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عنوان فارسی مقاله:
ارتباط تنوع فلور بومی ناهمگنی پوشش زمین و تهاجم گیاه در دره رودخانه
عنوان انگلیسی مقاله:
Linking the diversity of native flora to land cover heterogeneity and plant invasions in a river valley
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
Alien plant invasions promoted by human activities pose a growing problem for biodiversity conservation worldwide and have been recognized as a major driver of the decline of global species diversity (Mack et al., 2000). Alien plant species can significantly alter native ecosystems by their extensive use of resources, the accumulation of litter or salt, as well as by changing disturbance regimes, hydrology and nutrient cycling (Richardson et al., 2000). As a consequence, they negatively affect the richness, diversity and composition of native communities (Hejda et al., 2009), leading to the extinction of vulnerable native species and the homogenization of floras at different spatial scales (Wilcove et al., 1998; Schwartz et al., 2006). Therefore, alien plants are able to cause great environmental damage and generate considerable economic costs (Pimentel et al., 2005). However, the mechanisms responsible for alien plant invasions and their impact on natural ecosystems are complex and not fully understood. Identifying the determinants of alien species invasiveness is a challenging task, and predicting future invasion patterns is crucial for taking effective steps to prevent the further spread of biotic invaders (Mack et al., 2000). In this respect, particularly the issue of secondary invasions needs to be better understood (Pearson et al., 2016). Finally, increased knowledge about interactions between alien and native species at different scales is needed to mitigate the damage caused by alien plant invasions. Also extensive changes in the land-cover seriously affect biodiversity. Deforestation, urbanization and habitat fragmentation, are directly responsible for population declines and extinctions of native species (Wilcove et al., 1998; Jetz et al., 2007). Moreover, land cover changes in- fluence native species diversity indirectly by facilitating the dispersal of alien plants in altered environments (Blanchet et al., 2015). Considerable effort is currently devoted to assessing the risk of the spread of alien plants in various ecosystems (Chytrý et al., 2008) and to predict the impact of alien species due to future land-cover changes (Chytrý et al., 2012). River valleys are regarded as regional hotspots of biodiversity (Naiman et al., 1993; Ward et al., 2001), but they simultaneously have undergone severe human-induced changes for centuries (Décamps et al., 1988). The land-cover within river valleys has been altered as a result of river flow regulation and drainage, agricultural expansion, extensive building development etc. Hence, an extensive loss of natural riverine vegetation has been observed, locally reaching a decrease of up to 95% of its original coverage (Tockner and Stanford, 2002). River valleys are also especially susceptible to invasions by alien plant species (Pyšek and Prach, 1995). Flowing water acts as an effective dispersalagent supporting the downstream movement of plant diaspores, while natural fluctuations in the water level facilitate the colonization of river valleys by alien plants (Davis et al., 2000). Currently, several legislative efforts are being implemented in order to improve the conservation of riverine ecosystems (e.g. the EU Water Framework Directive). Furthermore, over 300 conservation projects in river valleys have been co-funded by LIFE (the EU financial instrument supporting nature conservation) since 1992 (http://ec.europa.eu/environment/life/). Despite the fact that invasions of alien plant species are a central threat to biodiversity, the relationships between alien and native plant species are poorly understood. Many theoretical and empirical studies suggested a negative relationship between plant diversity and vulnerability to invasions (e.g. Levine, 2000; Maron and Marler, 2008). Thus, invasions should be hampered at species-rich sites, mainly by the strong competition and domination of native species. An alternative hypothesis states that the richness of native and invasive plants are spatially synchronized by the same environmental factors (Deutschewitz et al., 2003), therefore, hotspots of native and invasive species may spatially overlap. In the case of river valleys, however, the plant diversity of native and alien species has been studied mainly in the narrow riparian zone (Renöfält et al., 2005a; Hejda and Pyšek, 2006), and few researches cover the adjacent habitats, and therefore, more studies are needed to disentangle these two alternative hypotheses concerning river valleys. In this study, we focused on the spatial distribution of vascular plant species richness within the San River Valley (south-eastern Poland). We considered three groups of species. The first group consisted of native vascular plants. Since many alien species pose no problem from a conservation point of view, the second studied group included only invasive alien species, which might have a negative impact on native plants (Hejda et al., 2009; Pyšek et al., 2012). Invasions of alien plants sometimes impact rare species more than common ones (e.g. when common species are strong competitors compared to rare species, see Powell et al., 2011) thus the third group consisted of red-listed species. Most of the research on plant diversity patterns is on a fine-scale (the community level) or a broad-scale (the level of geographical regions), creating a gap in knowledge about the diversity pattern of the intermediate scale (Heikkinen, 1996). We therefore, focused on the meso-scale and conducted our survey using data collected in 1 km × 1 km plots. First, we investigated the impact of land cover heterogeneity, and the river bed proximity on alpha-diversity of the three studied plant groups. We expected that a high proportion of forest should favor native species, while human settlements should promote invasive alien species (e.g. Kowarik et al., 2013). Moreover, we predicted that land cover diversity and proximity to the river are positively related to the species richness of both native and invasive plants. Second, we tested whether richness of natives and invasive species was related to the beta-diversity of the remaining studied groups. Third, we investigated the variation in gamma-diversity of the three plant groups in relation to the richness of native plants and invasive plants.
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کلمات کلیدی:
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