The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 18

Human commensal hypothesis (HCH)
The idea that the most successful invaders are commensal with humans and
thereby benefit from human disturbance and other activity is perhaps one of
the oldest and most cited 'rules of thumb' in invasion ecology. Indeed, many
of the most infamous invasive species, such as rats, pigeons, nonindigenous
garden and agricultural weeds, and feral populations of domestic animals, are
closely associated with humans and have evolved to benefit from human activ-
ity. There is no doubt that humans play a significant role in the repeated intro-
duction and spread of particular invaders, along with habitat modification
practices (e.g., conversion of old growth forest to agricultural land) much to
the benefit of particular invaders. The HCH is becoming regionalized (at least,
in North America) due to the fact that the HCH is altering “historical ranges of
variability (HRV)” in organizing (disturbance) factors such as fire and floods.
By modifying the HRV (via the HCH), species in these systems are no longer
adapted to soil/microclimatic conditions, reproduction is vastly affected, and
the propagule pressure from invaders is amplified in terms of the success of
these propagules (Tim Seastedt, personal communication ). However, invasion
of relatively undisturbed areas is an increasing trend that is poorly explained
by the HCH [130, 131]. Moreover, while introductions of NIS are by defini-
tion human-mediated, the spread, increase in abundance, and displacement of
native species exhibited by some invaders, are often not obviously facilitated
by human activity. For example, the overwhelming success of the Argentine
ant in North America is likely due to the formation of supercolonies.
Supercolonies are a collection of individual colonies that benefit from a
decrease in inter-colony aggression owing to a reduction in genetic diversity
and thus an increase in relatedness [132].
Environmental manipulation hypothesis (EMH)
Baldwin [133] proposed the weapon of mass destruction (WMD) hypothesis
to describe the role of allelochemicals in plant invasions. However, we suggest
the less pejorative and more inclusive term 'environmental manipulation
hypothesis' (EMH). This hypothesis suggests that invaders succeed because
they are able to alter their environment to suit their own needs at the expense
of native species. The literature on allelopathy is perhaps the best explored
example of this. One such example is the flavonoid (-)-catechin released by
Centaurea maculosa, which is a potent allelopathic agent against bunchgrass
species in North America [134] . Interestingly, the level of (-)-catechin in C.
maculosa soil from North America was twice the amount detected in soil sup-
porting C. maculosa in its native Europe. A related invader, Centaurea diffusa,
also has inhibitory effects on several grasses ( Festuca ovina, Koeleria
laerssenii and Agropyron cristatum ) in invaded North American habitats, com-
pared to those from its native Eurasia [135]. Vivanco et al. [136] isolated 8-