The Effects of Rabbit Grazing on the Grasslands of Macquarie Island

Abstract

It has been suggested by Elton (1958, pp. 77-81, 145) that populations of plants or animals containing a small number of species are less stable and more susceptible to external pressures than populations richer in species. Macquarie Island, isolated mid-way between New Zealand and Antarctica, may well be examined in this light. The small vascular flora of thirty-eight species (including three aliens) has only. recently been subjected to external pressures, notably heavy grazing by rabbits. Macquarie Island (Lat. 540 30' S., Long. 1580 57' E.) is about 21 miles long and 2 miles wide; its total area is 46 square miles. A narrow coastal strip rises through steep to precipitous slopes to the gentler sloping plateau at about 600-1000 ft. The highest point, Mt. Hamilton, is 1423 ft. Glacial topography is well developed, but there is now no permanent ice or snow. The climate is bleak and oceanic, with little sunshine and strong winds. Since the establishment of a permanent station on Macquarie Island in 1948 by the Australian National Antarctic Research Expeditions, the flora and fauna have received considerable attention. The report by Taylor (1955), based on field work in 1950-51, provides an account of the flora, vegetation and soils, and describes the effects of rabbit grazing. Introduced by sealers about 1880, rabbits have now spread along most of the coastal strip and adjacent slopes to within a few miles of North Head at the northern end of the island. Of the grassland, herbfield, fjaeldmark, bog and fen communities which comprise the vegetation, the first two have been most extensively grazed. The herbfields, characterized by Pleurophyllum hookeri, have been protected from serious erosion by their situation on flatter ground. On the grasslands, however, which occupy the steeper slopes up to 1000 ft, rabbit grazing has been followed by soil mass movement on an extensive scale. This paper is concerned with the grassland areas. The characteristic grassland dominants are Poa foliosa, a tall, coarse tussock grass 2-3 ft high, and to a less extent the forb Stilbocacrta Polaris. More locally, Polystichum vestitum and Poa hamiltoni are present as co-dominants. Owing to the density of the dominants, subordinate species are only sparsely developed. The floristic composition of the grassland communities in relation to the total vascular flora is shown in Table 1.