The response of plant community diversity to alien invasion: evidence from a sand dune time series

Abstract

This study examines the process of invasion of coastal dunes in north-eastern Italy along a 60-year time series considering alien attributes (origin, residence time, invasive status, and growth form strategy) and habitat properties (species richness, diversity and evenness, proportion of aliens, and proportion of focal species). Vegetation changes through time were investigated in four sandy coastal habitats, using a fine-scale diachronic approach that compared vegetation data collected by use of the same procedure, in four time periods, from the 1950s to 2011. Our analysis revealed an overall significant decline of species richness over the last six decades. Further, both the average number of species per plot and the mean focal species proportion were proved to be negatively affected by the increasing proportion of alien species at plot level. The severity of the impact, however, was found to be determined by a combination of species attributes, habitat properties, and human disturbance suggesting that alien species should be referred to as ''passengers'' and not as ''drivers'' of ecosystem change. Passenger alien species are those which take advantage of disturbances or other changes to which they are adapted but that lead to a decline in native biodiversity. Their spread is facilitated by widespread anthropogenic environmental alterations, which create new, suitable habitats, and ensure human-assisted dispersal, reducing the distinctiveness of plant communities and inducing a process of biotic homogenization.

Distribution of alien species along sand dune plant communities zonation

“…The level of invasion was set as the proportion of alien species in the community compared to its cumulative species pool (24,49).…”

“…This fact, and constant disturbances (natural and anthropogenic), make sand dunes a good object for the study of alien invasion and a rich body of literature has recently become available (22)(23)(24)(25)(26)(27)(28). Sand dunes are one of the most invaded habitats (29) but different habitats along a steep gradient are invaded differently (30).…”

“…The number of alien species reported in most studies of sand dunes is modest, although it can vary from 8 to 26 (even 50) (23)(24)(25)(26)56). It depends also on the methodology used; some studies mention only species found in vegetation plots (23,57), while others also report the results of floristic surveys (25,28).…”

“…We already know that new species are constantly invading sand dune communities and that changes in veg-etation composition occur over time (24). Neophytes first arrived in the Cakile maritima community but a few decades later, mobile and fixed dunes are most invaded (24).…”

“…Neophytes first arrived in the Cakile maritima community but a few decades later, mobile and fixed dunes are most invaded (24). As the number of archeophytes may be a good predictor of neophyte invasion risk (29), it is foreseen that sand dune communities rich in archaeophytes will suffer further invasion.…”