Trophic cascade
Encyclopedia
Trophic cascades occur when predators in a food web
suppress the abundance of their prey, thereby releasing the next lower trophic level
from predation
(or herbivory if the intermediate trophic level is an herbivore). For example, if the abundance of large piscivorous
fish is increased in a lake
, the abundance of their prey, zooplanktivorous
fish
, should decrease, large zooplankton
abundance should increase, and phytoplankton
biomass
should decrease. This theory has stimulated new research in many areas of ecology
. Trophic cascades may also be important for understanding the effects of removing top predators from food webs, as humans have done in many places through hunting and fishing activities.
A Top Down Cascade is a trophic cascade where the food chain or food web is disrupted by the removal of a top predator, or a third or fourth level consumer. On the other hand, a bottom up cascade occurs when a primary producer, or primary consumer is removed, and there is a diminishment of population size through the community. An example could include Paine's study from the University of Washington, where he removed several species in different plots along the North-Western United States coast line, and realized that Pisaster, a common starfish, when removed, created a top-down cascade which involved a surge in barnacle and mussel population, but a decrease in the populations of chitons, limpets, and whelks. This led to the conclusion that Pisaster was a keystone species in that food web.
, Frederick E. Smith and Lawrence B. Slobodkin
are generally credited with originating the concept of a trophic cascade, although they did not use the term. Hairston, Smith and Slobodkin argued that predators reduce the abundance of herbivores, allowing plant
s 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 pond
s reduced the abundance of zooplankton
, leading to an increase in the abundance of phytoplankton
.
Hairston, Smith and Slobodkin argued 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.
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:
and, especially, aquatic ecosystem
s. Some of the most famous examples are:
s lead Donald Strong to speculate that fundamental differences between aquatic and terrestrial food web
s made trophic cascades primarily an aquatic phenomenon. Strong argued that 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, however, 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 a recent meta-analysis
, trophic cascades were generally weaker in terrestrial ecosystems, meaning that changes in predator biomass resulted in smaller changes in plant biomass.
Food web
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
suppress the abundance of their prey, thereby releasing the next lower trophic level
Trophic level
The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek τροφή referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism...
from predation
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
(or herbivory if the intermediate trophic level is an herbivore). For example, if the abundance of large piscivorous
Piscivore
A piscivore is a carnivorous animal which eats primarily fish. Piscivory was the diet of early tetrapods , insectivory came next, then in time reptiles added herbivory....
fish is increased in a lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
, the abundance of their prey, zooplanktivorous
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, should decrease, large zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
abundance should increase, and phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
should decrease. This theory has stimulated new research in many areas of ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
. Trophic cascades may also be important for understanding the effects of removing top predators from food webs, as humans have done in many places through hunting and fishing activities.
A Top Down Cascade is a trophic cascade where the food chain or food web is disrupted by the removal of a top predator, or a third or fourth level consumer. On the other hand, a bottom up cascade occurs when a primary producer, or primary consumer is removed, and there is a diminishment of population size through the community. An example could include Paine's study from the University of Washington, where he removed several species in different plots along the North-Western United States coast line, and realized that Pisaster, a common starfish, when removed, created a top-down cascade which involved a surge in barnacle and mussel population, but a decrease in the populations of chitons, limpets, and whelks. This led to the conclusion that Pisaster was a keystone species in that food web.
Origins and theory
Nelson HairstonNelson Hairston
Nelson Hairston Sr. was a preeminent ecologist of the 20th century. 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
Lawrence B. Slobodkin
Lawrence B. Slobodkin was an American ecologist and Professor Emeritus at the Department of Ecology and Evolution, Stony Brook University, State University of New York. He was one of the leading pioneers of modern ecology...
are generally credited with originating the concept of a trophic cascade, although they did not use the term. Hairston, Smith and Slobodkin argued that predators reduce the abundance of herbivores, allowing plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s 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
Biocoenosis
A biocoenosis , coined by Karl Möbius in 1877, describes the interacting organisms living together in a habitat . This term is rarely used in English, as this concept has not been popularized in Anglophone countries...
