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

host environment, proposing that a lesser chance of substantial niche overlap exists under
these circumstances (Li and Moyle 1981). Infringement of non-indigenous species upon
niche space already allocated to native members of a native assemblage can disrupt the
structured evolution of that community. The effects of introduced shiner species on
native congenerics have been well documented: Mathur (1977), Cross et al. (1983), and
Walser et al. (2000).
The genetic capability of a species to adapt to a new area may determine whether
an introduced species will become established. Differences between coexisting species
today are primarily due to past competitive interactions (Harper et al. 1961, Surat et al.
1982). The genetic makeup of a newly introduced species population depends on the
genetic variability of those founding individuals. In a study of sympatric shiner species,
the genetic differences between populations in different drainages were hypothesized to
be due to founder populations from different genetic refugia (Dowling et al. 1989).
Genetic change, occurring within the founder populations introduced to new ecosystems,
is one mechanism involved in the common “lag phase” observed after an introduction
(Mooney and Cleland 2001). If the species is genetically capable of evolving
characteristics beneficial to the new environment, a subsequent population boom may
occur. This may lead to the competitive exclusion of a similar native species. Species in
the native assemblage may also evolve in response to an established introduced species
(Mooney and Cleland 2001). If this doesn’t occur, the native species may experience
dramatic population declines or extirpations due to the inability to withstand competitive
forces (Elton 1958, Mack et al. 2000). Geographically isolated species will be exposed to
different forces of selection, which often leads to differential adaptations (Harper et al.
12