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, phytoplankton and zooplankton 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 a 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 enhanced ecosystems. The study valued the potential storage between $205 million and $408 million dollars (US) on the European Carbon Exchange (2012).
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. 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.
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:
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:
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:
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. 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. 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. 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.
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.
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:
There are a number of other examples of trophic cascades involving large terrestrial mammals, including:
In addition to the classic examples listed above, more recent examples of trophic cascades in marine ecosystems have been identified:
The Atlantic cod (Gadus morhua) is a benthopelagic fish of the family Gadidae, widely consumed by humans. It is also commercially known as cod or codling. Dry cod may be prepared as unsalted stockfish, as cured salt cod or clipfish.In the western Atlantic Ocean, cod has a distribution north of Cape Hatteras, North Carolina, and around both coasts of Greenland and the Labrador Sea; in the eastern Atlantic, it is found from the Bay of Biscay north to the Arctic Ocean, including the Baltic Sea, the North Sea, Sea of the Hebrides, areas around Iceland and the Barents Sea.
The largest individual on record was 1.8 m (6 ft) long and weighed 96 kg (211 lb), but usually the cod is between 61 cm (24 in) and 1.2 m (4 ft) long and weighs up to 40 kg (88 lb). Males and females are similar in size and weight.Atlantic cod can live for 25 years, and usually attain sexual maturity between ages two and four, although cod in the northeast Arctic can take as long as eight years to mature fully. Colouring is brown or green, with spots on the dorsal side, shading to silver ventrally. A stripe along its lateral line (used to detect vibrations) is clearly visible. Its habitat ranges from the shoreline down to the continental shelf.
Several cod stocks collapsed in the 1990s (declined by >95% of maximum historical biomass) and have failed to fully recover even with the cessation of fishing. This absence of the apex predator has led to a trophic cascade in many areas. Many other cod stocks remain at risk. The Atlantic cod is labelled vulnerable on the IUCN Red List of Threatened Species.Camille Dungy
Camille T. Dungy (born 1972) is an American poet and professor.Cascade effect (ecology)
An ecological cascade effect is a series of secondary extinctions that is triggered by the primary extinction of a key species in an ecosystem. Secondary extinctions are likely to occur when the threatened species are: dependent on a few specific food sources, mutualistic (dependent on the key species in some way), or forced to coexist with an invasive species that is introduced to the ecosystem. Species introductions to a foreign ecosystem can often devastate entire communities, and even entire ecosystems. These exotic species monopolize the ecosystem's resources, and since they have no natural predators to decrease their growth, they are able to increase indefinitely. Olsen et al. showed that exotic species have caused lake and estuary ecosystems to go through cascade effects due to loss of algae, crayfish, mollusks, fish, amphibians, and birds. However, the principal cause of cascade effects is the loss of top predators as the key species. As a result of this loss, a dramatic increase (ecological release) of prey species occurs. The prey is then able to overexploit its own food resources, until the population numbers decrease in abundance, which can lead to extinction. When the prey's food resources disappear, they starve and may go extinct as well. If the prey species is herbivorous, then their initial release and exploitation of the plants may result in a loss of plant biodiversity in the area. If other organisms in the ecosystem also depend upon these plants as food resources, then these species may go extinct as well. An example of the cascade effect caused by the loss of a top predator is apparent in tropical forests. When hunters cause local extinctions of top predators, the predators' prey's population numbers increase, causing an overexploitation of a food resource and a cascade effect of species loss. Recent studies have been performed on approaches to mitigate extinction cascades in food-web networks.Cross-boundary subsidy
Cross-boundary subsidies are caused by organisms or materials that cross or traverse habitat patch boundaries, subsidizing the resident populations. The transferred organisms and materials may provide additional predators, prey, or nutrients to resident species, which can affect community and food web structure. Cross-boundary subsidies of materials and organisms occur in landscapes composed of different habitat patch types, and so depend on characteristics of those patches and on the boundaries in between them. Human alteration of the landscape, primarily through fragmentation, has the potential to alter important cross-boundary subsidies to increasingly isolated habitat patches. Understanding how processes that occur outside of habitat patches can affect populations within them may be important to habitat management.Ecological pyramid
An ecological pyramid (also trophic pyramid, Eltonian pyramid, energy pyramid, or sometimes food pyramid) is a graphical representation designed to show the biomass or bioproductivity at each trophic level in a given ecosystem.
