<i>PLANT COLONIZATION AND EXTINCTION PROCESSES IN GUADALENTÍN SALT STEPPES (SE SPAIN)</i>

Abstract

Lists of vascular plant species were compiled in 1991 and 1998 within Guadalentín salt steppe fragments (Region of Murcia, SE Spain), where human disturbance during the last 25 years has been substantial, leading to considerable changes in island sizes. Our aim was to determine whether equilibrium hypothesis explains the species-area relationship found in the study site and to elucidate the main factors determining colonization and extinction rates. Colonization and extinction rates between 1991 and 1998 were calculated in islands which have suffered less disturbance and related to area, percentage of change in the area of each patch and distance to the nearest potential sources of species. In the considered period, a differential extinction process associated with human disturbance has resulted in reductions in species number within each patch. Equilibrium theory must be taken into consideration to explain the species-area relations as there is a strong inverse correlation between extinction rates and area. Colonization rates are not related to island sizes but to the percentage of change in the area of islands that explains losses and increases in species number higher than those expected taking into consideration species-area relationships. Distance has no effect on colonization rates, as the fragments are isolated in a functional sense.