Biology Reference
In-Depth Information
neighboring grassland plants. Neighborhood plants included species
belonging to four major functional groups of native grassland herbs: C
3
and C
4
grasses, legumes, and nonlegumes. In both field and greenhouse
experiments, hawksbeard growth declined strongly as the diversity of
species forming its neighborhood increased. Diversity of different func-
tional groups of plants likewise appeared to limit the success of yellow
starthistle (
Centaurea solsticialis
) in microcosms of Mediterranean grassland
species in California (Dukes 2001, 2002).
Somewhat similar results have been obtained for communities of
aquatic organisms. Working with communities of sessile marine inverte-
brates in New England, Stachowicz et al. (1999) created artificial commu-
nities of one to four native invertebrates by placing together tiles, each 2
by 2 cm in size, on which individual species were growing. In an array of
five by five tiles, five additional tiles with a 1-week-old recruit of an alien
ascidian (
Botryllus schlosseri
) were interspersed. Survival of the alien ascid-
ian decreased in linear fashion from more than 60% to less than 20% with
an increase in diversity of the overall community.
Shea and Chesson (2002) developed a model that integrates many of
these relationships. They proposed that factors such as disturbance inter-
act with community organization to influence invasibility. For any given
level of supply of basic resources, the richness of a local community in
native species and the integrated pattern of resource use they exhibit lim-
its the ability of alien species to invade.When some factor has prevented
such a local community from reaching its greatest diversity, or when dis-
turbance interferes with resource use, opportunities for alien invasion
exist. In larger geographical regions, where major differences exist in basic
resource supply for communities, native communities contain more
species, but factors that create disturbance or interfere with community
organization also result in invasion by more aliens (fig. 7.1).These factors
may also interact with escape of alien species from natural enemies, as dis-
cussed later.
Another model, based on competition for resources by species in an
assemblage, also showed that scale can modify the invasibility of a com-
munity (Byers and Noonberg 2003). For a given number of resource
types, this model shows that increasing the number of species reduces
invasibility. Increasing the number of resource types, on the other hand,
increases invasibility.Thus, in both models (Shea and Chesson 2002; Byers
and Noonberg 2003), resource availability patterns strongly influence the
likelihood of invasion success.
