Biology Reference
In-Depth Information
European plants in Europe, California, and North and South Carolina,
and native North American plants in California, the Carolinas, and
Europe.They found no tendency for species to grow taller in areas where
they were alien, whether in North America or Europe. In fact, North
American natives tended to grow taller in California and the Carolinas.
Thus, growth, at least as measured by general information on height pub-
lished in regional floras, does not seem to be consistently greater for plants
in alien environments.
On the other hand, in many specific cases, reproductive performance
of alien plants has been shown to exceed that of native species that are
closely related or are their principal competitors. In Hawaii, fountain grass
(
Pennisetum setaceum
) has become an aggressive invader of dry habitats.
Fountain grass experiences much less seed and ovule damage by insects
and fungi than does its main native competitor, pili grass (
Heteropogon con-
tortus
) (Goergen and Daehler 2001a, 2001b). In Tennessee, the alien shrub
Ligustrum sinense
experiences much less foliage damage by insect herbi-
vores and realizes much greater fruit production than
Forestiera ligustrina
,
a similar native shrub of the same family (Morris et al. 2002).
Invasive Species and Mutualistic Relationships
The ability of many alien species to become established in new geograph-
ical regions depends on the establishment of beneficial relationships with
other organisms. For plants, these include organisms such as mycorrhizal
fungi, nitrogen-fixing microorganisms, pollinators, and seed dispersal
agents (Richardson et al. 2000). In Hawaii, for example, Carino and
Daehler (2002) found that a small, alien, nitrogen-fixing legume appeared
to facilitate the establishment of fountain grass (
Pennisetum setaceum
), a
highly invasive plant with serious impacts. For animals, mutualistic inter-
actions may develop with organisms that both provide food and are ben-
efited by animal activities. Mutualists may be former associates from the
native area or sometimes new associates from the region invaded. In Ore-
gon, for example, alien sunfish apparently facilitate the establishment of
the alien bullfrog (
Rana catesbeiana
) by reducing the density of dragonfly
larvae that attack small bullfrog tadpoles (Adams et al. 2003). Parasitic
species may likewise require the presence of specific hosts to become
established.
With the increase in kinds of organisms and numbers of species being
introduced to new regions, mutualistic and other interspecific interactions
appear to be playing a greater role in promoting the establishment and
