50 THE BIOGEOGRAPHY OF ISLAND LIFE
activities. Moreover, it may well be the case for
St Helena, as for some other island systems (e.g.
Easter Island—Flenley 1993), that some native and
endemic species became extinct before botanical
investigations were undertaken (Cronk 1989), so
that the known native biota is an incomplete
record of what once was present.3.3 Disharmony, filters, and regional biogeography
This section deals with the regional biogeograph-
ical setting of islands, beginning with some fairly
simple patterns, and progressing through increas-
ingly complex scenarios.Filtering effects, dispersal limits, and disharmony
Islands tend to have a different balance of species
compared to equivalent patches of mainland: they
are thus said to be disharmonic. There are two
aspects to this disharmony (Williamson 1981). First,
as Hooker noted, both the climate and the biota
of islands tend to be more polar than those of nearby
continents (Williamson 1981). Thus, the Canaries,
located west of the Sahara, have a generally
Mediterranean flora, Kerguelen in the Indian Ocean
is bleak and Antarctic-like for its latitude, and the
Galápagos, although equatorial, are closer to desert
(i.e. subtropical) islands, being influenced by
upwellings of cool subsurface waters. Hence, there is
a climatic ‘filter’ involved in the assemblage of
species on islands. Secondly, islands are disharmonic
in that effectively they sample only from the disper-
sive portion of the mainland pool: i.e. there is an
isolation or dispersal filter. This effect has of course,
to be distinguished from simple impoverishment,
i.e. it should not be merely a random subset of a
potential mainland pool that is missing.
It is hard to quantify the maximum dispersal
capability of particular taxa across oceans, although
we attempt just that in Table 3.1. At the core of the
problem are the following complications:
●Present-day distances may not reflect distances at
the time of colonization (Chapter 2, Myers 1991;
Galley and Linder 2006).●There is much variation in dispersal powers
between related species.
●Dispersal powers change through time following
island colonization.
Readers should be aware that some historical bio-
geographers regard extreme long-distance dispersal
with enormous scepticism, instead arguing for ‘vic-
ariant’ events, i.e. the past formation of barriers (by
a variety of geotectonic and eustatic processes) sep-
arating islands from previously connected land-
masses with which they share biogeographical
affinities (e.g. contrast the analyses of the biogeog-
raphy of the Falklands offered by Morrone 2005 and
McDowall 2005). However, the evidence for the
occurrence of long-distance dispersal is clearly
established. What remains at issue is not whether it
occurs at all, but exactly how far can it reach for
particular taxa, and what is the relative importance
of vicariance and dispersal for particular biogeo-
graphically challenging island groups like New
Zealand (see Winkworth et al. 2002)?
Table 3.1 provides an analysis at a coarse taxo-
nomic level, and it is important to realize that
within taxa such as birds, or higher plants, there is
considerable structure to the dispersal reach of
subtaxa (Fig 3.2). Thus, for instance, Krakatau,
sterilized in 1883 and isolated by only c.40 km of
ocean, is already home to one quarter of the ferns of
West Java, but lacks altogether certain tree families
and genera that are common on the mainland,
but have no obvious ocean-going dispersal
mechanisms (Whittaker et al. 1997). The partial
sampling of mainland pools is also exemplified by
the flora of Hawaii, which, relative to other tropical
floras, has few orchids, only a single genus of
palms (Pritchardia), and is altogether lacking in
gymnospermsand primitive flowering plant fami-
lies. The largest eight families of flowering plants
on Hawaii—Campanulaceae, Asteraceae, Rutaceae,
Rubiaceae, Lamiaceae, Gesneriaceae, Poaceae, and
Cyperaceae—together constitute over half of the
native species (Davis et al. 1995).
Many other examples could be given. For
instance, the Azores and the islands of Tristan da
Cuhna possess no land mammals or amphibians. In
this case, the disharmony is clearly related to dis-
persal ability. Tristan da Cuhna also lacks birds of