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.
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.
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. 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.
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 US$205 million and US$408 million on the European Carbon Exchange (2012).
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.Leopold, A. (1949) "Thinking like a mountain" in "Sand county almanac" 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. 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.
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. 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.
Examples
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: In North American lakes, piscivorous fish can dramatically reduce populations of zooplanktivorous fish; zooplanktivorous fish can dramatically alter freshwater zooplankton communities, and zooplankton grazing can in turn have large impacts on phytoplankton communities. Removal of piscivorous fish can change lake water from clear to green by allowing phytoplankton to flourish. In the Eel River, in Northern California, fish (steelhead and roach) consume fish larvae and predatory insects. These smaller predators prey on midge larvae, which feed on algae. Removal of the larger fish increases the abundance of algae. In Pacific kelp forests, sea otters feed on sea urchins. In areas where sea otters have been hunted to extinction, sea urchins increase in abundance and kelp populations are reduced. In popular media, the reintroduction of gray wolves (Canis lupus) to Yellowstone National Park is often celebrated as an example of a terrestrial trophic cascade in which the wolves "changed rivers" and "saved" the park. In reality, the Greater Yellowstone Ecosystem is too complex to serve as an example of a straightforward trophic cascade. The system is open, affected by conditions outside of the park, and it has been influenced by a range of human-made and natural, time-varying factors other than the reintroduction of wolves. Two dominant herbivores, American bison (Bison bison) and elk (Cervus canadensis), are predated on by five top predators, grizzly bears (Ursus arctos horribilis) and black bears (Ursus americanus), cougars (Puma concolor), wolves and human hunters in Montana. The ecosystem of Isle Royale Island is comparatively simple. Fifty years after moose (Alces alces) made their way to the island, wolves walked over winter ice in the late 1940s and began predating on the moose, keeping their number limited. In 1979 the wolf population crashed by 80% due to canine parvovirus, and the moose population exploded subsequently. After this event, the wolf population did not recover completely. Short term fluctuations in moose abundance were largely unrelated to wolves. Known factors include hot summers, both extremely cold winters and warm winters leading to severe tick infestations. Other factors remain unknown. Wolves were essential for keeping moose populations at a sustainable level over some time. The wolf population had dwindled down to two individuals, father and daughter, and the moose population was near 2000 individuals, when a project of relocating outside wolves to Isle Royale began in 2019. The long term moose-wolf study of Isle Royale continues in this new chapter of the moose-wolf saga. A "serendipitous natural experiment" took place in Banff National Park when the first wolf pack recolonized the Bow Valley in 1986. The valley includes areas of high and low human activity that corresponded to low and high wolf presence, respectively. The two areas showed differential rates of elk (Cervus canadensis) abundance, rates of wolf predation on elk, and a range of related environmental changes. High wolf activity increased aspen (Populus tremuloides) recruitment, willow (Salix spp.) production and beaver (Castor canadensis) lodge density. Riparian songbird density and diversity were also higher in high-wolf areas.
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.
Subsequent research has documented trophic cascades in terrestrial ecosystems, including:
In the coastal prairie of Northern California, yellow bush lupines are fed upon by a particularly destructive herbivore, the root-boring caterpillar of the lupine ghost moth Phymatopus californicus. Entomopathogenic nematodes kill the caterpillars, and can increase the survival and seed production of lupines. In Costa Rican rain forest, a clerid beetle specializes in eating ants. The ant Pheidole bicornis has a mutualistic association with Piper plants: the ant lives on the Piper and removes caterpillars and other insect herbivores. The clerid beetle, by reducing the abundance of ants, increases the leaf area removed from Piper plants by insect herbivores.