Energy flow, Food Chains and Food Webs Notes

 Learning objectives

At the end of this post, you should be able to:
- describe the flow of energy through living organisms referencing only to light energy from the sun and chemical energy in organisms and its transfer to the environment
- define producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
- define consumer as an organism that gets its energy by feeding on other organisms
- define the food chain as showing the transfer of energy from one organism to the next, beginning with the producer.
- construct simple food chains
- define food web as a network of interconnected food chains
- define herbivore as an organism that gets its energy by eating plants
- define carnivore as an animal that gets its energy by eating other animals
- define decomposer as an organism that gets its energy from dead or waste organic material
- define trophic level as the posítion of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass
- identify producers, primary, secondary, tertiary and quaternary consumers as the trophic levels in food chains, food web, pyramid of numbers and pyramids of biomass
- construct simple food chains
- describe how energy is transferred between trophic levels
- explain why food chains have usually fewer than five trophic levels
- describe and interpret pyramids of biomass
- explain why there is an increased efficiency in supplying green plants as human food compared to relative inefficiency in feeding crop plants to livestock that can be used as food.

 INTRODUCTION
All life depends upon a continuous supply of energy. Energy can never be produced/formed or made. Energy is released or given off by respiration in all living organisms. For the majority of organisms, such as most terrestrial and aquatic ecosystems, this energy comes from the sun. The energy flows in one direction through the ecosystems. Green plants and other photoautotrophs use sunlight to make food during photosynthesis. Plants are eaten by animals and other consumers, such as cows and goats. These animals are then eaten by other animals, such as lions and leopards. Each animal is dependent on another organism for its supply of nutrients. Some organisms, such as fungi and bacteria, break down the bodies of dead organisms so that the nutrients are recycled.

In todays post, we will study the way in which energy, in the form of food, is passed from one Iiving organism to another. We will also look at how each organism is dependent on another. Here are some important definitions used in this post:

Ecology
The study of interrelationships between organisms and their environment.

Ecosystem
The interaction of living (biotic) and non-living (abiotic) elements in any biological system.

 Trophic level
The position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass.

 Food chain
Shows the transfer of energy from one organism to the next, beginning with the producer.

 Food web
A network of interconnected food chains.

Producer
An organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.

 Consumer
An organism that gets its energy by feeding on other organisms.

Herbivore
An organism that gets its energy by eating plants (e.g. cattle, sheep).

Carnivore
An animal that gets its energy by eating other animals (e.g. eagle, lion).

Decomposer
An organism that gets its energy from dead or waste organic material (e.g. fungi, bacteria).

Detritivores/Scavengers
Animals that eat dead organic matter (e.g. vulture, jackal, earthworm).

When we study the way in which organisms interact with one another, we must also understand how they interact with the environment. This topic is a branch of Biology called Ecology. One very important way of studying living things is to study them in their natural surrounding/habitat/environment. Animals and plants do not live in complete isolation. They are affected by their surroundings, described as their environment. Their environment is also affected by them. The study of the interaction between living organisms and their environment is called ecology.

 Ecosystems
When you study ecology, naturally you will study a small area, such as a pond, an area of woodland or veld. One of these areas and the organisms that live in that area can be described as an ecosystem.

The living organisms in the pond, the water, the stones and the mud at the bottom, make up this ecosystem. Within an ecosystem, each living organism has its own life to live and role to play. In the pond, tadpoles, for example, do the following things:
• eat algae and other weeds
• disturb pebbles and mud at the bottom of shallow areas
• excrete ammonia into the water
• breathe in oxygen from the water
• breathe out carbon dioxide.
Another example of an ecosystem is a woodland. It is made up of different groups of plants and animals, for example, grasses, worms, ants, snails, snakes and birds.

 Energy flow
We need energy to grow, to stay warm and to move around. we get our energy from the food we eat. All of this energy originally came from the sun. Plants use light energy from the sun to make food by photosynthesis. Some of the energy is stored in the food, as chemical energy. When an animal eats a plant or other animals, both are taking in energy.

 Non-cyclical flow of energy
Unlike many other biological systems in which materials flow from one structure to another and back to the beginning, energy only flows one way. It does not flow backwards and return to the sun. We say that the energy flow through the ecosystem is non-cyclical.

Food chains and food webs
As we saw in the previous section, all living organisms need energy. They get energy from food, through respiration. All the energy in an ecosystem comes from the sun. Some of the energy in sunlight is captured by producers (plants), changed into chemical energy, and used to make food, such as glucose, starch, fats and proteins. These contain some of the energy from the sunlight. When the plant needs energy, it breaks down some of this food through respiration. Consumers (for example, animals) get their food, and therefore their energy, by eating plants, or by eating animals that have eaten plants.

