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Food Chains and webs

What you need to know

Reflections and Exam tips

 

Food Chains and webs

Simple Food Chains- Where does the energy come from?

A food chain shows feeding relationships:

 

Lettuce —› Rabbit —›Fox

  • Energy enters the food chain from the Sun into the lettuce.
  • The lettuce (a plant) is the producer, it produces chemical energy from sunlight during photosynthesis. This chemical energy now becomes available to other organisms in the food chain.

Every food chain begins with a plant/producer.

The rabbit is a primary consumer (first eater) while the fox is a secondary consumer (second aeter).

  • Each stage in a food chain is called a trophic level (feeding level).
  • The arrow shows the direction in which energy moves. The energy eaten is used to make new cells and body tissues. This is called biomass.
  • Biomass is the mass of living material in an organism. Only the energy stored as biomass can be passed along the chain as the organisms eat each other.

Energy loss along food chains

Not all the energy from the sun is passed along the food chain from left to right as edible biomass.

Consider this food chain:

Cabbage (1% energy from sun)—› Aphid (about 10% )—› Wasp (8%)—›Small bird (7%)—› Hawk

Plants use only a tiny percentageof the energy from the sun in photosynthesis. In turn, only a small percentage of this energy passes along as animals eat each other. The rest is wasted as:

  • Uneaten material (roots, bones etc)
  • Respiration - as Heat to keep the body temperature constant
  • Movement (in animals)
  • Waste - urine and faeces

This means that the longer the chain, the more energy is lost and the Biomass of the organisms gets less as you go to the right.

Reducing energy losses

Farmers use this knowledge to make sure that their animals grow faster (put on more biomass) and lose less energy. To do this they may:

  • Keep the houses warm- Limit energy loss through respiration.
  • Restrict movement of the animals by keeping them in small cages or pens- Limit energy loss through movement.
  • Feeding them mainly with food from plants.

This means that the animals don't waste food (money) to keep themselves warm or moving around unnecessarily.

Feeding them with plant food means they gain more energy from the food as no losses have occured.

Pyramids of Biomass

The enrgy losses in food chains can be represented using a pyramid of Biomass. This shows the total energy content at each trophic level with producers at the base.

The pyramid gets narrower towards the top because of energy losses. As a result, the number of top carnivores is also much smaller.

 

Decay processes

All organisms eventually die, and parts of them fall off during their lifetime for example leaves, fruits, hair, dead skin cells. What happens to these?

They are recycled by decomposers. Decomposers break down dead plant and animal material. They include Bacteria and Fungi. They use dead animal and plant material in respiration to produce energy. Remember the equation for respiration:

Glucose (or waste material) + Oxygen —› Carbon Dioxide + Water + Energy

And they use enzymes to speed up this process. So what conditions make it happen faster?

  • Presence of Oxygen
  • Moisture
  • Optimum temperature

The Carbon Cycle

Plants and animals consist mainly of the elements Carbon and Nitrogen. Carbon is found in carbohydrates, fats and proteins, while Nitrogen is found in Proteins. Upon death, these elements are recycled. They have to be recycled to keep the balance of nature.

Most GCSE (O'Level) syllabuses require you to know only the Carbon cycle.

You just need to understand the natural processes that keep the Carbon Balance and the Human factors that upset this balance.

The idea of 'wasted' energy is used to explain the decrease in numbers of animals along a food chain. It is also used to explain the decrease in the size of the pyramid upwards.

 

Make sure you can name the ways by which energy is lost/wasted.

 

More challenging exam questions ask you to explain farming practices using this idea.e.g. Why farmers may keep animals in cages/pens and not allow them to roam freely.

 

What issues does keeping animals in pens/cages raise?