Habitat associations of a rare South Australian sea star (Parvulastra parvivipara) and a co-occurring chiton (Ischnochiton variegatus): implications for conservation

Abstract

The sea star Parvulastra parvivipara is a small-range endemic known from only seven granitic headlands in South Australia. Evidence from previous studies suggests that its unstable boulder habitat will experience increased disturbance as sea levels rise. This study investigated distributional and ontogenetic patterns of P. parvivipara across different-sized boulders that vary in physical stability and susceptibility to disturbance. To develop a better understanding of community dynamics in P. parvivipara habitat, patterns were also assessed for a chiton with which the sea star commonly co-occurs (Ischnochiton variegatus), and assemblages of encrusting species were also measured, as these can vary amongst boulder sizes and influence mobile animals. Large boulders had many tubeworms, contributing to a significant boulder-size effect on encrusting species assemblage structure. P. parvivipara was most abundant on the smallest boulders; there was no evidence that abundances were influenced by encrusters. I. variegatus abundances were unaffected by boulder size, but a positive correlation between lengths of individuals and encruster cover occurred on the smallest boulders. Overall, physical habitat features, not encrusting biota, likely caused an association of P. parvivipara with small boulders, while I. variegatus displayed apparent ontogenetic use of encruster-related habitat features. As P. parvivipara uses mostly small boulders that are particularly susceptible to disturbances that are increasing with rising sea levels, future ecological restoration efforts to protect this rare species may be required. Such efforts can be properly informed about necessary habitat requirements from these results.