anthropogenic factors. Altitude and human-related 33factors play a significant role in the distribution of both naturalized and invasive species. Notably, 1 1

2 34the latter can potentially spread mainly in the alluvial basal areas. This study underpins the 35knowledge about alien species and their spatial distribution in Socotra Island. It provides a baseline 36for plant invasion management and contributes data for analyses of invasion processes on islands 37worldwide. 38 39Keywords Alien plants • Invasion • Islands • Naturalization • Species Distribution Models • Weeds

40

41Introduction

42 43Invasive alien plant species (IAPs) are widely recognized as one of the major threats to native 44biodiversity, particularly on oceanic islands (Denslow et al. 2009; Kueffer et al. 2010). Islands may 45be vulnerable to biological invasion for two main reasons. The first may be related to the 46geographical and historical isolation (Denslow 2003). This has led to the low diversity of resident 47biota, which is relatively less competitive because of the limited biotic interaction and adaptation to 48limited numbers of species through evolutionary time. As a result, IAPs may displace well adapted 49native species. Many IAPs are also dominant in their native range due to their (1) tolerance to a 50wide range of abiotic conditions, (2) use of a broad spectrum of resources, and (3) greater resistance 51to competition (Sax and Brown 2000). Thus, it also allows them to dominate in invaded 52environments. Moreover, dominating IAPs display a higher probability of dispersal as a result of 53their general abundance when compared to species that are rare and less widely distributed (Pyšek 54and Richardson 2006). 55 The second reason is linked to the socio-economic features of the islands. Hulme (2009) 56highlighted that in addition to biotic and environmental correlates, invasiveness depends on several 57other key factors such as the country's economy, the composition of its trade flows, its regulatory 58regimes and the history of land use change for agriculture, forestry and tourism. Islands tend to 59have high population density with intensive land uses and need for importing products. Dalmazzone 60(2000) found that imports for continental countries averaged 27% of their Gross Domestic Product 61in comparison to 43% for island countries. Moreover, the high level of islands invasion is also 62linked to the colonization by continental people associated with high rates of introduced species for 63food, wood and medicines, and as ornamentals (Guézou et al. 2010). 64 When plants are introduced to new environments, some of them will become naturalized, and 65of those some will become invasive (Richardson and Pyšek 2006). Recent studies of the invasion of 66islands by alien plant species has revealed some key trends in the invasion process (Trueman et al. 3 2

4 672010). Firstly, plant species biodiversity on islands has, on average, doubled due to the 68naturalization of alien species (Sax et al. 2002; Sax and Gaines 2008), which is higher than the 69naturalization in mainland environments (Stohlgren et al. 2008). As result, plant invasions can alter 70ecosystem processes, reducing the abundance or survival of native species (Mack et al. 2000). 71Caujapé-Castells et al. (2010) identified IAPs as an important threat to endemic plants on oceanic 72islands even if competition with invasive species may not be directly linked to extinction of 73indigenous species (Sax and Gaines 2008). 74 Several examples can be quoted about IAPs having serious impacts on islands: Miconia 75calvescens, has altered the forests of several Pacific Islands (Cuddihy and Stone 1990; Meyer and 76Florence 1996); Rubus spp. have invaded the Hawaiian Islands (Smith 1985), the Galápagos 77Archipelago (Guézou et al. 2010) and the Mauritius and Réunion Islands (Cronk and Fuller 2001); 78Lantana camara, the symbol of the worldwide result of ornamental plant introduction, has invaded 79native communities on the Galápagos Archipelago (Guézou et al. 2010), Hawaiian Islands (Smith 801985), Eastern Melanesia, the Cook and Society Islands of Eastern Polynesia (Mueller-Dombois 81and Fosberg 1998) and St. Helena Island (Kendle and Rose 2001). 82 The aim of this study was to: (1) revise a previous floristic survey on alien plant species on 83Socotra (Senan et al. 2010) in order to analyze the current stage of invasion; (2) analyze the envi- 84ronmental and anthropogenic factors influencing the spatial distribution of naturalized and invasive 85species; and (3) model the potential distribution of invasive species in order to assess the areas po- 86tentially most at risk from the current invasion. For points 2 and 3, we tested the applicability of 87novel statistical methods to analyze plant invasions. The results are discussed in the context of sug- 88gesting measures to help the control and management of alien plant species. 89 90Material and methods 91 92Study area 93 94Socotra Island (12°06’–12°42’ N and 52°03’–54°32’ E; Fig 1) is of continental origin and was 95joined to the Arabian plate not less than 15 Myr ago. The island is 130 km long and 40 km wide, 96and is characterized by a plateau ranging from 300 to 900 m composed of Cretaceous and Tertiary 97limestone. These carbonate formations overlie an igneous and metamorphic basement complex that 98crops out in the main Haghier mountains (1550 m; Beydoun and Bichan 1970). On the coastal 99plains, Quaternary and recent deposits of marine and fluvial origin overlie the older limestone. The 100climate is arid and influenced by the Indian Ocean monsoon system. The annual temperature is 28.9 5 3

6 101°C and the annual rainfall is c. 216 mm. Precipitation varies across the island depending on aspect 102and elevation. On the coastal plains, the annual precipitation may be less than 125 mm or absent al- 103together; however, in the Haghier mountains, fog-derived moisture may bring the total to excep- 104tional levels (c. 1000 mm; Scholte and De Geest 2010). 105 The interplay of the continental origin, climatic conditions and highly varied topography and 106geological substratum are responsible for the high level of endemism (Banfield et al. 2011): Socotra 107Island hosts 837 vascular plant species and with 308 of this species being endemics (36.8%). 108Socotra has been declared among the top five richest islands in the world in terms of biodiversity 109(Miller and Morris 2004). 110 Although there has been an indigenous community since the early Holocene, the people lived 111in relative isolation, engaging in trade in incense (mainly harvested from Boswellia elongata) and 112medicinal products (e.g. the sap from Aloe perryi), and in pastoral and fishing practices until the 113beginning of the 1990s, when many development projects began such as the construction of a new 114airport, sea port and paved roads. These, in turn, led to an increased movement of goods and 115population growth (now c. 50000 inhabitants, mainly concentrated in the settlements of Hadibou 116and Qalansiya), accompanied by the collapse of traditional land management activities, such as the 117regulation of tree cutting for timber and fuel, and the semi-nomadic transhumance system. This has 118led to the over-exploitation of woodlands and overgrazing of rangeland. Together with these 119factors, climate change and IAPs have been considered as the main threats to the native flora 120(Miller and Morris 2004; Attorre et al. 2007a, 2011; Scholte and De Gest 2010; Van Damme and 121Banfield 2011). 122 123 124Data set 125 126We divided Socotra Island into a grid of 42 quadrats each of 10 × 10 km. We surveyed a total of 36 127quadrats for alien plant species (c. 75% of the study area), excluding the least accessible ones. Due 128to logistic constraints and difficulties, surveys were conducted between 2006 and 2008 by foot 129along the main paths, and by car along the paved and unpaved roads (four-wheel drive). The latter 130survey was conducted keeping the vehicle speed between 8 and 16 kilometers per hour and 131recording alien species on both sides of the road. When an alien plant was encountered, GPS 132coordinates were recorded. We considered naturalized, casual and invasive alien plant species sensu 133Pyšek et al. (2004). We used data from a preliminary database of the alien flora of Socotra Island

