Impact of Impatiens glandulifera on riparian habitats in the Czech Republic and the UK
Effect of invasion by Impatiens glandulifera (Balsaminaceae, EPPO list of invasive alien plants) on the community and species composition of invaded riparian communities was studied on six rivers in the Czech Republic. The massive invasion of I. glandulifera is considered a conservation problem in riparian habitats. Riparian zones are unique and dynamic ecosystems with complex disturbance regimes and river bank communities are generally considered to be prone to plant invasions. The dominance of I. glandulifera has been reported to cause problems in stream management. Furthermore, being the tallest annual herbaceous plant in Europe (reaching up to 2.5 m in height), it is highly competitive and is considered to replace native flora in invaded sites.
A study by Hejda and Pyšek (2006) consisted of comparing both invaded and uninvaded plots and of removal experiments. It showed that invasion by I. glandulifera had no significant impact on species diversity and composition. Other major invasive species in Central Europe such as Heracleum mantegazzianum or Reynoutria spp., invading rapidly and building large stands with high cover in riparian habitats, have a very strong effect on the species diversity of invaded habitats. Pyšek and Pyšek (1995) showed that communities invaded by Heracleum mantegazzianum had 40.5% fewer species than not-invaded communities. Thus H. mantegazzianum produced a much greater impact upon species diversity than I. glandulifera. One possible reason for this difference could be the character of I. glandulifera cover; although this cover is high in invaded communities, it is not spatially homogenous as it is with Heracleum mantegazzianum and Reynoutria spp. The patches with lower I. glandulifera cover provide other species with an opportunity to survive in the invaded community. I. glandulifera did not seem to change soil characteristics even when growing with a high cover in invaded communities. Moreover, the effect of I. glandulifera on species composition was marginal: only the proportional covers, especially those of tall native nitrophilous dominant species, have slightly changed. It appeared that I. glandulifera merely took over the role of native tall nitrophilous dominants (Urtica dioica, Chaerophyllum bulbosum, Chenopodium album) while shorter species in the undergrowth were not principally affected. It was concluded that I. glandulifera exerted negligible effect on the characteristics of invaded riparian communities, hence it does not represent a threat to the plant diversity of invaded areas.
On the other hand, Hulme and Breller (2006) assessed the impact of I. glandulifera along the River Wear in the City of Durham (NE England, UK) with removal experiments. This study is one of the first to assess both the impact of the plant at the plot scale and at the community scale. Results show that I. glandulifera has a significant effect at both the local plot scale and the scale of the riparian community. Moreover, plant community response to I. glandulifera removal was rapid, with a significant increase in seedling recruitment resulting in an average increase of 4 species per m². However, all of the species encountered in the plots are widespread in the UK and the threat to any individual species may be small. The impact of I. glandulifera invasion was most marked for light demanding species. Furthermore, in the absence of I. glandulifera, other non-native species may comprise a greater proportion of the vegetation (Aegopodium podagraria, Myrrhis odorata), several of which may also exclude native species. That the same removal treatment did not result in a significant effect in the Czech Republic while it did in the UK may be attributed to the difference in cover of the invading species. While in the British study the cover varied from 80% to 100%, it only reached on average 43% in the Czech sites.
As a conclusion of both studies, while several authors recommend the removal of I. glandulifera, such action should be undertaken with care as it may only lead to a compensatory increase in the abundance of other non-native species and thus fail to achieve desired conservation goals.
Hejda M, Pyšek P (2006) What is the impact of Impatiens glandulifera on species diversity of invaded riparian vegetation? Biological conservation 132, 143-152.
Hulme P, Bremmer E (2006) Assessing the impact of Impatiens glandulifera on riparian habitats: partitioning diversity components following species removal. Journal of Applied Ecology 43, 43-50.
Pyšek P, Pyšek A (1995) Invasion by Heracleum mantegazzianum in different habitats in the Czech Republic. Journal of Vegetation Science 6, 711-718.