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Salmon are essential resource for bears because the survival of these large mammals depends on the amount of fat they can deposit in the late summer and fall.Onece bears enter their dens in the early winter, they neither eat nor drink for up to seven months.Yet bears are not true hibernators so they must produce body heat to survive throughout the cold months.In addition, females give birth and milk their young during this time.
Because the bears' the survival is closely tied to their physical condition in the autumn, natural selection favors those that get the most nourishment out of the fish they eat.And bears exhibit two types of behavior to this end.First, to avoid interference from othe bears, they often carry captured salmon to the riverbank or into the forest before eating.Bears are relatively solitary for most of their adult lives, expect when finding partners in spring and summer.When they gather in reverside areas to feed on salmon, they become aggressive.These confrontations can be either relatively harmless, resulting in one bear stealing a fish from another, or violent, ending in serious injury or death to one of the bears.Carrying the carcass into the forest out of sight of other bears isis a way to prevent confrontation.
The second important behavior is that the bears often eat only the most nourishing part of the salmon.If the salmon densities are high, it takes less than a minute to capture a fish in a small river, and under these conditions of plenty, the bears rarely eat the whole fish.An analysis of more than 20,000 carcasses revealed that bears consume about 25 percent of each captured salmon, selectively eating only the parts highest in fat content, such as the eggs. After consuming the tastiest parts, bears abandon the carcass and return to the river to catch another fish.Thus, bears kill far more salmon than they eat.
Why is this unusual feeding behavior important for the vitality of the ecosystem?After all, in the absence of bears, the salmon would still die after laying their eggs, and their carcasses would be eaten by birds, fish and insects in the river, and flushed out to the ocean.By killing many of the fatter salmon, carrying the fish to the forest, and then abandoning the carcass with most of the body remaining, bears make a tremendous amount of food and nourishment available to riverside plants and animals that would not otherwise have access to this resource.The bears are truly ecosystem engineers.
