Food Chains & Webs
- Organisms found in any ecosystem rely on a source of energy to carry out all their activies.
- The ultimate source of energy is the sunlight, which is converted to chemical energy by photosynthesising organisms.
- This chemical energy is then passed as food between other organisms.
- Organisms can be divided into three groups according to how they obtain their energy and nutrients. These three groups are: producers, consumers and decomposers.
- Producers are photosynthetic organisms that manufacture organic substances using light energy, water and carbon dioxide.
- Green plants are producers.
- Consumers are oganisms that obtain their energy by consuming other organisms rather than using the energy of sunlight.
- Animals are consumers.
- Those that directly eat producers are called primary consumers.
- Those that eat primary consumers are called secondary consumers.
- Those that eat secondary consumers are called tertiary consumers.
- Secondary and tertiary consumers are predators, but may also be scavengers or parasites.
- When producers and consumers die, the energy they contain can be used by a group of organisms that break down these complex materials into simple components again.
- They release valuable minerals and elements in a form that can be absorbed again by plants and so contribute to recycling.
- This is normally carried out by fungi and bacteria called decomposers and to a lesser extent by earthworms, known as detritivores.
- Food chains describes a feeding relationship in which producers are eaten by primary consumers, who in turn, are each by secondary consumers and so on.
- Each stage is known as a trophic level.
- Arrows on a food chain indicate the direction of energy flow.
- Most animals do not rely upon a single food source and within a single habitat many food chains will be linked together to form a food web.
- Food webs are very complex.
- Feeding relationships presented by food chains are not fix, they change depending on the time of year, age and population size of the organism.
Energy Losses in Food Chains
- Plants normally convert between 1 - 3% of the sun's energy available to them into organic matter, this is because:
- Over 90% of the suns energy is reflected back into space by clouds and dust or absorbed by the atmosphere.
- Not all wavelengths of light can be absorbed and used for photosynthesis.
- Light may not fall on a chlorophyll molecule.
- A factor, such as low carbon dioxide concentration, may limit the rate of photosynthesis.
- Gross production = the total quantity of energy that the plants convert to organic matter.
- However, plants use 20 - 50% of this energy in respiration.
Energy Losses in Food Chains 2
- Net production = the rate at which they store the energy.
- Net production = gross production - respiratory losses.
- Only 10% of the plants energy is passed on when consumed by the primary consumers to be used for their growth.
- Secondary and tertiary consumers transfer 10 - 20%.
- The low percentage of energy transferred at each stage is the result of the following:
- Some of the organism is not eaten.
- Some parts are eaten, but cannot be digested and so is lost in the faeces.
- Some of the energy is lose in excretory materials, e.g. urine.
Energy Losses in Food Chains 3
- Some energy losses occur as heat from respiration and directly from the body to the environment. These losses are high in mammals and birds, because of their high body temperature.
- It is relative inefficiency of energy transfer between trophic levels that explains why:
- Most food chains have only four or five trophic levels because insufficient energy is available to support a large enough breeding population at higher trophic levels.
- The total mass of organisms (biomass) is less at higher trophic levels.
- The total amount of energy stored is less at each level as one moves up the food chain.
Calculating the Efficiency of Energy Transfers
- The energy available is usually measured in kilojoules per square metre per year - kj m-2 year-1
- Energy transfer = energy available after the transfer, divided by, energy available before the transfer x 100
Pyramids of Number
- Usually the numbers of organisms at lower trophic levels are greater than the numbers at higher levels.
- There can be significant drawbacks using a number pyramid to describe a food chain:
- No account of size is take - one giant tree is treated the same as one tiny aphid for example.
- The number of individuals can be so great that it is impossible to represent them accurately on the same scale as other species in the chain.
Pyramids of Biomass
- This is more reliable, their biomass, which is the total mass of plants and/or animals in a particular place is used.
- The fresh mass isn't reliable due to the large quantity of water so it must be the biomass which is measured, but to measure dry mass the organism must be killed, it is usually only made on a small sample which may not be representative.
- Biomass is measured in grams per square metre.
- Pyramids of biomass only take into consideration the organisms which are present at the particular time; seasonal differences are not apparant.
Pyramids of Energy
- The most accurate representation of the energy flow through a food chain is to measure the energy stored in organisms.
- However, collecting the data can be difficult and complex.
- Data is collected in a given area for a set period of time, usually a year.
- These results are more reliable than biomass, because two organisms may have the same dry mass, but have different amount of energy, e.g. an organism with more fat would store more energy than one with less fat.
