"Producers, Consumers, and Decomposers"P1 Organisms that ar

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问题 "Producers, Consumers, and Decomposers"
P1 Organisms that are capable of using carbon dioxide as their sole source of carbon are called autotrophs (self-feeders), or producers. These are the plants. They chemically fix carbon through photosynthesis. Organisms that depend on producers as their carbon source are called heterotrophs (feed on others), or consumers. Generally, these are animals. From the producers, which manufacture their own food, energy flows through the system along a circuit called the food chain, reaching consumers and eventually detritivores. Organisms that share the same basic foods are said to be at the same trophic level. Ecosystems generally are structured in a food web, a complex network of interconnected food chains. In a food web, consumers participate in several different food chains, comprising both strong interactions and weak interactions between species in the food web.
P2 Primary consumers feed on producers. A Because producers are always plants, the primary consumer is called an herbivore, or plant eater. A carnivore is a secondary consumer and primarily eats meat. B A consumer that feeds on both producers (plants) and consumers (meat) is called an omnivore. C
P3 Detritivores (detritus feeders and decomposers) are the final link in the endless chain. D Detritivores renew the entire system by releasing simple inorganic compounds and nutrients with the breaking down of organic materials. Detritus refers to all the dead organic debris—remains, fallen leaves, and wastes—that living processes leave. Detritus feeders—worms, mites, termites, centipedes, snails, crabs, and even vultures, among others—work like an army to consume detritus and excrete nutrients that fuel an ecosystem. Decomposers are primarily bacteria and fungi that digest organic debris outside their bodies and absorb and release nutrients in the process. This metabolic work of microbial decomposers produces the rotting that breaks down detritus. Detritus feeders and decomposers, although different in operation, have a similar function in an ecosystem.
P4 An example of a complex community is the oceanic food web that includes krill, a primary consumer. Krill is a shrimplike crustacean that is a major food for an interrelated group of organisms, including whales, fish, seabirds, seals, and squid in the Antarctic region. All of these organisms participate in numerous other food chains as well, some consuming and some being consumed. Phytoplankton begin this chain by harvesting solar energy in photosynthesis. Herbivorous zoo-plankton such as krill and other organisms eat Phytoplankton. Consumers eat krill at the next trophic level. Because krill are a protein-rich, plentiful food, increasingly factory ships, such as those from Japan and Russia, seek them out. The annual krill harvest currently surpasses a million tons, principally as feed for chickens and livestock and as protein for human consumption.
Efficiency in a Food Web
P5 Any assessment of world food resources depends on the level of consumer being targeted. Let us use humans as an example. Many people can be fed if wheat is eaten directly. However, if the grain is first fed to cattle (herbivores) and then we eat the beef, the yield of available food energy is cut by 90% (810 kg of grain is reduced to 82 kg of meat); far fewer people can be fed from the same land area.
P6 In terms of energy, only about 10% of the kilocalories (food calories, not heat calories) in plant matter survive from the primary to the secondary trophic level. When humans consume meat instead of grain, there is a further loss of biomass and added inefficiency. More energy is lost to the environment at each progressive step in the food chain. You can see that an omnivorous diet such as that of an average North American and European is quite expensive in terms of biomass and energy.
P7 Food web concepts are becoming politicized as world food issues grow more critical. Today, approximately half of the cultivated acreage in the United States and Canada is planted for animal consumption—beef and dairy cattle, hogs, chickens, and turkeys. Livestock feed includes approximately 80% of the annual corn and nonexported soybean harvest. In addition, some lands cleared of rain forest in Central and South America were converted to pasture to produce beef for export to restaurants, stores, and fast-food outlets in developed countries. Thus, lifestyle decisions and dietary patterns in North America and Europe are perpetuating inefficient food webs, not to mention the destruction of valuable resources, both here and overseas.
Glossary
phytoplankton: a plant that lives in the sea and produces its own energy source
trophic level: category measured in steps away from the energy input in an ecosystem [br] According to paragraph 7, food webs are inefficient because

选项 A、consumers in developed nations prefer animal protein
B、politicians are not paying attention to the issues
C、there are not enough acres to grow crops efficiently
D、too much of the corn and soybean harvests are exported

答案 A

解析

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