Changes in the structure of the fine sand benthic community of the Bay of Seine: a comparison between species and functional diversity.
Résumé
While coastal ecosystems experience increasing pressures due to anthropogenic activities and
climate change, measurement of functional diversity based on the biological traits analysis of
species (BTA) is increasingly used as tool to assess ecosystem functioning and its responses to
disturbance. Based on data from a long-term monitoring of the benthic macrofauna community of
the eastern Bay of Seine (~ 50 stations from 1991 to 2016), an area exposed to numerous
anthropogenic disturbances, our current study aims to investigate the link between species and
functional diversity at local (α-diversity) and regional (ß-diversity) scales, as well as to assess
how changes in the community structure and species composition may have altered the
ecosystem functioning. A ‘traits-by-species’ matrix including seven traits (i.e. body size, life span,
feeding type, development mode, mobility type, sediment reworking type and tolerance to
disturbance) chosen according to their relevancy for three major ecosystem functions (i.e.
secondary production, nutrient cycling, resilience) was built. To cover the different facets of α-
diversity (i.e. richness, eveness and heterogeneity), a selection of species, taxonomic and
functional diversity indices were calculated at each station. For each campaign, the links between
these different metrics was determined from a Principal Component Analysis. The ß-diversity on
a regional scale was analyzed by applying clustering and non-parametric Multidimensional
scaling on both species and traits composition matrices (based on abundances of species and
traits). The results showed that population fluctuations of a few very abundant species led to the
major variations observed in the structure of the macrobenthic community in both taxonomic
and functional aspects. A certain redundancy was found among species and functional diversity
indices in terms of richness, evenness and heterogeneity. Likewise, at regional scale, similar
patterns were reported on the spatial structure of the community along an inshore-offshore
gradient described from species or traits composition. These results question the effort/profit of
the FD for providing new information to the understanding of community structure and
dynamics. To improve the measurements of functional diversity and facilitate the interpretation
of our results, some suggestions are proposed (e.g. measure functional diversity on biomass
rather than on density data; distinguish ‘effect traits’ and ‘response traits’). In parallel, a review of
more than 75 publications using BTA on benthic communities from early 2000s to present was
carried out and highlights large variations in the methods used in BTA in terms of the number of
traits, the selected traits, or the variable used to describe the relative abundance of traits (density
vs. biomass).