Life on Earth – Facts to Learn for National 5 Biology
A. Biodiversity and the distribution of Life
1. Biomes are regions of the planet which have similar climates and contain similar plant and
animal species.
2. Deserts, rainforests and tundra are all examples of biomes. Deserts are hot and dry so contain species that can withstand these conditions like camels and cacti. Rainforests are warm and humid and contain a rich diversity of plants and animal species.
3. An ecosystem consists of all the organisms living in an area and the non-living components
that they interact with. For example, in a pond ecosystem, snails crawl along the mud on
the pond bottom, fish swim in the water and pond skaters move over the water surface.
4. A stable ecosystem contains a wide variety of organisms. Plants in an ecosystem make
food by the process of photosynthesis and are referred to as producers. Animals eat plants
and other animals and so are called consumers. Bacteria and fungi act as decomposers,
breaking down dead plants and animals.
5. A stable ecosystem provides a range of habitats for organisms. For example, in a woodland ecosystem, birds nest in tree branches, squirrels live in holes in tree trunks and woodlice crawl around in the dead leaves on the soil surface.
6. All the different populations of organisms within an ecosystem make up the community of the ecosystem. For example in a loch community there will be populations of trout, populations of snails and populations of water beetles.
7. An organism’s niche describes the role it plays in the community of organisms found in the ecosystem. This includes the resources it uses and how it interacts with other organisms. For example, rabbits will live in burrows in areas where there is lots of grass for them to eat. They provide food for foxes and other predators and provide a home for parasites such as fleas.
8. No two species can occupy the same niche. One will eventually outcompete the other.
9. Biodiversity describes the number of different species living in an ecosystem.
An ecosystem with a rich biodiversity contains lots of species.
Biotic, abiotic and human influences are all factors that affect the biodiversity of an
ecosystem.
10.Anincrease in the human population has decreased the biodiversity of most ecosystems on Earth. Man has hunted some species of animals so that they have become endangered.
Man has also destroyed the habitats of many species, so that they are in great danger of becoming extinct.
11. Biotic factors are living factors such as disease and food availability which affect the abundance of organisms in an area.
Abiotic factors are non-living factors such as temperature and light availability which affect where organisms are found in an area.
12. Changes in abiotic factors such as soil pH and water temperature can reduce the
biodiversity of ecosystems. For example, if sea temperatures rise due to global warming
some cold water fish may be forced to move further North away from the oceans around
Britain.
13. Grazing is a biotic factor which increases species diversityin grasslands by preventing dominant species like grasses outcompeting species that are less abundant.
Predators can also increase species diversity by preventing prey animals from taking over an ecosystem and eating too many plants. For example, when predators such as wolves are removed from an area, deer numbers increase damaging the forest ecosystem.
B. Energy in Ecosystems
14. Food chains show the flow of energy through an ecosystem. Plants are producers,
converting light energy into chemical energy in their leaves when they photosynthesise.
Animals gain some of this energy when they eat the plants. Primary consumers eat plants
whilesecondary consumers eat the primary consumers.
15. Only around 10% of energy gained by an organism through feeding is used for growth. At
each level in the food chain 90% of energy is lost as heat, movement or in waste as
undigested materials. Decomposition also contributes to energy loss.
16. Food chains can be represented as pyramids which show a decrease at each stage of the chain. A pyramid of numbers shows the relative size of populations at each stage of a food chain. A pyramid of biomass shows the relative weight of populations at each stage of a food chain. A pyramid of energy shows the energy contained withinpopulations at each stage of a food chain.
17. Irregular pyramids of numbers can occur when one organism supports many organisms of another species.The upper stages of the food chain become wider than the lower stages, when drawn as a pyramid of numbers. For example, the leaves on one tree can feed many caterpillars while one hedgehog can feed many fleas which suck its blood. Irregular pyramids of numbers are better represented as either pyramids of biomass or energy.
18. Food webs show all the feeding relationships in an ecosystem. When the population of a plant or animal is reduced, for example by disease, other organisms in the food web are affected. The numbers of plants or animals that it feeds on increase as less are eaten, so more survive to breed. On the other hand, there is a decrease in the populations of animals which eat the diseased organism, as they have less available food.
19. Food webs show how animal speciescompete for food in an ecosystem. Animals compete for food, water, territory and mates while plants compete for light, soil minerals and water.
20. Interspecific competition is when individuals of different species compete for the same resource. For example, red and grey squirrels competing for tree seeds.
Intraspecific competition is when individuals of the same species compete for the same resource. For example, a herd of deer competing with each other for grass on a hillside.
21. Plants and animals require nitrogen in the form of nitrates in order to makeprotein to grow. Plant roots absorb nitrates, dissolved in water, from the soil. This is why gardeners and farmers add fertilisers containing nitrate to the soil. The crop plants absorb it and use it to grow and so give a higher yield.
22. Bacteria allow nitrogen to be naturally recycled in an ecosystem.
Decomposing bacteria break proteins in dead bodies and waste down to ammonium ions.
Nitrifying bacteria then convert the ammonium ions into nitrates.
Nitrogen-fixing bacteria can convert nitrogen gas directly into nitrates. These are found in the soil and in root nodules of certain plants like peas, beans and clover.
Denitrifying bacteria convert nitrates back into nitrogen gas.
C. Sampling techniques and measurement of abiotic and biotic factors
23. A quadrat can be used to sample plant species growing in a field. It must be placed randomly in the field and the number of squares containing each species of plant counted, rather than the number of plants. In order to produce reliable results at least ten quadrats should be used throughout the field.
