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This is Scientific American — 60-Second Science. I'm Jason G. Goldman.
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Predators like wolves affect their ecosystems by eating their prey. But a more subtle impact involves fear. Predators also terrify prey species. And when, for example, elk are hiding, they don't spend as much time eating leaves. The impact of a predator down through the food web all the way to plants is called a "trophic cascade."
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 foraging and hide instead.
"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 risky habitats, like river valleys or gorges."
Marine scientist Douglas Rasher, from the nonprofit Bigelow Laboratory for Ocean Sciences in Maine.
"So it got me thinking that maybe these shallow habitats might be the place where sharks have their most pronounced effects on the ecosystem."
At one time, researchers did not even think trophic cascades 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.
Here's how it works: when the tide rises, sharks make hunting raids into the shallow lagoons. The fish stop eating and hide instead. But during low tide, the predators are isolated in deeper waters, unable to access the reef-enclosed lagoons. That's when the fish can safely graze.
"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."
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.
For Rasher, these findings mean that the question is no longer whether sharks influence the dynamics of reef plant and animal communities, but instead under what conditions they do so.
"Predators can have important impacts on coral reefs, but we need to look carefully to determine when and where those important impacts exist."
Thanks for the minute for Scientific American — 60-Second Science. I'm Jason G. Goldman.
参考译文
这里是科学美国人——60秒科学。我是杰森·古德曼。
有一分钟时间吗？
像狼这样的捕食者通过吃掉猎物来影响生态系统。但还有一个更为微妙的影响与“恐惧”有关。捕食者也会恐吓猎物。比如，麋鹿在躲起来时所花费的时间没有吃叶子的时间多。捕食者沿食物链往下一路直达植物的影响被称为“营养级联”。
同时，南太平洋斐济群岛附近珊瑚礁上的鱼，通常以生长在珊瑚礁上的海藻为食。但是，当礁鲨从更深的海域出现时，这些小鱼最好停止觅食，马上躲起来。
“思考一下其他生态系统，比如狼对生态系统的影响并不会在每时每处都发挥作用。它们碰巧成为了在河谷或峡谷等危险栖息地生活的最广为人知的动物。”
缅因州非盈利比奇洛海洋科学实验室的海洋科学家道格拉斯·拉舍尔说道。
“所以这让我想到，这些浅层栖息地可能是鲨鱼对生态系统影响最为显著的地方。
研究人员甚至一度不相信营养级联存在于真实世界中，许多人至今仍在争论鲨鱼是否能推动珊瑚礁上的营养级联。通过观察斐济沃图阿海洋保护区的礁群，拉舍尔和团队发现，鲨鱼的确会影响珊瑚礁上的植物生长，而方法是吓跑草食性鱼类，使其无法进食。
以下是鲨鱼的做法：涨潮时，鲨鱼在较浅的环礁湖中捕食。鱼会停止进食并躲藏起来。但是在退潮时，捕食者被隔离在更深海域，无法进入被礁石围住的环礁湖。此时鱼儿们可以安全进食。
“这个系统中的所有鱼都对涨潮和落潮时间非常敏感。如果你坐在那里，观察涨潮落潮的这种变化，你会看到，开始退潮时，鱼儿们似乎知道，它们会冲出去，尤其是大型食草性动物。这的确可以预见。”
结果是，珊瑚礁的深层部分被鱼儿吃掉的面积更大，而海藻则会在珊瑚礁的较高地带自由生长，鱼儿只有在努力避免成为鲨鱼的午餐时才会接触这部分海藻。这项研究结果发表在《科学报告》期刊上。
对拉舍尔来说，这些发现意味着，问题不再是鲨鱼是否会影响珊瑚礁植物和动物群落的活力，而是它会在何种情况下产生影响。
“捕食者可以对珊瑚礁产生重要影响，但我们需要仔细观察，以确定这些重要影响发生的时间和地点。”
谢谢大家收听科学美国人——60秒科学。我是杰森·古德曼。
译文为可可英语翻译，未经授权请勿转载！
重点讲解
重点讲解：
1. emerge from 出现；露出；
例句：The missiles emerge from the underbelly of the transport plane.
导弹从那架运输机的底部飞了出来。
2. play out （使）（戏剧性的事件）逐渐发生；（使）展；
例句：And I honestly do not know how this is going to play out.
老实说，我不知道这会怎样发展下去。
3. go out （潮水）退去；
例句：The tide was going out.
正在退潮。
4. no longer 不再；已不；
例句：He's no longer a depressive character.
他不再是一个消沉的人。
