Biology Reference
In-Depth Information
success or failure of introduced fishes to become established in California
depends on whether or not abiotic conditions are appropriate, rather than
biotic characteristics of the recipient community.
On the other hand, in more stable and predictable environments, com-
munities do exhibit some degree of coevolved organization. This is
reflected in the presence of symbiotic relationships that are to some
degree species-specific (Richardson et al. 2000).These vary from species-
specific mutualisms, parasite-host interactions, and predator-prey relation-
ships to less specific food chain, pollination, seed dispersal, or other rela-
tionships in which interactions are limited to a certain subset of the total
community members. For plants, mycorrhizal fungi and nitrogen-fixing
microorganisms are symbionts of varying specificity that can influence the
success of alien plants.Absence of these symbionts may prevent the estab-
lishment of an alien in a new region. Host-specialized herbivores, preda-
tors, and parasites constrain the growth and productivity of many species
in their native regions. These relationships suggest that success of some
aliens is due to release from biotic constraints that exist in communities
of their native region—the so-called enemy release hypothesis. This is
also evident in the success of biological control strategies that involve
identification and importation of natural enemies of alien species from
their native regions. We examine the enemy release hypothesis in more
detail below.
Species Diversity and Invasion Resistance
In 1958, Charles Elton hypothesized that communities rich in species
were more resistant to invasion than less diverse communities. His
hypothesis has proved to be stimulating but very difficult to test. The
determinants of species diversity in biotic communities are complex and
include processes operating on temporal scales ranging from long evo-
lutionary time to short ecological time. A long history of evolution
under conditions favorable to growth of organisms favors high species
diversity. Tropical rain forests and coral reefs, for example, are ancient
ecosystem types that have accumulated species over long evolutionary
time. Other determinants of diversity are biogeographic factors, such as
geographic isolation. Islands, where only small populations can develop,
are subject to colonization and extinction dynamics that set a low evo-
lutionary limit to species richness. Still other determinants involve
events on an ecological time scale, especially disturbances such as fire,
flood, or drought. An intermediate frequency or intensity of distur-
