Alteration of the Breeding Habitat by Sea-Birds and Seals in Western Australia

The principles discussed in this paper apply over a wide area of temperate Australia and New Zealand and are concerned with the degeneration and regeneration of plant communities associated with fluctuating populations of sea-birds and seals. Actual examples are drawn from among twenty-three islands visited in the Indian Ocean off south-west Australia. These islands are scattered along approximately 500 miles (800 km) of coastline from Cape Leeuwin in the extreme south-west of the continent northwards to the coral islands of the Abrolhos Group in latitude 28? S. Their names and relative positions are shown on the accompanying map. The two southernmost islands are composed of granitic gneiss, one with a limestone capping. The three northernmost are composed largely of coralline limestone, the remainder of consolidated dune limestone or aeolianite. The larger of the limestone islands carry deposits of unchanged dune sand and the gneiss is partially covered by shell grit so that soils on all islands are calcareous. In size they range from a few square chains to several thousand acres, but only portions of the larger islands are occupied by nesting sea-birds. Ecological accounts of the various groups are in press or in course of preparation (Gillham and Storr, 1960, 1961); also McArthur (1957). The indigenous plants vary considerably in their tolerance of manuring and trampling by colonial sea-birds, and radical changes in the floristic composition of the vegetation are effected in the rookeries. Birds which nest on the ground surface or on top of the low maritime scrub are the most inimical to the indigenous plants, but burrowing birds also have a selective influence. The principal surface nesters (nomenclature according to Serventy & Whittell (1948)) are large pied cormorant (Phalacrocorax varius), silver gull (Larus novae-hollandiae), * Working in conjunction with the C.S.I.R.O. Wildlife Survey Section.