OCR Categories Archives: B3: Life on Earth

B3.3 What is the importance of biodiversity?

B3.3 What is the importance of biodiversity?

Organisms are classified into groups according to similarities and differences in characteristics including:

  • Physical features (e.g. flowers in flowering plants and the skeletons in vertebrates
  • DNA

The groups start off large with high numbers of organisms with a few features in common (e.g. KINGDOMS such as plants, animals) and reduce in size as they are sub-divided into much smaller groups containing organisms with more characteristics in common (e.g. SPECIES).

The classification of living and fossil organisms can help to:

  • Make sense o f the enormous diversity of organisms on Earth
  • Show the evolutionary relationships between organisms

BIODIVERSITY refers to the variety of life on Earth including:

  • The number of different species in an area
  • The range of different types of organisms, e.g. plants, animals and microorganisms
  • The genetic variation within species (how many different alleles there are)

The greater these are, the greater the BIODIVERSITY.

For humans biodiversity is vitally important because we need to exploit CROPS to feed a growing population. Plants are also the source of compounds that are effective against disease and genetic disorders. These could potentially be used as medicines.

The rate of extinctions on Earth has been increasing – this is likely to be due to human activity. Humans can cause extinctions directly or indirectly:


SUSTAINABILTY is about meeting the needs of people today without damaging the Earth for future generations.

Maintaining biodiversity to ensure the conservation of different species is one of the keys to SUSTAINABILITY.

In the past farmers used to grow a variety of different crops on a ranch. Hedgerows would separate the different parts of the farm. In the 20th century, techniques changed – giant fields made from many earlier fields joined together were planted with MONOCULTURES (a single variety of a crop). This technique was not sustainable because it reduced biodiversity affecting the whole food chain.

Virtually all products used in the industrialised world rely on oil and the products made from it. To IMPROVE SUSTAINABILTY, alternatives to oil need to be found. Packaging is used to attract the attention of consumers, as well as providing a way of keeping the product safe – packaging is often made from oil-based plastics.

Manufactures have to consider:

  • What materials should be used
  • How much energy is needed in the manufacturing process for a given packaging material
  • How much pollution will be produced as a result of manufacturing packaging

It is preferable to decrease the use of some materials, including packaging materials, even when they are biodegradable, because of:

  • Use of energy in their production and transport

Slow decomposition in oxygen deficient landfill sites


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B3.2 How has life on Earth evolved?

B3.2 How has life on Earth evolved?

Life on Earth began around 3500 million years ago. ALL life on Earth, including all life that is now extinct, evolved from very simple living things- so all organisms share a common ancestor. However there is variation between individuals of the same species and some of this variation is genetic so can be passed on to offspring.

Genetic variation is caused when changes called MUTATIONS take place in the genes. These changes are random and can be caused by background radiation and chemicals. MUTATIONS cause different proteins to be produces and this changes the function of the gene. If the mutations occur in the cells producing eggs in the ovaries and sperm in the testes, then the mutated genes may be passed on to the offspring. Sometimes the mutation causes new characteristics.

NATURAL SELECTION – The genetic variation between individuals in a species means that those with characteristics that improve their chances of survival in their physical environment are more likely to live to adulthood. When these individuals reproduce, they pass on the beneficial characteristics to their offspring.

SELECTIVE BREEDING – Where animals and plants with certain traits are deliberately mated together to produce offspring with certain desirable characteristics. This could be used to create new varieties of organisms or to increase the yield of animals and plants.

Differences between the two is that with selective breeding, it is humans who choose the desirable traits however with natural selection, it is the environment that determines the desirable traits.


The combined effects of the following can lead to the formation of new species:

  • Mutations
  • Natural selection
  • Environmental changes
  • Isolation – where individuals from one population are isolated from other populations so that they cannot meet to breed

This process is called EVOLUTION

Evidence for evolution is provided by:

  • The fossil record – fossils are the remains of plants or animals from thousands of years ago that are found in rock. Fossils indicate the history of species and can show the evolutionary changes in organisms over millions of years. The remains of the organisms are buried in the rock.
  • DNA organisms – similarities and differences in DNA can lead to the relationships being worked out between all life on Earth. Analysing the DNA of both living and fossilised specimens shows that there are similarities as well as differences. This can be used to chart the family tree of all life on Earth. The more shared genes organisms have, the more closely related they are.

Jean-Baptiste Lamarck was a scientist who proposed that the environment changed an organism. The organism then passed on the characteristic to their offspring, e.g. moles lived in the dark, so they lost their eyes as a result. This is called evolution through inheritance of acquired characteristics.

Charles Darwin devised a better explanation following many years of thought and collecting evidence. By collecting data, Darwin made the connection varieties, competition, the survival of the fittest and the passing on of desirable characteristics to the next generation.


