The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 20

[145] show that 68% of nonindigenous plants in Ontario are considered rela-
tively rare. Moreover, the majority of invasions do not seem to result in the
direct extinction of native species. Work done by Dov Sax and colleagues
[146, 147] revealed that despite global declines in species diversity, local
communities are increasing in richness. Further, the construction of the
Panama Canal resulted in a faunal interchange between isolated biotas [148].
This interchange has apparently not resulted in any species extinctions. In a
review by Davis [149], very few instances of species extinction caused by
competition from NIS have been reported. Davis also thought that neutral
dynamics may be, in part, an explanation for the lack of competition-induced
extinctions.
These observations may point to neutral rather than deterministic process-
es affecting the persistence and abundances of NIS. Within a neutral frame-
work, one would expect that adding species to a region only increases the
source pool in the birth-death probabilities (i.e., more species for the birth
part) and that no single species addition should have major impacts on the
whole community. However, this is demonstrably false as the most problem-
atic invasive species tend to be problematic in adventive populations else-
where (see IES, above). Further, one would expect long time lags in increas-
es in abundance, as random walk processes, on average, take many genera-
tions for sizable changes in population size. Other processes could potential-
ly explain many of the observations predicted by the NCD hypothesis.
Nevertheless, neutral models should be addressed as they may represent null
hypotheses and are potentially powerful explanations for some NIS observa-
tions at the community level.
Concluding remarks
In this essay, we have provided a brief review of the rapidly expanding litera-
ture on biological invasions. Many of the hypotheses explored above are not
mutually exclusive, and the processes that underlie them may act simultane-
ously, in concert or in opposition, to determine the ultimate success or failure
of an invader at each invasion stage. Though we have tried to be thorough,
experienced researchers in the field will no doubt formulate additional
hypotheses that warrant investigation. To date, most experiments have focused
on only a single hypothesis for a small subset of invasive species under partic-
ular environmental conditions. To arrive at a realistic understanding of the eco-
logical processes underlying biological invasions, a more integrative approach
is warranted - one that examines the relative importance of each processes for
a variety of species under myriad environmental conditions. This will prove to
be a daunting task indeed, but one that will perpetuate the expansion of the
field of invasion ecology for years to come, thereby ensuring plenty of fruitful
lines of research for the keen investigators of the future.