Relationship between species richness and morphodynamics in sandy beaches: what are the underlying factors?

One of the most documented generalisations in sandy beach ecology is the trend for species richness of macroinvertebrates to decrease along a morphodynamic gradient from the dissipative to the reflective condition. This widely documented pattern remains unexplained, because the responsible ecological factors and underlying biological processes are still unknown. It has been proposed that swash condition, which changes significantly along the morphodynamic gradient, is the key limiting factor underlying this biological pattern. In this study, I suggest that 2 other physical factors could also be involved: sediment grain size and erosion-accretion dynamics. This proposal is supported by a 2 yr study of 12 sandy beaches in Chile, which showed that grain size becomes coarser, erosion-accretion dynamics more intense, and swash frequency and velocity increase as morphodynamic conditions change from dissipative to reflective extremes. I propose that these 3 environmental factors, although physically interrelated, may have independent influences on different biological processes or species. I suggest therefore that the reduction of species towards the reflective extreme is caused by increasing environmental severity, which is generated by the action (of at least) the 3 physical factors mentioned above. This proposal constitutes the hypothesis of 'multicausal environmental severity', which agrees with the large interspecific variability in the life history and ecological traits of sandy beach communities, since it admits that species with different characteristics could be controlled by different limiting factors. Thus, the present hypothesis provides a more general and flexible framework for investigating the relationship between species richness and morphodynamics in sandy beaches.