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An ecosystem includes the living and the non-living parts of the environment. Ecology is a branch of Biology which involves studying interactions between living organisms in their natural environment. No living organism exists in isolation from other species - all living organisms depend on other organisms. In particular, animals need plants and plants also need animals, although their need is probably not as great.

Feeding relationships

The main interactions in an ecosystem involve food, which all living organisms need as fuel to respire for energy . Most of the principles have already been covered in studying the carbon and nitrogen cycles.
Green plants make their own food by the process of photosynthesis, in which they absorb some of the energy from sunlight. For this reason, green plants are called producers.

Animals, including Man, cannot make their own food - or absorb energy directly! Instead, they eat plants , or other animals. For this reason, animals are called consumers.

Animals which eat plants (sometimes called herbivores) are described as primary (1st) consumers, and animals which eat them (carnivores) are described as secondary (2nd) consumers. This can sometimes be extended to tertiary (3rd) and quaternary (4th) consumers. Each of these descriptions denotes a different trophic level.

Decomposers - bacteria and fungi - also obtain energy from dead animals and plants, and their products. In many ways they resemble animals, because they cannot make their own food, and instead rely on breaking down products originating from plants, using a similar digestion process. The main difference is that decomposers break down dead remains where they fall, from the outside (external digestion). In so doing, they cause decay, which gets rid of dead remains, and also recycle and release their constituent chemical elements, enabling plant and animal growth to continue.

The feeding relationships between producers and different consumers may be expressed in a variety of ways. In each case, organic food materials may be seen to pass on from one trophic level to another - one stage being food for the next. Another way of looking at it is in terms of energy flow - as the energy from sunlight, trapped in organic substances, is passed from one stage to the next.
Either way, the conversion is not very efficient, as there are large losses at each stage.

Organic materials and energy are wasted, in a number of ways:

- as heat, a by-product of respiration and all energy interconversions

- in indigestible materials, passed out unchanged from the digestive system of animals
- in movement, in order to get food

- etc.

Explain below in your own words what is meant by the following expression: All flesh is grass.

Plants contain organic substances which are passed on to herbivores and carnivores.

- Food Chains are simple statements showing (with arrows) the direction of movement of both food and energy between different trophic levels.

seaweed periwinkle gull

- Food Pyramids are illustrations of the "amount" of organisms at each trophic level. In extremely general terms, consumers get larger in size, but fewer in number as you go along the food chain. This, together with the inefficiency of energy transfer (loss of energy at each level), also partly explains why food chains usually cut off at 3 or 4 stages.
If you consider a pyramid of numbers, you may get confusing results, because sizes of organisms are not strictly comparable.
The best approach is to use pyramids of biomass, which include the total mass of living organisms at each trophic level.
These "pyramids" are usually drawn using boxes to represent the different trophic levels. If this is drawn to scale, the size (area of each box, or width if each is the same height) should be proportional to the "amount" of organisms at each level.
How are data for a pyramid of numbers obtained?
> counting organisms in a particular area

In what units is biomass measured?

> mass units - g/kg/tonnes - usually per unit area per year

- Food Webs are more realistic because they show alternative pathways for transfer of food and energy, effectively combining several food chains. They may also show that organisms can occupy different trophic levels, and which organisms are in competition with one another. For example, if one organism is reduced in numbers (for any reason), then organisms dependent on it for food will also tend to be reduced, whereas competitors will benefit.




Peri winkle















From the illustrations above, give examples of the following:

- tertiary consumers

>sparrowhawk >seal >seagull

- producers

>seaweed >oak tree

- primary consumers

>moth caterpillars >periwinkles / limpet

- an organism which can be a secondary or a tertiary consumer

>seagull / herring gulls

- a food chain with 4 trophic levels

oak -->moth caterpillars -->robin -->sparrowhawk

seaweed periwinkle/limpet octopus/crab/starfish seagull/seal

- things being acted upon by decomposers

>dead tree >apple

If the number of limpets in an area of seashore are reduced, which organism will probably:

- suffer directly from reduced food supply?

>starfish >crab >seal >seagull

- benefit, at least in the short term?

>seaweed >periwinkle / octopus

The following diagram shows the energy flow in a woodland food chain. At the base is the amount of light energy falling on the wood, and the square boxes showing the trophic levels are drawn with their areas proportional to the amounts of energy converted from the level below. Losses of energy due to respiration, and also energy lost to decomposers, are shown at each level.


What percentage of the light energy did the producers incorporate into their biomass?

Show your working.

> 80 000/ 1 500 000 x 100% = 5.3%

Suggest TWO reasons why a large amount of light energy was not incorporated into the producers' biomass.

> Some light is reflected away - especially green light

> Other energy wasted as heat - lost to the environment

Calculate the percentage efficiency of conversion of energy

from producers to primary consumers > 13000/80000x100%=16%

from primary consumers to secondary consumers >1500/13000x100%= 11.5%

from secondary consumers to tertiary consumers > 50/1500x 1005 = 3%

Why do you think food chains rarely have as many trophic levels as this?

> Very little energy is left at the top of the food chain.

What sort of units would the energy be expressed in?

> kilojoules per square metre per year?

What sort of time scale would be necessary to use in this sort of investigation? Explain why.

> (whole) years - because input of solar energy varies according to time of year, and so do quantities of biomass at each level.

Some components of a woodland food chain (Not to scale)

oak tree
blue tit
sparrow hawk
Sparrow hawk
carabid beetle
Ground beetle
(adult of) winter moth
(adult of) mottled umber moth
mottled umber mothh

The following information was collected by a class of pupils studying the numbers of animals feeding on an oak tree.
Animal Number
Food which each animal eats
winter moth larva 42 oak leaves
chiffchaff 6 mottled umber moth and winter moth larvae
mottled umber moth larva 51 oak leaves
vole 3 oak fruits
sparrowhawk 2 chiffchaffs and bluetits
bluetit 3 mottled umber moth and winter moth larvae
carabid beetle 5 mottled umber moth and winter moth larvae

Using the information above, name the animals in the feeding levels shown below, and work out how many animals were at each feeding level:

Trophic levels within the oak tree ecosystem
Feeding level Names of organisms Number Biomass
tertiary consumers sparrowhawk 2 1
secondary consumers chiffchaffs and bluetits, carabid beetles 6+3+5=14 6.6
primary consumers winter moth larva, mottled umber moth larva, vole 42+51+3=96 12.5
producers oak tree 1 177
In the space below, draw a pyramid of numbers of the plants and animals at the three feeding levels.
Try to do the same for a pyramid of biomass.


Suggest TWO reasons why the above information may not be truly representative of the pyramid of numbers for the oak tree ecosystem at the three feeding levels.

> children could have frightened away some animals so not counted them

> it was only a "snapshot" at a certain time - of day/ of year etc

- some species would fly and so numbers should be spread over several tree

This topic has connections with other BioTopics units on:-

Feeding relationships 
Food chains and webs
Examples of food chains
Measuring population sizes
Ecological pyramids
Conversion of biomass and energy
Predator prey interdependence
Natural Selection

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