Species Competition and Predation - Ecological Systems - page 244

Applications
These ideas have applications in a number of other areas beyond the basic concept
of how communities are structured, including invasion biology [ 91 , 92 ], conserva-
tion biology [ 93 , 94 ], and ecosystem functioning [ 95 ]. The focus here is on invasion
biology. The biosphere is undergoing vast changes as a result of human activities.
One activity is the introduction of new species to places where they were not
previously found either deliberately or as an accidental by-product of commerce.
Many transplanted species fail to perform well in a new environment, or if they do
perform well enough to establish self-sustaining populations (to “naturalize”), they
never become very abundant. However, a few species perform spectacularly well
and become major pests or weeds, often displacing native species [ 96 ]. Although
invasive species are not often responsible for regional extinction of native species,
they can displace them locally and dramatically change the character of local
communities. The local communities thus undergo a process of reassembly in
response to the arrival of these invasive species. The ability of alien species to
have these effects is often analyzed in terms of competition and predation, although
other mechanisms, such as facilitation of one species by another can be expected to
be important too [ 97 ]. The study of coexistence and exclusion mechanisms, as
discussed here, fundamentally involves the question of whether a species can
increase from very low density in the presence of other members of the guild, or
to “invade.” This invasibility analysis also applies to the question of how an alien
invader successfully enters a local guild, and whether it displaces existing
guild members.
The ability of an alien species to invade can be discussed in terms of the concept
of niche opportunities [ 91 ]. Fundamentally, a niche opportunity means sufficient
resources are available for the species in question to invade, given the risk from
natural enemies that it will encounter. To some extent, lower risk from natural
enemies can enable a species to invade at lower resource levels, as it would be able to
do so at lower reproduction or survival based on those resources. A surfeit of
resources is called a resource opportunity, while a low risk from natural enemies is
an escape opportunity. The overriding question in invasion biology is why native
species, in the eons of time, have not used up all opportunities to exploit a particular
environment. There are a number of potential answers to this question.
First the local environment may have changed as a result of human activities or
climate change, and the local community is therefore no longer well adapted to it,
allowing the potential for a species from elsewhere to be better adapted than local
species [ 91 , 98 ]. This idea of change, however, should not be confused with
a natural regime of disturbance or environmental variation that might be temporally
partitioned by the native species. Like other persistent features of the environment,
such environmental variation ought already be exploited by the native species, and
so should not provide new opportunities. Unless, the natural regime of environmen-
tal variation has changed, there is no change that should be expected to facilitate
invasion [ 98 ]. Pollution is an example of one common change that humans cause.