Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore).

The trophic cascade is an ecological concept which has stimulated new research in many areas of ecology. For example, it can be important for understanding the knock-on effects of removing top predators from food webs, as humans have done in many places through hunting and fishing.

A top-down cascade is a trophic cascade where the top consumer/predator controls the primary consumer population. In turn, the primary producer population thrives. The removal of the top predator can alter the food web dynamics. In this case, the primary consumers would overpopulate and exploit the primary producers. Eventually there would not be enough primary producers to sustain the consumer population. Top-down food web stability depends on competition and predation in the higher trophic levels. Invasive species can also alter this cascade by removing or becoming a top predator. This interaction may not always be negative. Studies have shown that certain invasive species have begun to shift cascades; and as a consequence, ecosystem degradation has been repaired.[1][2]

For example, if the abundance of large piscivorous fish is increased in a lake, the abundance of their prey, smaller fish that eat zooplankton, should decrease. The resulting increase in zooplankton should, in turn, cause the biomass of its prey, phytoplankton, to decrease.

In a bottom-up cascade, the population of primary producers will always control the increase/decrease of the energy in the higher trophic levels. Primary producers are plants and phytoplankton that require photosynthesis. Although light is important, primary producer populations are altered by the amount of nutrients in the system. This food web relies on the availability and limitation of resources. All populations will experience growth if there is initially a large amount of nutrients.[3][4]

In a subsidy cascade, species populations at one trophic level can be supplemented by external food. For example, native animals can forage on resources that don't originate in their same habitat, such as native predators eating livestock. This may increase their local abundances thereby affecting other species in the ecosystem and causing an ecological cascade. For example, Luskin et al. (2017) found that native animals living in protected primary rainforest in Malaysia found food subsidies in neighboring oil palm plantations.[5] This subsidy allowed native animal populations to increase, which then triggered powerful secondary ‘cascading’ effects on forest tree community. Specifically, crop-raiding wild boar (Sus scrofa) built thousands of nests from the forest understory vegetation and this caused a 62% decline in forest tree sapling density over a 24-year study period. Such cross-boundary subsidy cascades may be widespread in both terrestrial and marine ecosystems and present significant conservation challenges.

These trophic interactions shape patterns of biodiversity globally. Humans and climate change have affected these cascades drastically. One example can be seen with sea otters (Enhydra lutris) on the Pacific coast of the United States of America. Over time, human interactions caused a removal of sea otters. One of their main prey, the Pacific purple sea urchin (Strongylocentrotus purpuratus) eventually began to overpopulate. The overpopulation caused increased predation of giant kelp (Macrocystis pyrifera). As a result, there was extreme deterioration of the kelp forests along the California coast. This is why it is important for countries to regulate marine and terrestrial ecosystems.[6][7]

Predator-induced interactions could heavily influence the flux of atmospheric carbon if managed on a global scale. For example, a study was conducted to determine the cost of potential stored carbon in living kelp biomass in sea otter (Enhydra lutris) enhanced ecosystems. The study valued the potential storage between $205 million and $408 million dollars (US) on the European Carbon Exchange (2012).[8]

Origins and theory

Aldo Leopold is generally credited with first describing the mechanism of a trophic cascade, based on his observations of overgrazing of mountain slopes by deer after human extermination of wolves.[9] Nelson Hairston, Frederick E. Smith and Lawrence B. Slobodkin are generally credited with introducing the concept into scientific discourse, although they did not use the term either. Hairston, Smith and Slobodkin argued that predators reduce the abundance of herbivores, allowing plants to flourish.[10] This is often referred to as the green world hypothesis. The green world hypothesis is credited with bringing attention to the role of top-down forces (e.g. predation) and indirect effects in shaping ecological communities. The prevailing view of communities prior to Hairston, Smith and Slobodkin was trophodynamics, which attempted to explain the structure of communities using only bottom-up forces (e.g. resource limitation). Smith may have been inspired by the experiments of a Czech ecologist, Hrbáček, whom he met on a United States State Department cultural exchange. Hrbáček had shown that fish in artificial ponds reduced the abundance of zooplankton, leading to an increase in the abundance of phytoplankton.[11]

Hairston, Smith and Slobodkin feuded that the ecological communities acted as food chains with three trophic levels. Subsequent models expanded the argument to food chains with more than or fewer than three trophic levels.[12] Lauri Oksanen argued that the top trophic level in a food chain increases the abundance of producers in food chains with an odd number of trophic levels (such as in Hairston, Smith and Slobodkin's three trophic level model), but decreases the abundance of the producers in food chains with an even number of trophic levels. Additionally, he argued that the number of trophic levels in a food chain increases as the productivity of the ecosystem increases.


