Islands on Islands: Habitats on Inselbergs

Abstract

When seen from a distance, inselbergs seem to form homogeneous landscape features which mainly consist of large expanses of bare, dark colored rock (Fig. 4.1). However, a closer look reveals that they have to be considered as an ecosystem which comprises an unexpected amount of clearly distinguished vegetational habitat types representing “islands on islands” (Ornduff 1987). In the 19th century, the unusual appearance of inselbergs attracted the attention of famous naturalists, who provided the first descriptive accounts of their vegetation. Carl Friedrich Philipp von Martius (1842) and Alexander von Humboldt (1819) were amongst them. The latter discussed in detail the dark color of rocks in the Orinoco riverbed and of inselbergs in the same area. Humboldt assumed that the black color of the rocks was due to a sheath of manganese oxide and carbon. Today, however, we know that the occurrence of epi- and endolithic cyanobacteria is responsible for the dark coloration of these rocks. Among the first floristic studies concentrating on tropical inselbergs, one has to mention the work of Willis (1906) concerning vascular plants on Ceylonese rock outcrops. For a remarkably long period, the vegetation of inselbergs was simply denoted as lithophytic without any further differentiation, or it was described as “savanes-roche” due to a certain amount of physiognomic similarity with savanna vegetation (de Granville 1978).