Invasion of the Portuguese dune ecosystems by the exotic species Acacia longifolia (Andrews) Willd.: effects at the community level.

Pristine ecosystems are becoming rare along the Portuguese coastline because the sand dunes have been invaded by several exotic plant species, particularly Acacia species from Australia. At the beginning of the 20 century, a few Acacia species, e.g. Acacia longifolia (Andrews)Willd., were deliberately introduced into Portugal for dune stabilization. The invasion process is strongly correlated with the occurrence of fires, so there are long periods of stability interspersed with rapid increases in abundance of the exotic Acacia species. In this study, three sites, S.Jacinto, Palheirao and Quiaios, were monitored along the Portuguese coast to determine the impact of the invasions by A.longifolia. At each site, 12 permanent plots were established, six with and six without A. longifolia. The plots were monitored from winter 1998 to autumn 1999, and species numbers and plant cover were recorded. Shannon diversity indices and dominance-diversity curves confirmed that A. longifolia is changing the community composition and structure of the dune ecosystems promoting monospecific communities, decreasing species richness and increasing plant total cover.