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 (trophē) 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 is from the start of the chain is a measure of its trophic level. Food chains start at trophic level 1 with primary producers such as plants, move to herbivores at level 2, predators at level 3 and typically finish with carnivores or apex predators at level 4 or 5. The path along the chain can form a one-way flow, or a food "web." Ecological communities with higher biodiversity form more complex trophic paths.

Contents

Overview

The three basic ways organisms get food are as producers, consumers and decomposers.

Trophic levels can be represented by numbers, starting at level 1 with plants. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.

In real world ecosystems, there is more than one food chain for most organisms, since most organisms eat more than one kind of food or are eaten by more than one type of predator. A diagram which sets out the intricate network of intersecting and overlapping food chains for an ecosystem is called its food web.[3] Decomposers are often left off food webs, but if included, they mark the end of a food chain.[3] Thus food chains start with primary producers and end with decay and decomposers. Since decomposers recycle nutrients, leaving them so they can be reused by primary producers, they are sometimes regarded as occupying their own trophic level.[4][5]

Biomass transfer efficiency

Generally, each trophic level relates to the one below it by absorbing some of the energy it consumes, and in this way can be regarded as resting on, or supported by the next lower trophic level. Food chains can be diagrammed to illustrate the amount of energy that moves from one feeding level to the next in a food chain. This is called an energy pyramid. The energy transferred between levels can also be thought of as approximating to a transfer in biomass, so energy pyramids can also be viewed as biomass pyramids, picturing the amount of biomass that results at higher levels from biomass consumed at lower levels.

The efficiency with which energy or biomass is transferred from one trophic level to the next is called the ecological efficiency. Consumers at each level convert on average only about 10 percent of the chemical energy in their food to their own organic tissue. For this reason, food chains rarely extend for more than 5 or 6 levels. At the lowest trophic level (the bottom of the food chain), plants convert about one percent of the sunlight they receive into chemical energy. It follows from this that the total energy originally present in the incident sunlight that is finally embodied in a tertiary consumer is about 0.001 %[4]

Fractional trophic levels

Food webs largely define ecosystems, and the trophic levels define the position of organisms within the webs. But these trophic levels are not always simple integers, because organisms often feed at more than one trophic level.[6][7] For example, some carnivores also eat plants, and some plants are carnivores. A large carnivore may eat both smaller carnivores and herbivores; the bobcat eats rabbits, but the mountain lion eats both bobcats and rabbits. Animals can also eat each other; the bullfrog eats crayfish and crayfish eat young bullfrogs. The feeding habits of a juvenile animal, and consequently its trophic level, can change as it grows up.

The fisheries scientist Daniel Pauly sets the values of trophic levels to one in plants and detritus, two in herbivores and detritivores (primary consumers), three in secondary consumers, and so on. The definition of the trophic level, TL, for any consumer species i is:[5]

TL_i = 1 %2B \sum_j (TL_j \cdot DC_{ij})\!

where TL_i is the fractional trophic level of the prey j, and DC_{ij} represents the fraction of j in the diet of i.

In the case of marine ecosystems, the trophic level of most fish and other marine consumers takes value between 2.0 and 5.0. The upper value, 5.0, is unusual, even for large fish,[8] though it occurs in apex predators of marine mammals, such as polar bears and killer whales.[9]

Mean trophic level

In fisheries, the mean trophic level for the fisheries catch across an entire area or ecosystem is calculated for year y as:

TL_y = \frac{\sum_i (TL_i \cdot Y_{iy})}{\sum_i Y_{iy}}

where Y_{iy} is the catch of the species or group i in year y, and TL_i is the fractional trophic level for species i as defined above.[5]

It was once believed that fish at higher trophic levels usually have a higher economic value; resulting in overfishing at the higher trophic levels. Earlier reports found precipitous declines in mean trophic level of fisheries catch, in a process known as fishing down the food web.[10] However, more recent work finds no relation between economic value and trophic level;[11] and that mean trophic levels in catches, surveys and stock assessments have not in fact declined, suggesting that fishing down the food web is not a global phenomenon.[12]