A pyramid of energy shows how much energy is retained in the form of new biomass at each trophic level, while a pyramid of biomass shows how much biomass (the amount of living or organic matter present in an organism) is present in the organisms. There is also a pyramid of numbers representing the number of individual organisms at each trophic level. Pyramids of energy are normally upright, but other pyramids can be inverted or take other shapes.
Ecological pyramids begin with producers on the bottom (such as plants) and proceed through the various trophic levels (such as herbivores that eat plants, then carnivores that eat flesh, then omnivores that eat both plants and flesh, and so on). The highest level is the top of the food chain.
Biomass can be measured by a bomb calorimeter.Ecosynthesis
see Planetary ecosynthesis for Terraforming
Ecosynthesis is the use of introduced species to fill niches in a disrupted environment, with the aim of increasing the speed of ecological restoration. This decreases the amount of physical damage done in a disrupted landscape. An example is using willow in a stream corridor for sediment and phosphorus capture. It aims to aid ecological restoration which, is the practice of renewing and restoring degraded, damaged, or destroyed ecosystems and habitats in the environment by active human intervention and action. Humans ecosynthesis to make environments more suitable for life, through restoration ecology (introduced species, vegetation mapping, habitat enhancement, remediation and mitigation.)Environmental toxicology
Environmental toxicology is a multidisciplinary field of science concerned with the study of the harmful effects of various chemical, biological and physical agents on living organisms. Ecotoxicology is a subdiscipline of environmental toxicology concerned with studying the harmful effects of toxicants at the population and ecosystem levels.
Rachel Carson is considered the mother of environmental toxicology, as she made it a distinct field within toxicology in 1962 with the publication of her book Silent Spring, which covered the effects of uncontrolled pesticide use. Carson's book was based extensively on a series of reports by Lucille Farrier Stickel on the ecological effects of the pesticide DDT.Organisms can be exposed to various kinds of toxicants at any life cycle stage, some of which are more sensitive than others. Toxicity can also vary with the organism's placement within its food web. Bioaccumulation occurs when an organism stores toxicants in fatty tissues, which may eventually establish a trophic cascade and the biomagnification of specific toxicants. Biodegradation releases carbon dioxide and water as by-products into the environment. This process is typically limited in areas affected by environmental toxicants.
Harmful effects of such chemical and biological agents as toxicants from pollutants, insecticides, pesticides, and fertilizers can affect an organism and its community by reducing its species diversity and abundance. Such changes in population dynamics affect the ecosystem by reducing its productivity and stability.
Although legislation implemented since the early 1970s had intended to minimize harmful effects of environmental toxicants upon all species, McCarty (2013) has warned that "longstanding limitations in the implementation of the simple conceptual model that is the basis of current aquatic toxicity testing protocols" may lead to an impending environmental toxicology "dark age".Living machine
Living Machine is a trademark and brand name for a patented form of ecological sewage treatment designed to mimic the cleansing functions of wetlands. Similar to Solar Aquatics Systems, the latest generation of the technology is based on fixed-film ecology and the ecological processes of a natural tidal wetland, one of nature’s most productive ecosystems. The diversity of the ecosystem produced with this approach allows operational advantages over earlier generations of Living Machines and over conventional waste water treatment technologies.The Living Machine system was commercialized and is marketed by Living Machine Systems, L3C, a social benefit corporation based in Charlottesville, Va. The trademark Living Machine is owned by Dharma Group, LC, the parent company of Worrell Water Technologies.The Living Machine is an intensive bioremediation system that can also produce beneficial byproducts, such as reuse-quality water, ornamental plants and plant products—for building material, energy biomass, animal feed. Aquatic and wetland plants, bacteria, algae, protozoa, plankton, snails and other organisms are used in the system to provide specific cleansing or trophic functions. The tidal process operates outdoors in tropical and temperate climates. In colder climates, the system of tanks, pipes and filters may be housed in a greenhouse to prevent freezing and raise the rate of biological activity.
