Trophic level

First trophic level. The plants in this image, and the algae and phytoplankton in the lake, are primary producers. They take nutrients from the soil or the water, and manufacture their own food by photosynthesis, using energy from the sun.
Consumer categories based on material eaten (plant: green shades are live, brown shades are dead; animal: red shades are live, purple shades are dead; or particulate: grey shades) and feeding strategy (gatherer: lighter shade of each color; miner: darker shade of each color)
An energy pyramid illustrates how much energy is needed as it flows upward to support the next trophic level. Only about 10% of the energy transferred between each trophic level is converted to biomass.
Killer whales (orca) are apex predators but they are divided into separate populations that hunt specific prey, such as tuna, small sharks, and seals.
The mean trophic level of the world fisheries catch has steadily declined because many high trophic level fish, such as this tuna, have been overfished.
<center>Second trophic level
<center>Third trophic level
<center>Fourth trophic level
<center>Decomposers

Organism is the position it occupies in a food web.

- Trophic level

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Heterotroph

Organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter.

Cycle between autotrophs and heterotrophs. Autotrophs use light, carbon dioxide (CO2), and water to form oxygen and complex organic compounds, mainly through the process of photosynthesis (green arrow). Both types of organisms use such compounds via cellular respiration to both generate ATP and again form CO2 and water (two red arrows).
Flowchart to determine if a species is autotroph, heterotroph, or a subtype

Heterotrophs represent one of the two mechanisms of nutrition (trophic levels), the other being autotrophs (auto = self, troph = nutrition).

Food web

Natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community.

A freshwater aquatic food web. The blue arrows show a complete food chain (algae &rarr; daphnia &rarr; gizzard shad &rarr; largemouth bass &rarr; great blue heron)
A simplified food web illustrating a three trophic food chain (producers-herbivores-carnivores) linked to decomposers. The movement of mineral nutrients is cyclic, whereas the movement of energy is unidirectional and noncyclic. Trophic species are encircled as nodes and arrows depict the links.
A trophic pyramid (a) and a simplified community food web (b) illustrating ecological relations among creatures that are typical of a northern Boreal terrestrial ecosystem. The trophic pyramid roughly represents the biomass (usually measured as total dry-weight) at each level. Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Some ecosystems, such as many wetlands, do not organize as a strict pyramid, because aquatic plants are not as productive as long-lived terrestrial plants such as trees. Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy.
Multitrophic interaction: Euphydryas editha taylori larvae sequester defensive compounds from specific types of plants they consume to protect themselves from bird predators
Energy flow diagram of a frog. The frog represents a node in an extended food web. The energy ingested is utilized for metabolic processes and transformed into biomass. The energy flow continues on its path if the frog is ingested by predators, parasites, or as a decaying carcass in soil. This energy flow diagram illustrates how energy is lost as it fuels the metabolic process that transform the energy and nutrients into biomass.
An expanded three link energy food chain (1. plants, 2. herbivores, 3. carnivores) illustrating the relationship between food flow diagrams and energy transformity. The transformity of energy becomes degraded, dispersed, and diminished from higher quality to lesser quantity as the energy within a food chain flows from one trophic species into another. Abbreviations: I=input, A=assimilation, R=respiration, NU=not utilized, P=production, B=biomass.
Illustration of a range of ecological pyramids, including top pyramid of numbers, middle pyramid of biomass, and bottom pyramid of energy. The terrestrial forest (summer) and the English Channel ecosystems exhibit inverted pyramids.Note: trophic levels are not drawn to scale and the pyramid of numbers excludes microorganisms and soil animals. Abbreviations: P=Producers, C1=Primary consumers, C2=Secondary consumers, C3=Tertiary consumers, S=Saprotrophs.
A four level trophic pyramid sitting on a layer of soil and its community of decomposers.
A three layer trophic pyramid linked to the biomass and energy flow concepts.
Paleoecological studies can reconstruct fossil food-webs and trophic levels. Primary producers form the base (red spheres), predators at top (yellow spheres), the lines represent feeding links. Original food-webs (left) are simplified (right) by aggregating groups feeding on common prey into coarser grained trophic species.
An illustration of a soil food web.
A simplified version of a food web in the Gulf of Naples in eutrophic (Green) and oligotrophic (Blue) summer conditions. In the Green system state, both copepods and microzooplankton exert a strong grazing pressure on phytoplankton, while in the Blue state, copepods increase their predation over microzooplankton, which in turn shifts its predation from phytoplankton to bacterial plankton or picoplankton. These trophic mechanisms stabilize the delivery of organic matter from copepods to fish.
Victor Summerhayes and Charles Elton's 1923 food web of Bear Island (Arrows point to an organism being consumed by another organism).

Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs.

Autotroph

Organism that produces complex organic compounds using carbon from simple substances such as carbon dioxide, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis).

Overview of cycle between autotrophs and heterotrophs. Photosynthesis is the main means by which plants, algae and many bacteria produce organic compounds and oxygen from carbon dioxide and water ( green arrow ).
Flowchart to determine if a species is autotroph, heterotroph, or a subtype
Green fronds of a maidenhair fern, a photoautotroph

Primary producers are at the lowest trophic level, and are the reasons why Earth sustains life to this day.

Biomass (ecology)

Mass of living biological organisms in a given area or ecosystem at a given time.

