Trophic cascades are powerful indirect interactions that can control entireecosystems, occurring when atrophic level in afood 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 theirprey, thereby releasing the next lower trophic level frompredation (orherbivory if the intermediate trophic level is a herbivore).

The trophic cascade is an ecological concept which has stimulated new research in many areas ofecology. For example, it can be important for understanding the knock-on effects of removingtop predators fromfood webs, as humans have done in many places throughhunting andfishing.

Atop-down cascade is a trophic cascade where the top consumer/predator controls theprimary 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 largepiscivorous fish is increased in alake, the abundance of their prey, smaller fish that eatzooplankton, should decrease. The resulting increase in zooplankton should, in turn, cause thebiomass of its prey,phytoplankton, to decrease.

In abottom-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 asubsidy 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-raidingwild 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 andclimate change have affected these cascades drastically. One example can be seen withsea 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 ofgiant 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]

Aldo Leopold is generally credited with first describing the mechanism of a trophic cascade, based on his observations ofovergrazing of mountain slopes by deer after human extermination of wolves.^[9]Nelson Hairston, Frederick E. Smith andLawrence 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, allowingplants to flourish.^[10] This is often referred to as thegreen 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 shapingecological 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 aUnited States State Department cultural exchange. Hrbáček had shown that fish in artificialponds reduced the abundance ofzooplankton, leading to an increase in the abundance ofphytoplankton.^[11]

Hairston, Smith and Slobodkin feuded that the ecological communities acted asfood 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 ofproducers 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 theecosystem increases.

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

• InNorth American lakes,piscivorous fish can dramatically reduce populations of zooplanktivorous fish; zooplanktivorous fish can dramatically alterfreshwaterzooplankton communities, and zooplankton grazing can in turn have large impacts onphytoplankton communities. Removal of piscivorous fish can change lake water from clear to green by allowing phytoplankton to flourish.^[13]
• In theEel River, in NorthernCalifornia, fish (steelhead androach) consume fish larvae and predatoryinsects. These smaller predators prey onmidge larvae, which feed onalgae. Removal of the larger fish increases the abundance of algae.^[14]
• InPacifickelp forests,sea otters feed onsea urchins. In areas where sea otters have beenhunted toextinction, sea urchins increase in abundance andkelp populations are reduced.^[15][16]
• A classic example of a terrestrial trophic cascade is the reintroduction ofgray wolves (Canis lupus) toYellowstone National Park, which reduced the number, and changed the behavior, ofelk (Cervus canadensis). This in turn released several plant species from grazing pressure and subsequently led to the transformation of riparian ecosystems.^[17]

The fact that the earliest documented trophic cascades all occurred in lakes andstreams led a scientist to speculate that fundamental differences between aquatic and terrestrialfood webs made trophic cascades primarily an aquatic phenomenon. Trophic cascades were restricted to communities with relatively lowspecies 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 theprimary producer. Trophic cascades, Strong argued, may only occur in communities with fast-growing producers which lackdefenses against herbivory.^[18]

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

• In thecoastal prairie of Northern California,yellow bush lupines are fed upon by a particularly destructive herbivore, the root-boring caterpillar of the lupine ghost mothPhymatopus californicus.Entomopathogenic nematodes kill the caterpillars, and can increase the survival andseed production of lupines.^[19][20]
• InCosta Ricanrain forest, acleridbeetle specializes in eatingants. The antPheidole bicornis has amutualistic association withPiper plants: the ant lives on thePiper and removescaterpillars and otherinsect herbivores. The clerid beetle, by reducing the abundance of ants, increases the leaf area removed fromPiper plants by insect herbivores.^[21]

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.^[22] It was unclear if such damage would actually result in reduced plant biomass or abundance. In 2002 ameta-analysis found trophic cascades to be generallyweaker in terrestrial ecosystems, meaning that changes in predator biomass resulted in smaller changes in plant biomass.^[23] In contrast, a study published in 2009 demonstrated that multiple species of trees with highly varyingautecologies are in fact heavily impacted by the loss of an apex predator.^[24] Another study, published in 2011, demonstrated that the loss of large terrestrial predators also significantly degrades the integrity of river and stream systems, impacting theirmorphology,hydrology, and associated biological communities.^[25]

The critics' model is challenged by studies accumulating since the reintroduction ofgray wolves (Canis lupus) toYellowstone National Park. The gray wolf, after beingextirpated in the 1920s and absent for 70 years, wasreintroduced to the park in 1995 and 1996. Since then a three-tiered trophic cascade has been reestablished involving wolves,elk (Cervus elaphus), and woodybrowse species such asaspen (Populus tremuloides),cottonwoods (Populus spp.), andwillows (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 andrecruitment rates have significantly increased in some places within Yellowstone's northern range. This effect is particularly noted among the range'sriparian plant communities, with upland communities only recently beginning to show similar signs of recovery.^[26]

Examples of this phenomenon include:

