An Experimental Study of Vertical Habitat Use and Habitat Shifts in ...

exhibited by species involved; the presence of congeneric or taxonomic relatives in the
host assemblage; hybridization and introgression; number of invading individuals in the
source population and the frequency of those invasions; predator and parasite interactions
in the host environment; and anthropogenic disturbances.
The effects of introduced species on an ecosystem can be intensified if the
environment is under-saturated (Moyle 1986, Angermeier 1995, Angermeier and
Winston 1998, Case 1991). In theory, the greater the number of species in an
environment, the smaller the amount of existing niche space available to immigrants. As
the number of fish species decreases in an aquatic environment, the ability of immigrants
to become established increases (Magnuson 1976, Perry et al. 2002). The New River is
historically depauperate in native fish species (Jenkins et al. 1972, Hocutt et al. 1978,
Cincotta 1999, Wellman 2004). Isolated aquatic systems with natural barriers to
migration often have low levels of species diversity (Jenkins et al. 1972, Magnuson 1976,
and Sheldon 1988, Angermeier 1995). If niche partitioning is important in structuring
species assemblages then partitioning should increase as species number increases
(Schoener 1974). Therefore, resistance to invasion should also increases as species
richness increases (Elton 1958, Magnuson 1976, Angermeier and Winston 1998, Walser
et al. 2000). Similarly, disparity of freshwater habitat may limit resources and increase
the importance of resource partitioning, making competitive interactions much more
important (Ross 1986).
Released niche space and decreased diversity provides an opportunity for the
colonization of invasive species (Hocutt and Hambrick 1973). Simberloff (1981) stated
that areas possessing vacant niches due to impoverished species numbers are susceptible
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