Global warming and attendant permafrost thawing are altering plant growth, species composition, and terrestrial habitats in tundra communities. Climate change may also be responsible for anomalous dynamics of lemmings in recent years, and for apparent changes in habitat use. We do not know how widespread differences in habitat selection might be, nor do we know whether any differences are associated directly with habitat change or mediated indirectly via lemming dynamics. We attempt to answer these questions in three ways. First, we document habitat change in the central Canadian Arctic by examining long-term patterns in shrub growth and changes in habitat classification over a 14-year interval on 12 permanent study plots. Second, we construct isodars, lines that equalize fitness in paired habitats, to demonstrate that the relative abundances of brown and collared lemmings in sedge versus hummock habitats have changed through time. Third, we combine the results on changes in habitat and habitat selection in a path analysis to evaluate the relative roles of direct, versus indirect, influences on lemming habitat preferences.
Results/Conclusions
Shrub growth, and the proportion of sedge-dominated tundra, increased on our study plots in concert with global warming. Coincident with these effects, both lemming species increased their density-dependent preference toward hummock habitat. Indirect paths toward the changed preference, mediated via intra- and inter-specific competition, were greater in magnitude than were direct paths associated with habitat change. It thus appears that global warming is simultaneously altering arctic habitats, species interactions, and the selection of habitat by lemmings. New patterns in the use of space by these keystone species is also likely to alter selection gradients that forecast increased convergence in habitat use through time. Evolutionary consequences include the potential for these keystone species to compartmentalize lower trophic levels. Changes in habitat, and lemming habitat selection, can thus yield far-ranging consequences not only for lemming distribution and dynamics, but also for entire arctic ecosystems.