• A 2–3 fold increase indeciduous woody vegetation cover, mostly of willow, in theSoda Butte Creek area between 1995 and 1999.^[27]
• Heights of the tallest willows in theGallatin River valley increasing from 75 cm to 200 cm between 1998 and 2002.^[28]
• Heights of the tallest willows in the Blacktail Creek area increased from less than 50 cm to more than 250 cm between 1997 and 2003. Additionally, canopy cover over streams increased significantly, from only 5% to a range of 14–73%.^[29]
• In the northern range, tall deciduous woody vegetation cover increased by 170% between 1991 and 2006.^[30]
• In theLamar and Soda Butte Valleys the number of young cottonwood trees that had been successfully recruited went from 0 to 156 between 2001 and 2010.^[26]

Trophic cascades also impact thebiodiversity 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:

• Scavengers, such asravens (Corvus corax),bald eagles (Haliaeetus leucocephalus), and evengrizzly bears (Ursus arctos horribilis), are likely subsidized by the carcasses of wolf kills.^[31]
• In the northern range, the relative abundance of six out of seven native songbirds which utilize willow was found to be greater in areas of willow recovery as opposed to those where willows remained suppressed.^[30]
• Bison (Bison bison) numbers in the northern range have been steadily increasing as elk numbers have declined, presumably due to a decrease ininterspecific competition between the two species.^[32]
• Importantly, the number ofbeaver (Castor canadensis) colonies in the park has increased from one in 1996 to twelve in 2009. The recovery is likely due to the increase in willow availability, as they have been feeding almost exclusively on it. Askeystone species, the resurgence of beaver is a critical event for the region. The presence of beavers has been shown to positively impact streambankerosion,sediment retention,water tables,nutrient cycling, and both the diversity and abundance of plant and animal life among riparian communities.^[26]

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

• In bothZion National Park andYosemite National Park, the increase in human visitation during the first half of the 20th century was found to correspond to the decline of nativecougar (Puma concolor) populations in at least part of their range. Soon after, native populations ofmule deer (Odocoileus hemionus) erupted, subjecting resident communities of cottonwoods (Populus fremontii) in Zion andCalifornia black oak (Quercus kelloggii) in Yosemite to intensified browsing. This halted successful recruitment of these species except in refugia inaccessible to the deer. In Zion the suppression of cottonwoods increased stream erosion and decreased the diversity and abundance of amphibians, reptiles, butterflies, and wildflowers. In parts of the park where cougars were still common these negative impacts were not expressed and riparian communities were significantly healthier.^[33][34]
• Insub-Saharan Africa, the decline oflion (Panthera leo) andleopard (Panthera pardus) populations has led to a rising population ofolive baboon (Papio anubis). This case ofmesopredator release negatively impacted already decliningungulate populations and is one of the reasons for increased conflict between baboons and humans, as the primates raid crops and spreadintestinal parasites.^[35][36]
• In the Australian states ofNew South Wales andSouth Australia, the presence or absence ofdingoes (Canis lupus dingo) was found to be inversely related to the abundance of invasivered foxes (Vulpes vulpes). In other words, the foxes were most common where the dingoes were least common. Subsequently, populations of an endangered prey species, thedusky hopping mouse (Notomys fuscus) were also less abundant where dingoes were absent due to the foxes, which consume the mice, no longer being held in check by the top predator.^[37]

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

• An example of a cascade in a complex, open-ocean ecosystem occurred in the northwestAtlantic during the 1980s and 1990s. The removal ofAtlantic cod (Gadus morhua) and other ground fishes by sustainedoverfishing resulted in increases in the abundance of the prey species for these ground fishes, particularly smallerforage fishes and invertebrates such as the northernsnow crab (Chionoecetes opilio) and northernshrimp (Pandalus borealis). The increased abundance of these prey species altered the community ofzooplankton that serve as food for smaller fishes and invertebrates as an indirect effect.^[38]
• A similar cascade, also involving the Atlantic cod, occurred in theBaltic Sea at the end of the 1980s. After a decline in Atlantic cod, the abundance of its main prey, thesprat (Sprattus sprattus), increased^[39] and the Baltic Sea ecosystem shifted from being dominated by cod into being dominated by sprat. The next level of trophic cascade was a decrease in the abundance ofPseudocalanus acuspes,^[40] acopepod which the sprat prey on.
• OnCaribbeancoral reefs, several species of angelfishes andparrotfishes eat species ofsponges that lackchemical defenses. Removal of these sponge-eating fish species from reefs by fish-trapping and netting has resulted in a shift in the sponge community toward fast-growing sponge species that lack chemical defenses.^[41] These fast-growing sponge species are superior competitors for space, and overgrow and smother reef-building corals to a greater extent on overfished reefs.^[42]

Although the existence of trophic cascades is not controversial, ecologists have long debated how ubiquitous they are. Hairston, Smith and Slobodkin argued thatterrestrialecosystems, as a rule, behave as a threetrophic 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:

• Plants possess numerousdefenses against herbivory, and these defenses also contribute to reducing the impact of herbivores on plant populations.^[43]
• Herbivore populations may be limited by factors other than food or predation, such as nesting sites or available territory.^[43]
• For trophic cascades to be ubiquitous, communities must generally act as food chains, with discrete trophic levels. Most communities, however, have complexfood webs. In real food webs, consumers often feed at multiple trophic levels (omnivory), organisms often change theirdiet as they grow larger,cannibalism occurs, and consumers are subsidized by inputs of resources from outside the local community, all of which blur the distinctions between trophic levels.^[44]

Antagonistically, this principle is sometimes called the "trophic trickle".^[45][46]

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