Destruction of indigenous heath vegetaion in Victorian sea-bird colonies

Abstract

The sclerophyllous heath and scrub which occupies undisturbed granite coasts is essentially a vegetation of infertile, acid soils deficient in available phosphorus and nitrogen and sometimes in potassium and certain microelements. In sea-bird colonies soil reaction and fertility are altered by the deposition of guano, which is rich in phosphorus, nitrogen, and potassium, and the indigenous heath and scrub are destroyed. Mechanical disturbance and chemical changes hinder the accumulation of surface peat.

Toxicity of guano in summer leads to a specialized flora and limits the total number of species. Comparison of floristic data from sea-bird islands and nearby control areas with no nesting sea-birds illustrates the widely divergent nature of the flora in the two habitats.

Heath species are almost non-existent in the rookeries, and sclerophylls, switch plants, and woody plants much rarer than in the control areas. Conversely, succulents, annuals, biennials, and aliens are considerably more abundant in the rookeries. Man is the primary introducer of alien plants; but gulls are also important in this respect, and all colonial sea-birds make the habitat more suitable for invasion by ruderals in the autumn when mechanical disturbance ceases and guano becomes diluted to beneficial concentrations.

Where man has access to the rookeries large areas may become dominated by introduced weeds which are seldom able to establish themselves on unmodified heath soils.

The vegetation of sand dunes (to which few nutrients are being added and from which few have been leached) lies between that of the leached soils of heaths and the guano-rich soils of rookeries overlying granite in all the above characters, in spite of the markedly different physical nature of the substratum, and contains a higher proportion of creeping plants than either.

Guano and sea salt have parallel effects on vegetation, and the presence of sea-birds leads to a broadening of the coastal belt of salt-resistant plants and elimination of the indigenous, more inland type of flora. Pteridium esculentum and Leptospermum laevigatum are important species in the transition zone between either saline or guano-rich habitats and the surrounding heath.