12.7: Food Webs

Heterotrophic species from the smallest microbe to the largest whale depend on autotrophs to produce organic matter for food. Herbivores eat primary producers directly, whereas carnivores eat herbivores and other carnivores.

In the simplified food chain in Figure 12-10, photosynthetic organisms are eaten by herbivorous zooplankton, which are eaten by carnivores, which are eaten by larger carnivores, and so on. Each step in this food chain is a trophic level. As organisms at one trophic level are consumed by those at the next-higher trophic level, the ingested food biomass is not used entirely to create biomass of the consumer species at the higher trophic level. An average of only about 10% (varying from 1% to 40%) of food consumed at each level is used for growth. The remaining 90% is used during respiration to provide the consumer with energy or is excreted as waste. Clearly, the transfer of food energy in food chains is inefficient (CC15).

Figure 12-10. A simple food chain. Phytoplankton are eaten by herbivorous zooplankton, which are eaten by carnivorous zooplankton, which are eaten by small fishes, which are eaten by large fishes, which are eaten by seals, which are eaten by killer whales. Each step in this chain is one trophic level higher.

The consequences of food chain inefficiency are illustrated by the food chains in Figure 12-11. A good-sized tuna sandwich may contain about 100 g of tuna. For each 100 g of tuna produced, a tuna must eat about 1000 g of a large fish (e.g., mackerel). In turn, mackerel must eat 10 times their weight of smaller fishes (e.g., herrings), herrings must eat 10 times their weight of zooplankton (e.g., copepods), and copepods must eat 10 times their weight of phytoplankton (e.g., diatoms). The next time you eat a tuna sandwich, remember that phytoplankton synthesized about 1 million g (1 metric ton, or tonne) of phytoplankton biomass to produce your 100 g of tuna. In contrast, 100 g of sardines represent only 10 kg of primary production (Fig. 12-11b). We use ocean food resources 100 times more efficiently when we eat sardines instead of tuna.

Figure 12-11. The food chains leading to (a) tuna and humans, and (b) sardines and humans, assuming that the trophic efficiency at each trophic level is 10%. Phytoplankton must photosynthesize 1 kg of organic matter to feed the human 100 g of sardines, but must photosynthesize 1000 kg (1 tonne) of organic matter to feed the human 100 g of tuna. However, because sardine species feed on both phytoplankton and zooplankton, their overall trophic efficiency is somewhat less than depicted. In addition, some tuna feed on both small and large fishes, so their overall trophic efficiency is somewhat better than depicted.

Marine animals usually eat organisms that are not much smaller than themselves. Therefore, most large marine animals are high-trophic-level predators at the top of long food chains. Baleen whales are spectacular exceptions. The blue whale, the largest known animal, can be 30 m long and weigh more than 100 tonnes (Fig 12-25b). This magnificent creature eats only shrimplike crustaceans called krill (Fig 12-19b), each of which is only a few centimeters long. Thus, blue whales are at the same trophic level as herrings (Figs. 12-11a, 12-12).

Figure 12-12. Simplified representation of the pelagic food web of the Southern Ocean. The number beside each arrow represents the trophic level at which the consuming organism is feeding. Note that many species feed at more than one level. For simplicity in this diagram, a number of pathways have been left out. For example, pelagic fishes feed on various prey including phytoplankton, zooplankton, and krill.

Food relationships in the ocean are far more complex than the simplified food chains depicted in Figure 12-11 because they almost always include opportunistic carnivores that eat animals from several different trophic levels. The killer whale in the Antarctic marine ecosystem is a good example (Fig. 12-11). Complex food relationships, such as that shown in Figure 12-12, are called food webs. Food webs are further complicated by omnivorous species that will eat almost any living organism and sometimes even detritus. Detritus feeders eat particulate organic matter produced as waste products and/or dead tissues of organisms of all trophic levels. Certain detritus-based food webs are somewhat independent of phytoplankton-based food webs, although they depend ultimately on primary producers to synthesize organic matter that becomes their detrital food.

There are also microbial food loops. By some estimates, about one-half of the primary production in the oceans is performed by microscopically small phototrophic eukaryotes and bacteria. The microbial loop also uses dissolved and particulate organic matter. Heterotrophic bacteria and archaea can absorb this material and utilize it to grow. These organisms may then be consumed by protozoa and by other organisms that are themselves consumed by organisms in the higher levels of the food chain. Free-living heterotrophic bacteria in the water column help to recycle elements lost from the food web through the microbial loop, or by releasing them as they consume organic matter for energy and growth.