A haunting legacy from isoclines: Mammal coexistence and the ghost of competition

Abstract

Many of our concepts about species coexistence are rooted firmly in the analysis of competitor isoclines whose slopes estimate the average magnitude of competition. Realistically, competition will vary among habitats, and habitat selection will be a major contributor to coexistence. Competition will vanish to zero (the ghost of competition) when species occupy completely separate habitats, even though the potential for competition remains high. Competitive potential can be estimated from slopes of absolute isolegs that define boundaries between specialized use of a single habitat and the joint occupation of one or more additional habitats. Interpretations of current theories suggest, however, that we may seldom be able to plot isolegs because they represent a wall of competition that species seldom cross. If so, isoclines bend sharply at the isoleg, and population dynamics are restricted to the ghost region. But when competition is resolved by habitat selection, isoclines bend gradually, and the wall of competition disappears. The isolegs become visible through analysis of habitat isodars, lines that represent the set of each species' density such that expected fitness is equivalent in each occupied habitat. Preliminary analyses of rodent isodars agree with theory, reveal the ghost, and confirm a central role for density-dependent habitat selection in competitor coexistence.