Invasive plants: the process within wetland ecosystems - Invasive Plants: Ecological and Agricultural Aspects - page 98

expansion is from a high-density core surrounded by peripheral, low-density
zones. Expansion from this highly reproductive (sexual or clonal) core is often
constrained by resource limitations, such as topography and other biota.
Exotic species are not necessarily superior competitors; they are usually
separated by physiological requirements (e.g., deep soil water, nitrate, and
light) and could depress abundance and fecundity of exotic annual plants after
overcoming recruitment limitations. Exotics may dominate not because they
are physiologically better competitors, but rather as a result of prior distur-
bance and low dispersal abilities [35].
Effects on biological diversity
Major alterations and loss of biodiversity have occurred as invasive species
competitively suppress native species populations and alter habitats and
ecosystems [36-40]. Invasive species have been involved, often by influenc-
ing habitat conditions and availability, among about half of the species now
considered rare, threatened, endangered, or extinct [41]. More species diverse
ecosystems have greater temporal stability and exhibit positive diversity-pro-
ductivity relationships with less susceptibility to success of invasive species
[40]. Invasion rates increase markedly, however, once invasion progresses and
species diversity decreases.
Establishment of non-native species with the expansions of the ranges of a
species results in many ecological effects. Suppression of native species occurs
through predation, parasitism, disease, and especially competition. Invasive
species may hybridize with congeneric native species and thereby alter the
gene pool of the native species. Resulting hybrids and the invasive species can
displace native species (e.g., Spartina alterniflora [42]). Hybridization or
altering of selection pressures by changes in habitat can be powerful drivers of
evolutionary change. In particular, hybrids can be isolated reproductively from
native species, such as by polyploidy, and be highly competitive [42-43].
Competitive interactions
The effects of invasive plants on native species are enormously complex
because of the many interactive pathways that can and do occur [7, 44].
Usually exploitative competition (i.e., negative interactions resulting from uti-
lization of resources by a consumer and subsequent limitation of those
resources to other consumers) is given as a primary interactive mechanism
[45-46]. However, exploitative competition among plants is only one of many
interactive mechanisms. Non-exploitative mechanisms (i.e., not involving
resource consumption) can occur simultaneously [47-51]. For example, cer-
tain invasive species can release chemical compounds that have allelopathic
effects on selective species of the indigenous plant community [52]. The native