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

noting that the abundance of some species is more susceptible to, and thus regulated by,
environmental factors (Freeman et al. 1988). This is mainly due to morphological
features and the ability of benthic fish to hold position more efficiently than water
column species during increased flow. There is no doubt an abundance of instances
where environmental conditions have disallowed introduced species to colonize new
environments. However, it is very difficult to document an attempt at colonization by a
species if that species fails to become established before its presence can be recorded
(Williamson and Fitter 1996).
Suitable habitat availability can determine whether or not a species becomes
established and persists in a new environment (Courtenay and Hensley 1980).
Competitors will eventually exclude one another from habitats that are homogeneous
with respect to a limiting resource (Dueser and Porter 1986, Murray 1986). The number
of fish species an aquatic system can hold at equilibrium depends greatly on habitat size
and complexity (Harper et al. 1961, Magnuson 1976). For example, increased
environmental variability has been correlated with a decrease in niche overlap in
sympatric desert lizard species (Pianka 1974). Habitat complexity is often lower in small
aquatic environments than in large ones. Increased susceptibility to local extinctions is a
common result of this phenomenon (Magnuson 1976). With respect to introduced
species, mandatory habitat characteristics may already be utilized or completely absent in
the host environment. Thus, if an introduced species requires a particular habitat
characteristic colonization may not be feasible in the new area.
Low watershed connectivity or the degree of geographical isolation can limit the
ease of which an invasive species can reach a particular stream segment through natural
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