7 4

8 134that was compiled by integrating field surveys with published literature (Al Khulaid 2000; Miller 135and Morris 2004). 136 137Alien species features 138 139For each alien species, we estimated: the growth form, uses, year of introduction, area of origin, 140climatic characteristics and status (i.e. weed or not weed). These last three parameters serve as a 141metric of the possible potential economic or environmental effects of the alien species (Richardson 142et al. 2000). We extracted those data from the Global Compendium of Weeds 143(http://www.hear.org/gcw/), the Southern African Plant Invaders Atlas 144(http://www.agis.agric.za/agisweb/agis.html), the Germplasm Resources Information Network's 145(http://www.ars-grin.gov/) and other internet sources. Moreover, we classified each species as: herb, 146shrub, succulent, tree or vine. We further classified the species’ use as: edible, forage, medicinal, 147multi-purposes (fence, wood supply) or ornamental. Since determining the exact year of the 148introduction of each alien species was difficult as it relied on human perception, which varies 149according to the informant interviewed, we used a proxy measure such as the Minimum Residence 150Time (MRT; Rejmánek 2000). MRT (i.e. the year since the first species record) has been positively 151correlated with the current distribution of alien species on islands (Trueman et al. 2010) because the 152longer a species is present, the more likely it has the opportunity to naturalize and/or invade 153(Milbau and Stout 2008). 154 We analyzed the spatial distribution of the naturalized and invasive alien species in relation to 155both environmental and anthropogenic parameters. Because a correlation has been detected between 156alien species richness and/or naturalizations and human population size (Castro and Jaksic 2008) or 157other measures of human-related factors (e.g. propagule pressure or habitat disturbances; Chown et 158al. 1998; Denslow et al. 2009; Kueffer et al. 2010), we considered settlements, their population and 159roads. Those data were provided by the Environmental Protection Authority of Socotra. Roads data 160were integrated with information collected during the field campaign and from the photointerpreta- 161tion of two RapidEye image sets (5 × 5 m resolution) acquired in 2010. However, we did not distin- 162guish between road types because during our investigation several unpaved roads were actively 163paved. 164 We used climatic, topographical and geological data, in grid format with a spatial resolution 165of 100 m. We obtained mean annual temperature and annual precipitation maps by interpolating 166data recorded at 10 manual meteorological stations and calculating the average data for the 2000- 1672008 period using universal kriging, with a trend function defined on the basis of 9 5

10 168a set of covariates (altitude, slope, aspect, and distance to the coast; Attorre et al. 1692007b). We calculated a moisture index (Mi) based on: Mi = P/PET, where P is the mean annual 170precipitation and PET is the potential evapotranspiration. PET was calculated using the Jensen- 171Haise equation (Jensen and Haise 1963): PET=SR/2450*(0.025T + 0.08), where SR is the annual 172potential solar radiation (KJ), and T is the mean annual temperature (°C). 173 We extracted geological data using a simplified geological map (Beydoun and Bichan 1970), 174including granitic, limestone, alluvial and sand substrata. 175 176Data Analysis 177 178For the analysis of the distribution of naturalized species with respect to environmental and human 179factors, data were aggregated using a grid of 5 × 5 km resolution. These data were aggregated at a 180finer resolution than the 10 × 10 km for sampling procedure, because the latter was unlikely to 181efficiently detect the effects of environmental and human factors in the species patterns. Therefore, 182for each 5 × 5 km cell we calculated the number of naturalized and invasive species identified 183according to GPS coordinates, which we used as our response variable. As explanatory variables, 184we calculated the average value for all the environmental variables (altitude, slope, mean annual 185temperature, annual precipitation, and moisture index) and the density of settlements (settlements 186per Km2), population (people per Km2) and roads, paved and unpaved (Km of roads per Km2) 187standardized for the area of the cell. 188 The relationship was analyzed by using a two-part model (Di Lorenzo et al. 2011), which 189derives maximum likelihood simultaneously for a system of two models. The first model specified 190is a logistic regression model for the presence/absence of a species; the second is a multivariate 191linear regression model for the log-abundance conditionally on having a species present. We 192performed model selection using forward stepwise regression for both models. Using the two-part 193model, we could estimate simultaneously the effect of the covariates on a species presence or 194absence, and the possible different effects of covariates on abundance given that we have a species 195presence. The two parts of the model are expressed in a single joint likelihood, so that we avoided 196possible bias caused by ignoring abundance when predicting presence or absence, or when 197predicting abundance with an excess of zeros due to species absence in several cells. We must 198underline here that, the significance and the regression coefficients of roads may suffer from bias 199due to the nature of the sampling, since a vehicle was used to collect part of the data. This bias is 200nevertheless bound to affect the logistic part (probability of presence) much more than the 201regression part (abundance conditionally on a presence); where the latter may even be considered 11 6