An energy pyramid illustrates how much energy is needed as it flows upward to support the next trophic level. Only about 10% of the energy transferred between each trophic level is converted to biomass.
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Humans and their livestock represent 96% of all mammals on earth in terms of biomass, whereas all wild mammals represent only 4%.
Globally, terrestrial and oceanic habitats produce a similar amount of new biomass each year (56.4 billion tonnes C terrestrial and 48.5 billion tonnes C oceanic).
Grasses, trees and shrubs have a much higher biomass than the animals that consume them
Antarctic krill form one of the largest biomasses of any individual animal species.<ref name="NE97">{{cite book | vauthors = Nicol S, Endo Y |url=http://www.fao.org/documents/show_cdr.asp?url_file=//DOCREP/003/W5911E/w5911e00.htm |title=Fisheries Technical Paper 367: Krill Fisheries of the World |publisher=FAO |year=1997}}</ref>

An ecological pyramid is a graphical representation that shows, for a given ecosystem, the relationship between biomass or biological productivity and trophic levels.

Ecosystem

An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact.

Rainforest ecosystems are rich in biodiversity. This is the Gambia River in Senegal's Niokolo-Koba National Park.
Flora of Baja California Desert, Cataviña region, Mexico
Global oceanic and terrestrial phototroph abundance, from September 1997 to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential and not an actual estimate of it.
Sequence of a decomposing pig carcass over time
Biological nitrogen cycling
Loch Lomond in Scotland forms a relatively isolated ecosystem. The fish community of this lake has remained stable over a long period until a number of introductions in the 1970s restructured its food web.
Spiny forest at Ifaty, Madagascar, featuring various Adansonia (baobab) species, Alluaudia procera (Madagascar ocotillo) and other vegetation
A hydrothermal vent is an ecosystem on the ocean floor. (The scale bar is 1 m.)
The High Peaks Wilderness Area in the 6000000 acre Adirondack Park is an example of a diverse ecosystem.
The Forest Landscape Integrity Index measures global anthropogenic modification on remaining forests annually. 0 = Most modification; 10= Least.

G. Evelyn Hutchinson, a limnologist who was a contemporary of Tansley's, combined Charles Elton's ideas about trophic ecology with those of Russian geochemist Vladimir Vernadsky.

Apex predator

Predator at the top of a food chain, without natural predators.

The lion is one of Africa's apex land predators.
The saltwater crocodile is the largest living reptile and the dominant predator throughout its range.
The great white shark (bottom) was originally considered the apex predator of the ocean; however, the killer whale (top) has proven to be a predator of the shark.
The great skua is an aerial apex predator, both preying on other seabirds and bullying them for their catches.
The wolf is both an apex predator and a keystone species, affecting its prey's behaviour and the wider ecosystem.
Humans sometimes live by hunting other animals for food and materials such as fur, sinew, and bone, as in this walrus hunt in the Arctic, but their status as apex predators is debated.
Anomalocaris was an apex predator in the Cambrian seas.
Dogs have been used in hunting for many centuries, as in this 14th century French depiction of a boar hunt.
Tiger sharks are popular ecotourism subjects, but their ecosystems may be affected by the food provided to attract them.
The reintroduction of predators like the lynx is attractive to conservationists, but alarming to farmers.

Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic levels.

Ecological pyramid

A pyramid of energy represents how much energy, initially from the sun, is retained or stored in the form of new biomass at each trophic level in an ecosystem. Typically, about 10% of the energy is transferred from one trophic level to the next, thus preventing a large number of trophic levels. Energy pyramids are necessarily upright in healthy ecosystems, that is, there must always be more energy available at a given level of the pyramid to support the energy and biomass requirement of the next trophic level.
A pyramid of biomass shows the total biomass of the organisms involved at each trophic level of an ecosystem. These pyramids are not necessarily upright. There can be lower amounts of biomass at the bottom of the pyramid if the rate of primary production per unit biomass is high.
A pyramid of numbers shows the number of individual organisms involved at each trophic level in an ecosystem. The pyramids are not necessarily upright. In some ecosystems there can be more primary consumers than producers.

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.

Plant defense against herbivory

Plant defense against herbivory or host-plant resistance (HPR) describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores.

Foxgloves produce toxic chemicals including cardiac and steroidal glycosides, deterring herbivory.
Timeline of plant evolution and the beginnings of different modes of insect herbivory
Viburnum lesquereuxii leaf with insect damage; Dakota Sandstone (Cretaceous) of Ellsworth County, Kansas. Scale bar is 10 mm.
A plain tiger Danaus chrysippus caterpillar making a moat to block defensive chemicals of Calotropis before feeding
Persimmon, genus Diospyros, has a high tannin content which gives immature fruit, seen above, an astringent and bitter flavor.
The prickles on the stem of this raspberry plant, serve as a mechanical defense against herbivory.
Coconut palms protect their fruit by surrounding it with multiple layers of armor.
The large and directly defensive thorn-like stipules of Vachellia collinsii are also hollow and offer shelter for ants, which indirectly protect the plant against herbivores.
Illustration from the 15th-century manuscript Tacuinum Sanitatis detailing the beneficial and harmful properties of Mandrakes

One group of semiochemicals are allelochemicals; consisting of allomones, which play a defensive role in interspecies communication, and kairomones, which are used by members of higher trophic levels to locate food sources.

Fishing down the food web

Process whereby fisheries in a given ecosystem, "having depleted the large predatory fish on top of the food web, turn to increasingly smaller species, finally ending up with previously spurned small fish and invertebrates".

The mean trophic level of the world fisheries catch has steadily declined because many high trophic level fish, such as this tuna, have been overfished.
Fishermen are increasingly targeting lower trophic level fish, like these anchovies and other forage fish.
According to Daniel Pauly, if the trend continues, consumers may be eating jellyfish sandwiches.

Large predator fish with higher trophic levels have been depleted in wild fisheries.

Ecological efficiency

A diagram of energy transfer between trophic levels

Ecological efficiency describes the efficiency with which energy is transferred from one trophic level to the next.