The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 11

Superior competitor hypothesis (SCH)
David Tilman at the University of Minnesota proposed a mechanistic explana-
tion of species competition [67-69], which can potentially incorporate modern
notions of the niche [52]. He suggested that an invader could establish if it was
more efficient at obtaining limiting resources (symbolized as R*) than resident
species, resulting in suppression or extirpation of the less-fit native species.
This type of explanation can be seen as an alternative hypothesis to the RFH, as
R* explicitly assumes equilibrium dynamics. According to Grime [60; p. 40],
competition determines the addition or exclusion of species in productive com-
munities, while competition is unimportant in highly-infertile soils. Tilman [69,
70], however, believed competition to remain important under infertile condi-
tions. Under these conditions, the competitive success of a species is achieved
through the capacity of a species to reduce limiting nutrients in soils to level
under which other species cannot grow and reproduce. Grime [60] promoted
critical reexamination of Tilman's R* theory under field conditions, because
experiments supporting the R* theory could not discriminate between alterna-
tive mechanisms, such as nutrient loss via herbivory. Experiments of Wedin and
Tilman [71] showed that slow-growing plants in infertile conditions could
reduce the level of nitrogen to extremely low levels. However, they were unable
to show the extent to which the success of slow-growing plants was due to the
capturing of nitrogen at low levels or the ability to retain nitrogen because the
plants resisted to herbivores and pathogens [60; p. 43]. Shea and Chesson [49]
discussed Tilman's R* rule, predicting that invasion is favored when a resident
species' resource requirements (i.e., R*) is greater than an invader's R*. This
can happen in two situations: (1) when an invader has a higher resource acqui-
sition rate than that of resident species, and (2) if an invader has lower mainte-
nance requirement than that of resident species. It is important to note that
explanations of invader success involving SCH may still require one or more of
the other hypotheses presented in this paper. What resource-based explanations
often fail to address is why a species is a superior competitor. For example, an
NIS escaped from its natural enemies may appear the better competitor, when
really the mechanism lies in the release from enemies, not a change in resource
dynamics. Chase and Leibold [52] attempt to address this gap by incorporating
not only impacts on resource dynamics but also the effects of predation and par-
asitism. They also extend their discussion to include multi-species systems and
evolutionary processes. Nevertheless, the resource dynamics of invaders may
be contextual, relying on subsequent explanations that incorporate the impact
of the abiotic environment, other organisms and evolutionary dynamics.
Enemy release hypothesis (ERH)
As noted earlier, biological invasions result in a sub-sampling of invaders. Not
only does this sub-sampling occur at the level of species and populations, but