Vegetation of the Wessel and English Company Islands, North-eastern Arnhem Land, Northern Territory, Australia

Abstract

Vegetation patterning is described for two neighbouring continental island chains off Arnhem Land, Australia. A total of 684 species was recorded from 57 islands, with reasonably comprehensive lists for 36 of these islands. Almost 90% of the deviance in plant species richness was associated with island size. The richness of plants dispersed by sea and/or vertebrates was proportionally greater on small islands; that of relatively poor dispersers was greater on larger islands. Twelve vegetation communities were defined by classification of plant species composition in 226 50 × 50-m quadrats. There was no relationship between island size and species richness at the quadrat level, at least for the most extensive vegetation types. The vegetation of the islands is now composed of two main elements—an original set of communities (mostly of heath, tussock grassland and eucalypt open forest) whose species have poor inter-island dispersal, and a set of more recent colonists (mostly of strand, mangrove and coastal thicket communities). The former group is allied to the sandstone flora of western Arnhem Land, but is species-poor in comparison, as the islands lack the deep gorges which drive much of the species richness of western Arnhem Land. The latter group comprises many species with a broad geographic range across tropical coastal areas, including many species which have been shown elsewhere to be highly vagile. Many species of the former set appear to have been lost from the smaller islands. The island flora has been influenced by three sets of human managers. Aboriginal use of the islands has been long-standing and probably resulted in a fire regime which may have accentuated environmental patchiness. Centuries of wet-season use of some islands by Macassan trepang-harvesters, ending early this century, has apparently left few effects other than the persistence of occasional small populations of an introduced food tree. In contrast, European use of these islands has been minimal and fleeting, but is associated with the introduction of most weeds.