 Food chains
A food chain is the transter of energy from one organism to the next, beginning with the producer, example, Grass (producer) ----------> grasshopper (primary consumer) ----------> frog (secondary consumer) -------------> python (tertiary consumer) --------------> eagle (quaternary consumer)

Refering to the example i gave you above, a food chain always begins with green plants because these are the only organisms that can capture energy from sunlight. They are called producers, because they produce food. Animals are consumers. An animal that eats plants is a primary consumer because it is the first consumer in the food chain. It is said to be at the first trophic level. An animal that eats the primary consumer is a secondary consumer, and so on along the chain. The following terms are important.

Producer:
an organism that makes its own organic nutrients, usually using energy from the sun, through photosynthesis, using carbon dioxide, water, minerals and sunlight energy.
Consumer:
an organism that gets its energy by feeding on other organisms. The consumers are divided into herbivores and carnivores.
Herbivore:
an organism that gets its energy by eating plants, these are primary consumers that obtain their energy by feeding directly on green plants, the producers. Examples of herbivores are cattle, chickens, snails, elephants and zebra.
Carnivore:
an animal that gets its energy by eating other animals; these are secondary consumers that obtain their energy by feeding on herbivores. Examples of carnivores are humans, lions, crocodiles and snakes. Some carnivores obtain their energy by feeding on other carnivores, they are tertiary consumers. Tertiary consumers are the fourth link ot a food chain. Examples are birds of prey, such as eagles and hawks. Quaternary consumers are the fifth link of a food chain. They are usually the carnivores that are top predators. Examples are snakes, eagles, leopards and hawks.
Decomposer:
an organism that gets its energy from dead or waste organic material; these are microorganisms that break down the dead remains of animals and plants and release the nutrients into the soil. Examples of decomposers are bacteria and fungi.

Food webs
Food webs are more complicated. They show how most living things eat more than one kind of food. A food web has a network of interconnected food chains. This means that many different food chains are linked together to form a food web. The is a diagram showing a food web below.

Trophic levels
A trophic level is the position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass. In the trophic levels in food chains, food webs, pyramid of numbers and pyramids of biomass the producers, primary, secondary, tertiary and quaternary consumers are found at different levels. These different levels are described as trophic levels. Energy is transferred through a food web from one trophic level to the next higher level.

Energy losses in food chains
The plants in a field use a lot of sunlight energy, and convert it into chemical energy in food. Every time energy passes from one organism to the next in a food chain, quite a lot of energy is wasted. Plants lose energy from the sun by reflection and heat energy, which is 90% of the total amount that comes from the sun. Basically the plants obtain only 10% and they use this 10% for energy transfer; this means that only 10% flows from the producers, which are plants, to the primary consumers and only 10% of the total energy flows from the primary consumers to the secondary consumers. However, energy is lost at each level during respiration, excretion and movement. On average only about one-tenth of the energy from one trophic level is passed on to the next trophic level.

When herbivores eat the plants, they do not get all of this energy from the plants. Only a part of the energy in the plants is passed on to the herbivores. There are several reasons for this:
• The plants will have used up some of the energy for themselves.
• The herbivores may:
- not eat all of the plants
- not eat all parts of the plants, for example, they may only eat the leaves
- not be able to digest all parts of the plants. When carnivores eat the herbivores, again only some of the energy is transterred to the carnivores' bodies. There are several reasons for this:
• The herbivores will have used up some of the energy for themselves, for example, for moving or for keeping warm; this energy is lost from their bodies as heat.
• Some of the energy in the herbivores bodies will have been lost in urine and faeces.
• The carnivores may not eat all of the herbivores.
• The carnivores may not be able to digest all of the parts of the herbivores.
The loss of energy along the food chain also limits the length of it. There are rarely more than five links in a chain, because there is not enough energy left to supply the next link. Many food chains only have three links. It is an advantage for food chains to be as short as possible, as this reduces the amount of energy that is lost from the ecosystem.

Pyramid of numbers
If we looked at one square kilometre of veld in, say, Etosha National Park, we might find that within that area there are millions of grass plants and other plants, a few antelope, buftalo, dassies, and just one lion and one leopard. We could show this information in a diagram called a pyramid of numbers. Each level in the pyramid is calleda trophic level. Figure below shows a pyramid of numbers.

Why is the pyramid this shape? You may remember that energy is lost as it passes along a food chain. There is a lot of energy available for the plants in the grassland, because there is a lot of sunlight falling on them. However, not all of this energy is available for the herbivores, because a lot of it gets wasted. So there has to be fewer antelope than grass plants, because there is less energy available for them. The same happens at the next stage. There are fewer carnivores than herbivores, because a lot of energy is wasted between the herbivores and the carnivores. A leopard needs a very large area to hunt in, so as to find enough food to supply energy to stay alive. A pyramid of numbers gives an indication of the number of organisms making up a food chain, but it does not tell us much about size or energy relationships. For example, one very large oak tree can be the producer and provide energy for many caterpillars. These provide food for a few small birds. If we drew a pyramid of numbers for this food chain, it would look like the figure below.

