Ecological niche models and the geography of biological invasions: a review and a novel application - Invasive Plants: Ecological and Agricultural Aspects - page 39

Hypotheses supporting differences in niche occupancy
Limitations to dispersal in South America
The southern limit of RIFA may be imposed by limitations to dispersal, which
keep RIFA from occupying otherwise suitable habitats. There are no obvious
physical barriers that would preclude RIFA from dispersing southward. Rivers
pose only a minor obstruction to movement since RIFA can disperse and sur-
vive as a floating colony for several weeks and new colonies are established by
winged sexual females capable of dispersing several kilometers [23, 36].
Release from natural enemies in the US
RIFA may maintain populations in environmental conditions in its invaded
range that it does not in its native range due to release from adverse biotic
interactions. This explanation assumes that the ecological niche of RIFA
includes habitats found south of its native range, but biotic interactions exclude
it from these habitats. At least 30 species of natural enemies (including 18 spe-
cies of parasitic phorid flies) attack RIFA in South America [24]. Conversely,
only two or three natural enemies have been discovered in the US (and phorid
flies are being investigated as biological agents). Porter et al. [24] hypothe-
sized that an escape from natural enemies was responsible for observed differ-
ences in abundance of RIFA in North versus South America, although they did
not discuss the ability of RIFA to exploit different environmental conditions in
the US. Therefore, we cannot conclude from this study that enemy-release nec-
essarily allows RIFA to exploit different habitats in the US.
Lack of resistance to invasion in the US
Another mechanism akin to enemy-release is a lack of resistance to invasion
in the US by native ant faunas. There is evidence that chemical control pro-
grams, aimed at reducing the spread of RIFA, also decimated native ant popu-
lations, and consequently facilitated the spread of RIFA instead. The pesticide
reduced populations of both RIFA and native ants, but native ant populations
did not recover to their former dominant levels, while the abundance of RIFA
eventually exceeded native populations [23, 37].
Adaptation to new environments in the US
It is possible that the ecological niche of RIFA was not conserved when it
invaded the US, and invasive RIFA populations now are adapted to a new envi-
ronment. The founding populations of RIFA apparently passed through a bot-
tleneck that reduced genetic variation and altered the life history of invasive
RIFA relative to native RIFA, and resulted in differences in colony structure
between native and invading populations [38, 39].
These hypotheses are not exhaustive or mutually exclusive. Because detailed
study would be needed to fully evaluate them, we cannot conclude, based on
this study alone, which mechanisms are responsible for the differences in the
Forward- and Reverse-ENM predictions. However, the review and case study