The population dynamics of boreal mammals differ strikingly from those of mammals in temperate and tropical ecosystems in their extraordinary fluctuations in abundance (Elton 1924). These fluctuations lead to strong top-down direct effects in which herbivores reduce the biomass of their preferred foods, such as birch and willow, and predators reduce the biomass of herbivores (Chapter 13; Sinclair et al. 2000). These effects are clearly demonstrated in experiments that exclude herbivores or their predators. Some authors have argued that bottom-up influences of food supply on herbivores are negligible because food augmentation to herbivores in the presence of predators had no detectable effect in reducing herbivore decline (Sinclair et al. 2001). Several members of the mammalian herbivore guild are also important as a human subsistence resource. Dynamics of moose (Alces alces) and snowshoe hare (Lepus americanus) can be altered by human harvest. Overexploitation by humans may reduce moose populations to densities where they can be predator-limited—the so-called predator pit (Messier 1994). In this chapter, we present information on dynamics of some mammalian herbivores in the Alaskan boreal forest and potential drivers that are responsible for these dynamics. We omit discussions of the dynamics of porcupines (Keith and Cary 1991), red squirrels (Boonstra et al. 2001a), and beavers (Donkor and Fryxell 1999), as studies of these species have not been conducted in Alaska’s boreal forests. Moose are thought to have arrived in Alaska during the Illinoian glaciation, about 400,000 yr B.P. (Pewe and Hopkins 1967). They may have retreated to refugia in central Alaska during subsequent glacial advances (Peterson 1955) and expanded at times when climate was warmer. Moose populations in North America have more than doubled over the past 30–40 years, to approximately 890,000 animals (Kelsall 1987). The Koyukuk River drainage in the northern interior, for example, is presently known for its large moose populations. However, the oral tradition of moose hunting in the Koyukuk is relatively recent. Native elders recall that, in their youth, moose were extremely rare and that moose did not figure prominently in the local subsistence economy until the 1930s.
Microsite and climatic controls of tree population dynamics: an 18-year study on cliffs