The number of organisms in a food chain can be represented graphically in a pyramid. Each level in the pyramid represents the number of individuals at each trophic level (feeding level) in the food chain. In this example, one oak tree provides food for lots of caterpillars and a large number of caterpillars living in a single oak tree provide food for several blue tits. The pyramid of numbers usually shows that the number of organisms at each trophic level gets smaller towards the top. This particular case is an exception - one tree provides food for many caterpillars. It usually takes a large number of plants to provide sufficient energy for the consumers in the food chain.

The pyramid of numbers has shortfalls in the sense that all organisms are counted regardless of their size; the number of some individuals are so great that it is impossible to represent them accurately on the same scale as the other species in the food chain. There is no count for juveniles and other immature forms of a species whose diet and energy requirements may differ from the adult. These difficulties are corrected by the pyramid of biomass and energy.

Pyramid of biomass
A pyramid of biomass is a more accurate indication of how much energy is passed on at each trophic level. Biomass is the dry mass of all organisms estimated at each trophic level. Pyramids of biomass usually look like typical pyramids, with each level smaller than the one below it.

The biomass in each trophic level is always less than the trophic level below, because biomass is a measure of the amount of food available. When animals eat, only some of their food is converted into new tissue, the food for the next trophic level. Most of the biomass that animals eat is either not digested, or used to provide the energy needed for staying alive.

 Pyramid of energy
Probably the most accurate and valuable way ot presenting data relating to the organisms in a food chain is by means of a pyramid of energy. To do this, one has to find the amount of energy within each organism in the food chain. This is done by burning a dead organism from each trophic level and by recording the temperature rise for a given quantity of water. From this, one can work out the amount of energy in the organism. lf this figure is multiplied by the number of organisms at each trophic level, a pyramid of energy can be constructed, as shown in figure below.

 Energy flow and agriculture
Understanding how energy is passed along a food chain can be useful in agriculture. We can eat a wide variety of foods, and can feed at several different trophic levels. Which is the most efficient sort of food for a farmer to grow, and for us to eat?

The nearer to the beginning of the food chain we feed, the more energy there is available for us. This is why our staple foods, maize, wheat, rice and potatoes, are plants. When we eat meat, eggs or cheese or drink milk, we are feeding further along the food chain. There is less energy available for us from the original energy provided by the sun. Figure above shows how one hectare of land can produce enough cereal crop to supply food for 100 people. However, the same area of land could be used to produce enough grass to feed cows. These cows would produce food to feed only 10 people. Although there is far more energy in the grass than in the cattle, it is not available to humans. We simply cannot digest the cellulose in grass, so we cannot release the energy from it. The cattle turn the energy in grass into energy in protein and fat, which we can digest. However, there are many plant products that we can eat. Soya beans, for example, yield a high amount of protein, much more efficiently and cheaply than cattle or other animals. A change towards vegetarianism would enable more food to be produced on the Earth, if the right crops were chosen. By studying the energy relationships between organisms in a food chain, we can see that there is an increased efficiency in supplying green plants as human food. It is relatively inefficient, in terms or energy loss, to feed crop plants to animals.

Summary
- The sun is the principal source of energy in a biological system.
- An ecosystem is the physical environment and  all the organisms that live in it in any particular area.
- Energy enters an ecosystem as sunlight. The energy is used by plants in photosynthesis, to make carbohydrates and other substances.
- When animals eat plants, they obtain some of this energy. The energy flows between living organisms in the form of food.
- Producers are green plants. They produce food at the beginning of food chains.
- Consumers are animals. They obtain their energy from other organisms. Consumers can be either herbivores or carnivores.
- Herbivores feed on plants, carnivores feed on
other animals.
- Decomposers are bacteria and fungi. They obtain their energy when they break down the bodies of dead organisms.
- A food chain is a series ot organisms, connected by a flow of energy. Each organism is at a different trophic level.
- A food web is a number of interconnected food chains. A flow shovws energy is transferred from one organism to another. The arrows show the direction of energy flow.
- Energy is lost as it passes along a food chain. -There are therefore fewer organisms further up the chain.
- The interrelationships between organisms can be presented in terms of pyramids of numbers, biomass and energy. The most informative form of representation is probably the pyramid of energy.
- It is more energy-efficient for humans to eat green plants. It is relatively inefficient to feed crop plants to animals.

The End, Posted By Miss Elisabeth N.