Spread of non-indigenous macroalgae and disturbance: Impact assessment of the Costa Concordia shipwreck (Giglio Island, Italy) using the ALEX index

Abstract

The establishment and spread of Non-Indigenous Species (NIS) can be facilitated through perturbations that lower the resilience of native communities to invasion. After a disturbance to a rocky subtidal macroalgal assemblage in the Mediterranean Sea (Italy) by a shipwreck (Costa Concordia), we compared native and non-native seaweed community structure at the disturbed site and two reference sites using traditional abundance measures, in addition to employment of the ALien Biotic IndEX (ALEX). ALEX has been recently proposed as a potential multi-metric indicator to evaluate biological invasions in Mediterranean macroalgal assemblages, taking into account both the relative abundance of macroalgal NIS to native seaweed abundances and the invasive traits of those NIS. We found that macroalgal NIS were initially more abundant at the disturbed site, but four months later were decreased and there were no difference with the reference sites. Native macroalgal cover at the disturbed site was lower than at reference sites in the initial survey, but unexpectedly decreased significantly four months later. Despite the decline in the cover of macroalgal NIS, ALEX was consistently lower through time at the disturbed site, due to concurrent declines in native seaweed abundances. This suggests that evaluation of macroalgal NIS should not be based on cover of macroalgal NIS alone and that the relative cover of native species should also be considered. ALEX may represent a versatile tool to be employed in both monitoring programs and impact evaluation studies.