Saprotrophic Channels of Food Webs in the

Detritus in the sea is the nutritional base of some large animals and important fisheries, while on land only a limited number of species of small arthropods are detriti-vores, and fungi are the major consumers of wood detritus. Termites are one animal group that thrives on wood. Termites have invited themselves to this exclusive feast by evolving mutualistic interactions with bacterial and other symbionts, which live in the termite gut. These

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Figure 4 A larger fraction of primary productivity is consumed by aquatic than terrestrial herbivores. Algae and other phytoplankton are more nutritious to their consuming herbivores because they have more nitrogen per unit carbon than terrestrial plants, which produce large amounts of complex cellulose and lignin. The lower C/N ratio of aquatic primary productivity is more similar to animals and yields the higher nutritional quality. Reproduced from Shurin JB, Gruner DS, and Hillebrand H (2006) All wet or dried up? Real differences between aquatic and terrestrial food webs. Proceedings of the Royal Society B - Biological Sciences 273(1582): 1-9, figure 1.

microbes can digest complex cellulose and some lignin. Ruminant mammals are another group that uses bacterial symbionts to digest cellulose in plant cell walls, but they do not feed upon wood. However, ruminants have a distinctly biotrophic side and rely heavily upon the more nutritious living components of plant cells. The living components are mainly photosynthetic enzymes that are rich in usable nitrogen. Once plants die, bacteria quickly consume this nutritious fraction, stripping out much of the nitrogen. The remaining straw and wood comprise the lion's share of terrestrial detritus.

A simplified food web for the Bering Sea is shown in Figure 5. The autotrophic species are tiny algae of two kinds. About half of primary productivity of this ecosystem is due to phytoplankton, buoyant algae living in the upper few meters of the sea. The other half of primary productivity is due to ice algae, which live mostly just inside the lower surface of sea ice; they sometimes grow into long filaments extending downward into the water. Both of these algae have C:N ratios typical of oceanic primary producers. The arrows in Figure 5 indicate progress from algae to herbivores along the biotrophic channel of the food web of the Bering Sea. The herbivores are small pelagic crustaceans, decapods and copepods, which live high in the water column with their food. Also living just beneath the ice and near the sea surface

Saprotrophic

Figure 5 A simplification of the Bering Sea food web. One important point is that a substantial fraction of detritus reenters the biotrophic food web via deposit feeding invertebrates that live on the floor of the sea. Organisms such as basket stars, tunicates, bottom-dwelling shrimp, a variety of clams, sand dollars, and worms feed upon bacteria that consume detritus falling to the bottom, and these deposit feeders are themselves fed upon by crabs, ground fish, and marine mammals. On land, the lion's share of plant detritus remain in the saprotrophic web, and far less is cycled back into the biotrophic food web. 1, Ice algae; 2, phytoplankton; 3, copepods; 4, mysidsand euphausids; 5, medusae; 6, hyperid amphipods; 7, sea birds; 8 and 9, pelagic fishes; 10, walrus; 11, seals; 12, basket stars; 13, ascidians; 14, shrimps; 15, filter-feeding bivalves; 16, sand dollars; 17, sea stars; 18, crabs; 19, bottom-feeding fishes; 20, polychaetes; 21, predatory gastropods; 22, deposit-feeding bivalves. Reproduced from Trites AW (2003) Food webs in the ocean: Who eats whom and how much? In: Sinclair M and Valdimarsson G (eds.) Responsible Fisheries in the Marine Ecosystem, ch. 8, pp. 125-143, figure 8.1. Rome: FAO.

Figure 5 A simplification of the Bering Sea food web. One important point is that a substantial fraction of detritus reenters the biotrophic food web via deposit feeding invertebrates that live on the floor of the sea. Organisms such as basket stars, tunicates, bottom-dwelling shrimp, a variety of clams, sand dollars, and worms feed upon bacteria that consume detritus falling to the bottom, and these deposit feeders are themselves fed upon by crabs, ground fish, and marine mammals. On land, the lion's share of plant detritus remain in the saprotrophic web, and far less is cycled back into the biotrophic food web. 1, Ice algae; 2, phytoplankton; 3, copepods; 4, mysidsand euphausids; 5, medusae; 6, hyperid amphipods; 7, sea birds; 8 and 9, pelagic fishes; 10, walrus; 11, seals; 12, basket stars; 13, ascidians; 14, shrimps; 15, filter-feeding bivalves; 16, sand dollars; 17, sea stars; 18, crabs; 19, bottom-feeding fishes; 20, polychaetes; 21, predatory gastropods; 22, deposit-feeding bivalves. Reproduced from Trites AW (2003) Food webs in the ocean: Who eats whom and how much? In: Sinclair M and Valdimarsson G (eds.) Responsible Fisheries in the Marine Ecosystem, ch. 8, pp. 125-143, figure 8.1. Rome: FAO.

