Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands

The aim of this study is to investigate vegetation changes on small Mediterranean islands under the impact of the drastic expansion of the gull colony, at local scale over eleven years using a set of permanent plots. First, we focused on functional aspects of vegetation in addition to its specific composition with regard to the plant functional types (Raunkiaer growth forms and Grime life strategies) as indicators of vegetation changes. Second, we used STATICO analysis to investigate patterns of change in the relationship between environmental variables and floristic composition. Third, we quantified the changes in the abundance of plant functional types by applying a simple comparison test between the two observation dates. Fourth, we investigated the relationship between vegetation changes (species turnover, plant functional type dynamics, and species richness) and gull density by performing simple linear regression. Our results show that gull density did not evolve equally for all plots. For areas where gull density increased, we recorded ruderalization of the vegetation. Surprisingly, in areas where there was a decrease in gull density, no clear pattern of vegetation change was apparent. We observed a statistically significant increase in the number of plant species due only to the increase in ruderal and stress ruderal and geophyte species. Gull colonies were responsible for high species turnover between 1997 and 2008. The higher the density of gulls, the lower the species number in 1997 and 2008. For high gull nest densities, we observed a high proportion of ruderal plant species and a low proportion of stress tolerant species. Gulls induced an increase of stress-ruderal species. We show that nest density recorded in 1997 is mainly responsible for the changes in vegetation composition, species turnover and proportions of plant functional types. We noted that a decrease in gull nest density does not necessarily induce a return to previous vegetation composition patterns. This may be seen as evidence of the inertial nature of the changes in island vegetation in the face of strong changes in environmental conditions such as the recent drastic expansion of gull colonies. Garbage management policy can have a strong and long-term impact on remote ecosystems. (C) 2011 Elsevier Ltd. All rights reserved