Ecological Studies of Wolves on Isle Royale - Annual Report 1994-1995

Peterson, Rolf

In 1994-1995 the wolf population at Isle Royale remained stable, with 16 animals counted in January 1995, while the moose population increased to an estimated 2,400 animals. Annual wolf mortality (18%) was near the long-term average and reproduction fell back to the low level prevalent since the late 1980's. The wolf population is in an important transition as one generation is replaced by the next. After the 1995 winter study there remained only one wolf (a male) that had ever reproduced, yet there were 11 wolves less than four years old with ample numbers of males and females ready to assume alpha positions which permit reproduction. In the past year (January 1994 to January 1995) two aging alpha wolves died and another established alpha wolf perished during the 1995 study. Only one of the three territorial packs reproduced, yielding two surviving pups. Isle Royale is now free of important wolf diseases and wolf food supply (old moose) will increase steadily during the 1990's. We hope to resolve questions about the impact of genetic isolation on reproduction by monitoring the next generation. If the next generation of wolves reproduces at rates similar to the last, then genetic decay will probably remain the only reasonable explanation for low reproduction which is preventing expansion of wolf numbers. Growth of the moose population was furthered by "easy winters" during the past two years. There was little starvation mortality in spring 1994 and little is expected in 1995. During the 1995 winter study we recorded a few moose deaths from malnutrition and accidents, but wolf predation was responsible for almost 90% of the moose mortality we recorded. The wolf decline of the 1980's and 1990's revealed the importance of wolf predation in the dynamics of the Isle Royale "food chain". Released from top-down control by wolves, moose have now increased to historic high levels. Meanwhile, tree-ring studies of forest trees heavily browsed by moose have revealed that tree growth in the regenerating forest declined inconcert with the moose increase. The ecological significance of top carnivores in terrestrial food chains has probably been underestimated because they have been extirpated or reduced in number by human actions.