12 202free of bias. The use of a vehicle in fact may lead to oversampling of presences close to roads, but 203the measured abundance is not affected by this oversampling. 204 Potential distribution maps, with a resolution of 100 m, were obtained only for alien species 205with an invasive status by using species distribution models (SDMs). In this case, our response 206variable was the occurrences of invasive species, while as covariates we used climatic and 207topographical variables and the distance to the nearest settlement and road (using ArcGIS 9.3; ESRI 2082009). SDMs are based on the statistical relationship between occurrence data and environmental 209conditions and have been extensively used to predict spatial patterns of biological invasions and to 210prioritize locations for early detection and control of invasions. However, modeling the potential 211spread of invasive species is challenging because they violate the assumption of equilibrium with 212the environment (Hulme 2003). Moreover, species absence data are often unavailable or believed to 213be too difficult to interpret. This is the reason why the majority of the SDM studies use presence 214data only, such as the maximum entropy modeling (Václavík and Meentemeyer 2009; Jarnevich and 215Reynolds 2010), genetic algorithm for rule-set production (Schussman et al. 2006; Zhu et al. 2007) 216or variants of climatic envelope models including fuzzy envelope model (Robertson et al. 2004) or 217the Mahalanobis distance (Rouget et al. 2004; Baret et al. 2006). 218 In our study, we used a Random Forest (RF) model that implements the automatic 219combination of tree predictors (Breiman 2001). In RF, bootstrap samples are drawn to construct 220multiple trees and each tree is grown with a randomized subset of predictors. In our implementation 221we sampled 500 trees. This feature alleviates the problem of correlated variables because they may 222be extracted in turn, thus contributing to the aggregated tree model. Aggregation is obtained by 223averaging the trees. The predictors used were once again altitude, slope, mean annual temperature, 224annual precipitation, moisture index, density of settlements, population and roads. It is important to 225underline here that the RF technique automatically weights variables, so that unimportant variables 226may only seldom (or even never) be used by each tree. Trees are in fact grown sequentially starting 227from the most discriminatory predictors. The least discriminatory may not be used if the stopping 228criteria for growing the tree are satisfied before those are reached. This leads to an algorithm which 229seldom overfits data, creating no issues about model validation. The predictive ability (and hence, 230uncertainty associated with prediction through the RF) is assessed through the out-of-bag prediction 231error: each tree uses roughly two-thirds of the data points. The remaining one-third can be used as a 232test set. The final average proportion of misclassified observations in each test set provides a 233reasonable upper bound for the classification error. An additional useful feature of the RF algorithm 234is a measure of variable importance, derived from the contribution of each variable accumulated

13 7

14 235along all nodes and all trees where it is used. In order to implement the RF algorithm we used the 236function available in library randomForest (R Development Core Team, 2011). 237 Moreover, since presence-only models and models without dispersal information tended to 238over-predict the actual range of invasions (Václavík and Meentemeyer 2009), we used a model 239based on presence/absence data that has been already demonstrated its efficacy with respect to 240others in modeling the distribution of plant species (Scarnati et al. 2009). 241 242Results 243 244Alien species characteristics 245 246We found 88 alien species belonging to 35 families on Socotra Island (Appendix S1). Among those 247species, 18 are considered naturalized and 4 invasive. Dicotyledons represented 84% (74 species), 248monocotyledons 16% (14) with no gymnosperm and pteridophytes. Fabaceae (13 species), 249Solanaceae (8) and Cucurbitaceae (8) were the most represented families. Tree and herbaceous 250growth forms were the most abundant with 32 and 22 species, respectively. Edible species were the 251most common (49 species; 92 % are considered weeds worldwide), followed by the ornamentals 252(20 species; 90% are considered weeds worldwide; Fig. 2). The most represented area of origin was 253Asia (38) followed by Africa (29; Appendix S2). From the pool of alien species, we found a total of 25480 species that are considered weeds worldwide of which 53% (47) are adapted to grow in arid 255areas. According to the MRT criterion, the rate of species introduction increased dramatically 256starting in the 1990s (Fig. 3). The ground survey identified 825 locations with 22 naturalized 257species (25% from the pool of alien species). Among those, the most frequent species were the four 258considered invasive (21% from the pool of naturalized species) such as Argemone mexicana (140 259locations), Calotropis procera (153), Leucaena leucocephala (30) and Parkinsonia aculeata (39). 260 261Alien species distribution 262 263Alien species were recorded in 81 out of 173 quadrats of 5 x 5 km of resolution (Fig. 1). The most 264invaded areas were those adjacent to the major coastal settlements of Hadibou and Qualansiya. The 265two-part model highlighted that the presence of naturalized and invasive species was positively 266correlated with only two covariates: density of settlements and of roads (Table 1). The estimated 267log-odds were respectively 0.73 (p = 0.000902) and 0.22 (p = 0.0000028). In contrast, the richness 268of alien species was influenced positively by population density (an increase in richness of about 15 8

16 26980% for each additional unit of population density, p = 0.00000312) and roads (an increase of 7% 270for each additional unit of road density, p = 0.0203) and negatively by the altitude (a decrease of 2710.2% for each additional unit of altitude, p = 0.0153). The percent of increase/decrease is computed 272by exponentiating the coefficients estimated on the log-scale for the regression part of the two-part 273model. Notably, altitude is the only environmental parameter to have a significant influence on the 274presence/richness of alien species. 275 The spatial distribution of the most frequent invasive species was mainly determined by both 276road distance and altitude (Table 2). The climatic and topographical factors were less important, and 277the geological substrata were of minimal or no significance, probably due to the coarse spatial 278resolution of data. Accuracy of the four models based on species presence/absence was similar with 279Leucaena leucocephala as the worse (estimated error 19.3%) and Calitropis procera the best 280modeled species (9.3%). The potential distribution of the four species was similar, with their 281distribution limited in the basal alluvial plains. Each species may potentially cover from 15 to 25% 282of the study area (Fig. 4a-d). 283 284 285Discussion 286 287This study was carried out along roadsides and main paths of Socotra Island, where 88 alien species 288are currently hosted (Appendix S1). This represents only 10% of total native and endemic species 289(529 and 308 species respectively, Miller and Morris 2004). However, even if it can be 290hypothesized that the current number of alien species may be higher than that these figures are 291certainly small when compared with other oceanic islands, such as Easter Island, Desventuradas and 292Juan Fernàndez Archipelagos (Castro et al. 2008), tropical Indo-Pacific Islands (Mayer and 293Lavergne 2004), Azores Archipelago (Silva and Smith 2004), La Réunion Island (Baret et al. 2006) 294and the Galápagos Archipelago (Guézou et al. 2010). This result can be attributed to the interplay of 295historical, biogeographical and ecological factors. Socotra Island has been inhabited for at least 6 296millennia and the main economic activity was animal husbandry, based on a transhumance of goats, 297but also sheep on the limestone plateaus and cattle in the mountains (Naumkin 1993). Agricultural 298activity, the main source of alien species in oceanic islands, has been always limited to small home 299gardens. A Greek sailor around 60 A.D. reported: ‘the Island yields no fruit, neither vine nor grain’ 300(Mies and Beyhl 1996). Moreover, Balfour (1888) noted: ‘they (the Socotri) cultivate small tracts of 301ground near their houses, but are, as a rule, idle’. The only exceptions were the date palm 302plantations, which are currently an important source of income, and the cultivation of finger millet

