Effects of introduced plants on herbaceous stratum composition in wetlands

Background/Question/Methods

Both incidentally and deliberately, species have been introduced to ecosystems beyond their historical range boundaries.  While many introduced species are not significant threats to new ecosystems or may fail to survive, some become established and invasive, displacing native species, and cause severe environmental damage.   This study investigates effects of introduced plants on the biodiversity of wetland herbaceous stratum composition.  Introduced plants considered to have a high invasive potential to become widespread, abundant, and serious threat in natural area are referred to as “invasive” species, introduced plants with less invasive potential as “alien” species.  492 sampling sites situated within 40 wetlands in the northern Adirondack Mountains, New York were surveyed using a 1 m² quadrant.  All non-woody vegetation as well as all woody vegetation that is less than 1 m in height found in the quadrant were identified and their relative dominances recorded.  We hypothesized both invasive and alien plants decrease biodiversity of herbaceous community in wetlands and invasive species cause greater negative impacts than alien species.  

Results/Conclusions

A total of 39 introduced species were identified.  Among them, 10 species are considered invasive by the Adirondack Park Invasive Plant Program.  Purple loosestrife was the most abundant and was found at 35 of the 492 study sites with relative dominance up to 84%.  Common buckthorn was another aggressive invader and had completely covered the herb stratum at three of the 22 sites that it invaded. The results of this study indicated that the presence of introduced species, particularly alien species, might not, in general, decrease wetland biodiversity.  At sampling sites where a vertical structure was lacking and only herbaceous stratum was present, the number of both alien and invasive plant present increased the richness of the herbaceous stratum.  At sampling sites with shrubs/trees present, the number of alien plant species present increased richness in the herbaceous stratum, but invasive species had no effect.  In both sites with and without a tree/shrub stratum neither the relative dominance of alien nor invasive plants had a statistically significant effect on the richness or the diversity of plants in the herb stratum.  A possible explanation for the increase in diversity with increasing numbers of alien and invasive species might be that these introduced species, particularly the alien species, do not completely displace native species from their niches but instead coexist with the native species.