Environmental Engineering Reference
In-Depth Information
nearby locations (e.g. Europe) or from remote locations whose climate (and therefore
the invader's niche) matches what is found in Britain (Figure 2.3b). Note the small
number of alien plants from tropical environments; these species usually lack the
frost-hardiness required to survive the British winter.
Not all invaders cause obvious harm; indeed some ecologists distinguish exotic
species that establish without signifi cant consequences from those they consider
'truly invasive' - whose populations expand 'explosively' in their new environment,
with signifi cant impacts for indigenous species. Managers need to differentiate
among potential new invaders both according to their likelihood of establishing
should they arrive in a new region and in relation to the probability of having dra-
matic consequences in the receiving community. The likelihood of establishment is
largely dependent on niche requirements, relevant at the ecological scale of indi-
vidual organisms, and the subject of the present chapter. Community consequences
will be dealt with in Chapter 9 (Section 9.5) because these are relevant at the larger
scale of communities and ecosystems. Management strategies to control invaders
that have achieved pest status usually require an understanding at the intermediate
ecological scale of population dynamics and will be covered in Chapter 6.
In this section I describe a key process for predicting where invaders will do well
- ecological niche modeling (Section 2.2.1). Next I address the question whether it
is fundamental or realized niches that are being modeled (Section 2.2.2). Finally,
I consider how human disturbance may sometimes facilitate invasions
(Section 2.2.3)
2.2.1 Ecological
niche modeling -
predicting where
invaders will succeed
Ecological niche modeling has been applied to a diverse range of invading organisms
from aquatic and terrestrial plants, to fi sh, wood-boring beetles, butterfl ies and
vultures. As an example of the approach, let's turn to studies of four invasive plants
in North America. Hydrilla verticillata , an aquatic weed from South America, affects
lakes by forming dense canopies that often shade out native vegetation, interfere
with swimming and boating, and can clog power-generation intake pipes. Sericea
lespedeza , or Chinese bush clover, is a perennial legume native to eastern Asia that
can reduce or eliminate competing native plants, as can Elaeagnus angustifolia , or
Russian olive, a small thorny shrub native to southeastern Europe and western Asia.
Finally, Alliaria petiolata is a cool-season herb from Europe, known as garlic mustard
because the leaves give off a garlic odor when crushed. Once introduced to an area,
garlic mustard outcompetes many native springtime wild fl owers, and can adversely
impact rare native insects, such as the white butterfl y Pieris virginiensis , through the
loss of their food plants.
The fi rst step in modeling the ecological niches of these species is to obtain precise
locations ('geo-referenced') at which individuals are known to occur (from published
accounts and herbarium records). Next the niches are modeled using a computer
program that relates physicochemical characteristics at the known occurrence
points to those of points sampled randomly from the rest of the study region. This
plant study invoked a large number of 'niche dimensions' including elevation, slope
and aspect, annual averages of temperature (diurnal range, mean, minimum and
maximum), number of frost days and wet days, precipitation and solar radiation.
The best model for each species in its native range is then projected onto a map of
North America, using geo-referenced data on the physicochemical variables across
the continent, to provide predictions of potential distributions (Figure 2.4). These
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