17 9

18 303(Eleusine coracana), as the main cereal, nowadays completely abandoned and substituted in the diet 304of the local population by imported rice. 305 The harsh environmental conditions, seasonal isolation caused by the summer monsoons, 306malaria and the complex political situation prevented the island from being colonized by Europeans 307for centuries. The first short attempt was made by the Portuguese army in 1507 (Naumkin 1993). 308Successively, both the British and Russians armies used the island as a scientific base without 309substantially changing the traditional land use management. Furthermore, as for other oceanic 310islands of continental origin (Seychelles and New Caledonia), the “intrinsic” and strong resistance 311of Socotra to plant invasions can be hypothesized as linked to: (1) the remnant continental flora 312(Kueffer et al. 2010); (2) its old and nutrient-poor soils that may enhance the resistance to those 313alien species that are not specifically adapted to extreme soil conditions (Kueffer et al. 2008; 314Kueffer 2010); and (3) the high-pressure from grazing goats (Scholte et al. 2011). This seems to 315contradict other case studies of disturbances such as vegetation clearing and soil erosion caused by 316introduced livestock which especially favor the spread of unpalatable alien species (Walter and 317Lavin 2007). 318 Despite the peculiar characteristics of Socotra Island, a rapid increase of species introduction 319and naturalization started in the 1990s, after the unification of Yemen in 1990, which coincided 320with the socio-economic development. This, in turn, was triggered by the opening of the airport in 3212000 with a growth of trade and transport facilitations, and agricultural development projects 322leading to an increased introduction of alien species (i.e. propagule pressure). 323 This new invasion process is clearly at an early stage: the actual ratio between naturalized and 324native species is around 0.02 much lower than the 1:1 reached in other islands (Sax and Gaines 3252008). Classification based on the uses of alien species reflect also the current early stage of the 326invasion process being dominated by species mainly introduced as a source of food with a high 327number of potential weed species. With the ongoing socio-economic development, we may expect 328that in the near future the number of ornamental species will also increase as occurred, for instance, 329in La Réunion Island (Tassin et al. 2007) and the Galápagos Archipelago (Trueman et al. 2010). 330 The high number of species from Asia and Africa (Appendix S2) is the combined result of 331both cultural and economic links together with climate overlap. With regard to islands, it has been 332demonstrated that climatic pre-adaptation and/or matching is an important factor in plant invasions 333(Chown et al. 2005; Arteaga et al. 2009). However, Milton and Dean (2010) highlighted that 334invasive plants in arid areas, such as Socotra Island, are not all arid-adapted. Often, those species 335are linked to disturbance and facilitated by agricultural activities and their management can be 336complicated by the goods (edible) and services (ornamental) derived from them. 19 10

20 337 The spatial distribution of naturalized and invasive alien species is determined by the 338combined effect of environmental and anthropogenic factors. In particular, their presence/richness 339is positively correlated with the density of population, settlements, and roads (Table 1) while the 340spatial distribution of invasive alien species was primarily associated with the distance to roads 341(Table 2). Our results further corroborate the findings of other studies indicating that human- 342mediated propagule pressure and disturbance are key factors for the naturalization and invasion of 343plant species (Chow et al. 1998; McKinney 2002; Rodgers and Parker 2003; Dark 2004; Taylor and 344Irwin 2004; Denslow et al. 2009; Kueffer et al. 2010; Trueman et al. 2010). Bearing this in mind, 345special attention has to be devoted to the ongoing construction of the paved road network, which 346has already been subject to tensions regarding road design and route particularly with respect to the 347location of nature sanctuaries (Scholte et al. 2011). Our findings support the importance of roads in 348accelerating the spread of already invasive species or those that can become invasive in the future 349(Arévalo et al. 2005; Von der Lippe and Kowarik, 2007). Moreover, the improvement from 350unpaved to paved roads is expected to further facilitate the spread of IAPs due to an increasing 351traffic levels gradient and/or habitat alteration during road construction and maintenance (Parendes 352and Jones 2000; Trombulak and Frissell 2000; Gelbard and Benalp, 2003). 353 The outcome of the RF showed that the potential spread of the four invasive species will 354affect only the basal alluvial valleys (Fig. 4a-d) where Croton socotranus shrubland is the most 355common vegetation of the Island (Miller and Morris 2004). Our results do not provide any evidence 356of plant invasion in the Haghier mountains, which are dominated by Dracaena cinnabari and 357Pittosporum viridiflorum and considered to be one of the most important biodiversity hotspots and 358refugium for island endemics (Banfield et al. 2011; De Sanctis et al. 2012). However, a screening of 359potential invaders based on the climate matching and “invasiveness elsewhere” criteria (sensu 360Kueffer et al. 2010) is needed to determine future risks from the alien flora already present at the 361driest low elevation sites (where major settlements are located). This alien flora may spread and 362become invasive at more humid sites of the island (typically at higher elevations; Scholte and De 363Geest, 2010). 364 365Conclusions 366In our study, we used an approach that provided further insights into the plant invasion process on 367islands. At the same time, we highlighted specific features linked to the peculiar history and 368environment of Socotra Island. In particular, the island has been less affected by plant species 369invasions when compared with other oceanic islands. Nevertheless, the recent and rapid socio- 370economic development will likely foster the invasion process already seen on other oceanic islands 21 11

