Biology Reference
In-Depth Information
and the introduction of exotic plants has placed many endemic plants at
risk on many oceanic islands. In the Hawaiian Islands, for example, some-
where between 460 and 690 species of introduced plants have become
established in the wild (Mueller-Dombois and Loope 1990). The result-
ant vegetational changes, together with other anthropogenic pressures,
have driven about 177 native plants to extinction. With extinction of
these plants, many endemic arthropods are likely also to have become
extinct.
Individual island invaders can sometimes present major threats to
native plants. In Tahiti, for example, 40-50 of the island's 212 endemic
plant species are considered to be threatened by the spread of the South
American shrub,
Miconia calvescens
(Meyer and Florence 1996).This plant,
introduced in 1937 as an ornamental, has spread into the mountain
forests, where it forms nearly pure stands over about two-thirds of the
land area of the island. At least one endemic plant is believed extinct and
15 others critically endangered as a result.
Alien aquatic animals present competitive challenges to the survival of
native species.Among invertebrates, for example, introduced crayfish have
become serious threats to native crayfish in both Europe and North
America (Lodge et al. 2000). Only five native crayfish occur in Europe,
but four alien species have been introduced from North America and one
from Australia. These aliens, and crayfish diseases introduced with them,
have displaced European natives in thousands of locations.
North America has about 75% of the world's freshwater crayfish fauna,
about 333 species in all. Many of these species are threatened by a variety
of factors, including the spread of species to new waters by fishermen
using them as bait. Many of these North American species are quite local-
ized in distribution, however, and are thus at risk from the widespread
introduction of nonindigenous species. One of the most frequently intro-
duced crayfish,
Orconectes rusticus
, has extirpated native crayfish in many
locations in the midwestern United States (Perry et al. 2002).
North America also has the world's richest fauna of freshwater mus-
sels, some 296 species (Perry et al. 2002). Alien molluscs, particularly the
zebra mussel (
Dreissena polymorpha
) and its close relative, the quagga mus-
sel (
D. bugensis
), pose a serious threat to many native mussels, especially
large mussels of the family Unionidae (Strayer 1999). The most direct
impacts on unionids are through fouling, that is, growth of zebra and
quagga mussels on unionid shells. In the Rideau River in eastern Ontario,
Canada, for example, fouling by zebra mussels led to the extirpation of
three formerly abundant unionids between 1993 and 2000 (Martel et al.
