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

Abstract

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.

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