Environmental Engineering Reference
In-Depth Information
When this results in deposition of nitrogen in a nutrient-poor environment, it can
have a large effect on local plant communities, facilitating the invasion of species
adapted to exploit this resource [
99
]. The invasive species then potentially depress
other resources that the native species depend on, harming them.
Second, the native community might have not existed for very long, or in the
past was poorly connected to areas that might provide colonists or genetic diversity
allowing adaptation to exploit all opportunities well. In a sense, such a local
community is immature [
91
]. In these cases, niche opportunities exist, and species
from elsewhere might have the adaptations allowing them to exploit the local
environment more successfully than the native species. Islands that have been
devastated by invasions might be in this category [
96
,
100
], but so also might
aquatic systems which suffer from the problem that dispersal from one place to
another was historically much rarer than it is today. This idea has been applied on
continental scale, suggesting that the Eurasian continent being larger contains
species with stronger competitive ability than species in North America [
101
].
There is some evidence for this in plant species from Eurasian that use chemical
interference successfully in competition with North American species [
102
]. The
idea of biotic resistance says that localities with high species diversity ought to be
more difficult to invade [
103
,
104
], which makes sense to the extent that it means
that more niche opportunities are foreclosed by the adaptations present in the more
diverse community.
Third, invaders potentially arrive in the absence of their specialist natural
enemies. Even if diseases, parasites, or predators are introduced with an invader
at the same time, unless these natural enemies can exploit other species, they may
well be lost early in the introduction because the invader in question was initially at
too small a population size to sustain them. Such a species would have a strong
natural enemy escape opportunity that would give it an advantage in a novel
environment, especially if native species were attacked by natural enemies that
do not affect the invader [
97
,
105
]. Of course, native natural enemies do attack and
inhibit invaders to varying extents, reducing their success [
91
,
97
].
These various ideas can be analyzed within the Lotka-Volterra framework
presented here [
92
,
106
]. Fundamentally, if an invader has overall advantage such
as novel competitive weapons for interfering with other species, is less susceptible
to enemy attack, or is better overall in its adaptation to the environment, it will have
an average fitness advantage over native species (
k
ratios greater than 1), allowing it
to invade, with the potential, if this advantage is large enough, to exclude native
species, at least locally. Instead, it might not have an overall advantage, but simply
an advantage under specific conditions, leading to low niche overlaps,
r
, with
native species. This advantage might be that it can specialize better on
a particular resource or particular environmental conditions, arising spatially or
temporally, that native species are not fully exploiting. In many situations, it seems
that elements of both are likely. It might for instance weakly partition resources or
predators, but also have some fitness benefits, without average superiority
over natives, but together this might be sufficient for invasion. On the other hand,
the fact that even strongly successful invaders do not eliminate species in the
