interannual and an intraannual basis. However,
in some areas, the ocean currents and wind
currents are strongly directional and persistent. In
some cases, knowledge of current systems provides
a parsimonious explanation of biogeographical pat-
terning (Cook and Crisp 2005), as in the case of the
butterflies of the island of Mona in the Antilles. The
island is equidistant between Hispaniola and Puerto
Rico and is 62 km^2 in area. Its 46 species of butterflies
feature 9 subspecies in common with Puerto Rico
and none with the larger Hispaniola, whereas the
ratio of source island areas is 9:1 in favour of
Hispaniola. The explanation appears to lie in a
remarkably constant bias in wind direction, which is
from Puerto Rico towards Hispaniola (Spencer-
Smithet al. 1988). However, it should be recalled that
palaeoenvironmental data from islands such as
Hawaii and the Galápagos indicate that significant
changes in ocean and atmospheric circulation
patterns occur over time (Vitousek et al. 1995).
2.5 Natural disturbance on islands
One of the themes that we develop in this book is that
the significance of naturally occurring disturbances
has not been given due recognition in the develop-
ment of island biogeographical theory. We have
already seen that any ancient island will, over the
course of tens of thousands of years, have
experienced substantial environmental changes.
This section is concerned with shorter-term
changes in environment, such as the individual
volcanic eruptions that may intermittently add to
the bulk of an island while, at least temporarily,
reducing its plant and animal populations
(e.g. Partomihardjo et al. 1992; Hilton et al. 2003).
Ecologists have defined disturbance as ‘any rela-
tively discrete event in time that removes organ-
isms and opens up space which can be colonized
by individuals of the same or different species’
(Begonet al. 1990, p. 754). Such a definition is
rather inclusive and, in the context of island bio-
geography, it will be recognized that different
scales of perturbation are relevant in differing con-
texts (Fig. 2.20). Increasingly, ecologists are recog-
nizing that natural systems are largely structured
by disturbance (Pickett and White 1985; Huggett
1995). Often, a single event such as a hurricane
(tropical cyclone), will impact on both mainland
and island systems; however, because of the geo-
graphical and geological idiosyncrasies of small
islands and their location in oceans, their distur-
bance regimes when scaled to island size are atyp-
ical of continental land masses.
In order to incorporate the disturbance regime
into island biogeography, some form of classifica-
tory framework is required (Whittaker 1995).
Figure 2.20 represents an example of one attempt to
develop such a framework. Another scheme,
devised for the Caribbean islands and reproduced
in Table 2.5, indicates the main types of disturbance
phenomena, the area liable to be affected, the pri-
mary nature of the impact, the duration, and the
recurrence interval. In an approach owing much to
H. T. Odum, Lugo (1988) has classified the distur-
bance phenomena into five types, each character-
ized by their impact on energy transfers.●Type 1 eventschange the nature or magnitude of
the island’s energy signature before the energy can
be ‘used’ by the island; for example, shifts of
weather systems through ENSO events, leading to
droughts or heavy rainfall (Stoddart and Walsh
1992; Benton and Spencer 1995).
●Type 2 eventsare those acting on the major
biogeochemical pathways of an island, for
example as a result of changes caused by an
earthquake.
●Type 3 eventsare those that remove structure
from an island ecosystem, but without altering the
basic energy signature, so that recovery can proceed
rapidly after the event. Hurricanes are Type 3 events.
A well-researched example is Hurricane Hugo,
which caused complete defoliation of a large part of
the Luquillo forest on Puerto Rico in 1989, although
one might note that, despite a rapid ‘greening-up’ of
the forest, the signature of such an event will be evi-
dent in the unfolding vegetation mosaic for many
decades (Walker et al. 1996).
●Type 4 eventsalter the ‘normal’ rate of material
exchange between the island and either the ocean
or atmosphere. For instance, if the trade winds are
inhibited by changes in atmospheric pressure,
exchanges may be reduced.38 ISLAND ENVIRONMENTS