- Agricultural ecosystems are made up largely of domesticated animals and plants used to produce food for mankind.
- Agriculture tries to ensure that as much of the energy available by the sun as possible is transferred to humans.
- In effect, it is channelling the energy flowing through a food web into the human food chain and away from other food chains; this increases the productivity of the human food chain.
What is Productivity?
- Productivity is the rate at which something is produced.
- The rate at which plants assimilate chemical energy is called gross productivity; it is measured for a given area over a given period of time.
- Some of this chemical energy is utilised by the plant for respiration, typically 20%.
- The energy which is left after respiratory losses is known as net productivity; this is also the energy available to the next organism in the food chain.
- Net productivity = gross productivity - respiratory losses.
- Net productivity is important in agricultural ecosystems and is affected by two main factors:
- The efficiency of the crop at carrying out photosynthesis. This is improved if all conditions needed for photosynthesis is supplied at their optimum.
- The area of ground covered by the leaves of the crop.
Comparison of Natural & Agricultural Ecosystems
The main differences between natural and agricultural ecosystems in the energy input and productivity.
- Solar energy only - no additional energy input.
- Lower productivity.
- More species diversity.
- More genetic diversity within a species.
- Nutrients are recycled naturally within the ecosystem with little addition from the outside.
- Populations are controlled by natural means, such as competition and climate.
- It is a natural climax community.
- Solar energy plus energy from food (labour) and fossil fuels (machinery and transport)
- Higher productivity.
- Less species diversity.
- Less genetic diversity with a species.
- Natural recycling is more limited and supplemented by the addition of artificial fertilisers.
- Populations are controls by both natural means and by the use of pesticides and cultivation.
- Is an artificial community prevented from reaching its natural climax.
- In natural ecosystems the only source of energy is the sun.
- Most land in Britain would be covered by forest if left to develop naturally; this is known as a climax community.
- To maintain an agricultural ecosystem the climax is prevented from developing; this is done by excluding most of the species in that community, leaving only a particular crop that we are trying to grow.
- To remove or suppress unwanted species and to maximise growth requires an addition input of energy.
- This energy is used to plough fields, sow crops, remove weeds, suppress pest and diseases, feed and house animals, transport materials etc.
Energy Input 2
- This additional energy comes in two forms:
- Food - farmers and other people that work on the farm expend energy as they work. This energy comes from the food they eat.
- Fossil fuels - as farms have become more mechanised, energy has increasingly come from the fuel used to plough, harvest and transport crops, to produce and apply fertilisers and pesticides and to house, feed and transport livestock.
- Productivity in natural ecosystems in relatively low.
- The additional energy inputs added to agricultural ecosystems increases the productivity of a crop by reducing the effect of limiting factors on its growth.
- The energy is used to exclude other species means that the crop has little competition for light, carbon dioxide, water and minerals needed for photosynthesis.
- The ground is covered almost exclusively by the crop; fertilisers are added to provide essential ions, and pesticides used to destroy pests and prevent disease.
- Together these factors means that productivity is much higher in agricultural ecosystems than natural ones.
Chemical & Biological Control of Agricultural Pest
A pest is an organism that competes with humans for food or space, or it could be a danger to health.
Pesticides are poisonous chemicals that kill pests. They are used for the chemical control of pests.
- Herbicides kill plants.
- Fungicides kill fungi.
- Insecticides kill insects.
- Specific - so that it is only toxic to the organism at which it is directed. It should be harmless to other organisms, especially natural predators, earthworms and pollinating insects such as bees.
- Biodegradable - once applied, it should break down into harmless substances in the soil. It also needs to be chemically stable, so that it has a long shelf life.
- Cost-effective - development costs are high and new pesticides only remain useful for a limited time, due to the pest developing genetic resistance.
- Non-accumulating - it does not build up, either in specific parts of an organism or as it passes along food chains.
Adv & Disadv
Advantages of the chemical control:
- Acts immediately after application.
Disadvantages of chemical control:
- Always have some effect on the non-target species.
- Must be reapplied at intervals, making them very expensive.
- Pests develop genetic resistance, and new pesticides have to be developed.
- This is when the pest population is controlled by either natural predators or parasites of the pests; this is done by introducing them into the ecosystem and letting them naturally reproduce to a size that will control the pest species.
- The aim is to control the pest species, not to eradicate it; if we did this, there would be insufficient food for natural predators who would then eventually die.