Although the existence of trophic cascades is not controversial, ecologists have long debated how ubiquitous they are. Hairston, Smith and Slobodkin argued that terrestrial ecosystems, as a rule, behave as a three trophic level trophic cascade, which provoked immediate controversy. Some of the criticisms, both of Hairston, Smith and Slobodkin's model and of Oksanen's later model, were:

Antagonistically, this principle is sometimes called the "trophic trickle".[15][16]

Classic examples

Healthy Pacific kelp forests, like this one at San Clemente Island of California's Channel Islands, have been shown to flourish when sea otters are present. When otters are absent, sea urchin populations can irrupt and severely degrade the kelp forest ecosystem.

Although Hairston, Smith and Slobodkin formulated their argument in terms of terrestrial food chains, the earliest empirical demonstrations of trophic cascades came from marine and, especially, aquatic ecosystems. Some of the most famous examples are:

Terrestrial trophic cascades

The fact that the earliest documented trophic cascades all occurred in lakes and streams led a scientist to speculate that fundamental differences between aquatic and terrestrial food webs made trophic cascades primarily an aquatic phenomenon. Trophic cascades were restricted to communities with relatively low species diversity, in which a small number of species could have overwhelming influence and the food web could operate as a linear food chain. Additionally, well documented trophic cascades at that point in time all occurred in food chains with algae as the primary producer. Trophic cascades, Strong argued, may only occur in communities with fast-growing producers which lack defenses against herbivory.[22]

Subsequent research has documented trophic cascades in terrestrial ecosystems, including:

Critics pointed out that published terrestrial trophic cascades generally involved smaller subsets of the food web (often only a single plant species). This was quite different from aquatic trophic cascades, in which the biomass of producers as a whole were reduced when predators were removed. Additionally, most terrestrial trophic cascades did not demonstrate reduced plant biomass when predators were removed, but only increased plant damage from herbivores.[26] It was unclear if such damage would actually result in reduced plant biomass or abundance. In 2002 a meta-analysis found trophic cascades to be generally weaker in terrestrial ecosystems, meaning that changes in predator biomass resulted in smaller changes in plant biomass.[27] In contrast, a study published in 2009 demonstrated that multiple species of trees with highly varying autecologies are in fact heavily impacted by the loss of an apex predator.[28] Another study, published in 2011, demonstrated that the loss of large terrestrial predators also significantly degrades the integrity of river and stream systems, impacting their morphology, hydrology, and associated biological communities.[29]

The critics' model is challenged by studies accumulating since the reintroduction of gray wolves (Canis lupus) to Yellowstone National Park. The gray wolf, after being extirpated in the 1920s and absent for 70 years, was reintroduced to the park in 1995 and 1996. Since then a three-tiered trophic cascade has been reestablished involving wolves, elk (Cervus elaphus), and woody browse species such as aspen (Populus tremuloides), cottonwoods (Populus spp.), and willows (Salix spp.). Mechanisms likely include actual wolf predation of elk, which reduces their numbers, and the threat of predation, which alters elk behavior and feeding habits, resulting in these plant species being released from intensive browsing pressure. Subsequently, their survival and recruitment rates have significantly increased in some places within Yellowstone's northern range. This effect is particularly noted among the range's riparian plant communities, with upland communities only recently beginning to show similar signs of recovery.[30]

Examples of this phenomenon include:

Trophic cascades also impact the biodiversity of ecosystems, and when examined from that perspective wolves appear to be having multiple, positive cascading impacts on the biodiversity of Yellowstone National Park. These impacts include:

This diagram illustrates trophic cascade caused by removal of the top predator. When the top predator is removed the population of deer is able to grow unchecked and this causes over-consumption of the primary producers.

