Measuring non-biological diversity using commonly used metrics: Strengths, weaknesses and caveats for their application in beach litter management

Due to increasing worldwide anthropogenic pressure and in order to promote adequate environmental conservation strategies, quantification of non-biological diversity, such as considering marine and beach litter, is becoming increasingly useful. Information on beach litter in terms of richness and diversity may have a consistent influence regarding the evaluation of its pressure and impact on coastal ecosystems. Highlighted are strengths, weaknesses and caveats concerning the use of uni- and bi-variate diversity metrics applied to a class of man-made non-biological objects periodically accumulated on the beaches. Two case studies show evidence that the application of diversity metrics on non-biological objects may have different implications. In absence of a universal and standardized non-biological taxonomy, it is important to be cautious when comparing values obtained from non-living assemblages, in particular if different sites, time or operators are considered. Moreover, different indices provide different information. Therefore, users should pay particular attention on the application of diversity metrics, addressing specific research questions and avoiding automatic calculation of redundant and “magic” indices.