The Role of Biological Disturbance in Temperate Subtidal Encrusting Communities

Biological disturbance was investigated in three subtidal encrusting communities in the warm temperature region of northeastern New Zealand. The mode of operation of the major disturbance agents was established and their effect on encrusting organisms qualified where possible. The three communities were all dominated by crustose coralline algae, sponges, and ascidians. The urchin Evechinus chloroticus was the most widely influential of the grazers in these encrusting communities. At densities >6—7 urchins/m2 this species grazed over the entire surface area in any locality, modifying community structure and reducing diversity to form crustose coralline flats (often referred to as barren grounds). At densities below this level urchins usually confined their grazing to patches within the encrusting community. The mean rate of free space release by an individual urchin was different in the three communities; the rate was four times higher where the urchins occurred at high densities on a coralline flat community than in the other two areas. Larger urchins appeared to clear space at a rate 1.5 times higher than that of smaller urchins. A 2—yr exclusion experiment in a community heavily grazed by Evechinus resulted in an increase in cover of ephemeral algae and coralline turf algae, but no basic change in species composition. Large numbers of five species of herbivorous gastropods, Cellana stellifera, Cantharidus purpureus, Trochus viridis, Micrelenchus sanguineus, and Cookia sulcata were positively associated with high densities of Evechinus. Removal of high densities of Cellana resulted in an increase in the cover of ephemeral algae and coralline turf algae in spite of continued Evechinus grazing. At high densities the grazing fish Parika scaber occasionally fed in loose schools that removed large sponges and ascidiams and released free space at a rate of °. 02% of the surface per fish per year. This species also grazed selectively on small encrusting organisms in which case free space was not released. Exclusion of high densities of Parika for 2 yr resulted in a major shift in community structure from sponge/coralline domination to ascidian/sponge domination. Episodic outbreaks of a bacterial/fungal disease occasionally affected populations of long—lived sponges. In the summer of 1975, 18% of the population of Ancorina alata was destroyed and in 1976, 45% of Polymastia fusca degraded completely. These two episodes were the only extensive outbreaks of this disease in 10 yr of observation. The abundant grazing species investigated are generalist feeders that are capable of feeding on the entire size range of most encrusting species. Hence there is not escape from biological disturbance either in small or large size for encrusting organisms in these areas. The populations of the common grazers frequently fluctuate in numbers in a local area and their effect on the encrusting communities is not constant through time. The selective consequences of these features for long—lived encrusting organisms are discussed. It is suggested that the spreading "colonial" habit of many encrusting organisms is a response to these features of subtidal biological disturbance. Many spreading, encrusting sponges respond to grazing damage by rapid growth, up to 20 times the undisturbed growth rate, and this is thought to be a further mechanism to cope with Evechinus and Parika grazing. Finally, the influence of this disturbance on general community features such as diversity, dominance, and stability is discussed. Diversity was highest at intermediate levels of disturbance. A possible competitive dominant was identified but the disturbance regime did not allow monopolization of space by this species (the sponge Stylopus sp.), and it is suggested that this system is normally in a state of nonequilibrium.