22 371where a causal link between propagule pressure and human development has been identified 372(Kueffer et al. 2010). 373 Currently, despite the fact that there appears to be little awareness of the invasive species- 374related problems, based on the results of our study we raise several aspects of major concern: 375 • some of the worst invaders worldwide, such as Leucaena leucocephala, Nicotiana glauca, 376 Opuntia stricta, Parkinsonia aculeata, are already naturalized or even invasive (Appendix 377 S1) and urgent interventions are now required for their management; 378 • as new naturalizations from the pool of alien species are to be expected (time lag), timely 379 elimination of the worst species before they become invasive should be considered; 380 • specific indications should be included in the environmental impact assessments for paved 381 roads construction and maintenance in order to reduce the risk of spread of invasive species. 382In order to cope with IAPs we identify several key issues that should be addressed such as the need 383of the elaboration of an updated legal framework and the improvement of the local capacity for 384dealing with alien species. Moreover, a quarantine system to assess new arrivals and prevent 385potential problems and the implementation of pre-border interventions (e.g. weed risk assessment; 386Pheloung et al. 1999; Hulme 2012) would make it possible to introduce alien species while reducing 387the risk of invasions. 388 Within a pessimistic framework determined by limited human and financial resources, further 389weakened by the current political situation of Yemen, we can point out some positive aspects. 390These include a recent successful public awareness campaign about the threats posed by invasive 391species, and the creation of a plant nursery in order to provide native alternatives to alien 392ornamental species even though such intervention may carry its own risk (Milton and Dean 2010). 393 394 395Acknowledgments 396This work was supported by the Italian Development Cooperation and the Government of Yemen. 397 398References 399Al Khulaid AW (2000) Flora of Yemen. Sustainable Environmental Project; Taiz, Yemen 400Arévalo JR, Delgadoa JD, Otto R, Naranjoc A, Salasd M, Fernández-Palaciosa JM (2005) Distribution of alien vs. 401 native plant species in roadside communities along an altitudinal gradient in Tenerife and Gran Canaria (Canary 402 Islands). Perspect Plant Ecol Evol Syst 7:185-202 403Arteaga MA, Delgado JD, Otto R, Fernández-Palacios JM, Arévalo JR (2008). How do alien plants distribute along 404 roads on oceanic islands? A case study in Tenerife, Canary Islands. Biol Invasions 11:1071-1086 405Attorre F, Francesconi F, De Sanctis M, Alfo M, Bruno F (2007b) Comparison of different methods for the production 406 of climatic and bioclimatic maps at regional scale. Int J Climatol 27: 1825-1843 407Attorre F, Francesconi F, Taleb N, Scholte P, Saed A, Alfo M, Bruno F (2007a) Will dragonblood survive the next 408 period of climate change? Current and future potential distribution of Dracaena cinnabari (Socotra, Yemen). Biol 409 Conserv 138:430-439

23 12

24 410Attorre F, Taleb N, De Sanctis M, Farcomeni A, Guillet A, Vitale M (2011). Developing conservation strategies for 411 endemic tree species when faced with time and data constraints: Boswellia spp. on Socotra (Yemen). Biodiver 412 Conserv 20:1483-1499 413Balfour IB (1888) Botany of Soqotra. Transactions of the Royal Society of Edinburgh 31, pp 446 414Banfield LM, Van Damne K, Miller AG (2011) Evolution and biogeography of the flora of the Socotra Archipelago 415 (Yemen). In: Bramwell, D. (ed.) The Biology of Islands Flora, Cambridge University Press, UK, pp 197-225 416Baret S, Rouget M, Richardson DM, Lavergne C, Egoh B, Dupont J, Strasberg D (2006) Current distribution and 417 potential extent of the most invasive alien plant species on La Réunion (Indian Ocean, Mascarene islands). Austral 418 Ecol 31:747-758 419Beydoun ZR, Bichan HR (1970). The geology of Socotra Island, Gulf of Aden. Q J Geol Soc London 125:413-446 420Breiman L (2001) Random forests. Mach Learn 45:5-32 421Castro SA, Jaksic FM (2008) Role of non-established plants in determining biotic homogenization patterns in Pacific 422 Oceanic Islands. Biol Invasions 10:1299-1309 423Caujapé-Castells J, Tye A, Crawford DJ, Santos-Guerra A, Sakai A, Beaver K, Lobin W, Vincent Florens FB, Moura 424 M, Jardim R, Gómes I, Kueffer C (2010) Conservation of oceanic island floras: Present and future global 425 challenges. Perspect Plant Ecol Evol Syst. 12:107-129 426Chown SL, Gremmen NJM, Gaston KJ (1998) Ecological biogeography of Southern Ocean Islands: species-area 427 relationships, human impacts, and conservation. Am Nat 152:562-575 428Chown SL, Hull B, Gaston KJ (2005) Human impacts, energy availability and invasion across Southern Ocean Islands. 429 Global Ecol Biogeogr 14:521-528 430Cronk QCB, Fuller JL (2001) Plant Invaders: the Threat to Natural Ecosystems. Earthscan, London 431Cuddihy LW, Stone CP (1990) Alteration of Native Hawaiian Vegetation. University of Hawaii 432Dalmazzone S (2000) Economic Factors affecting vulnerability to biological invasions. In C. Perrings, M. Williamson 433 & S. Dalmazzone (Eds.) The Economics of Biological Invasions, Edward Elgar, Cheltenham, pp 17-30 434Dark SJ (2004) The biogeography of invasive alien plants in California: an application of GIS and spatial regression 435 analysis. Divers Distrib 10:1-9 436De Sanctis M, Adeeb A, Farcomeni A, Patriarca C, Saed A, Attorre F (2012) Classification and distribution patterns of 437 plant communities on Socotra Island, Yemen Appl Veg Sci in press 438Denslow JS (2003) Weeds in paradise: thoughts on the invasibility of tropical islands. Ann Mo Bot Gard 90:119–127 439Denslow JS, Space J, Thomas PA (2009) Invasive exotic plants in the tropical Pacific Islands. Biotropica 41:162-170 440Di Lorenzo B, Farcomeni A, Golini N (2011) A Bayesian Model for Presence-Only Semicontinuous Data, With 441 Application to Prediction of Abundance of Taxus Baccata in Two Italian Regions. J Agric Biol Envir S 16:339-356 442Esri (2009) ArcMap-ArcInfo version 9.3 software and manual. Ecological Systems Research Institute, Redlands, CA. 443Gelbard JL, Belnap J (2003) Roads as conduits for exotic plant invasions in a semiarid landscape. Conserv Biol 17:420- 444 432 445Guézou A, Trueman M, Buddenhagen CE, Chamorro S, Guerrero AM, Paola Pozo1, Atkinson R (2010) An Extensive 446 Alien Plant Inventory from the Inhabited Areas of Galápagos. PLoS ONE 5: e10276 447Hulme P E (2012) Weed risk assessment: a way forward or a waste of time? J Appl Ecol 49:10–19 448Hulme PE (2003) Winning the science battles but losing the conservation war? Oryx 37:178-193 449Hulme PE (2009) Trade, transport and trouble: managing invasive species pathways in an era of globalization. J Appl 450 Ecol 46:10-18 451Jarnevich CS, Reynolds LV (2010) Challenges of predicting the potential distribution of a slow-spreading invader: a 452 habitat suitability map for an invasive riparian tree. Biol Invasions 13:153-163 453Jensen ME, Haise HR (1963) Estimating evapotranspiration from solar radiation. J Irrig Drain Div 89:15-41 454Kendle AD, Rose JE (2001) Invasive plants on land recovering from desertification on Saint Helena Island. In: Brundu 455 G, Brock J, Camarda I, Child L, WadeM (eds), Species Ecology and Ecosystem Management, Leiden, The 456 Netherlands, pp 311–318 457Kueffer C (2010) Reduced risk for positive soil-feedback on seedling regeneration by invasive trees on a very nutrient- 458 poor soil in Seychelles. Biol Invasions 12:97-102 459Kueffer C, Daehler CC, Torres-Santana CW, Lavergne C, Meyer J, Otto R, Silva L (2010) A global comparison of plant 460 invasions on oceanic islands. Perspect Plant Ecol Evol Syst 12:145-161 461Kueffer C, Klingler G, Zirfass K, Schumacher E, Edwards P, Güsewell S (2008) Invasive trees show only weak 462 potential to impact nutrient dynamics in phosphorus-poor tropical forests in the Seychelles. Funct Ecol. 22:359-366 463Mack RN, Simberloff D, Lonsdale WM, Evans H, Clout M, Bazzaz FA (2000) Biotic invasions: causes, epidemiology, 464 global consequences, and control. Ecol Appl 10(3):689-710 465McKinney ML (2002) Influence of settlement time, human population, park shape and age, visitation and roads on the 466 number of alien plant species in protected areas in the USA. Divers Distrib 8:311-18 467Meyer JY, Florence J (1996) Tahiti’s flora endangered by the invasion of Miconia calvescens DC. (Melastomataceae). 468 J Biogeogr 23:775–781

