(20) Environmental Science

 Food Chains ,Food Webs,Energy Pyramid in Ecosystems- Video for Kids


What Happens to Energy in an Ecosystem?

CONCEPT 3-5 As energy flows through ecosystems in food chains and webs, the amount of chemical energy available to organisms at each succeeding feeding level decreases.

Energy Flows through Ecosystems in Food Chains and Food Webs

All organisms, whether dead or alive, are potential sources of food for other organisms. A caterpillar eats a leaf, a robin eats the caterpillar, and a hawk eats the robin. Decomposers consume the leaf, caterpillar robin, and hawk after they die and return their nutrients to the soil for reuse by producers.

A sequence of organisms, each of which serves as a source of food for the next, is called a food chain. It determines how chemical energy and nutrients move from one organism to another and return their nutrients to the soil for reuse by producers through thetrophic levels in an ecosystem along the same pathways-primarily through photosynthesis, feeding, and decomposition.

In natural ecosystems, most consumers feed on more than one type of organism, and most organisms are eaten by more than one type of consumer. Because of this, organisms in most ecosystems form a complex network of interconnected food chains called a food web. Trophic levels can be assigned in food webs just as in food chains. Food chains and webs show how producers, consumers, and decomposers are connected to one another as energy flows through trophic levels in an ecosystem.

Usable Energy Decreases with Each Link in a Food Chain or Web

Each trophic level in a food chain or web contains a certain amount of biomass, the dry weight of all organic matter contained in its organisms. In a food chain or web, chemicalenergy stored in biomass is transferred from one trophic level to another.

Energy transfer through food chains and food webs is not very efficient because, with each transfer, some usable chemical energy is degraded and lost to the environment as low-quality heat, as a result of the second law of thermodynamics. In other words, as energy flows through ecosystems in food chains and webs, there is a decrease in the amount of chemical energy available to organisms at each succeeding feeding level (Concept 3-5).

The percentage of usable chemical energy transferred as biomass from one trophic level to the next is called ecological efficiency. It ranges from 2% to 40% (that is, a loss of 60–98%) depending on what types of species and ecosystems are involved, but 10% is typical.

Assuming 10% ecological efficiency (90% loss of usable energy) at each trophic transfer, if green plants in an area manage to capture 10,000 units of energy from the sun, then only about 1,000 units of chemical energy will be available to support herbivores, and only about 100 units will be available to support carnivores.

The more trophic levels there are in a food chain or web, the greater is the cumulative loss of usable chemical energy as it flows through the trophic levels. The pyramid of energy flow illustrates this energy loss for a simple food chain, assuming a 90% energy loss with each transfer.


Energy Flow and the Second Law of Thermodynamics

Explain the relationship between the second law of thermodynamics and the flow of energy through a food chain or web.

Energy flow pyramids explain why the earth can support more people if they eat at lower trophic levels by consuming grains, vegetables, and fruits directly rather than passing such crops through another trophic level and eating grain eaters or herbivores such as cattle. About two-thirds of the world’s people survive primarily by eating wheat, rice, and corn at the first trophic level, mostly because they cannot afford meat.

The large loss in chemical energy between successive trophic levels also explains why food chains and webs rarely have more than four or five trophic levels. In most cases, too little chemical energy is left after four or five transfers to support organisms feeding at these high trophic levels.


Food Webs, Tigers, and Insects

(a) Why there are not many tigers in the world and (b) why there are so many insects (Core Case Study) in the world.

Some Ecosystems Produce Plant Matter Faster Than Others Do

The amount of life (biomass) that a particular ecosystem can support is determined by the amount of energy captured and stored as chemical energy by the producers of that ecosystem and how rapidly they can produce and store such chemical energy. Gross primary productivity (GPP) is the rate at which an ecosystem’s producers convert solar energy into chemical energy as biomass. It is usually measured in terms of energy production per unit area over a given time span, such as kilocalories per square meter per year (kcal/m2/yr).

To stay alive, grow, and reproduce, producers must use some of the chemical energy stored in the biomass they make for their own respiration. Net primary productivity (NPP) is the rate at which producers use photosynthesis to produce and store chemical energy minus the rate at which they use some of this stored chemical energy through aerobic respiration. In other words, NPP = GPP - R, where R is energy used in respiration.

NPP measures how fast producers can provide the nutrients or chemical energy stored in their tissue that is potentially available to other organisms (consumers) in an ecosystem.

Primary productivity is similar to the rate at which you make money, or the number of dollars you earn per year. Net primary productivity is like the amount of money earned per year that you can spend after subtracting your work expenses such as transportation, clothes, food, and supplies.

Ecosystems and life zones differ in their NPP. Despite its low NPP, the open ocean produces more of the earth’s biomass per yearthan any other ecosystem or life zone, simply because there is so much open ocean.

As we have seen, producers are the source of all nutrients or food in an ecosystem for themselves and for consumer organisms. Only the biomass represented by NPP is available as nutrients for consumers, and they use only a portion of this amount. Thus, the planet’s NPP ultimately limits the number of consumers (including humans) that can survive on the earth. This is an important lesson from nature.

Peter Vitousek, Stuart Rojstaczer, and other ecologists estimate that humans now use, waste, or destroy about 20–32% of the earth’s total potential NPP. This is a remarkably high value, considering that the human population makes up less than 1% of the total biomass of all of the earth’s consumers that depend on producers for their nutrients. These scientists contend that this is the main reason why we are crowding out or eliminating the habitats and nutrient supplies of so many other species and degrading or destroying some of the ecosystem services they provide.

Generalized pyramid of energy flow showing the decrease in usable chemical energy available at each succeeding trophic level in a food chain or web. In nature, ecological efficiency varies from 2% to 40%, with 10% efficiency being common. This model assumes a 10% ecological efficiency (90% loss in usable energy to the environment, in the form of low quality heat) with each transfer from one trophic level to another. Question: Why is a vegetarian diet more energy efficient than a meat-based diet?

Estimated annual average net primary productivity in major life zones and ecosystems, expressed as kilocalories of energy produced per square meter per year (kcal/m2/yr). Question: What are nature’s three most productive and three least productive systems?


Resource Consumption

What might happen to us and to other consumer species as the human population grows over the next 40-50 years and per capita consumption of resources such as food, timber, and grassland rises sharply? What are three ways to prevent this from happening?


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