. 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 pond
Pond
A pond is a body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens, water features and koi ponds; all designed for aesthetic ornamentation as landscape or architectural...
s reduced the abundance of zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
, leading to an increase in the abundance of phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
.
Hairston, Smith and Slobodkin argued 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
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
increases.
Criticisms
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 levelTrophic level
The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek τροφή referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism...
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:
- PlantPlantPlants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s possess numerous defensesPlant defense against herbivoryPlant defense against herbivory or host-plant resistance describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants use several strategies to defend against damage caused by herbivores...
against herbivory, and these defenses also contribute to reducing the impact of herbivores on plant populations. - Herbivore populations may be limited by factors other than food or predation, such as nesting sites or available territory.
- For trophic cascades to be ubiquitous, communities must generally act as food chains, with discrete trophic levels. Most communities, however, have complex food webs. In real food webs, consumers often feed at multiple trophic levels (omnivory), organisms often change their dietDiet (nutrition)In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...
as they grow larger, cannibalismCannibalism (zoology)In zoology, cannibalism is the act of one individual of a species consuming all or part of another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded for more than 1500 species...
occurs, and consumers are subsidized by inputs of resources from outside the local community, all of which blur the distinctions between trophic levels.
Classic examples
Although Hairston, Smith and Slobodkin formulated their argument in terms of terrestrial food chains, the earliest empirical demonstrations of trophic cascades came from marineMarine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
and, especially, aquatic ecosystem
Aquatic ecosystem
An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems....
s. Some of the most famous examples are:
- In North AmericaNorth AmericaNorth America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
n lakes, piscivorousPiscivoreA piscivore is a carnivorous animal which eats primarily fish. Piscivory was the diet of early tetrapods , insectivory came next, then in time reptiles added herbivory....
fish can dramatically reduce populations of zooplanktivorous fish, zooplanktivorous fish can dramatically alter freshwaterFreshwaterFresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
zooplanktonZooplanktonZooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
communities, and zooplankton grazing can in turn have large impacts on phytoplanktonPhytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
communities. Removal of piscivorous fish can change lake water from clear to green by allowing phytoplankton to flourish. - In the Eel RiverEel River (California)The Eel River is a major river system of the northern Pacific coast of the U.S. state of California. Approximately 200 miles long, it drains a rugged area in the California Coast Ranges between the Sacramento Valley and the ocean. For most of its course, the river flows northwest, parallel to the...
, in Northern CaliforniaCaliforniaCalifornia is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, fish (steelhead and roachCalifornia roachThe California roach, Hesperoleucus symmetricus, is a cyprinid fish native to western North America and abundant in the intermittent streams throughout central California. It is the sole member of its genus....
) consume fish larvae and predatory insectInsectInsects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s. These smaller predators prey on midgeMidgeA midge is a very small, two-winged flying insect. "Midge" may also refer to:-Real:* Midge Costanza , American politician* Mildred Gillars , aka "Midge", American broadcaster of Nazi propaganda during World War II...
larvae, which feed on algaeAlgaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
. Removal of the larger fish increases the abundance of algae. - In PacificPacific OceanThe Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
kelp forestKelp forestKelp forests are underwater areas with a high density of kelp. They are recognized as one of the most productive and dynamic ecosystems on Earth. Smaller areas of anchored kelp are called kelp beds....
s, sea otterSea OtterThe sea otter is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg , making them the heaviest members of the weasel family, but among the smallest marine mammals...
s feed on sea urchinSea urchinSea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
s. In areas where sea otters have been huntHuntingHunting is the practice of pursuing any living thing, usually wildlife, for food, recreation, or trade. In present-day use, the term refers to lawful hunting, as distinguished from poaching, which is the killing, trapping or capture of the hunted species contrary to applicable law...
ed to extinctionLocal extinctionLocal extinction, also known as extirpation, is the condition of a species which ceases to exist in the chosen geographic area of study, though it still exists elsewhere...
