Biology Reference
In-Depth Information
ages of alien plants on oceanic islands tend to be in the tropics, a pattern
opposite to that seen in the representation of alien plants in continental
floras. Patterns of human occupation, landscape disturbance, and plant
introduction vary greatly in tropical island and continental areas, but the
great difference in representation of alien species suggests that islands do
indeed offer less resistance to invasion. A similar pattern exists for alien
animals.The Hawaiian Islands provide one of the best-documented cases.
Alien vertebrates in Hawaii include 8 amphibians, 15 reptiles, 21 mam-
mals, and 28 birds; alien invertebrates number more than 2,500 species
(Cox 1999).
Invasive Species and Escape from Coevolved
Relationships
Only recently has attention been given to the possible role of coevolu-
tionary adjustments among community members in creating resistance to
invasion by alien species. In experiments examining species richness and
invasibility, the assumption is usually that if many species use resources
completely, it is because they have been sorted by ecological processes
rather than adjusted to each other by evolutionary selection. Indeed, in
many experiments, the species mix employed includes alien species that
obviously have not had time to coevolve with natives.
Many examples are now available of alien species that have escaped
coevolved relationships. Classical biological control, the identification and
release of natural enemies of a pest, rests on this fact. Many pest species,
both plants and animals, have been partially or largely controlled by the
importation and release of such enemies. In this effort, of course, exten-
sive research must be done to show that the probability that the intro-
duced biological control agent will itself cause damage is very small (see
chapter 17).
Several studies now suggest that many alien species have experienced
release from restrictions that exist in their native region, an idea known as
the enemy release hypothesis (Keane and Crawley 2002).This hypothesis
suggests that plants that invade distant geographical areas have usually left
most of their specialist herbivores behind and should not be quickly col-
onized by specialist herbivores in the new region. Although they may be
attacked by generalist herbivores in the new region, generalist impacts
should be heavier on native plants than the invading aliens.A survey of 13
studies that quantified herbivore impacts and differentiated between spe-
cialist and generalist herbivores supported these predictions (Keane and
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