Inferring Competitive Behavior from Population Census and Habitat Data

Abstract

Theories of density-dependent habitat selection predict that competitive interference should produce curvilinear relationships when the population densities in adjacent habitats are graphed against each other. We tested the theory by manipulating population densities of brook trout (Salvelinus fontinalis) in four separate natural-stream enclosures containing flat and riffle habitats. Brook trout distributions in streams typically reflect a size-based dominance hierarchy. As predicted by theory, interference among individuals was revealed by curved regressions of density in the paired habitats (isodars). Brook trout exhibited no strong preference for either habitat at low population densities. Habitat preferences emerged clearly at higher densities, apparently reflecting the distribution of suitable sites in the habitat pairs. Predicted bodysize distributions between flat and riffle were mirrored consistently by those observed in the population. The isodar analysis, based only on density data, revealed the competitive behaviors that underlie brook trout distributions in real streams.

Keywords:

• density-dependent habitat selection
• isodar
• brook trout
• Salvelinus fontinalis
• interference competition
• preemptive interference
• competitive behavior