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

Introduction
Non-native fishes negatively impact native populations through multiple
pathways (Meffe 1985, Ross 1991, Mooney and Cleland 2001), including competition for
food or space (Hocutt and Hambrick 1973, Mathur 1975, Walser et al. 2000). Following
ecological theories of competitive exclusion (Hardin 1960), limiting similarity
(MacArthur and Levins 1967) and niche (Pielou 1972, Pianka 1974), competitive
interactions are expected between non-native species and closely-related native species.
Aquarium-based studies of vertical segregation in minnows have emphasized expected
similarities within behaviors and habitat use of closely-related species (i.e., an historical
ecology emphasis on phylogenetic constraints on habitat use, Gorman 1988, 1992). Also,
researchers have emphasized morphological correlates on use and segregation of habitats
among closely-related species, such as mouth position and its deterministic influence on
benthic, pelagic, or surface foraging (Mendelson 1975, Surat et al. 1982, Page and
Swofford 1984). Alternatively, some closely-related species integrate as mixed-species
shoals, such as minnows (Cyprinidae), and this strategy may improve foraging success or
predator avoidance (Mendelson 1975, Magurran and Pitcher 1983, 1987). Multispecific
aggregations suggest a historic legacy of strong behavioral relationships amongst
cyprinid guilds. Both mutual responsiveness and selective forces, required to reduce
competition and maintain species identity, are required for mixed-species shoals to
function (Mendelson 1975). However, little is known of the occurrence and
consequences of mixed-species shoals of native and non-native minnows. Given recent
conservation concerns of fish introductions and impacts of non-natives (Rahel 2000),
additional research needs focused on native/non-native species interactions.
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