Reisewitz Shauna E, Estes James A, Simenstad Charles A
Institute of Marine Sciences, Long Marine Laboratory, 100 Shaffer Road, Santa Cruz, CA 95060, USA.
Oecologia. 2006 Jan;146(4):623-31. doi: 10.1007/s00442-005-0230-1. Epub 2005 Sep 27.
Although trophic cascades-the effect of apex predators on progressively lower trophic level species through top-down forcing-have been demonstrated in diverse ecosystems, the broader potential influences of trophic cascades on other species and ecosystem processes are not well studied. We used the overexploitation, recovery and subsequent collapse of sea otter (Enhydra lutris) populations in the Aleutian archipelago to explore if and how the abundance and diet of kelp forest fishes are influenced by a trophic cascade linking sea otters with sea urchins and fleshy macroalgae. We measured the abundance of sea urchins (biomass density), kelp (numerical density) and fish (Catch per unit effort) at four islands in the mid-1980s (when otters were abundant at two of the islands and rare at the two others) and in 2000 (after otters had become rare at all four islands). Our fish studies focused on rock greenling (Hexagrammos lagocephalus), the numerically dominant species in this region. In the mid-1980s, the two islands with high-density otter populations supported dense kelp forests, relatively few urchins, and abundant rock greenling whereas the opposite pattern (abundant urchins, sparse kelp forests, and relatively few rock greenling) occurred at islands where otters were rare. In the 2000, the abundances of urchins, kelp and greenling were grossly unchanged at islands where otters were initially rare but had shifted to the characteristic pattern of otter-free systems at islands where otters were initially abundant. Significant changes in greenling diet occurred between the mid-1980s and the 2000 although the reasons for these changes were difficult to assess because of strong island-specific effects. Whereas urchin-dominated communities supported more diverse fish assemblages than kelp-dominated communities, this was not a simple effect of the otter-induced trophic cascade because all islands supported more diverse fish assemblages in 2000 than in the mid-1980s.
尽管营养级联效应——顶级捕食者通过自上而下的作用力对营养级逐渐降低的物种所产生的影响——已在多种生态系统中得到证实，但营养级联效应在其他物种和生态系统过程方面更广泛的潜在影响却尚未得到充分研究。我们利用阿留申群岛海獭（海獭属）种群的过度捕捞、恢复及随后的崩溃，来探究海带森林鱼类的数量和饮食是否以及如何受到将海獭与海胆和肉质大型藻类联系起来的营养级联效应的影响。我们在20世纪80年代中期（当时海獭在其中两个岛屿数量众多，在另外两个岛屿数量稀少）和2000年（此时海獭在所有四个岛屿都变得稀少之后），对四个岛屿的海胆（生物量密度）、海带（数量密度）和鱼类（单位努力捕获量）进行了测量。我们的鱼类研究聚焦于岩鱼（六线鱼属），它是该区域数量上占主导的物种。在20世纪80年代中期，海獭种群密度高的两个岛屿拥有茂密的海带森林、相对较少的海胆以及丰富的岩鱼，而在海獭稀少的岛屿则出现了相反的模式（大量海胆、稀疏的海带森林以及相对较少的岩鱼）。到了2000年，在海獭最初稀少的岛屿，海胆、海带和岩鱼的数量总体上没有变化，但在海獭最初数量众多的岛屿，已转变为无海獭系统的典型模式。尽管由于强烈的岛屿特异性影响，很难评估这些变化的原因，但在从20世纪80年代中期到2000年期间，岩鱼的饮食发生了显著变化。虽然以海胆为主的群落比以海带为主的群落支持更多样化的鱼类组合，但这并非海獭引发的营养级联效应的简单结果，因为所有岛屿在2000年比20世纪80年代中期都支持更多样化的鱼类组合。
