Tide, time and space: Scales of variation and influences on structuring and sampling beach clams

Abstract

Scales of spatio-temporal variability in the densities and sizes of organisms need to be quantified so that the relative importance of different ecological processes that structure assemblages can be determined and future sampling strategies optimized. This study examined variability in the densities and size compositions of the beach clam, Donax deltoides, across tide stages and various hierarchical scales of time (day, week, month, season) and space (patch, area, beach). Densities of clams in the swash zone were most often least at high tide, which may be a consequence of their restricted shoreward movements and upper beach distributions. Regardless, the size compositions of sampled clams were the same across all tide stages. Future quantitative population sampling could be done across a six-hour window around low tide, providing ample time to access and sequentially sample many locations along a beach. The components of variation in densities of clams were in all but a couple of cases, greatest at the smallest temporal and spatial scales examined; among days and among replicate samples within a patch. Variation in size compositions was also greatest at the smallest spatial scale examined (among patches), but this was not the case for time. Size compositions were relatively stable across days and the influences of other temporal scales were inconsistent, suggesting that ecological processes influencing size compositions operate at different scales to those influencing densities. Nevertheless, the general predominance of small-scale variation was probably a consequence of fine-tuned, local-scale responses of clams to the variable and dynamic physical and biogenic features of the swash zone habitat. These results concur with studies in other habitats that show small-scale temporal and spatial variability is a dominant feature of aquatic benthic organisms and challenges the paradigm that ocean beach ecology is primarily driven by large-scale hydrodynamic forces. Small-scale patchiness of clams in time and space needs to be taken into consideration in future sampling strategies for population and impact assessments. Moreover, the ecological processes that drive small-scale temporal and spatial structuring of organisms on ocean beaches require greater investigation.