The current extinction of many of Earth's large terrestrial carnivores has left some extant prey species lacking knowledge about contemporary predators, a situation roughly parallel to that 10,000 to 50,000 years ago, when naive animals first encountered colonizing human hunters. Along present-day carnivore recolonization fronts, brown (also called grizzly) bears killed predator-naive adult moose at disproportionately high rates in Scandinavia, and moose mothers who lost juveniles to recolonizing wolves in North America's Yellowstone region developed hypersensitivity to wolf howls. Although prey that had been unfamiliar with dangerous predators for as few as 50 to 130 years were highly vulnerable to initial encounters, behavioral adjustments to reduce predation transpired within a single generation. The fact that at least one prey species quickly learns to be wary of restored carnivores should negate fears about localized prey extinction.

Large herbivores affect the morphology and productivity of their food plants, which in turn affects food availability and foraging efficiency. These effects can occur directly, through tissue removal, or indirectly through additions of nutrients in dung and urine. Although influences of herbivores on plant communities are well established for grazers, less is known about the effects of browsers. We studied how tree morphology (number of shoots, shoot mass, and mean yearly height increase) and production of winter browse of Scots pine (Pinus sylvestris) and birch (Betula pubescens and B. pendula) responded to simulated moose herbivory in Swedish boreal forests. We simulated browsing, defecation, and urination to represent effects of four levels of moose density in each of eight exclosures. To be able to study the whole range of moose impact and obtain better mean estimates from the ecosystem perspective, the eight exclosures were placed along a forest productivity gradient. Treatments were applied for three consecutive years. For both birch and pine, the mean average number of shoots per tree and the mean yearly height increase decreased with increasing biomass removal. Mass of individual shoots increased for birch, but not for pine. The mean yearly production of winter browse (per m 2 ) was significantly reduced by biomass removal. The responses of birch followed a threshold model predicting no effect up to a low to moderate level of biomass removal, and thereafter a linear decrease, although the model describing constant linear decrease not could be ruled out (model selection based on Akaike Information Criterion). The response of pine followed a threshold model. We concluded that winter browse might be in shortage at high browsing pressure over extended time, but most woody plants eaten by moose should sustain browsing pressure at moose densities common in many areas in Sweden.

The seasonal composition of and the annual variation in the diet of the brown bear Ursus arctos in the Pasvik Valley, northeastern Norway, were estimated based on the analysis of 137 bear scats. The importance of moose Alces alces and reindeer Rangifer tarandus in the diet was given special attention, because results from Russia suggest that brown bears are generally more carnivorous in the north. Ungulates, especially adult moose, comprised the most important food item for bears in the Pasvik Valley during spring and summer, contributing 85 and 70% of the Estimated Dietary Energy Content (EDEC), respectively. During autumn, when the bears have to build up fat reserves and increase lean body mass for hibernation, berries were the most important food item, contributing 49% of the EDEC, but ungulates were still important, contributing 30% of the EDEC. Insects and vegetation were of low importance in all seasons. The proportion of ungulates in the diet of brown bears in the Pasvik Valley was considerably higher than farther south in Scandinavia, and this regional difference is important concerning bear and moose management in northern areas.

To understand how large herbivores interact with their food resources and the ecosystem, it is necessary to understand plant growth responses to biomass loss. Tolerance to biomass loss, and the ability to regrow new photosynthetic tissue after herbivore attack, is one such important mechanism. Here we experimentally studied how year-round browsing corresponding to four different levels of moose, Alces alces , population density affected total annual browse biomass production, which also serves as a proxy for production of new photosynthetic tissue, in birch (Betula pubescens and B. pendula ) and Scots pine, Pinus sylvestris , along a habitat productivity gradient. Our results revealed that habitat productivity index was the most important variable determining annual browse biomass production for birch, and that a high moose density and biomass removals of 25 Á40% annually actually could stimulate birch above-ground productivity in more productive sites. Contrary to birch, pine was not affected by habitat productivity index, and appeared to have much lower regrowth capacity than birch. For pine, percent biomass removed was the most important factor affecting annual browse biomass production, which decreased with increasing levels of biomass removal. We also tested if birch and pine responded to biomass removal by allocating more resources to browse biomass production at in the lower portion of the stem (0 Á0.5 m), which was protected from biomass removal in winter. Birch did not allocate more resources to biomass production in the lower portion of the stem, whereas pines subjected to high biomass removal had more biomass in the lower portion of the stem than those protected from losses. Our study thus shows that plant responses can be linear or non-linear along the range of herbivore densities we studied, and that responses can change from negative to positive along underlying habitat productivity gradients.

To understand how large herbivores interact with their food resources and the ecosystem, it is necessary to understand plant growth responses to biomass loss. Tolerance to biomass loss, and the ability to regrow new photosynthetic tissue after herbivore attack, is one such important mechanism. Here we experimentally studied how year-round browsing corresponding to four different levels of moose, Alces alces , population density affected total annual browse biomass production, which also serves as a proxy for production of new photosynthetic tissue, in birch (Betula pubescens and B. pendula ) and Scots pine, Pinus sylvestris , along a habitat productivity gradient. Our results revealed that habitat productivity index was the most important variable determining annual browse biomass production for birch, and that a high moose density and biomass removals of 25 Á40% annually actually could stimulate birch above-ground productivity in more productive sites. Contrary to birch, pine was not affected by habitat productivity index, and appeared to have much lower regrowth capacity than birch. For pine, percent biomass removed was the most important factor affecting annual browse biomass production, which decreased with increasing levels of biomass removal. We also tested if birch and pine responded to biomass removal by allocating more resources to browse biomass production at in the lower portion of the stem (0 Á0.5 m), which was protected from biomass removal in winter. Birch did not allocate more resources to biomass production in the lower portion of the stem, whereas pines subjected to high biomass removal had more biomass in the lower portion of the stem than those protected from losses. Our study thus shows that plant responses can be linear or non-linear along the range of herbivore densities we studied, and that responses can change from negative to positive along underlying habitat productivity gradients.