Trophic cascade

Under a trophic cascade refers to a through the food chain mediated change in the production of an ecosystem through the influence of predators ( predators ) to herbivores.

Background

Within an ecosystem, the production and sales material generally depends on Trophieniveau. Green plants have a higher production than herbivores, such as predators higher first-order, etc. This food pyramid is clear from the associated with each consumption process conversion losses. The organisms of the higher trophic are therefore limited in their production, normally by the food supply from the underlying Trophieniveau (ie their food or prey ). So there is more green plants than herbivores, more herbivores as predators, etc. The conditions in the production between the trophic usually stand to each other in a fixed ratio, typically about 1:10, which corresponds to 90 % loss during the transition to a higher level.

Notwithstanding this ratio is observed in many studied ecosystems that actually observed ratios of this model differ. The density of herbivores is then not limited here "from below" ( by their food ) but "from above" ( by their predators ) or regulated. This herbivores are rare, as would be expected according to the food supply. Is now the density of herbivores reduced below its expected value, the of them eaten green plants can better develop and build a higher biomass. This means indirect, through its influence on the herbivore, determined in such a system the robbers crop production with critical (instead of predators could, of course, diseases or parasites are ). The habitat includes, for example, then more plant biomass than another similar production in which there is not such an effect. This indirect effect exerted by the predators on primary producers, is called " trophic cascade ".

The influence of trophic cascades is actually of course not limited to ecosystems with three trophic. Even robbers higher levels ( second, third, etc. order predators ) can trigger through their influence on organisms of the amount under the same trophic cascade them levels. Would a Spitzenprädator the fourth level (ie, a robber, the other, usually smaller predator hunts ) greatly reduce the density of the Raiders of the underlying level, the herbivores may be more common than expected, and thus again reducing the plant biomass strong. Since in real ecosystems, the number of trophic energy reasons usually at three, four and more rarely with hardly ever is longer, but such effects play a much smaller role in practice.

Is in an ecosystem trophic cascade effect, one can prove this experimentally. This requires the density of the predator, from which extends the cascade, reduce experimental. In the case where a cascade is effective, not only the biomass should now be influenced its prey (ie, increase ), but also that in the trophic level including change significantly. For example, the stock of green plants could be significantly reduced if a predator such as is removed by hunting, from the system. Does not happen, no trophic cascade was effective. In this case it would be likely that the system is controlled from the bottom, for example, by the nutrient content, the much more limited production of the plants, as it fortune the herbivores in this case. Such " experiments" can also be unintentional when an ecosystem has been altered by human influence.

Case Studies

Trophic cascades have been demonstrated by ecologists in a variety of ecosystems, or at least made ​​probable. In a series of classic experiments Carpenter and Kitchell could in several adjacent small lakes turbid water with high Phytoplanktongehalt or clear water with little phytoplankton produce by the fact that they (the self- other fish, but no plankton eat ) the density of predators, increased or decreased. These results were confirmed in numerous other studies. On the flat rocky coast of St.Margarets Bay, Nova Scotia, it was shown that the carp harvesting of lobsters has led to the disappearance of the extensive kelp forests. Reason for this was the spread of sea cucumbers, which graze on the algae and had been previously held by the lobsters short. Although it is generally assumed that trophic cascades are more important and more common in aquatic ecosystems, there are also numerous examples of terrestrial systems

Importance

About the general influence of trophic cascades in ecosystems, there is no consensus within the environmental science. Although the existence of the phenomenon is generally hardly deny its relevance is unclear. While some researchers see cascades in almost all ecosystems at work, they are rather rare exceptions according to different view. Partly the influence of cascade was probably made while no such effects were detected in many other cases in numerous studies. As effects that counteract the action of cascades in natural systems, were among others identified: strong spatial heterogeneity of the habitat, highly interconnected food webs ( instead of simple food chains ), low productivity and nutrient contents, low efficiency of herbivore or predator species.