132 COMMUNITY ASSEMBLY AND DYNAMICS
Box 5.3 The Krakatau eruption of 1883: a dramatic start to a ‘natural experiment’
The Krakatau group has undergone repeated
phases of volcanic activity. In 1883 it consisted of
three islands, arranged in a caldera which resulted
from prehistoric eruptive activity. The largest
island of the group became active in May of that
year, commencing a sequence that ended on
27 August in exceptionally destructive eruptions.
Two-thirds of the island disappeared, vast
quantities of ejecta were thrown into the
atmosphere, and a series of huge waves (tsunami)
generated in the collapse resulted in an estimated
36 000 human casualties in the coastal
settlements of Java and Sumatra flanking either
side of the Sunda Strait some 40 km or so from
Krakatau. The meteorological and climatological
effects of the eruption were observable across the
globe and the islands excited considerable and
lasting scientific interest from a number of
disciplines (Thornton 1996). Each of the three
Krakatau islands was entirely stripped of all
vegetation. The main island, now known as
Rakata, lost the majority of its land area, but all
three islands also gained extensive areas of new
land resulting from the emplacement on to the
pre-existing solid rock bases of great thicknesses
of pyroclastic deposits. Estimates of ash depths
are of the order of 60–80 m and to this day
almost all of the area of the three islands remains
mantled in these unconsolidated ashes, with
relatively little solid geology exposed at the
surface. No evidence for any surviving plant or
animal life was found by the scientific team led by
Verbeek later that year, and in May 1884 the only
life spotted by visiting scientists was a spider. The
first signs of plant life, a ‘few blades of grass’,
were detected in September 1884.
The efficacy of the destruction has been hotly
debated (Backer 1929; Docters van Leeuwen
1936). It is conceivable that some viable plant
propagules might somehow have survived to be
uncovered by later erosion of the ash mantle
(Whittakeret al. 1995). However, there is no
evidence for survival, and indeed the densest
populations of early plant colonists were located
onterra nova. Any viable rain forest propagules
uncovered by erosion of the ash would in any
case have emerged into a vastly different and
hostile environment, and their successfulestablishment would thus have been entirely
remarkable. The islands can be taken to have
been as near completely sterilized as makes no
practical difference. The fauna and flora have thus
colonized since 1883 from an array of potential
source areas, the closest of which, the island of
Sebesi, is 12 km distant. All the nearest land
areas, including Sebesi, were also badly impacted
by the 1883 eruptions.
Here was a group of three islands, each
mantled in sterile ashes and in time receiving
plant and animal colonists. The potential of the
islands as a natural experiment in dispersal
efficacy and ecosystem recovery was appreciated,
although, unfortunately, only botanists seized the
opportunity at an early stage, with surveys in
1886 and 1897. Since then, numerous scientific
teams have worked on Krakatau, to varying ends,
but along the way accumulating a remarkable
(if imperfect) record of the arrival, succession and
turnover of plant species and many groups of
animals (Whittaker et al. 1989; Thornton 1996;
Whittakeret al. 2000).
It is important to appreciate the dynamism of
the platform upon which ecosystem assembly has
taken place. The early pace of erosion of the ash
mantle must have been dramatic. It created
a deeply dissected ‘badlands’ topography which
remains geomorphologically highly dynamic.
The extensive new territories around the coasts
were also subject to extreme rates of attrition,
and steep cliffs formed rapidly around much of
the shoreline. Shallow shelving beaches, which
provide the most favourable points for the
colonization of many plant and some animal
species, are restricted in their extent. In 1927,
after nearly 46 years of inactivity, a new island
began to form in the centre of the 1883 caldera,
finally establishing a permanent presence, Anak
Krakatau, in 1930. Through intermittent activity
it grew by the mid-1990s to an island of over
280 m in height and 2 km in diameter (Thornton
et al. 1994). Over this period it caused significant
and widespread damage to the developing
forests of two of the three older islands, Panjang
and Sertung, but only indirectly impacted on
Rakata (Whittaker et al. 1992b; Schmitt and
Whittaker 1998).