Exotic plants may alter other ecosystem proper-
ties, such as fire regimes or hydrological conditions.
Examples of each can be drawn from Hawaii.
Bunchgrass (Schizachyriumspp.) increases the flam-
mability of the vegetation, and has prevented
regeneration of native species that are not adapted
to burning. Andropogon virginicusis another exotic
grass, considered to be one of the most threatening
of aliens. It not only carries fire, but also alters
hydrological properties. It occurs in disturbed
grassland and scrub on the island of Oahu in areas
on red clay soils where native forest vegetation has
been replaced by introduced woody and herba-
ceous plants. Its seasonal pattern of growth and
tendency to form dense mats of dead matter are
resulting in increasing runoff and accelerated ero-
sion (Cuddihy and Stone 1990).
11.7 How fragile and invasible are island ecosystems?
It has become axiomatic to represent oceanic islands
as fragile ecosystems, which, because of their evolu-
tion in isolation from continental biotas, are particu-
larly susceptible to the introduction of alien species.
Are island systems really particularly fragile or have
humans just hit them especially hard? D’Antonio
and Dudley (1995) contend that the data support
two propositions. First, island ecosystems do typi-
cally have a higher representation of alien species in
their biota than do mainland systems. Secondly, the
severity of the impacts of invasions appears in gen-
eral to increase with isolation.
Cronk and Fuller (1995) suggest six reasons why
oceanic islands may be more susceptible to
invasion:
●Species poverty. This may mean that there is
more vacant niche space and less competition from
native species. This is plausible, particularly where
humans have disturbed habitats, but it is only one
part of the picture.
●Evolution in isolation. It is often assumed that
island species are competitively inferior to conti-
nental species as a resut of their long evolutionary
isolation. The case of Myrica faya, transported from
one set of oceanic islands, in Macaronesia, to
another, Hawaii, illustrates that the ‘continentality’
of the land mass from which a species hails may not
be of great moment: simply to be from a different
biogeographical pool may be sufficient. However,
the evolution of flora and fauna in isolation, often
without adaptation to grazing, trampling, or preda-
tion by land mammals has led to the loss of defen-
sive traits in many oceanic island endemics,
expressed in plants as the absence of typical grazing
adaptations such as spines, thorns, and pungent leaf
chemicals. In the presence of browsers these plants
are at a competitive disadvantage and tend to dis-
appear, as was the fate of many endemic plant
species on St Helena following the introduction of
goats shortly after the island’s discovery in AD1502.
Similarly, many island endemic animals lack a fear
of predators, and some birds have reduced flight or
are flightless, and build their nests on the ground.
These are traits that may have no competitive dis-
advantage when faced with a species of the same
trophic level, but which have proven disastrous
when faced with terrestrial vertebrate predators.
●Exaggeration of ecological release. Alien species
generally arrive on islands without their natural
array of pests and diseases, and this provides them
with an advantage over native species. A similar
ecological release mechanism is proposed as part of
the taxon cycle in Chapter 9, and indeed also
applies to many introductions to continents.
●Early colonization. Particularly in the Atlantic,
Caribbean, and Indian Oceans, islands were the first
landfalls and first colonies of the Europeans, hence
they have had a long history of disturbance and of
introductions. Although this is true, this line of rea-
soning fails to account for the severity of impact of
particular introductions such as the brown tree
snake on Guam or Miconiaon Tahiti: the outcome
has been a particular property of the species
involved rather than the length of time over which
disturbance and introductions have occurred.
●Small scale. The small geographical size of
islands means that their history is concentrated in a
small area, within which there are few physical
features large enough to prevent exploitation,320 ANTHROPOGENIC LOSSES AND THREATS TO ISLAND ECOSYSTEMS