Spatial and temporal patterns of abundance and the effects of disturbance on under-boulder chitons

Abstract

The spatial and temporal patterns of abundance of 8 species of chiton found under boulders were documented at sites in the mid- and low-shore regions of an extensive boulder-field in the Long Reef Aquatic Reserve. The mean number of species and mean densities of Ischnochiton australis (Sowerby , 1840), Ischnochiton elongates (Blainville, 1825), Ischnochiton versicolor (Sowerby, 1840), Cryptoplax mystica (Iredale & Hull , 1925), Callistochiton antiquus (Reeve, 1847), Ischnochiton smaragdinus (Angus, 1894) and Acanthochiton spp. did not vary through time during the 6 month study, conducted over the autumn and winter months. However, their abundances varied at small (areas 3 m apart) and large (sites 60 m apart) spatial scales in the mid- and low-shore regions. Abundances ranged between 0 and 28 individuals 1000 cm-2 of boulder surface over the study period. Relationships between the abundances, sizes and numbers of species and the size of the boulder and/or the underlying sediments were rare, contrasting with suggestions made in early taxonomic studies. However, the sizes of I. australis in the low-shore and I. elongatus in the mid-shore were positively correlated with boulder size and the density of I. elongatus was positively correlated with the proportion of fine grained sediment. A manipulative field experiment was done to examine possible sampling-induced changes in abundances. The mean number of species, total number of individuals and mean densities of Ischnochiton australis, Ischnochiton elongatus, Ischnochiton versicolor all declined following the overturning of boulders. The experiment also showed that despite the alterations to the habitat caused by our sampling, the abundances of the under-boulder chitons were not significantly altered . Therefore our data documenting patterns of abundance in space and time were not confounded by sampling-induced disturbance.