The initial development of the technology in the United States is generally credited to Dr. John Todd, an ecological designer, and evolved out of the bioshelter concept developed at the now-defunct New Alchemy Institute. The Living Machine system falls within the emerging discipline of ecological engineering, and many systems using earlier generations of the technology are built without being dubbed a Living Machine.Mesopredator release hypothesis
The mesopredator release hypothesis is an ecological theory used to describe the interrelated population dynamics between apex predators and mesopredators within an ecosystem, such that a collapsing population of the former results in dramatically-increased populations of the latter. This hypothesis describes the phenomenon of trophic cascade in specific terrestrial communities.
A mesopredator is a medium-sized, middle trophic level predator, which both preys and is preyed upon. Examples are raccoons, skunks, snakes, cownose rays, and small sharks.Nelson Hairston
Nelson Hairston Sr. (16 October 1917 – 31 July 2008) was an American ecologist. Hairston is well known for his work in ecology and human disease. In the field of ecology he is famous for championing the idea of the trophic cascade, on which he published the provocative “Green World Hypothesis” with colleagues Frederick E. Smith and Lawrence B. Slobodkin. Nelson was also deeply interested in the factors that control human disease and was an adviser to the World Health Organization for many years.Non-trophic networks
Any action or influence that species have on each other is considered a biological interaction. These interactions between species can be considered in several ways. One such way is to depict interactions in the form of a network, which identifies the members and the patterns that connect them. Species interactions are considered primarily in terms of trophic interactions, which depict which species feed on others.
Currently, ecological networks that integrate non-trophic interactions are being built. The type of interactions they can contain can be classified into six categories: mutualism, commensalism, neutralism, amensalism, antagonism, and competition.
Observing and estimating the fitness costs and benefits of species interactions can be very problematic. The way interactions are interpreted can profoundly affect the ensuing conclusions.Northern Highland
The Northern Highland is a geographical region in the north central United States covering much of the northern territory of the state of Wisconsin.
The region stretches from the state border with Minnesota in the west to the Michigan border in the east, and from Douglas and Bayfield Counties in the north to Wood and Portage Counties in the south. While most of northern Wisconsin is within the Northern Highland region, a short belt of land along the coast of Lake Superior is not included in the area, and is instead part of the Lake Superior Lowland. Outside Wisconsin the highland stretches northward in Canada through the Upper Peninsula of Michigan and the Canadian Shield in Northern Ontario and Quebec to Labrador and Hudson Bay.Piscivore
A piscivore is a carnivorous animal that eats primarily fish. Piscivorous is equivalent to the Greek-derived word ichthyophagous. Fish were the diet of early tetrapods (amphibians); insectivory came next, then in time, reptiles added herbivory.Some animals, such as the sea lion and alligator, are not completely piscivorous, often preying on aquatic invertebrates or land animals in addition to fish, while others, such as the bulldog bat and gharial, are strictly dependent on fish for food. Humans can live on fish-based diets as can their carnivorous domesticated pets, such as dogs and cats. The name "piscivore" is derived from the Latin word for fish, piscis. Some creatures, including cnidarians, octopuses, squid, spiders, sharks, cetaceans, grizzly bears, jaguars, wolves, snakes, turtles, and sea gulls, may have fish as significant if not dominant portions of their diets.
