Ecology Theory

Trophic level

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Schematic trophic levels for simple terrestrial and aquatic ecosystems.

A trophic level is a set of species occupying one level of the ecological food chain. The lowest trophic level, Level 1, is that set of organisms that can manufacture their energy and biomass using only sunlight, drawing inorganic nutrients from the soil or water. Feeding upon this lowest tier of organisms are Level 2 organisms in the food chain, often classified as herbivores. The concept of a food web was clearly articulated as early as 1880, but it was not until 1942 that Lindeman introduced the term trophic level to describe the ecological hierarchy of consumption. To allow for the fact that most higher level taxa consume more than one species of prey, the concept of trophic levels other than whole numbers is allowed.

The trophic level theory is an elegant formalism that assists in explaining many ecological relationships; however, there are a number of criticisms of trophic theory, noting departures of the real world from simplified methods that the trophic theory suggests.

Terminology

Firstly it is desirable to clarify terminology differences that arise in the literature surroundinging trophic levels. These chiefly stem from the way that community ecologists discuss the trophic levels in opposition to the viewpoint of ecosystem ecologists. Community ecologists tend to name the trophic levels after their community member groups (e.g. plants, herbivores, carnivores); ecosystem ecologists tend to prefer class names by functional level when addressing trophic levels (e.g. primary producers, primary consumers, secondary consumers). In the present article, both sets of terms will be applied, choosing the most appropriate way of looking at trophic levels for each point discussed.

Primary producers

caption Giant Kelp juvenile, with a bottom holdfast. Source: Steinhart Aquarium, San Francisco, California

The lowest trophic level is typically populated by primary producers, those organisms who can synthesize the entirety of their biomass solely from primary energy (such as sunlight) and primary nutrients extracted from the environment (e.g. nitrate, phosphate). These Level 1 organisms are typically plants and algae. They are also often termed autotrophs.

Primary producers may be either terrestrial or aquatic. Common aquatic species include phytoplankton, diatoms and much larger species such as kelp. Terrestrial species include the whole gamut of angiosperms and gymnosperms as well as ferns, mosses, lichens and other non-vascular plants.

Many of the primary producers in aquatic media are small, or even microscopic organisms; however, some of these primary producers are very large aquatic plants, such as the giant kelp that can rise tens of meters tethered to the floor of continental shelves, growing with the power of sunlight filtering through the epipelagic zone..

 

Herbivores

caption Pygmy elephants are some of the largest herbivores. Kinabatangan River basin, Sabah, Malaysia. @ C.Michael Hogan Level 2 of the trophic chain consists of organisms that consume primary producers. Such species are often termed herbivores. Herbivores may be very small organisms such as zooplankton that graze on microscopic plants such as phytoplankton; however, herbivores also include many enormous mammals such as rhinoceri and elephants.

If significant numbers of the herbivore trophic level are removed, plant growth may exceed the balance found in the previous natural environment. For example the removal of herbaceous fishes by overfishing and the loss of the herbaceus sea urchin, Diameda antillarum, has caused an increase in the standing crop of algae on many coral reefs in the Caribbean Sea. Conversely, a sudden and large increase in the population size of herbivores, often caused by an decrease in the population of their predators, can greatly reduce the standing biomass of plants. The invasion of introduced herbivores can produce adverse effects on the native plant community.

Carnivores

Levels 3 and higher of the trophic chain consist of carnivores, animals that feed on other animals. Carnivores may vary greatly in size and feeding strategy, ragning from large and fierce animals such lions and sharks to small internal parasites.   Carnivores may play important roles in the ecosystem by controlling the population sizes of herbivores who ultimately influence the biomass of plants in an ecosystem.  For example, the loss of wolves in Yellowstone National Park in the United States allowed the elk population to increase so much that they greatly affected the growth of willows.  

Decomposers

Decomposers, also known as detrivores, are a special group of organisms that are capable of breaking down dead or dying tissue of other species. Unlike scavengers, who ingest dead biomass for internal digestion, the decomposers are capable of breaking down cells of other organisms using biochemical reactions that convert the prey tissue into metabolically useful chemical products, without need for internal digestion. The most common decomposers are fungi, bacteria and archaea, the latter two being micro-organisms.

caption Decomposer species lentinula edodes. Source: Frank Enstoen Thus, some decomposers can be considered Level 2 organisms, in the case that they consume dead plant material. However, some decomposers may occupy a higher trophic level, if they consume dead animals of a higher trophic level.

