Assessing the impact of Impatiens glandulifera on riparian habitats:: partitioning diversity components following species removal

1. Attempts to quantify the impact of non-native plants on the vegetation of invaded habitats have often been criticized because of the frequent use of observational assessments (invaded/uninvaded comparisons) focused on local- rather than community-level effects. This study highlights how removal experiments combined with partitioning of diversity components across spatial scales can provide key insights into invasive species' impacts. 2. Impatiens glandulifera (Balsaminaceae) is a widespread invasive riparian weed, yet few quantitative assessments of its impact on natural vegetation exist. Thus replicated experiments were used to compare the impact of Impatiens removal on alpha, beta, gamma components of species richness, diversity and evenness in open riparian habitats in north-east England. 3. Plant community response to Impatiens removal was rapid, with a significant increase in seedling recruitment resulting in an average increase of four species m(-2). The impact of Impatiens invasion was most marked for light-demanding species. Eight non-native species (in addition to Impatiens) occurred in the community and these responded proportionally more to Impatiens removal than native species. As a result the proportion of non-native species in removal plots was higher than in invaded plots. 4. The increase in alpha richness and diversity following Impatiens removal was matched by a similar increase in the gamma components. Thus the effect of Impatiens was significant at both the local plot scale and at the scale of the riparian community, although the former effect was more marked. Species accumulation curves suggested that extensive Impatiens stands may reduce species richness by as much as 25%. 5. Synthesis and applications. Removal experiments appear particularly well suited to assess the impacts of herbaceous non-native plants on ruderal communities. By partitioning diversity components, impacts were consistently assessed at local and community scales and, when combined with analysis of species accumulation curves, provided a comprehensive assessment of impact on the plant community. The approach adopted in this study highlights that although Impatiens reduces native species diversity in open and frequently disturbed riparian vegetation, many of the species negatively influenced by Impatiens are widespread ruderal species. Furthermore, management may lead to a compensatory increase in the abundance of other non-native species and thus fail to achieve desired conservation goals.