Similarly, the introduction of an exotic plant can
alter the insular networks in varying ways. The
new plant can offer the native dispersers new
resources that will produce positive effects for
both of them, but simultaneously have negative
consequences for the native plants that have to
compete for resources with the exotics. For
instance, the introduction of the prickly pears
(genusOpuntia) on the Canaries has provided
native dispersers, including ravens and endemic
lizards, a new fleshy fruit to feed on, with a much
larger year-around availability than the native
resources (Fig. 11.2). The spread of Opuntiahas
thus been aided by native lizards and ravens, so
thatOpuntiaspecies have successfully invaded
large areas of Canarian coastal scrub rich in
endemic Euphorbia and other plant genera
(Nogaleset al. 1999, 2005; Valido et al. 2003).
Finally, it is often the case that interactions
between two or more exotic species are crucial to
the spread of problematic invasives. For example,
within the Galápagos, introduced mammals are
entirely responsible for the spread of the intro-
duced guava (Psidium guajava), one of a number of
ecosystem-transforming non-native plant species
(e.g. Henderson et al. 2006).
Hybridization with native species
The introduction to an island of an allopatric con-
gener, which is not fully reproductively isolated
from the native taxon, can lead to a process of
introgressive hybridization and the genetic dilu-
tion and eventual disappearance of the native
form. This introgression process is currently
affecting the Canarian endemic palm Phoenix
canariensis, the plant symbol of the Canaries, as a
result of the introduction of its closest relative, the
date palm Phoenix dactylifera. P. canariensisgrows
wild in the lowlands of each of the main islands of
the archipelago, whereas P. dactyliferais a fruit
palm, of unknown origin, that has been cultivated
for at least 5000 years in the Arabian peninsula
and North Africa. Although it was introduced
before the Spanish settlement of the Canaries, the
problem of introgression is essentially a recent
one, following the importation of massive num-
bers of date palms to decorate the new tourist
resorts and paved roads across the archipelago
(Morici 2004). Hybrid swarms are now found all
over the archipelago, especially in the tourist hon-
eypot areas on the eastern islands. In addition to
hybridization, the endemic palm is also threat-
ened by a lethal pest, the weevil Rhynchophorous
ferrugineus, introduced with date palms imported
from the Middle East. As a result of these pres-
sures, this particular plant symbol of the Canaries
faces an uncertain future. Genetically pure stands
of the Canarian palm populations remain in the
interiors of Gran Canaria and La Gomera, far
away from the resorts (González-Pérez et al. 2004),
providing potential seed supply for future conser-
vation programmes. A similar process of intro-
gression is under way on the Cape Verde islands,
between the endemic palm Phoenix atlanticaand
the date palm (Morici 2004).Disease
The problem of disease is closely associated with
exposure to exotic competitors and, indeed, the
separation of exotic microbes as a category from
other introduced organisms is largely arbitrary. The
most striking exemplification of the impact of dis-
ease on insular populations has, without doubt,300 ANTHROPOGENIC LOSSES AND THREATS TO ISLAND ECOSYSTEMS
806040200
SONDJ FMAMJ J ASNeochamaeleaWithaniaRubia Opuntia
PlocamaMonth% of ripe fruitsFigure 11.2Availability of ripe fruits (% of the crop size) in a
Canarian subdesert coastal scrub, Teno Bajo, Tenerife, for September
1993 through August 1994. The presence of the exotic invasive
cactusOpuntiahas altered the pattern of fruit availability for the
local frugivore community, and this in turn may impact on the
dispersal services provided to the native plant species. (From Valido
et al. 2003)