FiB index

Since biomass transfer efficiencies are only about 10 percent, it follows that the rate of biological production is much greater at lower trophic levels than it is at higher levels. Fisheries catches, at least to begin with, will tend to increase as the trophic level declines. At this point the fisheries will target species lower in the food web.[13] In 2000, this led Pauly and others to construct a "Fisheries in Balance" index, usually called the FiB index.[14] The FiB index is defined, for any year y, by[5]

FiB_y = log((Y_y/(TE)^{TL_y})/(Y_0/(TE)^{TL_0}))

where Y_y is the catch at year y, {TL}_y is the mean trophic level of the catch at year y, Y_0 is the catch and {TL}_0 the mean trophic level of the catch at the start of the series being analyzed, and TE is the transfer efficiency of biomass or energy between trophic levels.

The FiB index is stable (zero) over periods of time when changes in trophic levels are matched by appropriate changes in the catch in the opposite direction. The index increases if catches increase for any reason, e.g. higher fish biomass, or geographic expansion [5] Such decreases explain the “backward-bending” plots of trophic level versus catch originally observed by Pauly and others in 1998.[13]

See also

References

  1. ^ Butz SD (2002) Science of Earth Systems Page 537. Cengage Learning. ISBN 9780766833913
  2. ^ Dover CV (2000) The ecology of deep-sea hydrothermal vents Page 117. Princeton University Press. ISBN 9780691049298
  3. ^ a b Lisowski M, Miaoulis I, Cyr M, Jones LC, Padilla MJ, Wellnitz TR (2004) Prentice Hall Science Explorer: Environmental Science, Pearson Prentice Hall. ISBN 9780131150904
  4. ^ a b American Heritage Science Dictionary, 2005. Houghton Mifflin Company.
  5. ^ a b c d e Pauly, D.; Palomares, M.L. (2005). "Fishing down marine food webs: it is far more pervasive than we thought". Bulletin of Marine Science 76 (2): 197–211. http://www.seaaroundus.org/Journal/2005/FishingDownMarineFoodWebItisfarMorePervasive.pdf. 
  6. ^ Odum WE and Heald EJ (1975) "The detritus-based food web of an estuarine mangrove community". Pages 265–286 in L. E. Cronin, ed. Estuarine research. Vol. 1. Academic Press, New York.
  7. ^ Pimm, SL; Lanton, JH (1978). "On feeding on more than one trophic level". Nature 275 (5680): 542–544. doi:10.1038/275542a0. http://www.nature.com/nature/journal/v275/n5680/abs/275542a0.html. 
  8. ^ Cortés, E (1999). "Standardized diet compositions and trophic levels of sharks". ICES J. Mar. Sci. 56 (5): 707–717. doi:10.1006/jmsc.1999.0489. 
  9. ^ Pauly, D; Trites, A; Capuli, E; Christensen, V (1998). "Diet composition and trophic levels of marine mammals". ICES J. Mar. Sci. 55 (3): 467–481. doi:10.1006/jmsc.1997.0280. 
  10. ^ Millennium Ecosystem Assessment (2005) Ecosystems and Human Well-being: Synthesis Island Press. Page 32–33.
  11. ^ Sethi, SA; Branch, TA; Watson, R (2010). "Fishery development patterns are drive by profit, not trophic level". Proceedings of the National Academy of Sciences 107 (27): 12163–12167. doi:10.1073/pnas.1003236107. 
  12. ^ Branch, TA et al.; Watson, Reg; Fulton, Elizabeth A.; Jennings, Simon; McGilliard, Carey R.; Pablico, Grace T.; Ricard, Daniel; Tracey, Sean R. (2010). "Trophic fingerprint of marine fisheries". Nature 468 (7322): 431–435. doi:10.1038/nature09528. PMID 21085178. 
  13. ^ a b Pauly, D; Christensen, V; Dalsgaard, J; Froese, R; Torres, FC Jr (1998). "Fishing down marine food webs". Science 279 (5352): 860–863. doi:10.1126/science.279.5352.860. PMID 9452385. 
  14. ^ Pauly, D; Christensen, V; Walters, C (2000). "Ecopath, Ecosim and Ecospace as tools for evaluating ecosystem impact of fisheries". ICES J. Mar. Sci. 57 (3): 697–706. doi:10.1006/jmsc.2000.0726. 

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Modelling ecosystems – trophic components
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