The Sequence of Change
Amongst the first colonists, as on the older Krakatau islands, wind-dispersed plants were particularly important. These included some ferns, grasses and herbs, and even the odd tree species. Contrary to the received ideas about succession, ecosystems very often do not build up from lower to higher life forms. They do not usually go from lichens and mosses, to herbs, to shrubs, to end with trees. And as we shall shortly see, this idealized sequence is inapplicable to Krakatau.
The successions on the Krakatau islands have been fairly speedy affairs, unlike some volcanic successions (e.g. Surtsey or Mt. St. Helens). There are two reasons for this. First, the islands are in the humid tropics, where growth rates and soil-forming processes are generally rapid. Second, unlike many parts of Hawaii, for instance, the substrate is predominantly fine ash, easy to root in, with large surface area for weathering to release vital minerals, and altogether a far better growing medium than the solid flows of lava. Lava is colonized only slowly, and the contrast between rapid colonisation of ashes and slow development on lava has been evident with respect to the post-1960 lava flows on Anak Krakatau itself. On Anak Krakatau, most vegetation has colonised the ash fields and beaches. Probably the two most prominent pioneers of the interior have been the wild relative of the sugar cane Saccharum spontaneum -- a tall, clump-forming grass -- and the Australian pine (not in fact a member of the pine family at all) Casuarina equisetifolia -- a fast growing, and quite tall tree. Both are wind dispersed. In the case of Casuarina, it probably colonised the island by sea, as its fruits float well and can survive immersion. But once established, the light, small seeds are easily blown around the interior. It is a nitrogen-fixing species, and this may account in large part for its success as a colonizer, as nitrogen is thought to be the key limiting nutrient early in a succession of this kind. The pioneers of the interior thus included grasses, some creepers, ferns, and prominently, a tree species!
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