Energy transfer describes the changes in energy (a state function) that occur between organisms within an ecosystem. Living organisms are constantly changing as they grow, move, reproduce, and repair tissues. These changes are fueled by energy. Plants, through photosynthesis, capture some of the sun’s radiant energy and transform it into chemical energy, which is stored as plant biomass. This biomass is then consumed by other organisms within the ecological food chain/web. A food chain is a sequence of organisms that are connected by their feeding and productivity relationships; a food web is the interconnected set of many food chains.
Energy transfer is a one-way process. Once potential energy has been released in some form from its storage in biomass, it cannot all be reused, recycled, or converted to waste heat. This means that if the sun, the ultimate energy source of ecosystems, were to stop shining, life as humans know it would soon end. Fortunately, every day, the sun provides new energy in the form of protons to sustain the food webs of Earth.
In 1927, British ecologist Charles Sutherland Elton (1900–1991) wrote that most food webs have a similar pyramidal shape. At the bottom, there are many photosynthetic organisms that collectively have a large biomass and productivity. On each of the following trophic levels, or feeding levels, there are successively fewer heterotrophic organisms, with a smaller productivity. The pyramid of biomass and productivity is now known as the Eltonian pyramid.
In 1942, after his death, American ecologist Raymond L. Lindeman (1915–1942) had a paper published, which was earlier rejected, that examined food webs in terms of energy flow. Lindeman proposed that, by using energy as the currency of ecosystem processes, food webs could be quantified. This allowed him to explain that the Eltonian pyramid was a result of successive energy losses associated with the thermodynamic inefficiencies of energy transfer among trophic levels.
Current research in ecological energy transfer focuses on increasing scientific understanding of the paths of energy and matter within grazing and microbial food webs. Rather little is understood about such pathways because of the huge numbers of species and their complex interactions. This understanding is essential for proper management of ecosystems. The fate and effects of toxic chemicals within food webs must be understood if impacts on vulnerable species and ecosystems are to avoided or minimized.
Energy transfers within food webs are governed by the first and second laws of thermodynamics. The first law relates to quantities of energy. It states that energy can be transformed from one form to another, but it cannot be created or destroyed. This law suggests that all energy transfers, gains, and losses within a food web can be accounted for in an energy budget.
The second law relates to the quality of energy. This law states that whenever energy is transformed, some of must be degraded into a less useful form. In ecosystems, the biggest losses occur as respiration. The second law explains why energy transfers are never 100% efficient. In fact, ecological efficiency, which is the amount of energy transferred from one trophic level to the next, ranges from 5 to 30%. On average, ecological efficiency is only about 10%.
Because ecological efficiency is so low, each trophic level has a successively smaller energy pool from which it can withdraw energy. This is why food webs have no more than four to five trophic levels. Beyond that, there is not enough energy to sustain higher-order predators.
A food web consists of several components; primary producers, primary consumers, secondary consumers, tertiary consumers, and so on. Primary producers include green plants and are the foundation of the food web. Through photosynthesis, primary producers capture some of the sun’s energy. The net rate of photosynthesis, or net primary productivity (NPP), is equal to the rate of photosynthesis minus the rate of respiration of plants. In essence, NPP is the
Biomass— Total weight, volume, or energy equivalent of all living organisms within a given area.
Ecological efficiency— Energy changes from one trophic level to the next.
First law of thermodynamics— Energy can be transformed but it cannot be created nor can it be destroyed.
Primary consumer— An organism that eats primary producers.
Primary producer— An organism that photosynthesizes.
Second law of thermodynamics— When energy is transformed, some of the original energy is degraded into less useful forms of energy.
profit for the primary producer, after their energy costs associated with respiration are accounted for. NPP determines plant growth and how much energy is subsequently available to higher trophic levels.
Primary consumers are organisms that feed directly on primary producers, and these comprise the second trophic level of the food web. Primary consumers are also called herbivores, or plant-eaters. Secondary consumers are organisms that eat primary consumers, and are the third trophic level. Secondary consumers are carnivores, or meat-eaters. Successive trophic levels include the tertiary consumers, quaternary consumers, and so on. These can be either carnivores or omnivores, which are both plant- and animal-eaters, such as humans.
Much of the food web’s energy is transferred to the often overlooked microbial, or decomposer, trophic level. Decomposers use excreted wastes and other dead biomass as a food source. Unlike the main, grazing food web, organisms of the microbial trophic level are extremely efficient feeders. Various species can rework the same food particle, extracting more of the stored energy each time. Some waste products of the microbial trophic level re-enter the grazing part of the food web and are used as growth materials for primary producers. This occurs, for example, when earthworms are eaten by birds.
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