The ecological effects of piscivores can extend to other food chains. In a study of cutthroat trout stocking, researchers found that the addition of this piscivore can have noticeable effects on non-aquatic organisms, in this case bats feeding on insects emerging from the water with the trout.There exists classifications of primary and secondary piscivores. Primary piscivores, also known as "specialists", shift to this habit in the first few months of their lives. Secondary piscivores will move to eating primarily fish later in their lifetime. It is hypothesized that the secondary piscivores' diet change is due to an adaptation to maintain efficiency in their use of energy while growing.Planktivore
A planktivore is an aquatic organism that feeds on planktonic food, including zooplankton and phytoplankton.Productivity (ecology)
In ecology, productivity refers to the rate of generation of biomass in an ecosystem. It is usually expressed in units of mass per unit surface (or volume) per unit time, for instance grams per square metre per day (g m−2 d−1). The mass unit may relate to dry matter or to the mass of carbon generated. Productivity of autotrophs such as plants is called primary productivity, while that of heterotrophs such as animals is called secondary productivity.Salt marsh die-off
Salt marsh die-off is a term that has been used in the US and UK to describe the death of salt marsh cordgrass leading to subsequent degradation of habitat, specifically in the low marsh zones of salt marshes on the coasts of the Western Atlantic. Cordgrass normally anchors sediment in salt marshes; its loss leads to decreased substrate hardness, increased erosion, and collapse of creek banks into the water, ultimately resulting in decreased marsh health and productivity.
Die-off can affect several species of cordgrass (genus Spartina), including S. alterniflora, S. densiflora, and S. townsendii. There are several competing hypotheses predicting the causes and mechanisms of salt marsh die-off throughout the western Atlantic. These hypotheses place different emphasis on the effects of top-down or bottom-up processes for salt marsh die-off. Combined with salt marsh dieback of the high marsh, salt marsh die-off is a serious threat to the ecosystem services that marshes provide to local coastal communities.Soil food web
The soil food web is the community of organisms living all or part of their lives in the soil. It describes a complex living system in the soil and how it interacts with the environment, plants, and animals.
Food webs describe the transfer of energy between species in an ecosystem. While a food chain examines one, linear, energy pathway through an ecosystem, a food web is more complex and illustrates all of the potential pathways. Much of this transferred energy comes from the sun. Plants use the sun’s energy to convert inorganic compounds into energy-rich, organic compounds, turning carbon dioxide and minerals into plant material by photosynthesis. Plant flowers exude energy-rich nectar above ground and plant roots exude acids, sugars, and ectoenzymes into the rhizosphere, adjusting the pH and feeding the food web underground.Plants are called autotrophs because they make their own energy; they are also called producers because they produce energy available for other organisms to eat. Heterotrophs are consumers that cannot make their own food. In order to obtain energy they eat plants or other heterotrophs.Trophic
Trophic, from Ancient Greek τροφικός (trophikos) "pertaining to food or nourishment", may refer to:
Trophic factor receptor
Trophic level index
Trophic state index
Trophic webWilliam J. Ripple
William J. Ripple is a Distinguished Professor of Ecology at Oregon State University in the Department of Forest Ecosystems and Society. He is a widely published researcher and a prominent figure in the field of ecology. He is best known for his research on terrestrial trophic cascades, particularly the role of the gray wolf (Canis lupus) in North America as an apex predator and a keystone species that shapes food webs and landscape structures via “top-down” pressures.
Ripple heads the Trophic Cascades Program at Oregon State University, which carries out several research initiatives such as the Aspen Project, the Wolves in Nature Project, and the Range Contractions Project. He has a Ph.D. from Oregon State UniversityRipple was the lead author on the "Global Scientists' Warning to Humanity: A second Notice", published on November 13, 2017. This article includes 15,364 scientist co-signatories from 184 countries. The article suggests "To prevent widespread misery and catastrophic biodiversity loss, humanity must practice a more environmentally sustainable alternative to business as usual."