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Sharks Rule the Reef's Underwater Food Chain

Predators² like wolves affect their ecosystems⁴ by eating their prey⁵. But a more subtle impact involves fear. Predators also terrify prey species. And when, for example, elk⁶ are hiding, they don't spend as much time eating leaves. The impact of a predator¹ down through the food web all the way to plants is called a "trophic cascade⁷."

像狼这样的掠夺者，通过吃猎物来影响自己的生态系统。但更微妙的影响是恐惧。掠食者也害怕猎物。例如，当麋鹿躲藏时，不会花太多的时间吃叶子。捕食者通过食物网的影响被称为“营养级联”。

Meanwhile, fish at a coral reef near the Fiji archipelago in the South Pacific generally graze on the seaweeds that grow on the reef. But when reef sharks emerge from deeper waters, it's best to quit foraging⁸ and hide instead.

与此同时，南太平洋斐济群岛附近的珊瑚礁上的鱼，通常以珊瑚礁上生长的海藻为食。但是，当鲨鱼从更深的海域出现时，这些鱼类会停止觅食而选择躲藏起来。

"Thinking about these other ecosystems, like wolves, their effects in their ecosystems don't play out in all places in all times. They happen to be most pronounced in risky⁹ habitats, like river valleys or gorges¹⁰.”Marine¹¹ scientist Douglas Rasher, from the non-profit Bigelow Laboratory for Ocean Sciences in Maine.

“考虑到狼等其他生态系统的例子，鲨鱼在生态系统中的作用在任何时候都不会在所有地方发挥出来，而是在高风险的栖息地，如河谷或峡谷中发挥作用。”缅因州非盈利Bigelow海洋科学实验室海洋科学家Douglas Rasher说。

“So it got me thinking that maybe these shallow habitats might be the place where sharks have their most pronounced effects on the ecosystem³."

“所以这让我思考，这些浅海栖息地，可能是鲨鱼对生态系统影响最大的地方。”

At one time, researchers did not even think trophic cascades¹² even existed in the real world, and many still debate whether sharks can drive trophic cascades on coral reefs. By observing reef communities in Fiji's Votua Marine Reserve, Rasher and his team discovered that sharks do in fact influence plant growth on the reefs—by scaring the herbivorous fish away from eating them.

研究人员曾经一度认为，现实世界中还没有营养级联，许多人仍然在争论鲨鱼是否能驱动珊瑚礁上的营养级联。Rasher和团队通过观察斐济Votua海洋保护区的珊瑚礁群落，发现鲨鱼的确影响了珊瑚礁上的植物生长- 通过吓走草食性鱼类，进而鱼类无法吃到珊瑚。

Here's how it works: when the tide rises, sharks make hunting raids into the shallow lagoons¹³. The fish stop eating and hide instead. But during low tide, the predators are isolated¹⁴ in deeper waters, unable to access the reef-enclosed lagoons. That's when the fish can safely graze.

以下工作原理：当潮水涨起时，鲨鱼会在浅水湖中狩猎。鱼停止进食和躲藏。但在低潮期间，食肉动物被隔离在更深的水域，无法进入珊瑚礁封闭的泻湖。而此时鱼可以安全吃食。

"All the fish in this system have a very keen sense of when the tide is coming up and when the tide is going out. If you just sort of sit there and watch through the transition, you see, particularly with the large herbivores, as the tide starts to drop they seem to know it, and they jet. And it's really predictable."

“这个系统中的所有鱼，都对潮汐起落有非常敏锐的感觉，如果坐在那里观察，你会发现，特别是对于大型食草动物，浪潮开始下降时，它们似乎知道这一点，就会喷涌而出，这是真的可以预见的。“

The upshot is that the deeper parts of the reef are more extensively grazed, while seaweed grows more freely on the higher parts of the reef that are accessible to the fish only when they are focused on avoiding becoming a shark's lunch. The results are in the journal Scientific Reports.

结果是礁的较深部分，被更加广泛地放牧，而海藻只能在避免成为鲨鱼午餐的时候，才能自由生长。该研究结果发表在《科学报告》杂志上。

For Rasher, these findings mean that the question is no longer whether sharks influence the dynamics¹⁵ of reef plant and animal communities, but instead under what conditions they do so.

对于Rasher来说，这些发现意味着，鲨鱼不再是否影响礁石植物和动物群落的动态，而是在何种条件下才会起作用。

"Predators can have important impacts on coral reefs, but we need to look carefully to determine when and where those important impacts exist."

“掠夺者可以对珊瑚礁产生重要影响，但我们需要仔细研究，以确定这些重要影响的存在时间和地点。”