may be alternative stable states. These simple
analyses of the species lists from Krakatau clearly
show non-monotonic episodes in the early phases
of island successions. This implies that succession
is imposing structure that swamps the general sto-
chastic processes represented in the EMIB. Much of
the turnover is successional, and quite a lot
involves species which may well not have estab-
lished breeding populations. If the turnover attrib-
utable to the loss and gain of habitats and to
succession is set to one side, undoubtedly some
turnover remains. Precisely how much is real, in the
sense of extinction of established populations, is
very hard to quantify. Overall, turnover can be
judged to be heterogeneous rather than homoge-
neoussensuRey (1985).
The degree of organization in the Krakatau assembly process
Is the assembly of the Krakatau system governed
by chance, or is it deterministic and predictable in
its behaviour? It is difficult to provide definitive
answers to this loosely phrased question because of
the deficiencies of the historical data and because
there are not enough other similar ‘experiments’ to
compare it with. However, some interesting
insights have emerged from studies of the new
island, Anak Krakatau.
The emergence and development of the new
island has modified the conditions for arrival, mor-
tality, and survival of plants and animals within the
island group as a whole (Box 5.3; Bush and
Whittaker 1991, 1993). It has impinged upon devel-
opments on the other islands, not least through its
volcanic activity. Its own history of environmental
development has been distinctive, and its colonists
can be assumed to have been drawn predominantly
from within the archipelago, rather than from the
more distant source pools of the post-1883 period.
Indeed, some populations of animals move
between the islands in foraging. As an experiment
in island colonization, it thus cannot be viewed as a
proper replicate. Nevertheless, it is interesting to
note that there has been a bias in the early patterns
of species assembly on Anak Krakatau towards
those species which colonized the archipelago in
the early post-1883 phase (Thornton et al. 1992).
This applies to several groups of plants and
animals (e.g. birds, reptiles, and bats). The excep-
tion to the pattern was provided by the butterfly
data set. This was probably due to the dependence
of butterfly species on the availability of particular
habitats and food plants, which differed sufficiently
between old and new runs of the experiment to
buck the trends.
The recolonization of Anak Krakatau has not
been an uninterrupted process. Partomihardjo et al.
(1992) have analysed the succession of floras fol-
lowing, first, the island’s appearance in the centre
of the Krakatau caldera in 1930, and, subsequently,
following wipe-outs of the vegetation in 1932/3,
1939, 1953, and damaging, but not entirely destruc-
tive eruptions around 1972 (Thornton and Walsh
1992; Whittaker and Bush 1993). Surveys have been
carried out only intermittently, but we have in
essence three or four runs of the assembly experi-
ment on Anak Krakatau, for each of which a species
list is available. This rather small-scale natural
experiment has demonstrated a strong degree of
repetition, a temporal nestedness, with a core of
constant species, which make it back each time and
are added to (Whittaker et al. 1989; Partomihardjo
et al. 1992). The first assemblage was really just a
seedling flora, a few members of which were not
found in the second assemblage. Yet, of 32 species
identified from these two surveys, 30 recolonized
subsequently. The eruptions in 1972 are presumed
to have severely reduced the ‘third’ flora, yet all but
2 of its 43 species were recorded between 1979 and
1991.
If the Anak Krakatau flora is broken down into
arbitrary functional guilds, there are basically three
sets of species:1 Strandline species. These are the largest set, repre-
senting 58 of 125 spermatophytes found between
1989 and 1991. Propagules of these species are sea-
dispersed, and are produced in large numbers
locally on the other islands. This is a very consistent
set. Forty-two of these species have been found on
all four Krakatau islands.
2 Pioneers of interior habitats. These species are
wind-dispersed ferns and grasses, with a fewKRAKATAU—SUCCESSION, DISPERSAL STRUCTURE, AND HIERARCHIES 141