25 13

26 469Meyer JY, Lavergne C (2004) Beautes fatales: Acanthaceae species as invasive alien plants on tropical Indo-Pacific 470 islands. Divers Distrib 10:333-347 471Mies BA Beyhl FE (1996) The vegetation ecology of Soqotra. In: Proceedings of the First International Symposium on 472 Soqotra Island. Vol. I. UN Publications, New York, pp. 35–82 473Milbau A, Stout J (2008) Factors associated with alien plants transitioning from casual, to naturalized, to invasive. 474 Conserv Biol 22:308-317 475Miller A, Morris M. (2004) Ethnoflora of the Soqotra archipelago. Royal Botanic Garden Edinburgh, UK 476Milton SJ, W. Dean WRJ (2010) Plant invasions in arid areas: special problems and solutions: a South African 477 perspective. Biol Invasions 12:3935-3948 478Mueller-Dombois D, Fosberg FR (1998) Vegetation of the tropical Pacific Islands. In: Caldwell MM et al. (eds) 479 Ecological Studies, Vol 132. Springer-Verlag, New York 480Naumkin VV (1993) Island of the Phoenix. An Ethnographic Study of the People of Socotra. Ithaca Press, Reading, UK 481Parendes L A, Jones JA (2000) Role of light availability and dispersal in exotic plant invasion along roads and streams 482 in the H.J. Andrews Experimental Forest, Oregon. Conserv Biol 14:64-75 483Pheloung PC, Williams PA, Halloy SR (1999) A weed risk assessment model for use as a biosecurity tool evaluating 484 plant introductions. J Environ Manag 57:239-251 485Prasad AM, Iverson LR, Liaw A (2006) Newer classification and regression tree techniques: bagging and random 486 forests for ecological prediction. Ecosystems 9:181-199 487Pysek P, Richardson DM (2006) The biogeography of naturalization in alien plants. J Biogeogr 33:2040-2050 488Pyšek P, Richardson DM, Rejmánek M, Webster GL, Williamson M, Kirschner J (2004) Alien plants in check lists and 489 floras: towards better communication between taxonomists and ecologists. Taxon 53:131-143 490R Development Core Team (2011) R: a language and environment for statistical computing. R Foundation for 491 Statistical Computing, Vienna, Austria. Available online at http://www.R-project.org. 492Rejmánek M (2000) Invasive plants: approaches and predictions. Austral Ecol 25:497-506 493Richardson DM, Pyšek P (2006) Plant invasions: merging the concepts of species invasiveness and community 494 invisibility. Progr Phys Geogr 30:409–431 495Richardson DM, Pyšek P, Rejmánek M, Barbour MG, Panetta FD, West CJ (2000) Naturalization and invasion of alien 496 plants: concepts and definitions. Divers Distrib 6:93-107 497Robertson MP, Villet MH, Palmer AR (2004) A fuzzy classification technique for predicting species’ distributions: 498 applications using invasive alien plants and indigenous insects. Divers Distrib 10:461-474 499Rodgers JC, Parker KC (2003) Distribution of alien plant species in relation to human disturbance on the Georgia Sea 500 Islands. Divers Distrib 9:385-398 501Rouget M, Richardson DM, Nel JL, Le Maitre DC, Egoh B, Mgidi T (2004) Mapping the potential ranges of major 502 plant invaders in South Africa, Lesotho and Swaziland using climatic suitability. Divers Distrib 10:475-484 503Sax DF, Brown JH (2000) The paradox of invasion. Global Ecol Biogeogr 9:363-371 504Sax DF, Gaines SD (2008) Species invasions and extinction: the future of native biodiversity on islands. Proc Natl Acad 505 Sci USA 105:11490-11497 506Sax DF, Gaines SD, Brown JH (2002) Species invasions exceed extinctions on islands worldwide: a comparative study 507 of plants and birds. Am Nat 160:766-783 508Scarnati L, Farcomeni A, Attorre F, Francesconi F, De Sanctis M (2009) Comparison between potential distribution 509 model of forest species and fragmentation species from abundance data. Community Ecol 10:215-224 510Scholte P, Al-Okaishi A, Suleyman AS (2011) When conservation precedes development: a case study of the opening 511 up of the Socotra archipelago, Yemen. Oryx 45:401-410 512Scholte P, De Geest P (2010) The climate of Socotra Island (Yemen): a first-time assessment of the timing of the 513 monsoon wind reversal and its influence on precipitation and vegetation patterns. J Arid Environ 74:1507-1515 514Schussman H, Geiger E, Mau-Crimmins T, Ward J (2006) Spread and current potential distribution of an alien grass, 515 Eragrostis lehmanniana Nees, in the southwestern USA: comparing historical data and ecological niche models. 516 Divers Distrib 12:582-592 517Senan AS, Somashekar RK, Attorre F, Taleb N, Bruno F (2010) Exotic Species of Socotra Island, Yemen: a first 518 contribution. Ann Bot 1:85-96 519Silva L, Smith CW (2004) A characterization of the non-indigenous flora of the Azores. Biol Invasions 6:193–204 520Smith CW (1985). Impacts of alien plants on Hawaii’s native biota. In Stone CP and Scott JM (eds) Hawaii’s 521 Terrestrial Ecosystems: Preservation and Management. University of Hawaii, Honolulu, pp. 180-250 522Stohlgren TJ, Barnett DT, Jarnevich CS, Flather C, Kartesz J (2008) The myth of plant species saturation. Ecol Lett 523 11:313-322 524Tassin J, Triolo J, Lavergne C (2007) Ornamental plant invasions in mountain forests of Réunion (Mascarene 525 Archipelago): a status review and management directions. Afr J Ecol 45:444-447 526Taylor BW, Irwin RE (2004) Linking economic activities to the distribution of exotic plants. Proc Natl Acad Sci USA 527 101:17725–30

