Biologists who study nature's food chain agree aboutthe basic links in the chain: sunlight provides fuel forplants, plants are consumed by herbivores, andherbivores are eaten by carnivores. But researchersaren't sure about all the factors that control thechain.

研究自然食物链的生物学家一致认为食物链有一个基本的关联：阳光为植物提供养料，植物被食草动物吃，食草动物被食肉动物吃。但研究人员并不确定控制食物链的所有因素。

Traditionally, they have thought that food-chainpopulations are regulated from the bottom up.According to this idea, the availability of plants as food determines the size of animalpopulations in an area. But a study of a food chain in Isle Royal National Park, an island in LakeSuperior, has shown that food-chain control can work in both directions.

习惯上，他们认为食物链上的种群数量受到自下而上的控制。根据这种观念，可食用植物的面积决定了这个区域动物种群的大小。但位于苏必利尔湖上一个小岛上的皇家公园对食物链进行了一项研究，结果显示食物链的控制是双向的。

In the island's food chain, balsam firs are eaten by moose, which, in turn, are eaten by wolves.When researchers tracked the effects of the moose and wolves on the balsam firs, they actuallyfound that the number of wolves in the area affected the growth of the firs. During the study,researchers recorded the width of the tree rings of balsam firs. Each ring marked a year of thetree's growth.

在这个岛上的食物链中，香脂冷杉被驼鹿吃，反过来，驼鹿又被狼吃。当研究人员追踪驼鹿和狼对香脂冷杉的影响时发现，这片区域狼的数量实际上也会影响冷杉的生长。在研究过程中，研究人员记录了冷杉的年轮的宽度。每个年轮代表冷杉树一年生长了多少。

A narrow ring indicated that the tree's growth had been suppressed that year. The narrowrings appeared only after wolf populations decreased, allowing a greater number of moose tosurvive and eat more balsam firs. The researchers concluded that wolves play an important rolein maintaining the complex and delicate balance of the island's food chain. This food chainappears to be regulated from the top as well as from the bottom.

狭窄的年轮表示那一年树的生长受到抑制。但狭窄的年轮只出现在狼的数量减少之后。狼群数量减少使更多的驼鹿存活下来，它们会吃掉更多的冷杉树叶。研究人员总结到，狼群在保持这个岛上食物链的复杂性和微妙的生态平衡上扮演着重要的角色。因此食物链即受到来自底部的控制，也受到来自顶部的控制。