Ecological niche models and the geography of biological invasions: a review and a novel application - Invasive Plants: Ecological and Agricultural Aspects - page 29

Predicting invasions with ecological niche models: approach,
assumptions, and limitations
Approach
Predicting the distribution of invasive species using ENM has been described
elsewhere [8, 11], so here we provide only an overview of the technique and
its applications. ENM typically refers to niche modeling approaches that use
point occurrence data (in the form of latitude-longitude coordinates) and geo-
graphic datasets (in the form of digital maps that describe broad-scale envi-
ronmental characteristics) to produce an approximation of the ecological
niche. The ecological niche is defined as the range of environmental conditions
that delineate where a species is able to maintain populations without immi-
gration [12]. At the scales of analysis typical of ENM, modeled niche dimen-
sions are limited to those relevant to geographic ranges (e.g., macro-climate,
topography, and landcover). Ecological niche models, when projected onto
regions other than the one from which they were developed, can estimate the
potential range of an invader or forecast shifts in the distributions of species
under scenarios of environmental change. Niche models also have been used
to prioritize conservation goals, study the distribution of species, and investi-
gate patterns and mechanisms of biodiversity within the region from which
they are based [13].
Assumptions and limitations
ENM relies on a key assumption that the distribution of a species represents a
manifestation of the ecological niche. Recall that Hutchinson [14] suggested
that the boundaries of the ecological niche are set by the values at which a par-
ticular environmental factor becomes limiting and beyond which the species
cannot survive. ENM assumes that these limits on the ecological niche deter-
mine the geographic boundaries of the range of that species.
Hutchinson further defined the realized niche as the niche that a species
actually occupies because of the influence of biotic interactions that exclude
the species from parts of its ecological niche. Further, because limitations to
dispersal will prevent species from occupying all suitable habitats, they may
be absent from locales in addition to those excluded by biotic or environmen-
tal limitations. For these reasons, it is unlikely that a species will occupy its
entire niche. In terms of geographic ranges, it is unlikely that the current dis-
tribution of a species will encompass its entire potential distribution.
Therefore, niche models based on the current geographic distribution of a spe-
cies will reflect only a fraction of the potential conditions in which a species
could exist. Because niche models rely almost exclusively on abiotic factors as
modeled niche dimensions, the model will not include all parameters pertinent
to the distributions of species.