An isolated subpopulation of A. tenebrosa was studied, for three years, on the rocky intertidal coast at Kaikoura, New Zealand. The number of adults remained very stable: 94% of adults first seen in November 1973 survived until the observations were concluded. Adults died mainly from impact injuries caused by moving rocks and logs during gales. The number of juveniles in the colony fluctuated markedly from season to season, between 81 and 225 % of the number first seen. Most settlement was in the summer months, but some occurred throughout the year. The main causes of juvenile mortality were, in order of decreasing importance, failure to successfully attach leading to probable predation by benthic invertebrates, desiccation at low tides, dislodgement or crushing by grazing molluscs, impact injuries during gales, and exposure to combinations of excessive desiccation and high temperatures at low tides. Adult mortalities appeared to be independent of size. The rate of mortality of juveniles in their first 20 days after release from brooding adults is largely independent of juvenile size. After that, the mortality rate is negatively correlated with size until juveniles exceed about 15 mm column diameter: the smaller juveniles are more susceptible to both desiccation at low tide and to fatal interference from grazing molluscs.
Impacts of wetland afforestation on the distribution of benthic invertebrates in acid streams of Westland, New Zealand
