the consideration of turnover in Chapter 4, but it is
placed here because of the structural features
evident within it. An important caution applies to
these studies. The Krakatau islands are large, topo-
graphically complex, and sufficiently difficult to
explore that some parts have never, or only rarely,
been penetrated by scientists. For both plants and
animals, surveys span over 100 years of expedi-
tions, have involved considerable turnover in
personnel (and thus in expertise and methods), dif-
ferences in intensity and differing areas of search.
They have also taken place at irregular intervals.
For these reasons, we must be cautious in interpret-
ing changes in rates of immigration and extinction.
The rates calculated are, in any case, not strictly as
defined in the EMIB, rather they represent ‘arrival
in the lists’ and ‘departure from the lists’.
Whittakeret al. (1989) presented an analysis of
higher plant colonization for 1883–1983, which
illustrates the smoothing effect of calculating immi-
gration rates over greater intervals of time, ignoring
the intervening survey data (Fig. 5.7). The repeat
surveys of Docters van Leeuwen (1936) in 1919 and
1922 produce a pronounced spike in the immigra-
tion data (another occurs for the same reasons at
the end of the series), but the peak for the 1920s
remains even when this survey frequency effect is
removed. The low point, c.1951, reflects survey
deficiency. Bush and Whittaker (1991) therefore cal-
culated ‘immigration’ and ‘extinction’ rates for
plants using survey data grouped into adjacent
surveys, and ignoring the 1951 datum. Reasoning
that for most plants, especially large forest trees,
temporary disappearance from the lists was more
likely to reflect survey inefficiency than genuine
turnover, they provided upper and lower estimates:
first, the recorded turnover and, secondly,
assuming the minimum turnover allowable from
the data (Fig. 5.8) (see also Whittaker et al. 1989,
1992 a). The overall trend in species richness is also
shown in this figure, and although the lack of ade-
quate surveys between 1934 and 1979 makes the
precise shape of the curve unknowable, additional
exploration between 1989 and 1997 continued to
turn up additional species records. Providing
regional source pools are not destroyed by human
action, it seems probable from our analyses of
forest dynamics (e.g. Schmitt and Whittaker 1998;
Whittakeret al. 1998) that species number can con-
tinue to rise for some time to come.
Evident in the analyses of ‘immigration’ and
‘extinction’ rates is that discovery of new species on
Rakata peaked during the period of forest forma-
tion and closure, and the rate of loss of species from
the lists also has a peak, lagging behind the
‘immigration’ peak, and reflecting the loss of early
successional habitats. Further survey work between
1983 and 1994 allowed Whittaker et al. (2000) to
re-examine the patterns of species loss in the light
of an improved knowledge of the present day flora.
Their approach was to focus on the 325 speciesKRAKATAU—SUCCESSION, DISPERSAL STRUCTURE, AND HIERARCHIES 1370
1883 1903 1923 1943 1963 198324681012141618Survey pointsNew species per annumRakata raw data,
13 intervals
Rakata, 6 intervals
Rakata, 2 intervals
Rakata, Sertung
and Panjang, 2
intervalsYear
Figure 5.7Numbers of new species of higher plants recorded on
Krakatau over a variety of intervals, expressed as an annual rate. The
figures exclude re-invasions. (Redrawn and corrected from Whittaker
et al. 1989, Fig. 16.)