Ilkka Hanski and small mammals: from shrew metapopulations to vole and lemming cycles.
Résumé
Ilkka Hanski may be best known for his work on insect and metapopulation dynamics, but he also contributed significantly to small mammal research. In the early 1980s he became interested in shrew dynamics, energetics, and of course, shrew metapopulations. He aimed at understanding the population biological consequences of body size in different shrew species. Feeding habits and environmental stochasticity affect shrew species in profoundly different ways: due to their short survival time small species have high extinction rates but their dispersal and colonization capacity is high which enables them to survive as metapopulations. After Hansson and Henttonen reported the Fennoscandian gradients in vole dynamics in the mid-1980s, Hanski became interested in vole and lemming cycles. The first models on this were published with Henttonen and Hansson in 1991 where the roles of specialist and generalist predators were assessed. Later, the models were further developed with Korpimäki and Turchin, with model parametrization from Microtus biology and including both specialist mammalian predators as well as avian predators. A special case was the model with Henttonen on competing vole species with a shared predator (apparent competition), which was related to the long-term fading out of vole cycles in Finnish Lapland in the mid-1980s (which though returned in the early 2010s). Later Hanski became interested in the work of Sittler and Gilg in Greenland. Together they modelled the very simple vertebrate community and showed how stoats played a pivotal role in generating a population cycle in the collared lemming. In addition to these specific works, Hanski was leading collaborator in several reviews on small rodent cycles and predation. He intended to return to shrew biology, but that never realized. Hanski was a fearless field biologist, but he always aimed at understanding natural phenomena at more general, theoretical level.