Abstract
Species richness in sandy beaches is strongly affected by concurrent variations in morphodynamics and salinity. However, as in other ecosystems, different groups of species may exhibit contrasting patterns in response to these environmental variables, which would be obscured if only aggregate richness is considered. Deconstructing biodiversity, i.e. considering richness patterns separately for different groups of species according to their taxonomic affiliation, dispersal mode or mobility, could provide a more complete understanding about factors that drive species richness patterns. This study analyzed macroscale variations in species richness at 16 Uruguayan sandy beaches with different morphodynamics, distributed along the estuarine gradient generated by the Rio de la Plata over a 2 year period. Species richness estimates were deconstructed to discriminate among taxonomic groups, supralittoral and intertidal forms, and groups with different feeding habits and development modes. Species richness was lowest at intermediate salinities, increasing towards oceanic and inner estuarine conditions, mainly following the patterns shown for intertidal forms. Moreover, there was a differential tolerance to salinity changes according to the habitat occupied and development mode, which determines the degree of sensitivity of faunal groups to osmotic stress. Generalized (additive and linear) mixed models showed a clear increase of species richness towards dissipative beaches. All taxonomic categories exhibited the same trend, even though responses to grain size and beach slope were less marked for crustaceans and insects than for molluscs or polychaetes. However, supralittoral crustaceans exhibited the opposite trend. Feeding groups decreased from dissipative to reflective systems, deposit feeders being virtually absent in the latter. This deconstructive approach highlights the relevance of life history strategies in structuring communities, highlighting the relative importance that salinity and morphodynamic gradients have on macroscale diversity patterns in sandy beaches.
Highlights
Sandy shores are dynamic environments defined by the interaction between wave energy, tides and wind regimes, which produce a continuum from microtidal reflective beaches to macrotidal dissipative beaches [1]
A main paradigm in sandy beach ecology argues that macrofaunal richness increases from reflective to dissipative beaches [2,3,4,5,6,7,8,9]
Integrated patterns of species richness Macroscale distribution of the macrofauna in Uruguayan sandy beaches is determined by estuarine and morphodynamic gradients
Summary
Sandy shores are dynamic environments defined by the interaction between wave energy, tides and wind regimes, which produce a continuum from microtidal reflective beaches (narrow, steep and with coarse sand) to macrotidal dissipative beaches (wide, flat and with fine sand) [1]. Two recent meta-analyses on data coming from four continents confirmed that species richness increases from microtidal reflective beaches to macrotidal dissipative beaches and from temperate to tropical regions [9,10] These studies considered the macrofauna in an aggregated way and did not differentiate between taxa or other groups of species [11]. Most studies carried out at the community level rarely consider the characteristics of the species’ life cycle, their taxonomic affiliation, or the beach zone occupied by the different macrofaunal components This is of importance, taking into account that aggregated variables, such as species richness, reduce to a single value the particularities of a set of species that result from the interaction of biotic and abiotic processes. It is essential to consider the characteristics of each group in order to understand the observed patterns and to disentangle the drivers of diversity according to beach morphodynamics
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