Reinforcement of the Food Webs
Gradually, as the early vegetation develops, it provides the base of many food chains, as insects feed on the plants, and other insects, and vertebrates colonize and feed on the plants or on the plant-feeding insects. From time to time, passing fruit-eating birds and bats visit the island in search of food, depositing in their droppings the seeds eaten in their last port of call, often being one of the other Krakatau islands. These animal-dispersed seeds, if lucky enough to fall in the right place, germinate and establish, and when mature, begin to fruit -- fig trees being typical examples of the type of plants involved. At this point, the island provides a genuine attraction to birds and bats, whose visits increase in frequency, re-inforcing a cycle of seed dispersal, plant establishment, food provision, and increased visits by the dispersing animal in search of a meal. By the end of the 1980s, some 58 sea-dispersed plants were to be found on the island; 25 wind-dispersed flowering plants; 13 ferns (all being dispersed by microscopic, wind-dispersed spores); 26 animal-dispersed plants; and finally some 16 species brought in accidentally by visiting people (tourists, fishermen, and most likely scientists).
Thus, in total, more than 130 species of plants had colonized by 1990. And if you landed on the island and headed for the summit (not wise during the eruptions - as one visiting party found out with tragic consequences), you would walk first through a narrow fringe of coastal creepers and trees into the early stages of a mixed lowland forest, and out again through pioneering trees and grasses onto an increasingly bare cinder cone as you left the coastal strip behind. As you did so, you might catch a glimpse of some of the birds that had made their home on the island. By April 1991, as many as 43 species of birds had been recorded on Anak Krakatau since 1951, with about 33 species thought to be "resident."
The list of species was weighted in the early years towards shorebirds and insectivores -- the latter particularly abundant in the 1980s -- but towards the end of the 1980s, the system diversified still further, following on from the vegetation developments. In 1985, the first rain forest frugivores (fruit eating birds) were found on the island, co-incident with the first fruiting of fig trees. As the support-base increased and numbers of birds built up, predatory birds began to establish themselves. Indeed, it seems that at one point, the establishment of fruit-pigeons was set back by their predation by falcons that took up residence on the island. However, once over a critical threshold of fruit supply, and thus numbers of birds, islands can support predators without their knocking out elements of their own food supply, and then another phase of the building of food webs is complete. However, as the volcano has remained active it has also periodically wreaked destruction on these youthful ecosystems, and further eruptions in the early 1990s reduced the area of vegetation cover, eliminating in the process, large numbers of individual plants, and some species from the island.
These period setbacks to colonization and succession have been of great interest to the scientists working on the ecology of the islands. They have allowed them to monitor anew the different phases of the "natural experiment" as they are played out each time with subtle variations in starting conditions or circumstances. For instance, from this work, it has become apparent that some elements of the succession are more predictable. The coastal and early pioneering communities of plants are each highly efficient in dispersal and fairly specialized in the habitats that they can succeed in. Moreover, there are not vast numbers of species. The pattern of their colonization and of the developing vegetation communities in these early stages appears to repeat itself and is thus fairly predictable. Once the system gets to the stage of the later colonists of the interior, the bird- and bat- dispersed trees and shrubs in particular, the precise order of arrival seems much less certain. It depends on just where the frugivores had their last meal before setting off to Anak Krakatau, and which species of plants happened to be in fruit at the time. From a much larger pool of potential colonists, we still cannot predict which species will happen to arrive and in what particular order, and probably never will be able.
Dr. Robert J. Whittaker teaches Geography at St. Edmund Hall in the University of Oxford, and is a lecturer in the university's School of Geography. He has been working on the ecology of Krakatau for 20 years, and is an author of over 30 articles on the islands. He has written more widely on island ecology and is the author of Island Biogeography, Ecology, Evolution and Conservation, published by Oxford University Press in 1998. He edits a scientific journal of international distribution, "Global Ecology and Biogeography."
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