TOPIC 海胆对褐藻的影响

　　版本一：关于一种海洋生物(动物?)对一种海洋植物(类似海藻)数量的影响。首先说这种动物会吃这个植物 (记不太清了)，彻底或有选择性地破坏植物, 还有。。。人们发现当这种生物增多的时候，植物会减少或增长变缓。第二段提出不能简单的认为，植物的减少完全是这种生物的原因。貌似说，因为在这种生物的 predator增多的时候，植物并没有增多。所以人们提出另一个原因，环境变化。接着，又说在某个特定的地方，这种生物的数量和植物的数量确实密切相关。后面的。。。忘了。

　　版本二：某种动物(urchin?)对seawed的影响的观点。然后提出了一些weakness。。

　　忘了忘了。。。记不住啊。。

　　版本三：bio话题，说的一个海洋生物类似贝壳类的动物的数量和周围环境kelp以及各种捕食者的关系;海里的某种ur什么和kelp的关系。

　　Kelp forest

　　Classic studies in kelp forest ecology have largely focused on trophic interactions (the relationships between organisms and their food webs), particularly the understanding and top-down trophic processes. Bottom-up processes are generally driven by the abiotic conditions required for primary producers to grow, such as availability of light and nutrients, and the subsequent transfer of energy to consumers at higher trophic levels. For example, the occurrence of kelp is frequently correlated with oceanographic upwelling zones, which provide unusually high concentrations of nutrients to the local environment.This allows kelp to grow and subsequently support herbivores, which in turn support consumers at higher trophic levels. By contrast, in top-down processes, predators limit the biomass of species at lower trophic levels through consumption. In the absence of predation, these lower level species flourish because resources that support their energetic requirements are non-limiting. In a well-studied example from Alaskan kelp forests,sea otters (Enhydralutris) control populations of herbivorous sea urchins through predation. When sea otters are removed from the ecosystem (for example, by human exploitation), urchin populations are released from predatory control and grow dramatically. This leads to increased herbivore pressure on local kelp stands. Deterioration of the kelp itself results in the loss of physical ecosystem structure and subsequently, the loss of other species associated with this habitat. In Alaskan kelp forest ecosystems, sea otters are the keystone species that mediates this trophic cascade. In Southern California, kelp forests persist without sea otters and the control of herbivorous urchins is instead mediated by a suite of predators including lobsters and large fishes. The effect of removing one predatory species in this system differs from Alaska because there is redundancy in the trophic levels and other predatory species can continue to regulate urchins.However, the removal of multiple predators can effectively release urchins from predator pressure and allow the system to follow trajectories towards kelp forest degradation. Similar examples exist in Nova Scotia,South Africa, Australia and Chile.The relative importance of top-down versus bottom-up control in kelp forest ecosystems and the strengths of trophic interactions continue to be the subject of considerable scientific investigation.

　　The Disappearing Kelp Forest（Extension of JASON XIV: From Shore to Sea — Unit 4）

　　If you were to dive into the cool coastal waters of the Channel Islands, you might find yourself swimming through a magnificent underwater forest of enormousalga called kelp. Kelp is a type of seaweed that can grow almost two thirds of a meter per day. Kelp forests provide a home for hundreds of species, like sea urchins, brittle stars, and sea slugs.

　　Generally, an increase in sea urchin density at Johnson’s Lee South is associated with a decrease in kelp density. However, other factors in the ecosystem such as disease outbreaks, pollution, and other human activities may alter this relationship.. After severe El Niño events, large numbers of sea urchins appear in the kelp forest, which reduce kelp density.

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