Download This variation makes it possible for a population to evolve over time

Nat. 4 / 5 Biology
Life on Earth
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Use the following table as a checklist for your revision.
Remember to ask your teacher for help with anything you don’t understand.
Learning Outcomes/ Mandatory Course Key Content
1. Biodiversity and the distribution of life.
a. Biotic, abiotic and human influences are all factors that affect biodiversity
in an ecosystem.
b. Competition for resources, disease, food availability, grazing and
predation are biotic factors; Light intensity, moisture, pH and temperature are
abiotic factors.
c. Biomes are the various regions of our planet as distinguished by their
similar climate, fauna and flora.
Global distribution of biomes can be influenced by temperature and rainfall.
d. An ecosystem consists of all the organisms (the community) living in a
particular area and the non-living components with which the organisms
interact.
e. A niche is the role that an organism plays within a community. It includes
the use it makes of the resources in its ecosystem and its interactions with
other organisms in the community including competition, parasitism,
predation, light, temperature and nutrient avavilability
2. Energy in ecosystems
a. Definitions of other ecological terms including: species, population,
producer, consumer, herbivore, carnivore and omnivore.
b. In transfers from one level to the next in a food chain 90% of the energy is
lost as heat, movement or undigested materials.10% is used for growth.
c. Definitions and comparisons of pyramids of biomass, enrgy and numbers.
d. Competition in ecosystems. Interspecific competition occurs when
individuals of different species compete for the same resource in an
ecosystem. Intraspecific competition is when individuals of the same species
compete for exactly the same resources.
e. Nitrogen in ecosystems. Plant proteins are produced from nitrates. The
roles of nitrifying, denitrifying, root nodule and free-fixing soil bacteria.
Decomposers such as fungi and bacteria convert proteins and nitrogenous
wastes to ammonium compounds. These are converted to nitrites and
nitrates. Animals obtain nitrogen required to produce protein by consuming
plants.
3. Sampling techniques and measurement of abiotic and biotic factors.
a. Sampling of plants and animals using quantatitive techniques including
quadrats and pitfall traps.
b. Evaluation of limitations and sources of error in pitfall traps and quadrats.
c. Using and constructing paired statement keys to identify organisms.
d. Measuring abiotic factors including light intensity, temperature, pH
and soil mositure. Possible sources of error and how to minimise
them.
e. The effect of abiotic factors on the distribution of organisms.
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Learning Outcomes/ Mandatory Course Key Content
4. Adaptation, natural selection and the evolution of species
a. A mutation is a random change to genetic material. Mutations may be
neutral, confer an advantage or a disadvantage. Mutations are spontaneous
and are the only source of new alleles. Environmental factors such as
radiation, high temperatures and some chemicals can increase the rate of
mutations.
b. New alleles produced by mutation allow plants and animals to adapt to
their environment. These adaptations can be structural or behavioural.
Variation within a population makes it possible for a population to evolve
over time in response to changing environmental conditions.
c. Species produce more offspring than the environment can sustain. Natural
selection or survival of the fittest occurs when there are selection pressures.
The best adapted individuals survive to reproduce, passing on the favurable
alleles that confer a selective advantage.
d. Speciation occurs after part of a population becomes isolated by an
isolation barrier. These can be geographical, ecological or reproductive.
Mutations occur in each sub-population. Natural selection selects for
different mutations in each group, due to different selection pressures. Each
sub-population evolves until they become so genetically different they are
two different species.
5. Human Impact on the environment
a. Increasing human population requires an increased food yield.
b. Fertilisers can leach into fresh water increasing algal blooms. This
reduces light levels killing aquatic plants. These dead plants as well as dead
algae become the food for bacteria which increase greatly in number. The
bacteria use up large quantities of oxygen reducing the availability for other
organisms.
c. Indicator species are species by their presence or absence indicate
environmental quality/levels of pollution.
d. Pesticides sprayed onto crops can accumulate in the bodies of organisms
over time. As they are passed along food chains, toxicity increases and can
reach lethal levels.
e. Biological control may be an alternative to the use of pesticides. GM crops
may be an alternative to the use of fertilisers.
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Biodiversity and the Distribution of Life
Plants and animals interact with each other and with their environment. You
will study the feeding relationships between organisms, how the energy flows
through food chains and how nutrients are cycled. This knowledge will help
you predict how the environment might change over time, and how we as
humans can manage those changes, and how we can conserve and protect
plants and animals.
You need to be able to understand and apply the following terms and
definitions:
Biomes – large areas of the Earth which
have
similar
climatic
conditions,
particularly in terms of temperature and
rainfall, and because of this have similar
communities of flora (plants) and fauna
(animals).
Biomes can be grouped into 5 main
types:
aquatic,
deserts,
forests,
grasslands and tundra.
Ecosystem – a natural biological unit
made up of all the living organisms (the community) and the non-living
surroundings with which the organisms interact.
Examples of ecosystems include heather moorland, Caledonian forest, coral
reefs, etc.
Abiotic Factors – non-living factors that affect ecosystems. Examples include
rainfall, temperature, soil pH, light intensity, moisture, etc.
Biotic Factors – living factors that affect ecosystems. Examples include food
availability, grazing, predation, competition for resources and disease.
Niche – the role that an organism plays within its community. It includes the
use it makes of the resources available including light, temperature and
nutrient availability, and its interactions with other organisms including
competition, parasitism and predation.
Exam Tip – If you’re asked to describe the niche of an animal give the following information:
 what it eats,
 what eats it,
 what its habitat is.
You will usually get the information from a food web.
Energy in Ecosystems
You should be familiar with the following definitions that you covered in S3
Biology:
A group of organisms that can mate together to
Species
produce fertile offspring.
Population
Producer
Consumer
A group of organisms of the same species.
An organism, usually a plant that can make its own
food.
A general name for an organism unable to make its
own food and dependent upon a ready-made food
supply
Herbivore
An animal that feeds on plant material only.
Carnivore
An animal that feeds on animal material only.
Omnivore
An animal that eats both plant and animal material.
Energy Loss in Food Chains
In this food chain when the grass seeds are eaten by the vole, energy is
transferred from the grass seeds to the vole. When the vole is eaten by the
Barn owl energy is again transferred.
As energy flows through the food chain there is a loss of energy because:

