Everything that happens in an ecosystem depends on the initial importing of energy and nutrients by primary producers. These are called autotrophs, meaning 'self-feeders'. As ecologists assign every organism in an ecosystem to a feeding level, or trophic level, depending on whether it is a producer or a consumer and on what it eats or decomposes, producers belong to the first trophic level. The autotrophs are capable of synthesizing organic molecules from inorganic precursors, and of storing biochemical energy in the process. Organisms able to manufacture complex organic molecules from simple inorganic compounds (water, CO2, nutrients) are found today primarily in three important groups: multicellular plants, chiefly in terrestrial environments but with some representatives in aquatic systems, algal protists (many different groups), and blue-green bacteria (also called cyanobacteria) in aquatic settings. The process by which they do this usually is photosynthesis, and as its name implies, photosynthesis requires light. For completeness, we should mention the pathway known as chemosynth-esis. Some producer organisms, mostly specialized bacteria, can convert inorganic nutrients to organic compounds without the presence of sunlight. There are several groups of chemosynthetic bacteria in marine and freshwater environments, particularly those rich in sulfur or hydrogen sulfide gas. Like chlorophyll-bearing plants and other organisms capable of photosynthesis, chemo-synthetic organisms are autotrophs.
Many organisms can only obtain their energy by feeding on other organisms. These are called primary consumers, or heterotrophs, meaning 'other-feeders'. Primary consumers belong to the second trophic level and they include consumers of any organism in any form: plants, animals, microbes, even dead tissue. There are several classes of consumers, depending on their food sources: herbivores (plant-eaters) feed directly on producers and carnivores (meat-eaters) feed on other consumers. The secondary consumers feed only on primary consumers, and they belong to the third trophic level. Most secondary consumers are animals but with few exceptions like the Venus' flytrap (carnivorous plant). Tertiary (higher-level) consumers feed only on other carnivores. Omnivores are consumers that eat both plants and animals, for example, pigs, rats, foxes, bears, cockroaches, and humans. These consumers typically hunt and kill live prey. On the contrary, the so-called scavengers feed on dead organisms that have either already been killed by other organisms or have died naturally, for example, vultures, flies, crows, hyenas, some species of shark, beetles, and ants. Detritivores (decomposers and detritus feeders) live off detritus, parts of dead organisms, and cast-off fragments and the waste of living organisms. Decomposers, mostly bacteria and fungi, are consumers that recycle organic matter in ecosystems by breaking down dead organic material (detritus) to get nutrients and release the resulting simpler inorganic compounds into the soil and water, where they can be taken up as nutrients by producers. This is also known as biodegradability. Detritus feeders, such as crabs, carpenter ants, termites, earthworms, and wood beetles, extract nutrients from partly decomposed organic matter.
Both producers and consumers use the chemical energy stored in glucose and other compounds to fuel their life processes. In most cells, this energy is released by the chemical process of aerobic respiration, which uses oxygen to convert organic nutrients back into carbon dioxide and water.
The net chemical change for aerobic respiration is the opposite of that for photosynthesis, which takes place during the day, when sunlight is available, whereas aerobic respiration can happen during the day or at night. Some decomposers get the energy they need through the breakdown of glucose (or other nutrients) in the absence of oxygen. This form of cellular respiration is called anaerobic respiration or fermentation. Instead of carbon dioxide and water, the end products of this process are compounds such as methane gas (CH4), ethyl alcohol (C2H6O), acetic acid (the main component of vinegar, C2H4O2), and hydrogen sulfide (H2S, when sulfur compounds are broken down).
The survival of any individual organism depends on the flow ofmatter and energy through its body. However, organisms in an ecosystem survive primarily through a combination of matter recycling and one-way energy flow. Decomposers complete the cycle of matter by breaking down detritus into inorganic nutrients that are usable by producers. Each type oforganism in an ecosystem uniquely plays a role in this flow ofenergy and in the flow and eventual recycling of matter.
Even if they feed on the same tissues (either plant or animal) as a source of energy, not all consumer organisms have the same efficiency of transforming energy consumed into secondary production. Of the food ingested by a consumer, a portion is assimilated across the gut wall, and the remainder is expelled from the body as waste products. Of the energy that is assimilated, some is utilized in respiration, while the remainder goes to production, which includes both the production of new tissues and reproduction. The ratio ofassimilation to ingestion, the assimilation efficiency, is a measure of the efficiency with which the consumer extracts energy from food. The ratio of production to assimilation, the production efficiency, is a measure of the efficiency with which the consumer incorporates assimilated energy into secondary production.
The ability of a consumer to convert the energy it ingests varies with species and the type of consumer. Insects that feed on plant tissues, such as grasshoppers, are more efficient producers than insects that feed on plant juices, such as aphids. Larval stages of insects are more efficient producers than the adult stage. Endotherms have a high assimilation efficiency, but because they use about 98% of that energy in metabolism, they have poor production efficiency. Ectotherms use about 79% of their assimilation in metabolism. They convert a greater portion of their assimilated energy into biomass than do endotherms. The difference, however, is balanced by assimilation efficiency. Ectotherms have an efficiency of around 30% in digesting food, whereas endotherms have an efficiency of around 70%.
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