Abstract
The impact of invasive species is often difficult to assess due to species × ecosystem interactions. Impatiens glandulifera heavily invaded several habitat types in Central Europe but its impact on native plant communities is rated ambiguously. One reason could be that the impact differs between habitat types or even between environmentally heterogeneous patches (micro-habitats) within one habitat type. In the present study a vegetation survey was performed within heterogeneous riverside habitats in Germany investigating the impact of I. glandulifera on native vegetation in dependence of environmental conditions. The vegetation was recorded in summer and spring because of seasonal species turnover and thus potentially different impact of the invasive plant. We found that the cover of I. glandulifera depended on environmental conditions resulting in a patchy occurrence. I. glandulifera did not have any impact on plant alpha-diversity but reduced the cover of the native vegetation, especially of the dominant species. This effect depended on micro-habitat and season. The native vegetation was most affected in bright micro-habitats, especially those with a high soil moisture. Not distinguishing between micro-habitats, plant species composition was not affected in summer but in spring. However, environmental conditions had a higher impact on the native vegetation than I. glandulifera. We conclude that within riparian habitats the threat of I. glandulifera to the native vegetation can be rated low since native species were reduced in cover but not excluded from the communities. This might be due to patchy occurrence and year-to-year changes in cover of I. glandulifera. The context-dependency in terms of micro-habitat and season requires specific risk assessments which is also an opportunity for nature conservation to develop management plans specific to the different habitats. Particular attention should be given to habitats that are bright and very wet since the effect of I. glandulifera was strongest in these habitats.
Highlights
Biological invasions are one aspect of anthropogenic global change
I. glandulifera plants were higher than the resident vegetation if they reached more than 20 % cover, while in spring I. glandulifera was always lower than the resident vegetation (Suppl. material 2)
Impatiens glandulifera had no impact on plant diversity but on plant cover We found that I. glandulifera reduced the cover of the resident vegetation but it had no impact on species composition in summer or on plant species alpha-diversity at all
Summary
Invasive species can alter ecosystems processes, change native community structure and reduce diversity (Ehrenfeld 2010, Vilà et al 2011). Ecosystems in turn affect invasion processes (Kueffer et al 2013) as site specific conditions can modify the performance of the invader and its interaction with other organisms. Since such species × ecosystem interactions (Kueffer et al 2013) make it difficult to generalize the impact of an invasive species on native ecosystems it is important to understand such context-dependencies. Knowledge of species × ecosystem interactions helps to understand species invasions and allows nature conservation authorities to develop more targeted management plans, prioritizing those habitats where an invasive species should have highest impact
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