There are a number of other examples of trophic cascades involving large terrestrial mammals, including:

Marine trophic cascades

In addition to the classic examples listed above, more recent examples of trophic cascades in marine ecosystems have been identified:

See also


  1. ^ Kotta, J.; Wernberg, T.; Jänes, H.; Kotta, I.; Nurkse, K.; Pärnoja, M.; Orav-Kotta, H. (2018). "Novel crab predator causes marine ecosystem regime shift". Scientific Reports. 8 (1): 4956. Bibcode:2018NatSR...8.4956K. doi:10.1038/s41598-018-23282-w. PMC 5897427. PMID 29651152.
  2. ^ Megrey, Bernard and Werner, Francisco. "Evaluating the Role of Topdown vs. Bottom-up Ecosystem Regulation from a Modeling Perspective" (PDF).((cite web)): CS1 maint: multiple names: authors list (link)
  3. ^ Matsuzaki, Shin-Ichiro S.; Suzuki, Kenta; Kadoya, Taku; Nakagawa, Megumi; Takamura, Noriko (2018). "Bottom-up linkages between primary production, zooplankton, and fish in a shallow, hypereutrophic lake". Ecology. 99 (9): 2025–2036. doi:10.1002/ecy.2414. PMID 29884987. S2CID 46996957.
  4. ^ Lynam, Christopher Philip; Llope, Marcos; Möllmann, Christian; Helaouët, Pierre; Bayliss-Brown, Georgia Anne; Stenseth, Nils C. (Feb 2017). "Trophic and environmental control in the North Sea". Proceedings of the National Academy of Sciences. 114 (8): 1952–1957. doi:10.1073/pnas.1621037114. PMC 5338359. PMID 28167770.
  5. ^ Luskin, M. (2017). "Cross-boundary subsidy cascades from oil palm degrade distant tropical forests". Nature Communications. 8 (8): 2231. Bibcode:2017NatCo...8.2231L. doi:10.1038/s41467-017-01920-7. PMC 5738359. PMID 29263381.
  6. ^ Zhang, J.; Qian, H.; Girardello, M.; Pellissier, V.; Nielsen, S. E.; Svenning, J.-C. (2018). "Trophic interactions among vertebrate guilds and plants shape global patterns in species diversity". Proceedings of the Royal Society B: Biological Sciences. 285 (1883): 20180949. doi:10.1098/rspb.2018.0949. PMC 6083253. PMID 30051871.
  7. ^ "University of Kentucky Lecture Notes".
  8. ^ Wilmers, C. C.; Estes, J. A.; Edwards, M.; Laidre, K. L.; Konar, B. (2012). "Do trophic cascades affect the storage and flux of atmospheric carbon? An analysis of sea otters and kelp forests". Frontiers in Ecology and the Environment. 10 (8): 409–415. doi:10.1890/110176. ISSN 1540-9309. S2CID 51684842.
  9. ^ Leopold, A. (1949) "Thinking like a mountain" in "Sand county almanac"
  10. ^ Hairston, NG; Smith, FE; Slobodkin, LB (1960). "Community structure, population control and competition". American Naturalist. 94 (879): 421–425. doi:10.1086/282146. S2CID 84548124.
  11. ^ Hrbáček, J; Dvořakova, M; Kořínek, V; Procházkóva, L (1961). "Demonstration of the effect of the fish stock on the species composition of zooplankton and the intensity of metabolism of the whole plankton association". Verh. Internat. Verein. Limnol. 14: 192–195.
  12. ^ Oksanen, L; Fretwell, SD; Arruda, J; Niemala, P (1981). "Exploitation ecosystems in gradients of primary productivity". American Naturalist. 118 (2): 240–261. doi:10.1086/283817. S2CID 84215344.
  13. ^ a b Murdoch, WM (1966). "Community structure, population control, and competition – a critique". American Naturalist. 100 (912): 219–226. doi:10.1086/282415. S2CID 84354616.
  14. ^ Polis, GA; Strong, DR (1996). "Food web complexity and community dynamics". American Naturalist. 147 (5): 813–846. doi:10.1086/285880. S2CID 85155900.
  15. ^ Eisenberg, Cristina (2011) "The Wolf's Tooth: Keystone Predators, Trophic Cascades, and Biodiversity pp. 15. Island Press. ISBN 978-1-59726-398-6.