27 14

28 528Trombulak SC, Frissell CA (2000) Review of ecological effects of roads on terrestrial and aquatic communities. 529 Conserv Biol 14:18-30 530Trueman M, Atkinson R, Guézou A, Wurm P (2010) Residence time and human-mediated propagule pressure at work 531 in the alien flora of Galápagos. Biol Invasions 12:3949-3960 532Václavík T, Meentemeyer RK (2009) Invasive species distribution modeling (iSDM): Are absence data and dispersal 533 constraints needed to predict actual distributions? Ecol Model 2120(23):3248-3258 534Van Damme K, Banfield L (2011) Past and present human impacts on the biodiversity of Socotra Island (Yemen): 535 implications for future conservation. Biodiversity Conservation in the Arabian Peninsula Zoology in the Middle East 536 3:31-88 537Von der Lippe M, Kowarik I (2007) Long-distance dispersal of plants by vehicles as a driver of plant invasions. 538 Conserv Biol 21:986-96 539Walter HS, Levin GA (2007) Feral sheep on Socorro Island: facilitators of alien plant colonization and ecosystem 540 decay. Divers Distrib 14:422-431 541Zhu L, Sun OJ, Sang W, Li Z, Ma K (2007) Predicting the spatial distribution of an invasive plant species ( Eupatorium 542 adenophorum) in China. Landscape Ecol 22:1143-1154 543 544 545 546 547

29 15

30 548 Table 1 Two-part model on abundance of alien species. Parameters for the regression part are on log-scale Logistic part parameter log-odds Std. Err. p-value Intercept -1.4846 0.2844 <0.001 Settlement density 0.7344 0.2212 <0.001 Road density 0.2225 0.0531 <0.001 coefficien Regression part parameter Std. Err. p-value t Intercept -6.3604 0.2531 <0.001 Population density 0.6255 0.1412 0.001 Road density 0.0693 0.0292 0.0203 Altitude -0.0018 0.0007 0.0153 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570

31 16

32 571 Table 2 Error rate and variable importance predicted by RF Argemone Calotropis Leucaena Parkinsonia Species mexicana porcera leucocephala aculeata Error rate (%) 16.0 9.3 19.3 13.6 Distance to road (m) 0.140 0.134 0.128 0.137 Distance to Settlement (m) 0.028 0.041 0.038 0.038 Altitude (m) 0.071 0.067 0.060 0.057 Slope (°) 0.023 0.022 0.044 0.039 Annual Precip. (mm) 0.042 0.025 0.027 0.021 Mean annual Temp. (°C) 0.014 0.012 0.008 0.009 PET (mm) 0.031 0.035 0.025 0.018 Moisture index 0.035 0.018 0.024 0.021 Sand 0.000 0.000 0.000 0.001 Alluvial 0.003 0.001 0.003 0.006 Limestone 0.010 0.008 0.015 0.012 Vulcanic 0.003 0.002 0.002 0.002 572 573

33 17

34 574Figure Captions 575 576Figure 1. Study area, its location: 380 km south of Ras Fartak on the Gulf of Aden coast of Yemen and 230 km east of 577Cape Guardafui in Somalia (inset), and the location of major settlements; details of the distribution of alien plant 578species are shown. Number of alien species is reported for 5 x 5 km grid cells used for the statistical analysis. 579 580Figure 2. The total number of alien species recorded per uses, classified according to weed status worldwide. 581 582Figure 3. MRT for alien species according to the known year of introduction. 583 584Figure 4. Occurrences (black triangles) and potential distribution (green colour) of the four invasive alien species: A – 585Argemone Mexicana; B – Calitropis procera; C – Leucaena leucacephala; D – Parkinsonia aculeate. 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604

