Sharks Rule the Reef’s Underwater Food Chain
珊瑚礁表面食物链的独裁者：鲨鱼
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."
像狼这种猎食者通过捕食猎物来影响生态圈。但是这其中也有更加细微的担忧掺和在里面，捕食者可能会对被捕食者的种群造成负面影响，例如，当麋鹿藏匿起来时，它们在这段时间是不进食植物叶子的。猎食者通过食物网总是对植物造成影响称之为“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 foraging and hide instead.
与此同时，通常来说，南太平洋裴济群岛附近躲藏在珊瑚礁里面的鱼类以生长在珊瑚礁表面的海藻为生。但是当礁鲨潜入更深的海水时，这些鱼类避免被捕食的最好办法就是躲藏起来放弃觅食。
"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.”
“想想其他的这种生态系统，像狼，在它们的生态系统中，不会每时每刻在任何地方猎食，而是经常会出现在所谓的危险地带，如河谷或者峡谷当中”
Marine scientist Douglas Rasher, from the non-profit Bigelow Laboratory for Ocean Sciences in Maine.
海洋科学家Douglas Rasher，来自缅因州非营利性机构，Bigelow海洋科学实验室。
“So it got me thinking that maybe these shallow habitats might be the place where sharks have their most pronounced effects on the ecosystem."
“所以这种情况让我认为，也许这些浅海栖息地是鲨鱼对生态系统影响最大的区域。”
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.
有一段时间，科学家们甚至不认为营养级联在现实世界中存在着，许多人仍然在争论鲨鱼是否能驱动珊瑚礁上的营养级联。通过观察斐济Votua海洋保护区的珊瑚礁群落，Rasher和他的团队发现鲨鱼的确会对生长在珊瑚礁表面的植物造成影响，即通过吓走食草性鱼类从而远离它们。
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.
原理是这样的：当涨潮的时候，鲨鱼会在浅水湖区狩猎，这些鱼会停止进食藏匿起来。而当低潮期间，这些猎食者会被隔离在深水区，无法靠近这些珊瑚礁封闭的泻湖，这样这些鱼就可以安全地进食这些藻类。
"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."
“在这个系统中，所有的鱼类都会对何时涨潮，何时退潮极其敏感。如果你坐在那儿观察那些过渡期，尤其是那些大型的食草鱼类，你会发现，当开始退潮的时候，它们似乎就知道了，然后蜂拥而出，完全在预料之中!”
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，深处的礁被吃的面更广，礁上面更深处的海藻却生长的更加茂盛，原因是它们更容易接触到且在专注于进食的时候，不被鲨鱼当作美味的午餐吃掉。这个发现被发表在《科学报道》上。
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.
对Rasher来说，这些发现意味着这个问题不再是鲨鱼是否会影响到珊瑚礁表面植物的动态以及动物群落，而是在什么样的条件下，它们会那样做。
"Predators can have important impacts on coral reefs, but we need to look carefully to determine when and where those important impacts exist."
“猎食者可能会对珊瑚礁造成重大影响，然而我们需要仔细确定这些重大的影响存在于何时何地。”
来源:科学美国人
