THE AGENCIES OF DESTRUCTION 299populations have been responsible for the local
extinction of land iguanas in the Galápagos.
Even invertebrates have led to the extinction of
island endemics, notably the losses of all endemic
species of Partulaland snails following the mis-
guided introduction of the carnivorous snail
Euglandina rosea to the island of Moorea
(Williamson 1996). Similarly, exotic ants introduced
to the Galápagos and Hawaii (from which they are
naturally absent) predate endemic invertebrates
and are an increasing cause for concern in both
archipelagos. Browsing animals such as goats and
rabbits have also led to losses (Cronk 1989; Johnson
and Stattersfield 1990). Examples blamed on the
rabbit include the Laysan rail (Porzanula palmeri)
and Laysan millerbird (Acrocephalus familiaris), both
of which disappeared during the early twentieth
century because rabbits denuded their island.
Changes to pollination and dispersal networks
Species dispersing spontaneously or being intro-
duced by humans to remote islands leave behind
their usual web of interacting species, which may
include pollinators, dispersers, competitors, food
plants, prey, predators, parasites, and pathogens
(Janzen 1985, Olesen and Valido 2004). If they can-
not withstand the loss of evolutionary ‘partners’
they may fail to establish on the island, but at least
a fraction of introduced species find a new web of
interacting species they can interact with success-
fully. Indeed, as discussed in Chapter 7, there is
some evidence that pollinator networks tend to be
more generalist on remote islands, involving super-
generalist endemic species that readily include
non-native species into their networks, thus aiding
their establishment (Olesen et al. 2002).
The introduction of exotic pollinators such as the
honey bee (Apis mellifera), continental bumblebees
(Bombusspp.), or wasps (Vespulaspp.) on to an
island, will have consequences for both native plant
and pollinator communities, and these conse-
quences may be positive or negative for the island
species. If the exotic pollinator is more effective in
transporting pollen than the native pollinators,
then native (but also exotic) plants will increase
their fruit and seed production in relation to native
plants not pollinated by the exotic species.
Conversely, if it is less effective in pollinating a par-
ticular plant than a displaced native pollinator, then
this may reduce seed set. For instance, introduced
honey bees have been found to compete with the
endemic Canarian bumblebee (Bombus canariensis),
resulting in diminished pollen transfer success for
several Canarian endemic plants (such as Echium
wildpretii) (Valido et al. 2002).
The introduction of predators or disease organ-
isms that knock out native pollinator networks can
be expected to have significant negative conse-
quences for native plant species. For instance, the
introduction in Hawaii of predatory insects such as
Vespula pensylvanicaand of diseases, like avian
malaria, have extinguished entire groups of pollina-
tors, such as 52 endemic species of Nesoprosopisbees
(Vitousek et al. 1987a), and many endemic bird
species, respectively. The disappearance of exclusive
pollinators is considered responsible for the extinc-
tion of many plants pollinated by them, including
some 30 endemic Hawaiian Campanulaceae species
(Cox and Elmqvist 2000).
The introduction of exotic dispersers can have
positive consequences on native (but also on
exotic) plant species. For example, the Balearic
endemic plant Cneorum tricoccon, was originally
dispersed by Podarcis lilfordilizards, which occupy
elevations of less than 500 mASL). Following the
introduction to Majorca of an alternative disperser,
the European pine marten (Martes martes),Cneorum
begin to colonize altitudes up to 1000 mASL,
driven by the wider altitude range exploited by the
marten (Traveset and Santamaría 2004).
Conversely, the introduction of less-effective dis-
persers can result in negative consequences for
native species. For example, two introduced mam-
mal species, rabbit (Oryctolagus cuniculus) and
Barbary ground squirrel (Atlantoxerus getulus),
now compete for the fruits of the endemic shrub
Rubia fruticosain Fuerteventura (Canaries) with the
indigenous dispersers (native birds and lizards).
The seeds are dispersed less effectively by the
mammals and have a lower viability. In this case
there are thus negative consequences both for the
native plant (poorer dispersal services) and for the
native frugivores (competition for a food resource)
(Nogaleset al. 2005).