Piddocks (Mollusca: Bivalvia: Pholadidae) increase topographical complexity and species diversity in the intertidal

Bioerosion increases the topographic complexity of soft rock habitats, thereby increasing species diversity. This increase in species diversity may either be associated with an increase in complexity or may simply be a consequence of the increase in available surface area for colonisation. The influence of habitat modification by piddocks on intertidal species richness was investigated through field survey using fractal geometry to assess topographical complexity. The relationship between topographical complexity and species diversity was examined using the species spacing technique, which uses fractal dimensions to normalise the species richness data in relation to topographical complexity. Six sites were chosen, comprising either clay or chalk substratum, which had a range of rock hardness. Through their rock-boring activities, piddocks significantly increased the topographical complexity of the shore. Associated with this increase was an increase in species richness at all sites. Using species spacing, at 5 of the 6 sites, the increased species richness was found to be area-independent, with more species being observed than would be expected for a simple increase in surface area alone. However, piddocks are also known to significantly increase the erosion of soft rock habitats, many of which are regarded as being of particular conservation importance because of their rarity within Europe. Piddocks thus increase intertidal biodiversity while at the same time significantly contributing to erosion of the substratum.