Tritrophic

 Fish at higher trophic levels usually have a higher economic value, which can result 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.^[20] However, more recent work finds no relation between economic value and trophic level;^[21] 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.^[22] However Pauly et al. note that trophic levels peaked at 3.4 in 1970 in the northwest and west-central Atlantic, followed by a subsequent decline to 2.9 in 1994. They report a shift away from long-lived, piscivorous, high-trophic-level bottom fishes, such as cod and haddock, to short-lived, planktivorous, low-trophic-level invertebrates (e.g., shrimp) and small, pelagic fish (e.g., herring). This shift from high-trophic-level fishes to low-trophic-level invertebrates and fishes is a response to changes in the relative abundance of the preferred catch. They consider that this is part of the global fishery collapse,^[17][23] which finds an echo in the overfished Mediterranean Sea.^[24]

Humans have a mean trophic level of about 2.21, about the same as a pig or an anchovy.^[25][26]

FiB index

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Since biomass transfer efficiencies are only about 10%, it follows that the rate of biological production is much greater at lower trophic levels than it is at higher levels. Fisheries catch, 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.^[23] In 2000, this led Pauly and others to construct a "Fisheries in Balance" index, usually called the FiB index.^[27] The FiB index is defined, for any year y, by^[8]

${\displaystyle Fi{B}_y=\log \frac{Y_y/(TE{)}^{T{L}_y}}{Y_0/(TE{)}^{T{L}_0}}}$

where ${\displaystyle Y_y}$ is the catch at year y, ${\displaystyle T{L}_y}$ is the mean trophic level of the catch at year y, ${\displaystyle Y_0}$ is the catch, ${\displaystyle T{L}_0}$ the mean trophic level of the catch at the start of the series being analyzed, and ${\displaystyle TE}$ 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.^[8] Such decreases explain the "backward-bending" plots of trophic level versus catch originally observed by Pauly and others in 1998.^[23]

Tritrophic and other interactions

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Further information: Tritrophic interactions in plant defense

One aspect of trophic levels is called tritrophic interaction. Ecologists often restrict their research to two trophic levels as a way of simplifying the analysis; however, this can be misleading if tritrophic interactions (such as plant–herbivore–predator) are not easily understood by simply adding pairwise interactions (plant-herbivore plus herbivore–predator, for example). Significant interactions can occur between the first trophic level (plant) and the third trophic level (a predator) in determining herbivore population growth, for example. Simple genetic changes may yield morphological variants in plants that then differ in their resistance to herbivores because of the effects of the plant architecture on enemies of the herbivore.^[28] Plants can also develop defenses against herbivores such as chemical defenses.^[29]