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 segreg ation, their competition might, like a ghost, be invisible because the spec ies occupy separate habitats. Population fluctuations should often bring th e species into competition in jointly occupied habitats where their competi tion can be measured by habitat isodars (lines or planes of density where t he expected fitness of individuals is the same in all occupied habitats). W e tested the theory by calculating isodars for two species of lemmings with distinct habitat preferences When population densities are high, both habi tats are occupied by both species But as densities decline, habitat isodars suggest that the joint dynamics of each species pass through a region wher e each occupies a separate habitat (the ghost of competition). the competit ion was asymmetrical. The density of collared lemmings in their preferred h abitat was reduced as the density of brown lemmings increased in the same h abitat. But collared lemmings had no direct competitive effect on brown lem mings. The interspecific effect from brown lemmings was comparable to - pos sibly even in excess of - intraspecific competition for habitat. The asymme tric competition for habitat yields spectacular new kinds of isolegs catego rizing habitat competition between co-existing species. Although current ev idence implicates competition, the patterns are also consistent with appare nt competition driven by specialized predators. Regardless of whether lemmi ng habitat use reflects true or apparent competition, the associated densit y dependent differences in habitat preference are likely to have major cons equences for the nonstable dynamics of lemmings and non-linear lemming isoc lines.