Because abundances typically vary at a range of spatial scales, the size of the sam-
pling unit selected to sample populations is very important in identifying patterns
of abundance. For example, consider a population of polychaetes or other small
benthic animals that typically aggregate in clusters, about 10 cm in diameter, with
the clusters spaced about 20–30 cm apart (Fig. 1.5a). Therefore, some patches of
sediment have very large numbers of animals, whereas other patches have very
few or no animals at all. Sampling with very large cores or quadrats, for example
50 cm×50 cm, will give the impression that the animals are very regularly spaced
throughout the site. Each quadrat will probably sample one cluster, with perhaps a
few individuals from adjacent clusters, giving very similar measures of abundance
in each replicate (Fig. 1.5b). This is due to the fact that this size of sampling unit is too large to measure the spatial pattern because the ecological processes causing
these patterns are operating at scales of 30 cm or smaller.