are the primary carnivores, amphipods and jelly fish, which eat the herbivores. Note that decapods eat both phytoplankton and herbivorous copepods. The decapods are omnivores, which consume species both at the next trophic position as well as species at least one trophic position removed. Omnivory is common in nature and is one of the two kinds of links that weave the threads of food chains into fabrics of food webs. Polyphagy is another process that makes food webs from food chains. It happens when a species eats more than one species at the next trophic position. The first kind of pelagic fish in Figure 5 is omnivorous; it eats decapods and copepods. Sea birds and seals are prominent omnivores and secondary consumers in the biotrophic chain of the Bering Sea.

Detritus is the fate of a large minority if not a majority of primary production in the Bering Sea and in other places in the ocean. The phytoplankton and ice algae that are not consumed by herbivores settle to the bottom of the sea and enter the saprotrophic chain. Detritus fuels a rich and productive saprotrophic web in the Bering Sea. The primary beneficiaries of detritus are bottom-dwelling invertebrates, including basket stars, tunicates, bottom-dwelling shrimp, a variety of clams, sand dollars, and worms. The species that consume these are sea stars, predatory gastropods, and bottom-dwelling fish in the case shown in Figure 5. The higher consumers, which specialize on secondary detrital consumers, include sea stars, crabs, and walrus. Seals are a fascinating connection between the biotrophic and saprotrophic web in these data. Top-down and bottom-up forces are intertwined with unknown relative strengths in the Bering Sea food web.

Detritus is the entire source of energy for the most extensive habitat on Earth, the aphotic abyssal plains of the oceans, which are too deep for light penetration and photosynthesis. These profundal depths are home to extraordinary biodiversity that runs on quite low rates of energy input from above. A diverse community of mesopelagic middlemen serves as trophic merchants between the photic and aphotic abyss of the oceans. Many of these migrate as far as 500 m daily in a nightly trek for the higher concentrations of energy in the photic zones followed by daily retreat to the safety from predation of the lightless depths: krill, copepods, shrimp, ostracods, amphipods, jellyfish, siphonophores, comb jellies, larvaceans, ptertopods, squid, arrow worms, and fish comprise the community of circadian trophic go-betweens. Whales and large fish dive to feed upon these dense photically fueled populations that take refuge in the mesopelagic zones.

Although the food webs of the rich mesopelagic communities are little studied, they must contribute substantially to the energy budgets of the profundal species below, which do not make the trek to the surface. Below about 500 m, falling oxygen tensions contribute to the slower pace of life, and organisms that live deep in the sea have lower metabolic rates than those living at the surface. Benthic organisms of the vast abyssal plains, from 1000 to 11 000 m, include meiofauna that consume detri-tivorous bacteria, which are the living base of the profundal food web. These bacteria subsist upon the extraordinarily lean fare of the light particulate and dissolved organic material falling from above. This saprotrophic source fuels a biotrophic web of many species but largely unknown interactions. The meiofauna are consumed by deposit feeders, brittlestars, crustaceans, clams, polychaetes, and sea cucumbers. Fish are extremely rare but diverse. They are predators of rare prey and have elaborate adaptations for reduction of metabolic rate: loss of the swim bladder, soft bones, loss of scales, few spines, weak musculature, and watery flesh. These are patient predators of lassitude that wait long periods for the rare prey item. Top-down forces are yet to be studied in the abyssal food webs, but it is a good bet that fishing will rapidly deplete these food webs.

Increasingly heavy fishing by humans may be reducing top-down forces in marine food webs. Fishing tends to remove large predators that have strong top-down effects. The statistical patterns indicating top-down effects, negative correlations between primary productivity and catches of large fish, are the same in fisheries and lakes with four trophic links, referred to above. The large fish suppress numbers of small fish, which allows zooplankton to thrive and suppress phytoplankton density. Recent research suggests that these negative correlations switch to positive as the large predators are fished down.

See also: Trophic Structure.

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Responses

  • leland
    Is phytoplankton a saprotrophic?
    4 years ago

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