Physical and biotic factors limiting the distribution and abundance of littoral molluscs on Macquarje Island (sub-Antarctic)

A number of factors have been examined for their influence on the limits of upper distribution and abundance of six species of adult molluscs in the rocky littoral zone of Macquarie Island. The molluscs were Amphineura: Plaxiphora aurata (Spalowsky), Hemiarthrum setulosum Dall; Gastropoda: Patinigera macquariensis (Finlay), Cantharidus (Plumbelenchus) coruscans (Hedley), Laevilittorina caliginosa (Gould), Kerguelenella lateralis (Smith), and the factors included habitat, temperature, desiccation, salinity, food, predation, and reproduction. Macquarie Island is mid-way between the southern temperate zone and the Antarctic and has an equable, oceanic climate. Because of differences in both geography and environmental stability, the results allow important comparisons to be made with those on the effects of factors limiting the distribution of littoral molluscs elsewhere. The effects of physical factors are similar to those examined in other regions as shown by tolerance to temperature, desiccation, and salinity. Habitat and zonal sequence of species are related to tolerances with the notable exception of the heat resistance of chitons which, when compared with that of gastropods, is greater than their distribution would suggest. In the field, the temperatures and desiccation of the animal bodies and the surrounding salinities do not reach critical lethal levels but the debilitating effects in combination with other adverse factors can be lethal — in particular, a combination of high temperatures and predation. Biotic factors have considerable influence on the distribution and abundance of the species and this may reflect the comparative stability of the physical environment. The aspects of the biotic factors which are important in this regard include morphology, mobility, food preferences, the presence of the large kelp Durvillea antarctica (Chamisso) Hariot, predation by birds and by isopods on enfeebled animals, and a brooding mode of reproduction. In all species, no single factor limits the upper distribution and factors vary in their importance for each species. Synergistic effects of factors are limiting and the effective combinations differ for each species. This stresses the danger of placing too much emphasis on results from studies of a single factor.