The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 7

to analyze species patterns [42]. An alternative approach has been to compare
native and introduced populations of the same species - a biogeographical
approach. Conversely, invasibility studies have taken a community-level, or
biotic approach, by examining the species composition (e.g., the number of
parasites or predators) of invaded habitats, or an environment-based approach,
by examining abiotic characteristics (e.g., physicochemical characteristics)
associated with invasibility. Sub-sampling at each stage may therefore occur at
either the taxonomic or biogeographical level, and filtering mechanisms may
be biotic or abiotic. Table 1 summarizes 15 hypotheses, derived or inferred
from the invasion ecology literature, that attempt to explain the success or fail-
ure of invaders. These hypotheses are discussed in greater detail below.
However, it is important to note that these hypotheses should not be considered
mutually exclusive, as the relative importance of each may vary among habi-
tats, invaders, and invasion stage (Fig. 1). Although we often do not explore
the matter explicitly, it is worth considering the stage(s) at which each hypoth-
esis is most likely to act, whether each hypothesis predicts success or failure
of an invader at that stage, and how the processes underlying these hypotheses
may interact with each other in synergistic or antagonistic ways.
Biotic resistance hypothesis (BRH)
The BRH is a widely-cited hypothesis to explain patterns of invasion but iron-
ically does not predict why invaders succeed; rather it describes why they are
likely to fail. The late Charles S. Elton of Oxford University first became inter-
ested in biological invaders while studying the effects of rodents on England's
food supply during World War II [43]. Elton [31] first formalized the idea of
invasibility, which is based on niche theory and Lotka-Volterra-type competi-
tive systems. Since its inception [44], the niche has been premised on a sup-
posed inability for ecologically similar species to coexist [45-47]. Niche the-
ory, as formalized by Hutchinson [48], suggests that each species occupies a
position in 'niche space' defined by its resource requirements. One key pre-
diction of the BRH then, first noted by Elton [31], is that opportunities for
invasion should decrease as the number of resident species increases. This is
because niche space becomes filled up as species invade, leaving fewer
resources available for future invaders.
Niche-type explanations for invasions are still being promoted today [49],
but it is also important to note that such explanations are not universally
accepted [50, 51]. Recently, Chase and Leibold [52] have tried to recast the
niche concept to include more ecologically complex dynamics than just
resource use (see SCH below). Not only has the concept been questioned but
so has the pattern. The classic proposition that more diverse communities are
less invasible has subsequently been challenged, and remains an area of much
debate [53, 54]. More recent reincarnations of the BRH have focused on the
role of resident enemies (i.e., predators, parasites, or pathogens), and the def-