|
This influx of energy from the ocean can have an extraordinary effect on freshwater systems because the nutrient composition of the fish and their densities are so great. For example, an adult male chum salmon on the spawning grounds contains an average of 130 grams of nitrogen, 20 grams of phosphorus（磷）and more than 20,000 kilojoules of energy in the form of protein and fat. Multiplying the average nutrient composition of salmon by the total number of returning fish, we found that a 250-meter reach of stream in southeastern Alaska received more than 80 kilograms of nitrogen and 11 kilograms of phosphorus in the form of chum salmon tissue in just over a month. ( n" X! F4 @3 W+ o* l- G2 G* d
/ m# O" W, q9 W/ A( `
The behavior of the bears that feast on the salmon-brown bears-is the other part of the equation. Salmon are a crucial resource for the bears because the survival and reproductive success of these large mammals depend on the amount of fat they can deposit in the late summer and fall. Once bears enter their dens in the early winter, they neither eat nor drink for up to seven months. Yet bears are not true hibernators-their body temperature stays above surrounding levels-so they must produce body heat to maintain metabolic（新陈代谢的）functions throughout the cold months. In addition, females give birth and lactate during this time.
/ j, W* i0 S4 v2 h2 Y4 _" a! u
# i# d5 u, [" D. }/ ~ Because the bears’ survival and reproductive success are 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 behaviors to this end. First, to avoid interference from other bears, they often carry captured salmon to the stream bank or into the streamside forest before eating. Bears are relatively solitary for most of their adult lives, save for a few weeks of courtship in spring and summer. When they aggregate in riverside areas to feed on salmon, they become aggressive. These confrontations can be relatively benign, resulting in one bear stealing a fish from another, or violent, ending in serious injury or death to a bear or its offspring. Carrying the carcass into the forest out of sight of other bears is a way to avoid confrontation.
% q* m) g. y7 J) r: N. c # K% k" X$ G* B. T8 C
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 a bear less than a minute to capture a fish in a small stream, and under these conditions of plenty, the bears rarely eat the whole fish. An analysis of more than 20,000 carcasses revealed that bears consumed about 25 percent of each captured salmon, selectively eating only the parts highest in fat content, such as the eggs. In fact, it is common for bears to carry a carcass to the stream bank and not eat a bite after finding it is a male salmon or a female that has already spawned her eggs.
9 Z# {( r* O Q5 e
( f0 B: `, c3 n8 Z, ~/ U After consuming choice morsels（一口, 少量）, bears abandon the carcass and return to the stream to spear another fish. Thus, bears kill far more salmon than they eat. At a small stream in southeastern Alaska, for instance, we observed a 200-kilogram female brown bear capture more than 40 chum salmon during several feeding periods over the course of eight hours. She removed over 143 kilograms of salmon from the stream but consumed only a small fraction of it. j2 S2 S/ c: Y! j# K3 `
/ e% H4 h) R' r
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 following spawning, and their carcasses would be eaten by birds, fishes and insects in the streams, decomposed by microbes and flushed out to the ocean. By killing many of the fatter salmon, carrying the nutrient-loaded fish to the forest, and abandoning the carcass with most of the species remaining, bears make a tremendous amount of food and nutrients available to streamside plants and animals that would not otherwise have access to this resource. The bears are truly ecosystem engineers: they deliver marine-derived nutrients to the river side system.
6 }; {* j+ {2 _
# `$ g% }. s' |3 N4 O9 j The spread occurs because many different animals make use of the protein and fat in the abandoned fish. Flies, beetles, and other insects colonize the carcasses almost immediately and deposit their eggs there. Gulls, ravens, crows, jays, magpies, mink, marten, and other species of birds and mammals readily and often quickly make a meal of the carcasses. 外语学习网; g9 k; P8 J- }. i" B3 x
* y2 }& y- Q L6 Q9 W; D$ a A creature does not have to consume the salmon directly to benefit from the ocean’s gift. The insects that colonize carcasses are devoured in turn by wasps, birds and other insect-eaters, including small mammals such as voles and mice that eat not only the insects but the carcasses themselves. We have found that densities of insect-eating songbirds can be higher along salmon streams than along waterways that do not support spawning salmon, suggesting that the bird communities respond to the abundance of insects produced by the harvest of salmon carcasses. 考试用书
2 m/ h' e$ T5 b/ P7 f 0 M$ A$ w4 D5 A" B; }
In the longer term, the feeding of all these animals, together with leaching by rain and microbial activity, breaks down the carcasses, making the nitrogen, phosphorus and other nutrients available to streamside plants. Plant growth in northern forests is often limited by either nitrogen or phosphorus, and thus the bears’ feeding activities may influence growth rates of many plant species in these areas. Not surprisingly, one study found that growth of Sitka spruce, the dominant streamside tree in the area, was three times greater along salmon streams than along nonsalmon streams. In several studies, researchers correlated the amount of salmon-derived nitrogen or carbon directly with the movements of bears, providing further evidence that their feeding behavior is the mechanism that delivers the salmon nutrients to riverside plants. |
发表于 2012-8-14 10:30:44
楼主