The role of biological disturbance in determining the organisation of sub-tidal encrusting communities in temperate waters

Abstract

Biological disturbance was found to be one of the most important mechanisms control1ing community organisation in the temperate sub-tidal region. The different types of biological disturbance structuring three encrusting communities were investigated on the east coast of Northland, New Zealand. The operation of each type was determined and the rate of disturbance measured. Experimental exclusion treatments were set up to demonstrate the effects of the major disturbance agents on community structure. The urchin Evechinus chloroticus was the most abundant agent of biological disturbance and affected the widest spectrum of encrusting organisms. The abundant balistid fish Navodon scaber was another major agent of biological disturbance in this region. Disturbance of algal populations also resulted from a guild of abundant herbivorous gastropods. Two episodes of fungal/bacterial infection degraded numbers of the large sponges Ancorina alata and Polymastia fusca. The operation of the different disturbance agents was found to be generally unpredictable in both time and space. There was no escape from biological disturbance for encrusting organisms in either small or large size. Re-occupation processes on patches of free primary space were investigated both experimentally and by using settlement plates (artificial free space patches). Recruitment was found to be irregular in space and time, especially for long-lived sessile organisms. Settlement processes as well as growth and mortality of newly settled organisms were investigated with a view to understanding community development. Only one verifiable example of substrate preparation or biological succession was found to operate in the communities studied. It is postulated that community organisation is flexible and not rigidly directed along a single successional pathway. Multiple developmental pathways and multiple stable configurations are possible in the same locality, resulting from the operation of different disturbance regimes.