The Dynamics of Habitat Use in a Guild of Ozark Minnows

The underlying causes of habitat use in an assemblage of Ozark minnows were investigated in an observational field study. Six habitat variables were used: depth of water, current speed, substrate size, lateral distance from stream edge, vertical position in the water column, and presence in pool, riffle, or raceway. Analysis focused on the six numerically dominant species that used a common set of pool microhabitats and thereby formed a natural habitat guild. Observations on habitat use were conducted so as to record simultaneous habitat use by all species at each sample point. This technique made possible an analysis of interspecific interactions over habitat use. The six species were segregated primarily by the vertical position variable (X overlap = 0.160). Subdivision of pool habitat data into two depth zones yielded subsets that were homogeneous with respect to all habitat variables except vertical position. Within these data subsets, interactions between species pairs over use of vertical position were examined. Species whose distributions shifted significantly toward those of heterospecifics were considered associations while distributions that shifted away were considered dissociations. Of 60 possible vertical interactions, 33 were significant, and of these approximately two—thirds were associations and one—third were dissociations. The subdivision of data also permitted an analysis of horizontal interactions between species pairs over the sample points. This analysis showed that guild members were significantly clumped into multispecies groupings. Analysis of species—pair interactions within these groupings showed that of 60 possible interactions, 37 were significant and of these approximately half were associations and half were dissociations. A pattern of vertical and horizontal interactions was evident between the two depth zones. In the shallower zone, associations were the predominant interaction in the vertical dimension, but the number of associations and dissociations was nearly equal in the horizonal dimension. In the deeper zone, vertical associations still predominated, though to a lesser extent, and associations predominated in the horizontal dimension. Several explanations for these patterns were proposed. Some associative interactions may facilitate opportunistic feeding behaviors, and others may promote interspecific schooling as an antipredation tactic. Dissociations probably promote habitat segregation among guild members. A balance of these two types of interactions may be critical to guild maintenance. The frequent interactions over habitat use indicate that habitat use among guild members is highly variable; the overall pattern of segregation results from a combination of habitat selection as modified by interspecific interactions over time and space. This view contrasts with the more conventional interpretation of segregational patterns in which shifts or differences in resource use are assumed to be relatively static and interactions and competition among species are reduced or eliminated. the dynamic nature of habitat partitioning observed in this minnow assemblage is consistent with the requirement of ecological flexibility in the characteristically unstable and unpredictable environments of streams.