35 18

36 605 606

607 608Fig. 1 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624

37 19

38 625 626

627 628Fig. 2 629 630 631 632 633 634 635 636 637

39 20

40 638 639

640 641Fig. 3

41 21

42 642 643

644 645

646 647

648 649Fig. 4a-d

43 22

44 650Appendix S1 651Complete list of the alien vascular plant species encountered in Socotra Island. Species names. The super- 652script refers to the plant species status (* Naturalized, ** Invasive). Use: E) edible; F) forage; M) medici- 653nal; MP) multi-purposes; O) ornamental. Growth form: H) herb; S) shrub; Su) succulent; T) tree; V) vine. 654Year of introduction. 655 Growth Family Scientific name Use Year form Malvaceae Abelmoschus esculentus (L.) Moench E V 2003 Fabaceae Acacia ehrenbergiana Hayne O S 2002 Acacia nilotica (L.) Willd. ex Delile subsp. in- Fabaceae O T 2003 dica Fabaceae Acacia tortilis (Forssk.) Hayne* O T 2001 Fabaceae Albizia lebbek (L.) Benth.* MP T 1993 Amaryllidaceae Allium cepa L. E H 2003 Amaryllidaceae Allium fistulosum L. E H 2003 Amaryllidaceae Allium chinense G. Don E H 2003 Amaranthceae Amaranthus cruentus L. E H 2003 Annonaceae Annona squamosa L. E T 2003 Apiaceae Apium graveolens L. E H 2003 Papaveraceae Argemone mexicana L.** M H <1978 Meliaceae Azadirachta indica A. Juss.* MP T 1993 Asteraceae Blainvillea acmella (L.) Philipson E H 2003 Nyctaginaceae Bougainvillea spectabilis Willd. O S 2004 Brassicaceae Brassica juncea (L.) Czern. E H 2003 Apocynaceae Calotropis procera (Aiton) W. T. Aiton** M S <1978 Solanaceae Capsicum annuum L. E H 2003 Solanaceae Capsicum frutescent L. E H 2003 Caricaceae Carica papaya L. E T 1992 Celastraceae Catha edulis (Vahl) Forssk. ex Endl. E S 2007 Apocynaceae Catharanthus roseus (L.) G. Don* O H 2005 Poaceae Chloris jubaensis Cope* F H 1984 Cucurbitaceae Citrullus lanatus (Thunb.) Matsum. & Nakai E V 2003 Rutaceae Citrus aurantifolia (Christm.) Swingle E T 2004 Rutaceae Citrus sinensis (L.) Osbeck E T 2004 Arecaceae Cocos nucifera L. O T 2004 Malvaceae Corchorus olitorius L. E H 2003 Boraginaceae Cordia sinensis Lam. O T 2002 Cucurbitaceae Cucumis anguria L. E V 2003 Cucurbitaceae Cucumis metuliferus E. Mey. ex Naudin E V 2003 Cucurbitaceae Cucumis sativus L. E V 2003 Cucurbitaceae Cucurbita maxima Duchesne E V 2003 Cucurbitaceae Cucurbita moschata Duchesne E V 2003 Cucurbitaceae Cucurbita pepo L. E V 2003 Apiaceae Daucus carota L. E H 2003 Poaceae Eleusine coracana (L.) Gaertn. E H <1978 Moraceae Ficus carica L. E S 2004 Fabaceae Glycine max (L.) Merr. E S 2003 Asteraceae Helianthus annuus L. O S 2003 Malvaceae Hibiscus sabdariffa L. M H 2003 Arecaceae Hyphaene thebaica (L.) Mart. MP T 2005 Convolvu- Ipomoea aquatica Forssk.* O V 2004 laceae Convolvu- Ipomoea batatas (L.) Lam. E V 2003 laceae

45 23

46 Euphorbiaceae Jatropha curcas L.* MP S 2005 Fabaceae Lablab purpureus (L.) Sweet E V 2004 Asteraceae Lactuca sativa L. E H 2003 Lythraceae Lawsonia inermis L.* MP S 2003 Fabaceae Leucaena leucocephala (Lam.) de Wit** MP T 2003 Rosaceae Malus sylvestris (L.) Mill. E T 2004 Meliaceae Mangifera indica L. E T 1998 Fabaceae Medicago sativa L. F H 2003 Lamiaceae Mentha x piperita L. M H 2005 Cucurbitaceae Momordica charantia L. E V 2003 Moringaceae Moringa oleifera Lam.* MP T 2003 Moraceae Morus nigra L. E T 2006 Musaceae Musa x paradisiaca L. E H 1998 Apocynaceae Nerium oleander L. O S 2003 Solanaceae Nicotiana glauca Graham* O S 2007 Lamiaceae Ocimum basilicum L. E H 2003 Cactaceae Opuntia ficus-indica (L.) Mill. O Su 2004 Cactaceae Opuntia stricta (Haw.) Haw.* O Su 2004 Amaryllidaceae Pancratium maximum Forsk. O H 2004 Fabaceae Parkinsonia aculeata L.** O T 2004 Poaceae Pennisetum purpureum Schumch. F H <1978 Fabaceae Phaseolus vulgaris L. E V 2003 Arecaceae Phoenix dactylifera L. E T <1978 Fabaceae Pithecellobium dulce (Roxb.) Benth.* O T 1978 Fabaceae Prosopis juliflora (Sw.) DC.* O T 1998 Myrtaceae Psidium guajava L.* E T 2001 Punicaceae Punica granatum L. E S 2004 Brassicaceae Raphanus sativus L. E H 2003 Rutaceae Ruta chalepensis L. M H 2005 Sapindaceae Sapindus mukorossi Gaertn. O T 2001 Solanaceae Solanum aethiopicum L. E H 2003 Solanaceae Solanum lycopersicum L. E S 2003 Solanaceae Solanum melongena L. E H 2003 Solanaceae Solanum nigrum L. E H 2003 Solanaceae Solanum macrocarpon L. E H 2003 Poaceae Sorghum bicolor (L.) Moench E S 1998 Asteraceae Tagetes minuta L.* M H 2005 Asteraceae Tagetes patula L. O H 2006 Combretaceae Terminalia catappa L.* O T 1998 Malvaceae Thespesia populnea (L.) Sol. ex Corrêa* O T 1998 Typhaceae Typha domingensis Pers.* F H 1993 Fabaceae Vigna unguiculata (L.) Walp. E V 2003 Vitaceae Vitis vinifera L. E V 2004 Poaceae Zea mays L. E H 2002 656

47 24

48 657 658Appendix S2 Origins of alien plant species of Socotra Island. Numbers in parenthesis after each region show the total 659number of alien species from the region and the percentage present in the flora of Socotra Island. 660

49 25

50