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Densities of many organisms, particularly invertebrates, are inherently patchy from place to place, with much variability at small spatial scales. Intertidal and shallow subtidal boulder fields in New South Wales, Australia, support a diverse suite of species, many of which have very patchy distributions among boulders. In this study, the purple urchin Heliocidaris erythrogramma was very patchily distributed at small spatial scales, being abundant under some boulders and absent from others. This pattern was found consistently at different times and in boulder fields tens of kilo- metres apart. Experiments were done to identify mechanism(s) that may establish and maintain the observed distribution. Small-scale variability in distribution was established early in the animals' life-cycle, because Heliocidaris recruits were more abundant on boulders with adult urchins than on boulders without adult urchins. Manipulative experiments indicated that the pattern of dispersion was also maintained by the behavioural responses of juvenile and adult urchins to either feature(s) of boulders or to cues associated with conspecifics. When Heliocidaris were transplanted to boulders originally without urchins, they left at a greater rate than did urchins on boulders that originally had urchins but from which urchins had been removed. In addition, juvenile and adult urchins colonised more boulders with urchins than boulders without urchins. Behavioural responses to unidentified but specific requirements of habitat provided by some boulders may thus be important in maintaining non-random densities of urchins among boulders.

Small-scale variability in the dispersion of the sea urchin Heliocidaris erythrogramma among boulders