Energy considerations

Ecologists often evaluate the energy flow among various trophic levels, particularly with respect to efficiency of energy conversion from one production or consumption level to the next. Typically there is a substantial loss of energy and biomass as one views successively higher trophic levels. This observation can be best understood by noting all the processes of energy transfer among each connection in the trophic chain (or food chain). For example, the lowest level of primary producers is rather efficient at combining sunlight with carbon dioxide and nutrients to produce plant or algal biomass; a variant of this process, also rather efficient is for some deep sea extremophiles (known as chemolithotrophs), who obtain energy by the chemical oxidation of inorganic compounds and can grow in the absence of light.

Much of the plant biomass is wasted, in the sense that it is not consumed by herbivores; instead, considerable portions die and decay, feeding the decomposers. Thus, only a fraction of the biomass store of primary producers ends up being consumed by the next higher trophic level. In the case of herbivores, again much of the built up herbivore population dies and decays without feeding a higher trophic level; however, additional losses of energy are sustained at the herbivore level through (a) Respiration loss (an intrinsic ongoing energy cost of survival for all herbivores; (b) Expended locomotion energy loss (again a major expenditure for all but the most sedentary of aquatic creatures); (c) Non-assimilation losses (e.g. urine, feces, bleeding, sloughing of skin and hair, saliva loss).

The ratio of net energy (or biomass) production at one level to net production at the next higher level is called the conversion efficiency. As a result of all these cumulative losses, the energy conveyed from the herbivore trophic level to the carnivore level is greatly reduced. It is typical for an order of magnitude less energy (or biomass) to result in each successively higher trophic level. The most efficient trophic energy transfer is probably consumption of phytoplankton by zooplankton, which process can be approximately forty percent energy efficient. At the opposite end of the efficiency scale is consumption by herbivores of plants, which have large quantities of undigestible biomass; in such cases up to two orders of magnitude of energy loss may be sustained in such a trophic cascade. In most terrestrial ecosystems the maximum number of trophic levels is four or five; however, there are many aquatic systems which manifest many more trophic levels due to the presence of successive levels of carnivory with a great variety of faunal sizes.

Ecological pyramid

An ecological pyramid is an idealized construct to illustrate how energy, biomass, and population size, vary between trophic levels in an ecosystem. Conventionally, this concept is illustrated by placing primary producers at the bottom of a diagram and the highest trophic levels at the top of the diagram. The size of the portion of the diagram associated with each trophic level is intended to depict the amount of energy, biomass, or number of individuals found in each trophic level. 

Energy flows through the food web beginning at the base of such a hypothetical pyramid, by photosynthesis, and then moving up to higher trophic levels. The concept of an ecological pyramid can often be useful in visualizing a trophic chain; however, caution must be used, lest oversimplification may presume that all trophic chains have more member species at lower levels. Such a conventional notion is often, but not always, true.

Problems with the trophic theory

As elegant as the trophic formalism first appeared to ecologists, it generated numerous critiques and dilemmas for 20th century ecologists. Even with the following critiques and inherent inconsistencies of the trophic theory, the trophic level concepts remain useful to ecologists, so long as their application is used judiciously. The chief criticisms of trophic theory were:

A. Certain herbivores and decomposers can feed on the same Level 1 of fallen leaf or plant material; thus species of very different metabolisms and lifestyles must both be assigned to the same Level 2 consumers.

B. Some organisms undergo life cycle changes, consuming at different trophic levels for different life stages. For example some lacewing species consume pollen as adults, while preying on smaller insects when in their larval stage.

C. The concept of omnivory inherently muddles the trophic theory, since omnivores can consume from different trophic levels. Hence their assignment to a single trophic level is impossible.

D. Trophic theory inherently oversimplfies the richness of ecological connections in a real world ecosystem, connections which are intrinsically almost impossible to diagram in completeness.

See also

References

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Glossary

Citation

Hogan, C. (2012). Trophic level. Retrieved from http://www.eoearth.org/view/article/171874

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