, sea urchins increase in abundance and decimate kelpKelpKelps are large seaweeds belonging to the brown algae in the order Laminariales. There are about 30 different genera....
.
Terrestrial trophic cascades
The fact that the earliest documented trophic cascades all occurred in lakes and streamStream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
s lead Donald Strong to speculate that fundamental differences between aquatic and terrestrial food web
Food web
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
s made trophic cascades primarily an aquatic phenomenon. Strong argued that trophic cascades were restricted to communities with relatively low species diversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
, 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 prairieCalifornia coastal prairieCalifornia coastal prairie, also known as northern coastal grassland, is a grassland plant community of California and Oregon in the Temperate grasslands, savannas, and shrublands Biome...
of Northern California, yellow bush lupines are fed upon by a particularly destructive herbivore, the root-boring caterpillar of the ghost moth. Entomopathogenic nematodeEntomopathogenic nematodeEntomopathogenic nematodes are soil-inhabiting, lethal insect parasitoids that belong to the phylum Nematoda, commonly called roundworms. The term entomopathogenic comes from the Greek word entomon, meaning insect, and pathogenic, which means causing disease...
s kill the caterpillars, and can increase the survival and seedSeedA seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
production of lupines. - In Costa RicaCosta RicaCosta Rica , officially the Republic of Costa Rica is a multilingual, multiethnic and multicultural country in Central America, bordered by Nicaragua to the north, Panama to the southeast, the Pacific Ocean to the west and the Caribbean Sea to the east....
n rain forest, a CleridCleridaeCleridae are a family of beetles of the superfamily Cleroidea. They are commonly known as checkered beetles. The Cleridae family has a worldwide distribution, and a variety of habitats and feeding preferences....
beetleBeetleColeoptera is an order of insects commonly called beetles. The word "coleoptera" is from the Greek , koleos, "sheath"; and , pteron, "wing", thus "sheathed wing". Coleoptera contains more species than any other order, constituting almost 25% of all known life-forms...
specializes in eating antAntAnts are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...
s. The ant Pheidole bicornis has a mutualistic association with PiperPiper (genus)Piper, the pepper plants or pepper vines , are an economically and ecologically important genus in the family Piperaceae...
plants: the ant lives on the Piper and removes caterpillarCaterpillarCaterpillars are the larval form of members of the order Lepidoptera . They are mostly herbivorous in food habit, although some species are insectivorous. Caterpillars are voracious feeders and many of them are considered to be pests in agriculture...
s and other insectInsectInsects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
herbivores. The Clerid beetle, by reducing the abundance of ants, increases the leaf area removed from Piper plants by insect herbivores.
Critics pointed out, however, 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 a recent meta-analysis
Meta-analysis
In statistics, a meta-analysis combines the results of several studies that address a set of related research hypotheses. In its simplest form, this is normally by identification of a common measure of effect size, for which a weighted average might be the output of a meta-analyses. Here the...
, trophic cascades were generally weaker in terrestrial ecosystems, meaning that changes in predator biomass resulted in smaller changes in plant biomass.
See also
- Cascade effect (ecology)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, or forced to coexist...
- Food webFood webA food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
- Lake ecology
- Mesopredator release hypothesisMesopredator release hypothesisThe mesopredator release hypothesis is a relatively new hypothesis from 1988 which describes the phenomenon of trophic cascade in certain terrestrial communities. It states that as top predators decline in an ecosystem, an increase in the populations of mesopredators occurs...
- Population dynamics of fisheriesPopulation dynamics of fisheriesA fishery is an area with an associated fish or aquatic population which is harvested for its commercial or recreational value. Fisheries can be wild or farmed. Population dynamics describes the ways in which a given population grows and shrinks over time, as controlled by birth, death, and...
- Soil food webSoil food webThe 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....
- Stream ecology