Life on Earth (part 2)

 Life on Earth(part 2)

                                                                 

Sub-Topic 2: Energy in Ecosystems

 When an organism is removed from a food web all other organisms are affected, you should be able to explain particular examples. The more links in a food web the more stable it is – that is, a change in the population of one organism has less effect on the others. 

As energy is transferred from one level to the next in a food web, 90% of the available energy is lost. This is because the energy is being used up by the organism keeping themselves warm, moving and producing body parts (such as bone) which cannot be digested.

 Only 10% of the energy is passed on. This 10% comes from the growth and repair of new body tissue, which is then eaten.

Food chains and pyramids.

 A pyramid of numbers can be constructed from a food chain. 

•  The purpose is to represent the number of organisms present at each stage in the food chain.
•  By counting the organisms and representing the number as the area of a box you get: 
•  Some food chains begin with a large producer (such as an oak tree or a rosebush), this would give an odd shape if presented as a pyramid of numbers: 
•  A better way of representing this food chain is in a pyramid of biomass. o Biomass is the total mass of the organisms at each level in the food chain. o A pyramid of biomass for the above example would look like this:
• A 3rd way of presenting the same food chain is to use a pyramid of energy. This would look similar to the pyramid of energy, but would be measured in Kilojoules, as it demonstrates the total energy available at each level in the food chain.

Population size

The number of organisms in a population depends on:

•   Birth rate
•   Death rate In any population

 where there are no limits on the size of the population the birth rate will exceed the death rate and population numbers will increase.

• Space 
• Food supply
• Water supply
•  Disease
•  Predators
•  Build up of toxic waste in the environment

No population can continue growing forever. At present global growth of the human population is not severely restricted by any of these factors however lack of food, fresh water and/or disease can increase the death rate on a local basis. 

•  Eventually one or more limiting factors will stop the increase in numbers. 
•  This produces the ideal growth curve
•  In the first part from day 1 to 6 organism is settling in and beginning to reproduce. 
•  In the middle from day 6 to day 12 the population is reproducing rapidly 
•  At the end from day 12 to day 19 the population is being limited by one of the factors above.

Nutrient Recycling

 Some resources are in short supply for all organisms e.g. carbon and nitrogen.

 These need to be recycled. Dead organisms, both animals and plants, are broken down by decomposers (bacteria and fungi) and the chemicals bound up in their remains are released for reuse.

•   Nitrogen gas is common in the air but is chemically inert (unreactive and useless to most organisms 
• Some species of bacteria can fix nitrogen into useful chemical nitrates 
•  Some plants called legumes e.g. peas and beans have nitrogen fixing bacteria living in root nodules (swellings on the roots) 
•  Plants can absorb nitrates through their roots to make protein 
•  Animals eating the plants can convert this protein into their own protein 
•  When protein is excreted or the animals die decomposers (bacteria and fungi) change the protein into ammonia in the soil 
• Soil bacteria convert the ammonia into nitrites which is then converted into nitrates in the soil 

Some soil bacteria called denitrifying bacteria change soil nitrates into nitrogen gas 

Food Chains and Food Webs 

Every ecosystem is made up of various populations of organisms, all depending on each other in order to survive. There is certain vocabulary used to describe the relationships which these organisms have with one another: 

•  Population The number of organisms of the same species which live in a particular habitat. 
•  Producers Green plants that produce their own food by photosynthesis.
•  Consumers Animals which have to eat other organisms to live. 
•  Herbivore Animals which eat only plants. 
• Carnivore Animals which eat only meat. 
• Omnivore Animals which eat both plants and animals.

Competition 

When two organisms or populations of organisms need the same, or similar, resources there will be competition Competition can be for: Usually the less successful population will become extinct (but only within that ecosystem).

□ Water

□ Light 

□ Space

□ Food

□ Mates (Sexual Partners)

Inter-specific competition is where individuals of different species are competing for the same (or similar) resources e.g.

 o Red and grey squirrel 

 o Brown and rainbow trout 

Intra-specific competition is where individuals of the same species compete for exactly the same resources e.g.

o Feeding in grasshoppers 

o Territorial behaviour in pigeons

 o Growth of cress seedlings

 Experiment to illustrate competition within a plant species.

 Beaker A produced seedlings that were longer with larger leaves. This is because each seedling had more space, light and water available to it.

Bioaccumulation 

Pesticides are used in agriculture to control pest populations. The use of pesticides can result in bioaccumulation. Bioaccumulation is the increase in concentration of a substance in an organism over time. If the pesticides are sprayed onto crops which then make their way into the food chain, the pesticides are carried with the crops and as they pass up the food chain their toxicity increases.