Measuring the ghost of competition: Insights from density-dependent habitat selection on the co-existence and dynamics of lemmings

When interspecific competitors resolve their co-existence by habitat segregation, their competition might, like a ghost, be invisible because the species occupy separate habitats. Population fluctuations should often bring the species into competition in jointly occupied habitats where their competition can be measured by habitat isodars (lines or planes of density where the expected fitness of individuals is the same in all occupied habitats). We tested the theory by calculating isodars for two species of lemmings with distinct habitat preferences. When population densities are high, both habitats are occupied by both species. But as densities decline, habitat isodars suggest that the joint dynamics of each species pass through a region where each occupies a separate habitat (the ghost of competition). The competition was asymmetrical. The density of collared lemmings in their preferred habitat was reduced as the density of brown lemmings increased in the same habitat. But collared lemmings had no direct competitive effect on brown lemmings. The interspecific effect from brown lemmings was comparable to – possibly even in excess of – intraspecific competition for habitat. The asymmetric competition for habitat yields spectacular new kinds of isolegs categorizing habitat competition between co-existing species. Although current evidence implicates competition, the patterns are also consistent with apparent competition driven by specialized predators. Regardless of whether lemming habitat use reflects true or apparent competition, the associated densitydependent differences in habitat preference are likely to have major consequences for the nonstable dynamics of lemmings and non-linear lemming isoclines.