Impacts of invasive alien plants in Mediterranean habitats
Data on impacts are crucial to justify the invasiveness of an alien species, in particular in the framework of risk analysis. To measure how impacts vary according to the characteristics of invaders and recipient communities, the vegetation of invaded and non-invaded plots was sampled for 8 different invasive species in a variety of habitats within the French continental Mediterranean region. The results showed that although, on average, the presence of invasive alien plants is associated with a significant change in resident community diversity and composition, the magnitude of some impacts vary, both according to invader identity, and also according to invasive population characteristics and features of the recipient community.
Most of the observed impact variation was species-specific, with the greatest effects on community-level metrics (indicators) found for Carpobrotus spp. (Aizoaceae) in sand dune communities and Reynoutria x bohemica (Polygonaceae, EPPO List of Invasive Alien Plants) in river bank communities. Ambrosia artemisiifolia (Asteraceae, EPPO List of IAP) which is mainly known for its impacts on human health and crop yield, does not exhibit a significant impact on biodiversity in invaded communities of river banks. Even though Artemisia verlotiorum (Asteraceae) has been the object of less attention by land managers, the study shows that this plant has a comparable or even stronger community-level impact than R. x bohemica in pioneer riparian forest. The study also confirmed the major impacts of Baccharis halimifolia (Asteraceae, EPPO A2 List), especially on therophytes and on endemic saltmarsh species. It was also observed that invasive alien species with an annual life cycle (e.g. A. artemissifolia and Impatiens balfourii, Balsaminaceae) have a lower effect, probably because even dense populations of these species reach lower cover and do not form homogeneous stands and therefore lead to less competition for resources. In contrast, rhizomatous perennials (e.g. A. verlotiorum and R. x bohemica) and to a lesser extent creaping perennials, caused the largest effects.
The results also showed that for a given invasive alien plant, all habitats did not undergo the same level of impacts. A rough analysis showed that habitats with sparse vegetation, such as sand dunes invaded by Carpobrotus spp. and Amorpha fruticosa or river banks invaded by R. x bohemica are more susceptible to impacts than more densely vegetated habitats.
Sources
Fried G, Laitung B, Pierre C, Chagué N ; Panetta FD (2013) Impact of invasive plants in Mediterranean habitats: disentangling the effects of characteristics of invaders and recipient communities. Biological Invasions. DOI 10.1007/s10530-013-0597-6