  16. ^ Barbosa P and Castellanos I (Eds) (2005) Ecology of Predator-Prey Interactions pp. 306, Oxford University Press.ISBN 9780199883677.
  17. ^ Carpenter, SR; Kitchell, JF; Hodgson, JR (1985). "Cascading trophic interactions and lake productivity". BioScience. 35 (10): 634–639. doi:10.2307/1309989. JSTOR 1309989.
  18. ^ Power, ME (1990). "Effects of fish in river food webs". Science. 250 (4982): 811–814. Bibcode:1990Sci...250..811P. doi:10.1126/science.250.4982.811. PMID 17759974. S2CID 24780727.
  19. ^ Szpak, Paul; Orchard, Trevor J.; Salomon, Anne K.; Gröcke, Darren R. (2013). "Regional ecological variability and impact of the maritime fur trade on nearshore ecosystems in southern Haida Gwaii (British Columbia, Canada): evidence from stable isotope analysis of rockfish (Sebastes spp.) bone collagen". Archaeological and Anthropological Sciences. In Press (X): XX. doi:10.1007/s12520-013-0122-y. S2CID 84866250.
  20. ^ Estes, JA; Palmisano, JF (1974). "Sea otters: their role in structuring nearshore communities". Science. 185 (4156): 1058–1060. Bibcode:1974Sci...185.1058E. doi:10.1126/science.185.4156.1058. PMID 17738247. S2CID 35892592.
  21. ^ Weston, Phoebe (2022-06-23). "'People may be overselling the myth': should we bring back the wolf?". The Guardian. Retrieved 2022-06-24.
  22. ^ Strong, D. R. (1992). "Are trophic cascades all wet? Differentiation and donor-control in speciose ecosystems". Ecology. 73 (3): 747–754. doi:10.2307/1940154. JSTOR 1940154.
  23. ^ Strong, D. R.; Whipple, A. V.; Child, A. L.; Dennis, B. (1999). "Model selection for a subterranean trophic cascade: Root-feeding caterpillars and entomopathogenic nematodes". Ecology. 80 (8): 2750–2761. doi:10.2307/177255. JSTOR 177255.
  24. ^ Preisser, E. L. (2003). "Field evidence for a rapidly cascading underground food web". Ecology. 84 (4): 869–874. doi:10.1890/0012-9658(2003)084[0869:fefarc];2.
  25. ^ Letourneau, D. K.; Dyer, L. A. (1998). "Experimental test in lowland tropical forest shows top-down effects through four trophic levels". Ecology. 79 (5): 1678–1687. doi:10.2307/176787. JSTOR 176787.
  26. ^ Polis, G. A.; Sears, A. L, W; Huxel, G. R.; et al. (2000). "When is a trophic cascade a trophic cascade?". Trends in Ecology & Evolution. 15 (11): 473–475. doi:10.1016/s0169-5347(00)01971-6. PMID 11050351.((cite journal)): CS1 maint: multiple names: authors list (link)
  27. ^ Shurin, J. B.; Borer, E. T.; Seabloom, E. W.; Anderson, K.; Blanchette, C. A.; Broitman, B; Cooper, S. D.; Halpern, B. S. (2002). "A cross-ecosystem comparison of the strength of trophic cascades". Ecology Letters. 5 (6): 785–791. doi:10.1046/j.1461-0248.2002.00381.x.
  28. ^ Beschta, R.L., and W.J. Ripple. 2009. Large predators and trophic cascades in terrestrial ecosystems of the western United States Biological Conservation. 142, 2009: 2401–2414.
  29. ^ Beschta, R. L.; Ripple, W. J. (2011). "The role of large predators in maintaining riparian plant communities and river morphology". Geomorphology. 157–158: 88–98. doi:10.1016/j.geomorph.2011.04.042.
  30. ^ a b c Ripple, W. J.; Beschta, R. L. (2012). "Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction". Biological Conservation. 145: 205–213. doi:10.1016/j.biocon.2011.11.005. S2CID 9750513.
  31. ^ Groshong, L. C. (2004). Mapping Riparian Vegetation Change in Yellowstone's Northern Range using High Spatial Resolution Imagery (MA Thesis). Eugene, Oregon, USA: University of Oregon.
