Since 1972 we have monitored population densities of red-backed voles (Myodes rutilus) ,deer mice (Peromyscus maniculatus) ,and meadow voles (Microtus spp.) in the pristine boreal forest of the Kluane region of Canadas Yukon.Boom-bust dynamics is characteristic of voles,which follow 3-4 year population cycles,but not of deer mice.In this poster we describe the mechanisms driving the dynamics of this suite of small mammals in northern Canada.Red-backed voles are a dominant species in these forests and have displayed a 3-4 year cycle since 1972.For most of this time period specialist predators like weasels (Mustela) were largely absent from this area,and predation thus seems unlikely to be a major cause of population changes.We can predict density changes in these voles from berry production and a combination of food supply fluctuations and social dynamics appear to drive changes in numbers.Deer mice are primarily seed-eaters and their numbers have changed dramatically.From 1972 to 1984 they were a dominant species in these forests,but in 1990 they completely disappeared for 5 years and were at low abundance until 2005 when they returned to moderate abundance.We have no explanation for this disappearance.Microtus voles are more patchily distributed in the Kluane boreal forest,primarily because they are grass-eaters in a forest habitat.The 4 species of Microtus in this area-M.oeconomus,M.pennsylvanicus,M.miurus,M.longicaudus-independently oscillate in numbers,but the combined result is a clear 3-4 year cycle.