LAST JULY, Ian Thornton – emeritus professor of zoology at La Trobe University in Melbourne – was dropped by helicopter to spend an hour on the volcanic island of Anak Krakatau in the Sunda Strait between Java and Sumatra. It was enough time to watch an eruption and its impact.
The Indonesian authorities were wary of letting him stay longer. Anak Krakatau has been almost continuously in eruption since November 1992. The week before, an American tourist had been killed and her five companions injured by flying rock.
But Thornton finds it hard to keep away from the island. Anak Krakatoa and three nearby islands provide a perfect natural laboratory to study how tropical rainforest is assembled and how islands are colonised by plants and animals. They also are a natural test for one of the most influential models of modern zoology, the theory of island biogeography proposed by American biologists Robert MacArthur and Edward Wilson in 1967.
Studying how plants and animals colonise islands is directly relevant to land reclamation, rehabilitation and reafforestation. An important global impact of forestry, agriculture and industrialisation has been to chop vast tracts of habitat into what amounts to islands. The theory of island biogeography makes predictions about how many species these islands of habitat can support. This information is crucial to the planning of national parks and reserves.
The island experiment, which began for Thornton about 10 years ago, really had its origins a century earlier in the events of 27 August 1883, when the island of Krakatoa blew apart in the most devastating volcanic eruption in recorded history. But it also created three small islands devoid of life – Rakata, Sertung and Panjang. Anak Krakatau (Child of Krakatoa) pushed out of the sea in 1930 and extinguished all life on itself again when it erupted in 1952 and 1953.
“What makes this area even more exciting is that we have an experiment within an experiment,” says Thornton. The three older islands have been colonised for more than a century, but life on Anak Krakatau has only arrived since 1953. So, not only is the colonisation experiment being run in four different places, but it also being run at two different times, with Anak Krakatau being secondarily colonised from the three neighbouring islands.
Thornton and a group of zoological colleagues from La Trobe, together with biologists from the Bogor Zoological Museum and the Bogor Herbarium in Indonesia, have formed the nucleus of a team which has surveyed the animal life on the Krakatau islands over the course of six expeditions beginning in 1984. They plan to go again in the next two months.
The La Trobe team has compared results from their surveys of Anak Krakatau – biologically just over 40 years old – with records from the surrounding islands which were colonised 70 years earlier. The similarities they found were striking. The pattern of early colonisation does not happen by chance, but is determined by the innate capacity of species to disperse.
According to the theory of island biogeography, islands are capable of catering for a limited number of species. The theory predicts what that number is and how rapidly it will be reached. MacArthur and Wilson argued that the birds of Krakatau had achieved their limit about 25 years after the 1883 eruption. The results of the La Trobe team do not support this. The number of bird species is still rising after 110 years, although nearing its limit.
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