Ecology of some New Zealand seabird colonies
Effects of colonial seabirds on the vegetation amongst which they nest fall into three main categories: (a) trampling and track formation, (b) burrowing; (c) manuring. Other biotic influences, such as the grazing mammal and man with inevitable following of alien weeds, have more general implications but also involve specific aspects. For example, the degree of mammalian grazing and trampling affect<, the 'burrowing birds, and many alien plants appear to compete more adequately than indigenous ones in the heavily manured soil of bird colonies, their local distribution being assisted by gulls.
The effects of bird trampling are largely mechanical and the lush growths induced by guano are especially susceptible to breakage. Penguins breeding considerable distances from the sea may form well-defined tracks to the nests, sometimes bare, sometimes floored by nitrophiles such as Tetragonia trigyna. On steep tracks trampling may be localised at halting places-bare areas surrounded by guano-splashed coprophiles. Mutton birds nesting in bush often drop vertically through the trees but take off by flapping their way along well-marked flight tracks to the cliff edge, breaking the marginal ferns and fouling the peaty soil.