PATTERNS OF LOSS ACROSS ISLAND TAXA 319Box 11.4 The invasion of Myrica fayain the Hawaiian islands
Of the more than 13 000 plant species that have
been introduced to Hawaii,c.900 have became
established in the wild, and over 100 of them
have became serious pests (Cox 1999). Among
the more problematic tree species are guava
(Psidium cattleianum), also a serious pest in the
Galápagos, miconia (Miconia calvescens), tropical
ash (Fraxinus uhdei) and, especially, the fire tree
(Myrica faya, also known as Morella faya).Myrica
fayais an interesting example of an oceanic island
endemic species being introduced to other
oceanic islands on the other side of the globe
with a similar environment. A valuable constituent
and early pioneer tree within protected forests in
the moist zones within Macaronesia, it has
become a pernicious forest weed in Hawaii
(Cronk and Fuller 1995).
In the Canaries, Myrica fayais the only
nitrogen-fixing tree species of the community, in
symbiotic association with the actinomycete
Franckia, and it exhibits a persistent pioneer
strategy (Fernández-Palacioset al. 2004b). It is a
dioecious species some 20 m in height, possessing
medium-size fleshy fruits dispersed by indigenous
birds, such as the blackbird (Turdus merula).
Myricamaintains a local soil seedbank, and
indeed its seeds germinate only after the
appearance of a large canopy gap. Thus, Myrica
plays an important successional role in the return
of forest to previously cleared areas, but unlike
some other Canarian woody pioneers (Ericaspp.)
also persists after the gap is filled through its
ability to re-sprout via suckers from the base
(Fernández-Palacios and Arévalo 1998).
Myrica fayawas introduced to Hawaii as early as
AD1880 and it was used extensively in
reforestation, especially during the 1920s and
1930s. In Hawaii its fruits are consumed and seeds
dispersed by a large number of both native
(Phaeornis obscurus—Hawaiian thrush) and
introduced birds (e.g. Zosterops japonica—the
Japanese white-eye, and Carpodacus mexicanus—
house finch), as well as feral pigs. Between the
1960s and 1980s, aided by its ability to fix
nitrogen, Myricarapidly invaded native forests of
Metrosideros polimorpha, which it out-competed
in reaching the forest canopy and forming pure
stands (Vitousek and Walker 1989). Myricabecame
invasive within the Hawaiian Volcanoes National
Park in 1961 and has now spread over more than
35 000 ha. Today the new Myrica–Metrosideros
forest extends across a belt of 400–1200 mASLon
the southern slopes of Hawaii (the Big Island),
particularly on recent ( 1000 years) lava flows
(Mueller-Dombois and Fosberg 1998), and it is a
well-established and problematic invasive in all of
the main Hawaiian islands.
Cronk and Fuller (1995) listed the following
ecological characteristics of the fire tree as possible
reasons for its success as an invader: prolific seed
production, long-distance dispersal by birds,
nitrogen fixation, possible production of
allelopathic substances that may inhibit potential
native competitors (specifically Metrosideros
polymorpha) and the formation of a dense canopy
under which native species are unable to
regenerate. The impacts of Myricaon soil
properties are particularly important, with knock-
on consequences for ecosystem form and
function. Vitousek et al. (1987b) calculate that the
nitrogen availability in an unaltered Hawaiian
forest is about 5.5 kg/ha per year (mainly through
the fixation ability of the native Acacia koa). In
stands invaded by Myricathis may be increased up
to 23.5 kg/ha per year. These substantially altered
edaphic conditions, in combination with the
activities of feral pigs, have combined to enable
the invasion of many continental plant species.
Options for the control of Myricainfestation
have been the subject of considerable research
efforts, particularly in the search for a safe
biological control agent. However, the most
effective means of control at present appears to
be cutting and the use of foliar applications of
herbicide, both of which are only really practical
for dealing with newly establishing populations.
The species continues to spread throughout the
archipelago.