The grazer-algae interaction in the intertidal zone on New Zealand rocky shores

Abstract

The effect of shore level on algal diversity, grazing mollusc biomass and grazing mollusc diversity was investigated at Leigh and on the Otago Peninsula, New Zealand. Algal diversity generally increased from high to low shore levels whilst grazer biomass increased from high to mid-shore levels decreasing at low shore levels on six of the eight shores examined. Grazer diversity varied independently of shore level and algal diversity, and was related to habitat diversity. Removal of grazers at Pipikaretu had little or no effect on the algal community structure except at the lower shore level. At this level the algal species diversity of the ungrazed area increased and remained higher than the control area for the duration of the experiment. Much of the difference between ungrazed and control community diversity is attributable to the persistence of seasonal species in the ungrazed area. It is suggested that at the shore levels studied, the harsh physical environment could suppress competitive interactions between algae, so that no species is capable of exploiting a reduction in grazing pressure. Longer term studies are, however, needed to test this hypothesis. At Quarantine Point, removal of grazers allowed settlement of mud on rock surfaces, reducing the abundance of low-habit and encrusting algae.