  32. ^ Ripple, W.J.; Beschta, R.L. (2004). "Wolves, elk, willows, and trophic cascades in the upper Gallatin Range of Southwestern Montana, USA". Forest Ecology and Management. 200 (1–3): 161–181. doi:10.1016/j.foreco.2004.06.017.
  33. ^ Beschta, R.L.; Ripple, W.J. (2007). "Increased Willow Heights along northern Yellowstone's Blacktail Deer Creek following wolf reintroduction". Western North American Naturalist. 67 (4): 613–617. doi:10.3398/1527-0904(2007)67[613:iwhany];2. S2CID 85023589.
  34. ^ a b Baril, L. M. (2009). Change in Deciduous Woody Vegetation, Implications of Increased Willow (Salix spp.) Growth for Bird Species Diversity and Willow Species Composition in and around Yellowstone National Park's Northern Range (MS). Bozeman, USA: Montana State University.
  35. ^ Wilmers, C.C.; Crabtree, R. L.; Smith, D. W.; Murphy, K. M.; Getz, W. M. (2003). "Trophic facilitation by introduced top predators: grey wolf subsidies to scavengers in Yellowstone National Park". Journal of Animal Ecology. 72 (6): 909–916. doi:10.1046/j.1365-2656.2003.00766.x.
  36. ^ Painter, L. E.; Ripple, W. J. (2012). "Effects of bison on willow and cottonwood in northern Yellowstone National Park". Forest Ecology and Management. 264: 150–158. doi:10.1016/j.foreco.2011.10.010.
  37. ^ Ripple, W.J.; Beschta, R.L. (2006). "Linking a cougar decline, trophic cascade, and catastrophic regime shift in Zion National Park". Biological Conservation. 133 (4): 397–408. doi:10.1016/j.biocon.2006.07.002.
  38. ^ Ripple, W.J.; Beschta, R.L. (2008). "Trophic cascades involving cougar, mule deer, and black oaks in Yosemite National Park". Biological Conservation. 141 (5): 1249–1256. doi:10.1016/j.biocon.2008.02.028.
  39. ^ Estes, James A.; et al. (2011). "2011. Trophic Downgrading of Planet Earth". Science. 333 (6040): 301–306. Bibcode:2011Sci...333..301E. CiteSeerX doi:10.1126/science.1205106. PMID 21764740. S2CID 7752940.
  40. ^ Prugh, Laura R.; et al. (2009). "2009. The Rise of the Mesopredator". BioScience. 59 (9): 779–791. doi:10.1525/bio.2009.59.9.9. S2CID 40484905.
  41. ^ Letnic, M.; Dworjanyn, S.A. (2011). "Does a top predator reduce the predatory impact of an invasive mesopredator on an endangered rodent?". Ecography. 34 (5): 827–835. doi:10.1111/j.1600-0587.2010.06516.x.
  42. ^ Frank, K. T.; Petrie, B.; Choi, J. S.; Leggett, W. C. (2005). "Trophic Cascades in a Formerly Cod-Dominated Ecosystem". Science. 308 (5728): 1621–1623. Bibcode:2005Sci...308.1621F. doi:10.1126/science.1113075. ISSN 0036-8075. PMID 15947186. S2CID 45088691.
  43. ^ Alheit, J; Möllmann, C; Dutz, J; Kornilovs, G; Loewe, P; Mohrholz, V; Wasmund, N (2005). "Synchronous ecological regime shifts in the central Baltic and the North Sea in the late 1980s". ICES Journal of Marine Science. 62 (7): 1205–1215. doi:10.1016/j.icesjms.2005.04.024.
  44. ^ Mollmann, C.; Muller-Karulis, B.; Kornilovs, G.; St John, M. A. (2008). "Effects of climate and overfishing on zooplankton dynamics and ecosystem structure: regime shifts, trophic cascade, and feedback loops in a simple ecosystem". ICES Journal of Marine Science. 65 (3): 302–310. doi:10.1093/icesjms/fsm197.
  45. ^ Loh, T.-L.; Pawlik, J. R. (2014). "Chemical defenses and resource trade-offs structure sponge communities on Caribbean coral reefs". Proceedings of the National Academy of Sciences. 111 (11): 4151–4156. Bibcode:2014PNAS..111.4151L. doi:10.1073/pnas.1321626111. ISSN 0027-8424. PMC 3964098. PMID 24567392.
  46. ^ Loh, T.-L.; et al. (2015). "Indirect effects of overfishing on Caribbean reefs: sponges overgrow reef-building corals". PeerJ. 3: e901. doi:10.7717/peerj.901. PMC 4419544. PMID 25945305.