Geoscience Reference
In-Depth Information
Krakatoa revisited: successional re-invasion
after volcanic disturbance in 1883
KEY CONCEPTS
Recolonization by vegetation of the island group of Krakatoa, Indonesia, which was completely sterilized by
catastrophic volcanic eruptions in 1883, has been studied by many ecologists since the early twentieth century. The
English biogeographer Robert J. Whittaker (not to be confused with the American ecologist Robert H. Whittaker)
studied processes of recolonization and succession in relation to the dispersal mechanisms of potential invading
plants, and their biological interactions, and has been able to extend MacArthur and Wilson's (1967) theory of island
biogeography.
Figure 20.9 summarizes a model of recolonization resulting from a synthesis of past and recent research (Whittaker
1998). A key distinction is between the dispersal of propagules of the invading flora by sea (thalassochorous), animals
(zoochorous) and wind (anemochorous). The model also distinguishes between strand-line habitats in the outer circle
and inland habitats. Colonization in phase 1, 1883-97, is achieved mainly by plants that disperse their seeds by sea
water. However, the rate of invasion by thalassochorous incomers levels off by 1920, since when it has remained
relatively uniform. The input of anemochorous plants continues to increase, but the rate of increase slows down
after the 1920s. In contrast the contribution of animal-dispersed plants has continued to grow at a rapid rate. The
contrasts in the richness of animal species between Krakatoa and similar neighbouring islands which were not affected
by the volcanic eruptions are striking. Biodiversity in bird populations is only slightly less, but species richness in
poorly dispersing groups like non-flying mammals is much lower than on similar nearby islands. Some of these
mammals may be absent because of poor physical opportunities for dispersal, but others may be absent because
their host plant has not been able to recolonize successfully.
The fourth figure illustrates the principal constraints on future recolonization by the three dispersal types. Although
much of the flora and fauna has managed to re-establish in 100 years of the data set, Whittaker's estimate is that it
will take thousands of years to re-establish the pre-eruption equilibrium, if it can be achieved at all. Studies of remote
Pacific islands lead to the view that bats and non-flying mammals never reach equilibrium. Dispersal is so slow for
these groups that frequent disturbances like fires and hurricanes have a high probability of causing extinctions over
time.
contents increase in step with the increase in clay minerals,
synthesized from the products of rock weathering, and
organic colloids, formed from humification. The late
successional community witnesses the arrival of the first
tree seedlings such as birch ( Betula spp.), rowan ( Sorbus
aucuparia ) and ash ( Fraxinus excelsior ). Eventually deeper-
rooting trees of the climax vegetation , e.g. oak ( Quercus
spp.) colonize
Because weathering of hard consolidated rocks
proceeds slowly, the lithosere takes hundreds of years to
reach a climax condition. The key variables are, first, the
susceptibility of the rock to weathering and, second, the
weathering intensity as influenced by climate. Soil-
forming processes in other primary successions involve
either stabilization of sand (psammoseres) or siltation of
water bodies (hydroseres and haloseres). These physical
processes are generally more rapid, and the corresponding
metals, especially iron and aluminium. They also add
organic material to the thin, raw soil, and in so doing
increase its water-holding capacity and nutrient cycling.
Several generations of these organisms provide humic
remains, which in turn can be invaded by a second-stage
community of prostrate mosses. Once deeper depressions
have been hollowed out, thick cushion mosses can fill
them and continue the processes of soil formation. Fine
mineral particles are released by weathering, and often
also blown or washed into the site. The surface is
becoming less susceptible to drought as the depth of
water-holding soil increases ( Figure 20.10 ).
The third-stage community consists of hardy grasses
like sheep's fescue ( Festuca ovina ) and annual herbs. The
fourth-stage community will consist of shrubs such as
bramble ( Rubus fruticosus ) and dog rose ( Rosa canina ).
Soil is thickening continually, and water and nutrient
 
 
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