Energy flow, Food chains and food webs

  • The source of energy for all life on earth is the sun.
  • The sun provides light energy and heat energy.
  • Heat energy is used by cold blooded organisms to keep their body warm.

The non-cyclical nature of energy flow

  • Energy cannot ‘flow’ in a cycle, because it cannot be recycled as the materials can.
  • The sun supplies light energy which is absorbed by the chlorophyll during photosynthesis where light energy is converted to chemical energy in food molecules.
  • The chemical energy in the form of food is passed onto animals when they feed on plants and uses some of the energy for metabolic processes and some energy is lost to the environment (as heat from respiration and during excretion)
  • In this way the sun energy enters then flows through the ecosystem and gradually lost to the environment as it passes from one organism to another.
  • Energy does not get passed back to the producers. The lost energy cannot be recycled.

The non-cyclical nature of energy flow

Food chains

  • A food chain is a sequence of organisms starting with a photosynthesizing organism through which energy is passed as one organism is eaten by the next in sequence.
  • Usually all food chains begin with a photosynthesizing organism called the producers.
  • Producers are organisms which manufacture and supply energy rich foods made by photosynthesis to all organisms in their food chain.
  • Eg.: Grass  →       grasshopper  →           snake      →         hawk

Food Chain

  • The organisms which rely on the energy supplied by the producer in particular food chain are called consumers.
    • Consumers which feed directly on the producers are called primary consumers. Sometimes they can be herbivores as well.
    • Consumers which feed directly on the primary consumers are called secondary consumers.
    • Consumers which feed on the secondary consumers are called tertiary consumers
  • All consumers above the level of herbivores are called carnivores.
  • When organisms in a food chain die, they are decomposed by saprotrophic organisms usually bacteria and fungi.
  • Decomposers are organisms which release enzymes to break down large molecules in dead organic matter into smaller ones which can then be recycled.
  • In a food chain the row indicates the flow of energy from one organism to another
  • In this food chain;
    • The producer is the grass
    • Primary consumer and herbivore is the grasshopper
    • Secondary consumer is the snake
    • grasshopper and snake are the consumers.
  • The position occupied by an organism on a food chain is called trophic level.
  • Grass is in the first trophic level, grasshopper in the second trophic level and snake in the third trophic level.
  • When all three organisms die decomposers act upon the dead organic matter.

nutrients cycle

Energy loss along food chains

  • Some of the energy locked away by the producers is released by the producer itself through the process of respiration.
  • Some of the energy is used by producers in the process of cell division, growth and reproduction.
  • A lot of energy is still present when the plant dies and is then available to decomposers.
  • So only about 10% of a plants available energy is passed onto an herbivore which eats it.
  • Herbivores the release the energy by respiration and use it for growth, movement and as heat to maintain body temperature.
  • Much of the energy is still present in the faces of herbivores and some in nitrogenous waste.
  • This is available to decomposers.
  • At each trophic level the consumer lose about 90% of the energy consumed mainly as heat energy in respiration and a small amount lost through urine and feces.
  • Therefore successive members of the food chain incorporate only 10% of the energy available in the organism they consume in to their body as new tissues

Energy loss along food chains

Longer food chains versus shorter food chains

  • The longer the food chains the more energy will be lost and less will be available for high consumers.
  • Shorter food chains help to conserve energy and are much more energy-efficient than long ones.
  • In shorter food chains organisms have enough energy to survive.

Food web

  • A food web is made up of interlinked food chains involving organisms within the same ecosystem.
  • If one organism in the food web is affected then it affects the other organisms too.
  • Foodwebs are easily unbalanced, especially if one population of organisms in the web dies or disappe This may happen for a number of reasons, including:
    • over–predation or hunting
    • Disease
    • Pollution
    • Use of pesticides
    • Lack of food (or other resources)
    • Emigration.

food web

Ecological pyramids

Ecological pyramids are quantitative ways of representing relationships between organisms in an ecosystem, built up from bars stacked above another. It is standard practice to place producers at the bottom of the pyramid, with primary consumer, secondary consumers, tertiary consumers and top carnivores above them. The width of each bar should be proportional to the quantity that is being displayed, and height is usually the same for each bar. It is possible to produce pyramids to describe either food chain or a food web.

Pyramid of numbers

Pyramid of numbers is the graphical representation of number of individuals per unit area of various trophic levels stepwise with producers forming the base and top carnivores at the tip.

The number of organism at each trophic level in a food chain can be represented by a bar chart called pyramid of numbers. Each block represents trophic level. The width of each block represents number of organism at the trophic level in the food chain. Generally when moving from producers to primary consumers to top consumers the number of organisms at each trophic level decreases. This is due to the large amount of energy loss at each trophic level.

Sometimes the pyramid of numbers is not in pyramid shape, e.g. one big plant could support many insects, so the base of the pyramid in narrower than 2nd trophic level.

A pyramid of numbers involving parasites at the end of a food chain will have a wide apex at the top of the pyramid.

Pyramid of numbers

Problems with pyramid of numbers

  • The size of the producer varies and yet one grass plant is given the same status as one tree.
  • The range of numbers from the producers to the top carnivores may be so great that it is impossible to represent the pyramid to scale.
  • To overcome these problems a pyramid of biomass may be used.

Pyramid of biomass

Sometimes a pyramid of numbers is not the best way to represent a food chain. In this case a pyramid of biomass is a better diagram to use. It shows the total mass of organisms at each stage of a food chain.In general, all producers have a higher biomass than the primary consumer, so a pyramid will always be produced.

The total energy (and biomass) present at a lower tier of the pyramid, must be greater than the higher tiers in order to support the energy requirements of the subsequent organisms.

Pyramid of biomass

Other pyramid shapes

Sometimes the pyramid of numbers doesn’t look like a pyramid at all. This could happen if the producer is a large plant such as a tree, or if one of the animals is very small.  Remember, though, that whatever is the situation, the producer still goes at the bottom of the pyramid.