Indigenous Fishes of Western North America and the Hypothesis of Competitive Displacement: Meda fulgida (Cyprinidae) as a Case Study

Abstract

ern North America by the nonnative red shiner (Cyprinella lutrensis) seems linked with the dramatic decline of a threatened cyprinid (spikedace, Medafulgida) native to the Gila River subbasin. The mechanism by which red shiner impacts spikedace is unknown. Two hypotheses have been offered: displacement of the native through competitive interaction with invader, and replacement of native by nonnative as a result of environmental perturbation. To ascertain whether spikedace was being actively displaced by red shiner, we compared niche requirements of each in syntopy, allotopy, and disjunct allopatry. Fishes were collected by seining one to three sites in each of six different stream reaches; and current velocity, substrate particle size, and water depth were measured at each site. Red shiner occupied similar microhabitat whether allopatric, allotopic, or syntopic with spikedace. Spikedace occupied the same microhabitat when allopatric or allotopic to red shiner. However, spikedace syntopic with red shiner displayed a niche shift into currents significantly swifter than those selected when in isolation. Displacement of spikedace by red shiner suggests negative interspecific interactions potentially detrimental to the indigenous species.