Patterns of variation in the climates, soils and vegetation of some subantarctic and antarctic islands

Patterns of variation in the climate, soils and vegetation among 19 sites from 3 subantarctic and one maritime antarctic island were defined by principal component analyses (PCA) of abiotic and vegetation variables. The sites were classified on the same parameters by nearest neighbour/group mean clustering. The first two components from the PCA of the abiotic (climate and soil) data separated the islands along a climatic gradient, partly related to latitude. Sites within each island were separated along parallel series by edaphic factors and exposure. The third and fourth components reflected changes in soil chemistry caused by manuring by seabirds and seals. Cluster analysis on the abiotic variables showed that the main division was between high- and low-latitude islands, with a strong tendency to ‘chaining’. PCA of the vegetation data resulted in few conspicuous or simple patterns but three environmental trends were indicated, which corresponded approximately to components II, III and IV from the PCA of the abiotic data. Animal manuring induces vegetation successions parallel to the soil changes noted in the abiotic analyses and these combine with radiative successions from fellfields to all other vegetation types to produce a complex, and somewhat ambiguous, pattern of sites. Cluster analyses on the botanical variables produced clear groupings but these varied markedly according to the life-form category used to classify two of the plant species. This instability reflects the wide ecological amplitude of the phanerogamic species, with single species often filling what would be a variety of niches in less isolated ecosystems. The results also emphasize the importance of bryophytes on the southern subpolar islands. The two sets of analyses, taken together, emphasize the special characteristics of subantarctic islands found in previous bipolar comparisons and also indicate the extreme sensitivity of these ecosystems to the introduction of alien species