Trophic cascade

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A trophic cascade is understood as a change in the production of an ecosystem mediated via the food chain through the influence of predators on herbivores .

background

Within an ecosystem, the production and the metabolism generally depend on the trophic level . Green plants have a higher biomass production than herbivores, this higher than first-order predators etc. This food pyramid results from the conversion losses associated with every consumption process. The organisms of the higher trophic levels are therefore usually limited in their own production by the food supply from the trophic level below (i.e. their food or prey). So there are more green plants than herbivores, more herbivores than predators, etc. The ratios in production between the trophic levels are usually in a fixed relationship to one another, typically about 1:10, which corresponds to 90% loss when moving to a higher level.

Deviating from this relationship, however, one observes in many examined ecosystems that the actual relationships deviate from this model. The density of herbivores is then not limited or regulated "from below" (through their food), but "from above" (through their predators). As a result, herbivores become rarer than would be expected from the food supply. If the density of the herbivores is now reduced below their expected value, the green plants they eat can develop better and build up a higher biomass. That means: indirectly, through their influence on the herbivores, the predators have a decisive influence on the plant production in such a system (instead of predators there could of course also be diseases or parasites). The biotope then contains z. B. more plant biomass than another comparable productivity in which such an effect does not exist. This indirect effect that the predators have on primary producers is called the “trophic cascade”.

The influence of trophic cascades is of course not limited to ecosystems with three trophic levels. Predators of higher levels (predators of the second, third, etc. order) can also trigger trophic cascades through their influence on organisms of the respective levels below them. If a top predator of the fourth level (i.e. one predator who hunts the other, usually smaller predators) would greatly reduce the density of predators of the level below, the herbivores could become more frequent than expected, and thus the plant biomass could again be greatly reduced. Since in real ecosystems the number of trophic levels is mostly three, less often four and hardly ever higher for energetic reasons, such effects play a much smaller role in practice.

If a trophic cascade is effective in an ecosystem, this can be proven experimentally. To do this, one has to experimentally reduce the density of the predator from which the cascade starts. In the case where a cascade is effective, not only should the biomass of its prey be affected (i.e. increase), but that at the trophic level below should also change significantly. For example, the population of green plants could decrease significantly if a predator, e.g. B. by hunting, is removed from the system. If this does not happen, no trophic cascade has been in effect. In this case it would be more likely that the system is controlled from below, e.g. B. by the nutrient content, which limits the production of the plants much more than the herbivores can in this case. Such “experiments” can also take place unintentionally when an ecosystem has been altered by human influence.

Case studies

Trophic cascades have been demonstrated by ecologists in a variety of ecosystems. In a series of classic experiments, Carpenter and Kitchell were able to create turbid water with high phytoplankton content or clear water with little phytoplankton in several adjacent small lakes by increasing or decreasing the density of predatory fish (which themselves eat other fish but not plankton) . These results have been confirmed in numerous other studies. On the flat rocky coast of St.Margarets Bay, Nova Scotia, it has been shown that fishing for lobsters has led to the disappearance of the vast kelp forests. The reason here was the spread of sea ​​cucumbers that graze the seaweed. The sea cucumbers were kept short by the lobsters for so long. It is believed that trophic cascades are more significant and occur more frequently in aquatic ecosystems than in terrestrial systems. At least numerous examples show that.

Trophic cascades can also be triggered by behavior changes. In landscapes of western North America, such as in Yellowstone National Park was observed around for decades a continuous decline of riparian vegetation of the American aspen ( Populus tremuloides ), for which, among other factors, strong grazing by elk was -Hirsche responsible that by grazing of seedlings prevented the rejuvenation. Since the resettlement of the wolf in 1994/1995, a recovery of the riparian forest has been observed. The vegetation structure of the area also depends on the presence of the wolf, although as a predator it only has an indirect effect on it. The large herbivores such as elk or bison are kept in constant motion by the wolves and no longer stand in one and the same place for days. They no longer eat every young tree shoot and avoid these habitats because they do not want to fall victim to the wolves. The cascade effect is effective here, even though the wolves can actually prey only a few elk.

meaning

There is still no agreement within ecological science about the general influence of trophic cascades in ecosystems. Although the existence of the phenomenon is generally not disputed, its relevance is unclear. While some researchers see cascades at work in almost all ecosystems, others believe that they are rare, exceptional cases. In numerous studies, the influence of cascades was made likely, while in many other cases no such effects could be demonstrated. As effects that counteract the action of cascades in natural systems, u. a. identified: strong spatial heterogeneity of the habitat, strongly networked food webs (instead of simple food chains), low productivity and nutrient content, low efficiency of herbivore or predator species.

Individual evidence

  1. ↑ Stephen R. Carpenter, James F. Kitchell: Consumer Control of Lake Productivity. In: BioScience. Vol. 38, no. 11 (How Animals Shape Their Ecosystems), 1988, pp. 764-769.
  2. ↑ Michael T. Brett, Charles R. Goldman: A meta-analysis of the freshwater trophic cascade. In: Proceedings of the National Academy of Sciences USA. Vol. 93, 1996, pp. 7723-7726.
  3. ↑ Overview In: JK Pinnegar, NVC Polunin, P. Francour, F. Badalamenti, R. Chemello, ML Harmelin-Vivien, B. Hereu, M. Milazzo, M. Zabala, G. D'Anna, C. Pipitone: Trophic cascades in benthic marine ecosystems: Lessons for fisheries and protected-area management. In: Environmental Conservation. 27 (2), 2000, pp. 179-200.
  4. ↑ Donald R. Strong: Are Trophic Cascades All Wet? Differentiation and Donor-Control in Speciose Ecosystems. In: Ecology. Vol. 73, No. 3, 1992, pp. 747-754.
  5. ↑ J. Halaj, DH Wise: Terrestrial trophic cascades: trickle how much do they? In: American Naturalist. 157 (3), 2001, pp. 262-281.
  6. ↑ Cristina Eisenberg, S. Trent Seager, David E. Hibbs (2013): Wolf, elk, and aspen food web relationships: Context and complexity. Forest Ecology and Management 299: 70-80. doi: 10.1016 / j.foreco.2013.01.014 .
  7. ↑ John W. laundre, Lucina Hernandez, Kelly B. Altendorf (2001): Wolves, elk, and bison: reestablishing the "landscape of fear" in Yellowstone National Park, USA Canadian Journal of Zoology 79 (8): 1401-1409. doi: 10.1139 / z01-094
  8. ↑ Emma Marris: Ecology: The fairy tale of the wolf. on: Spektrum.de , April 9, 2014.
  9. ↑ Jonathan B. Shurin, Elizabeth T. Borer, Eric W. Seabloom, Kurt Anderson, Carol A. Blanchette, Bernardo Broitman, Scott D. Cooper, Benjamin S. Halpern: A cross-ecosystem comparison of the strength of trophic cascades. In: Ecology Letters. 5, 2002, pp. 785-791. doi: 10.1046 / j.1461-0248.2002.00381.x
  10. ↑ ET Borer, EW Seabloom, JB Shurin, KE Anderson, CA Blanchette, B. Broitman, SD Cooper, BS Halpern: What determines the strength of a trophic cascade? In: Ecology. 86, 2005, pp. 528-537. doi: 10.1890 / 03-0816