24. A pitfall trap can be used to sample small invertebrates such as beetles which live on the soil surface in an area. Pitfall traps should have holes in their base so that water can drain out of them. A number of traps should be set in an area to produce reliable results.
25. Biologists use paired-statement keys to identify the species living in an ecosystem.
When designing a paired-statement key the number of pairs of statements used in the key
should be one less than the number of species being identified.
26. Abiotic factors can be measured using a meter. These factors include light intensity,
temperature, pH and soil moisture.
Light and moisture meters are easy to use. It is important not to shade the light sensor and
you must wipe the probe dry after each soil moisture reading is taken. A number of
readings should be taken in an area and an average calculated. This ensures that the results
arereliable.
27. Abiotic factors affect the distribution of organisms in an area. For example, grass will
not grow in a dense woodland because it is too dark and dry.
D. Adaptation, natural selection and the evolution of species
28. A mutation is a random change in a gene. Mutations can naturally occur and they are usually a disadvantage to the organism. Environmental factors such as radiation and chemicals can increase the rate of mutation.
29. Sometimes a mutation can give an organism an advantage over other organisms. For example it might give the organism resistance to a disease that the other organisms do not have. This means that the organism will survive to breed and pass on the gene for disease resistance. Darwin called this natural selection or survival of the fittest.
30. Natural selection occurs because more offspring are produced than the environment can sustain. Only the organisms that are best adapted to their environment survive to reproduce and pass on the gene that gave them their selective advantage.
31. Variation within a population makes it possible for a population to evolve over time in response to changing environmental conditions. Darwin observed this in the Galapagos Islands where different species of finch had evolved different shapes of beak in order to feed. For example, the woodpecker finch had a pointed beak for extracting insects from tree bark while the large ground finch had a thick beak for crushing seeds.
32. The creation of new species is called speciation. This can only happen after a population of organisms becomes isolated from the rest of the species. A barrier must be present to prevent interbreeding between the two populations. The barrier could be geographic such as a river or desert dividing the populations. It could be ecological such as changes in temperature or pH in an area between the populations. It could even be reproductive with different populations not being attracted by each other’s courtship behaviour.
33. If the isolated populations then experiencedifferent environmental conditions natural selection will follow a different path in each population. Different mutations will be selected and gradually the two populations will become genetically different. They are classified as new species when they can no longer breed with each other.
E. Human Impact on the Environment
34. The human population is growing very quickly and this means that man needs to produce
anincreased food yield on the planet.
35. Modern methods of farming include growing monocultures which are vast fields of one
crop species. This allows farmers to get the maximum yield of crop from the land but
hedges and walls are removed, meaning loss of habitats for wildlife. The crops need lots of
fertilisers and to be regularly sprayed with pesticides.
36. The crop grown can be a GM crop which has had a gene from another species inserted
into it. This gene may allow the crop to grow larger or have resistance to disease
organisms. Some crops have been genetically engineered to resist specific herbicides. This
means that the farmer can spray the crop with the herbicide and only kill weeds.
The worry with GM crops is that the GM plants crossbreed with wild varieties creating
varieties of weeds which are difficult to control.
37. Intensive farming involves growing more crops on land or quickly rearing lots of animals
in a small area. This is often referred to as factory farming and it involves the animals being
given lots of food while their movements are restricted e.g. chickens reared in cages. The
process has been criticised by animal welfare groups.
38. Monocultures require lots of fertilisers to help plants grow. Inorganic fertilisers contain
minerals such as nitrates which plants use to make proteins for root and shoot growth.
They also contain magnesium which plants use to make chlorophyll for absorbing light
energy.
39. Fertilisers can be washed out of the soil by heavy rain and end up in rivers and lochs. This is called leaching. The fertilisers stimulate the growth of algae in the water causing an algal bloom. This mass of green algae eventually dies and bacteria start to decompose it. The bacteria multiply quickly and use up the oxygen in the water. This leads to the death of fish and invertebrates that require oxygen.
40. Indicator species are species that by their presence or absence indicate the level of pollution. For example, freshwater invertebrates indicate the water quality. The presence of mayfly nymphs indicate that the water is unpolluted and has a high oxygen concentration while the presence of bloodworms indicate that the water is polluted and low in oxygen. Similarly, the variety of lichen growing in an area indicates the local air quality. Crusty lichens indicate polluted air rich in sulphur dioxide while shrubby lichens indicate clean air with little sulphur dioxide.
41. Pesticides are sprayed onto crops to kill pests. Insecticides kill insects while fungicides kill fungi. Herbicides kill weeds preventing them competing with the crop for minerals and light.One problem with some pesticides,like DDT, is that they are non-biodegradable and cannot be broken down in the environment. Instead they persist and can build-up in the bodies of animals over time. The pesticides are passed along the food chain, building up in concentration in the bodies of animals towards the top of the food chain. Eventually, they can reach toxic levels which killthe predators at the top of the food chain. This build-up in concentration of pesticides along the food chain is called bioaccumulation.
42. GM crops are one potential way of reducing overuse of pesticides in the environment. If the crop contains an inserted gene that makes it produce a chemical which kills the insect pest, then there is no need to spray the crop with insecticide.
43. Another way of reducing overuse of pesticides is to use biological control. This involves using one organism to control the spread of another. For example, the virus myxomatosis has been used to kill rabbits and so keep their numbers down. Another example involves introducing ladybirds into a Greenhouse of plants and using them to naturally kill aphids which damage the plants.