Darwin’s theory was proven to be better because there was no evidence or scientific mechanism for Lamarck’s inheritance of acquired characteristics. The scientific community, having repeated Darwin’s experiments and peer reviewed his work accepts Darwin’s explanation for evolution over Lamarck’s.



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B3.1 Systems in balance – how do different species depend on each other?

B3.1 Systems in balance – how do different species depend on each other?

A group of organisms that can breed together and produce fertile offspring is called a SPECIES. All things rely on their environment and other species to survive. The individuals in a species are adapted to living in their environment – ADAPTATIONS of living organisms to their environment increases the species’ chance of survival by making it more likely that individuals will survive to reproduce. If an organism reaches sexual maturity, it is more likely to pass on its genes, including the genes coding for its adaptations, to the next generation.

For example – POLAR BEAR they are well adapted for survival in the Arctic. They have:

  • A white appearance, as camouflage from prey on the snow and ice
  • Thick layers of fat and fur, for insulation against the cold
  • A small surface area to volume ration, to minimise heat loss
  • A greasy coat which sheds water after swimming


When the same RESOURCES (E.g. shelter, food, water, light availability, etc) are needed by different organisms in the same habitat then there is COMPETITION. The organisms that are most successful at competing survive and pass on those genes that code for adaptations – SURVIVAL OF THE FITTESET.

FOOD WEBS show how all the food chains in a given habitat are interrelated. It shows how the loss of one organism has an effect of other organisms in the food web. All the organisms in a food web are dependent on other parts of the web – this is called INTERDEPENDENCE. This means changes affecting one species in a food web could have an impact on other species that are part of the same food web.


Change in a n environment may cause the species to become extinct, for example if:

  • The environmental conditions change beyond its ability to adapt
  • A new species that is a competitor, predator or disease organism of that species is introduces
  • Another species (animal, plants or microorganisms) in its food web becomes extinct

Nearly all organisms are ultimately dependant on energy from the SUN. Energy from the sun enters the food chain when green plants absorb light in order to PHOTOSYNTHESISE. By the process of photosynthesis, organic compounds like glucose are made from carbon dioxide and water using this energy. Plants only absorb a small percentage of the sun’s energy for this process; this energy is stored in the chemicals which make up the plants’ cells.

Energy is transferred between organisms in an ecosystem:

  • When organisms are eaten
  • When dead organisms and waste materials are fed on by decay organisms. There are two types of decay organism:
    • DECOMPOSERS such as bacteria and fungi break down the dead materials and use the energy stored inside.
    • DETRITIVORES include animals such as earthworms and woodlice. These consume the DETRITUS (dead plants or animals and their waste), breaking it down into smaller particles that other detritivores and decomposers can use

In a food chain only around 10% of the energy is passed on to the next level. A large proportion of the energy is:

  • Loss to the environment as heat
  • Excreted as waste products
  • Trapped in indigestible materials such as bones, cellulose and fur

As less energy is transferred at each level of the food chain, the number of organisms at each level gets smaller.

The percentage of energy efficiency can be calculated using the formula

CARBON is a vital element for living things. It is used in all organic molecules, including sugars, proteins and amino acids. Carbon is recycled through the environment do that it is available for life processes. This can be seen in the CARBON CYCLE:

Plants and animals need NITROGEN to make proteins but they cannot get nitrogen directly from the air because, as a gas nitrogen is fairly unreactive. Plants are able to take up nitrogen compounds such as nitrates and ammonium salts from the soil.

Nitrogen, like carbon, has to be recycled to ensure that it is available for life processes. This can be seen in the NITROGEN CYCLE:


Nitrogen is recycled through the environment in the process of:

  • Nitrogen fixation (making nitrogen in the air) to form nitrogen compounds including nitrates
  • Conversion of nitrogen compounds to protein plants and animals
  • Transfer of nitrogen compounds through food chains
  • Excretion, death and decay of plants and animals resulting in release of nitrates into the soil
  • Uptake of nitrates by plants
  • Denitrification – this is where, in the absence of oxygen, denitrifying bacteria converts’ nitrates in dead plants and animal remains back into nitrogen gas. This completes the cycle, releasing nitrogen back into the atmosphere.



Biologists can measure changes in the environment by using indicators. These may be NON-LIVING or LIVING.


  • Nitrate levels can be measured using test kits with chemicals that change colour. The chemicals can then be matched against a chart indicating the amount of nitrate present in the sample
  • Temperature can be measured using a thermometer, or a data-logger, which is more accurate and reliable
  • Carbon dioxide levels can be measured using data-loggers


  • In the oceans, phytoplankton (plant) are useful for detecting the effect of temperature changes and for detecting changes in the food web
  • Lichens grow very slowly and are vulnerable to atmospheric pollutants and acid rain. A decline in their number can indicate pollution
  • River organisms, such as the larvae of mayfly (mayfly nymphs) can be used to indicate the quality of water. Mayfly nymphs can only live in clean river water with enough oxygen water. If a river has mayfly nymphs, then pollution levels will be low



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