Adv & Disadv
Disadvantages of the biological control are:
- They do not act quickly.
- The control organism may become a pest itself.
Advantages of the biological control are:
- Very specific to one pest.
- Once introduced, the control organism reproduces itself.
- Pest do not become resistant.
Intergrated Pest Control Systems
This system aims to integrate all forms of pest control rather than being reliant on one type. These systems can be effective with minimum impact on the environment. Integrated control involves:
- Choosing animal or plant varieties that suit the local area and are a pest-resistance as possible.
- Managing the environment to provide suitable habitats, close to the crops, for the natural predators.
- Regularly monitoring the crops for signs of pests so that early action can be taken.
- Removing pest mechanically, by hand-picking, vacuuming, erecting barriers, if pests exceed an acceptable population level.
- Using biological agents if necessary and available.
- Using pesticides as a last resort if pest pop. is uncontrollable.
How Controlling Pests affects Productivity
- Pests reduce productivity in agricultural ecosystems.
- Weeds compete with crop plants for water, mineral ions, carbon dioxide, space and light, which are often in limited supply. This means the rate of photosynthesis of the crop plant is reduced and therefore the productivity is decreased.
- Insect pests may damage the leaves of crops, limiting their ability to photosynthesise and this reducing their productivity.
- Alternatively, they may be in direct competition with humans, eating the crop itself. Many crops are now grown in monoculture, and this enables insects and fungal pests to spread rapidly.
- Pest of domesticated animals may cause disease; animals may not grow rapidly, be unfit for consumption or die - all which lead to reduced productivity.
How Controlling Pests affects Productivity 2
- The aim of pest control is to limit the effect of pest on productivity to a commercially acceptable level. In other works, to balance the cost of pest control with the benefits it brings.
- The problem with this is two different interests involved:
- The farmer who has to satisfy our demand for cheap food and still make a living.
- The conservation of natural resources, which will enable us to continue to have food in the future.
Intensive Rearing of Domestic Livestock
- This is designed to produce the maximum yield of meat, eggs and milk at the lowest possible cost.
- The aim is to reduce energy which is lost as heat during respiration, so more energy is available for growth.
- This is achieved by minimising energy losses during their lifetime.
- By minimising energy losses from domestic animals more of the food energy taken in is converted into body mass, ready to be passed onto the next link in the food chain, humans.
- This is achieved by keeping the animals in confined spaces, such as small enclosures, barns or cages.
- This increases energy conversion because:
- Movement is restricted and so less energy is used in muscle contraction.
Intensive Rearing of Domestic Livestock 2
- The environment can be kept warm in order to reduce heat loss from the body.
- Feeding can be controlled so that animals receive the optimum amount and type of food for maximum growth with no waste.
- Predators are excluded so that there is no loss to other organisms in the food web.
- Other means of improving the energy-conversion rate include:
- Selective breeding of animals to produce varieties that are more efficient at converting the food they eat into body mass.
- Using hormones to increase growth rates.
Features of Intensive Rearing of Livestock
Food is essential for life and with the ever-expanding human population, there is a pressure to produce more and more food intensively. The main features include:
- Efficient energy conversion - by restricting wasteful loss of energy, more energy is passed to humans along the food chains/
- Low cost - meat, eggs and milk can be produced cheaper like this than other methods.
- Quality of food - food quality is said to decrease.
- Use of space - uses less land and so less of the countryside is required for agriculture, leaving more natural habitats.
- Safety - smaller, concentrated units are easier to control and regulate, however, they are more vulnerable to rapid spread of diseases.
- Disease - infections can spread quickly and easily between the animals. Antibiotics are regularly given to animals to avoid this.
Features of Intensive Rearing of Livestock
- Use of drugs - over-use can lead to antibiotic resistance in the animals. This resistance can be transferred to bacteria that cause human diseases, making their treatment with certain antibiotics ineffective.
- Animal welfare - animals are kept unnaturally and this may cause stress, resulting in aggressive behaviour, to harm the other animals or themselves. Restricted movement can lead to osteoporosis and joint pain. The well-being of animals may be sacrificed for financial gain - this is seen as unethical to some people.
- Reduced genetic diversity - selective breeding is used to develop animals with high energy conversion rates and tolerance of confined conditions.
- Use of fossil fuels - high energy conversion rates are possible because fossil fuels are used to head buildings that house animals, help with the production of the materials used for these building and also transportation of animal feeds. The carbon dioxide emitted increases global warming.