CONTEMPORARY PROBLEMS ON ISLANDS 327Box 12.1 Invasive plants in the Canaries: the incorporation of prickly pear (Opuntia) into the
landscape
During the nineteenth century, several prickly pear
species (Opuntia, Cactaceae) of Mexican origin
were introduced into the Canaries, principally for
the natural red-purple colorant (carminic acid)
produced by the females of the parasitic insect
Dactylopius coccus(Coccoidea, Homoptera), also
introduced from Mexico, which feed on their
succulent leaf-shaped stems. True cacti are not
naturally found outside the New World. Opuntia
rapidly naturalized and became invasive within the
natural and semi-natural ecosystems of the
Canarian lowlands (Otto et al. 2001, 2006), where
they found similar arid, warm conditions to their
original distribution. One of the keys to the success
of prickly pears as invaders on the Canaries is the
year-round availability of their large, fleshy, sweet
fruits, which have proved attractive to both native
and exotic dispersers (Chapter 11). They also
regenerate and spread vegetatively, from falling
leaves taking root: behaviour not found in the
native Canarian Euphorbia, which are the
indigenous dominants of this scrub vegetation.
Finally, another clue to their success is that whereas
all but one species of Euphorbialose their leaves in
summer drought conditions, Opuntiaare still able
to photosynthesize, using the crassulacean acid
metabolism (CAM) photosynthetic pathway, which
again is not possessed by the local Euphorbia
species.
Although several prickly pear species grow wild
today on the Canaries (O. ficus-barbarica,
O.dillenii,O. tomentosa,O. tuna,O. robusta, and
O. vulgaris) (Izquierdo et al. 2004) only two of
them (O. ficus-barbaricaandO. dillenii) are widely
distributed.O. dilleniiform part of the subdesert
coastal scrub of the islands, whereas O. ficus-
barbaricathrives in the disturbed zones at mid
altitudes, where it forms dense, almost
monospecific patches. Their succulent leaves were
eaten by goats, providing them with a valuable
water intake, whereas their fleshy fruits are not
only eaten by humans, but also by endemic
Gallotializards (Valido and Nogales 1994)
and several native birds, especially the raven
(Corvus corax). Nogales et al. (1999) found O.
ficus-barbaricato be the most common
seed in the pellets regurgitated by ravens,
and both O. ficus-barbaricaandO. dillenii
showed improved germination after passing
through raven guts. Furthermore, Sturnus
vulgarishas been observed feeding on the
parasitic insect Dactylopius coccusgrowing
on the prickly pear (Martín and Lorenzo 2001).
Opuntiathus provides an illustration of how
exotic plant species can compete successfully
with native and endemic species, whilst in
this case simultaneously contributing to the
food resources of both native and exotic
animal populations.
BesidesOpuntia, other invasive plants
competing successfully in otherwise well-preserved
Canarian ecosystems include Pennisetum
setaceum,Nicotiana glauca, and Agave americana,
in the subdesert coast scrub; Cytisus scoparius,
Ulex europaeus,Tradescantia fluminensis,
Ageratina adenophora,Ailanthus altissima,
Zantedeschia aethiopica,Eschscholtzia californica
(Californian poppy) in the native laurel and pine
forest ecosystem and several Acaciaspecies at
different altitudes. Altogether, alongside
approximately 1300 native plant species in the
Canaries (see Table 3.2), Izquierdo et al. (2004) list
some 673 plant species as being introduced (or
probably introduced), of which 79 are currently
considered invasive. This amounts to 11.7% of the
total introduced set, in line with Williamson’s (1996)
so-called ‘tens rule’ (Chapter 11).
Today, after several centuries of presence in the
Canaries, prickly pears are considered part of the
Canarian landscape and no attempt has been
made to eradicate or control them. In contrast,
a lot of money has been invested in ineffective
efforts to control some other exotics, such as the
grassPennisetum setaceum.