Some parts of the body such as the cellulose in the cell walls of the
grass seeds, and the bone and hair in the vole, have little nutritional
value so may be left uneaten or are expelled as undigested materials
from the body

The energy is used for moving about.

In the case of warm blooded animals, energy is used for keeping
warm. As a result much of the energy is lost as heat.
In total about 90% of the energy taken in by an organism is used for heat,
movement and indigestible materials and so is lost. This leaves only 10% for
growth, to be built up into body parts that can be used by the next organism in
the food chain. Ecological Pyramids
A food chain can be represented quantitatively (with numbers) in the form of a
pyramid of numbers, below is one for the previous food chain. From this
graph we can see there are fewer Barn owls than voles; which makes sense
because a Barn owl must eat several voles to get enough energy in order to
survive.
The Barn owl food
chain is a typical food chain with
a large number of producers but decreasing numbers of consumers.
However, if the producer was a tree for example, followed by insects, then the
bottom bar would appear small as many organisms feed on one tree. In this
instance a pyramid of biomass is more useful as the tree is much larger. A
pyramid of biomass shows the total dry mass of the organisms at each link in
the food chain.
In some food chains both the pyramid of numbers and biomass show a
smaller producer bar. This is because some producers can reproduce very
quickly. In this case, a measurement of the total energy produced at each link
in the food chain will be more accurate. When we represent this information
in a pyramid of energy we always get a true pyramid shape.
The following diagram compares the pyramids of number, biomass and
energy for two different food chains. Note that it is only the pyramid of energy
that is truly pyramid shaped in both examples
Barn owl
Vole
Grass seeds
Competition
Habitats have limited amounts of the resources needed by living organisms.
Organisms must compete with others in order to get enough of these
resources to survive. If they are unsuccessful and cannot move to another
habitat, they will die.
Animals
Some of the resources that animals compete for:

food
water
space
mates



Plants
Plants make their own food using photosynthesis, so they do not need to
compete for food but they may compete for:




light
water
space
minerals
There are two types of competition:
Intraspecific Competition: This is when plants or animals of
the same species compete for exactly the same resources,
eg Barn owls living in the same area will compete for voles,
or wheat plants growing together in a field compete for
space. Because they compete for exactly the same
resources intraspecific competition is more intense than interspecific
competition.
Interspecific Competition: This is when plants or animals of different
species compete for the same resources, eg red and grey squirrels compete
for food, or oak trees and hazel trees growing in the same wood will compete
for light.
The Nitrogen Cycle
Nitrogen is essential for the formation of many chemicals in nature including
amino acids in proteins. The nitrogen cycle explains how nitrogen is recycled
in the environment.
Approximately 78% of the air is nitrogen gas. Because nitrogen is so unreactive, it cannot be used directly by plants to make protein. Only nitrates
are useful to plants, so plants are dependent on other processes to convert
nitrogen and other nitrogen containing compounds into nitrates in the soil.
Plants absorb nitrates from the soil and use these to build up proteins. The
plant may be eaten by an animal, and it in turn is used to produce animal
protein.
Urea and other waste material such as the dead bodies of plants and animals
is broken down by decomposers. This results in nitrogen being returned to
the soil as ammonia.
Ammonia is converted to nitrites and then to nitrates by nitrifying bacteria
in the soil.
Nitrogen gas can also be converted to nitrate compounds by nitrogen-fixing
bacteria. These bacteria can be found free-living in soil or in the root nodules
of leguminous plants such as peas, beans and clover. The plant can use the
nitrate to make protein.
In some conditions denitrifying bacteria in the soil break down nitrates and
return nitrogen to the air. This is usually in waterlogged soil.
Sampling Techniques and Measurement of Abiotic and Biotic
Factors
Sampling Techniques
If you are studying plants and animals in an ecosystem you may need to find
ways of capturing and counting them in order to estimate the population size..
There are many different ways to do this depending on the animal or plant you
are studying. These are known as sampling techniques.
A pitfall trap is used to collect animals that live on the soil surface and
amongst leaf litter.
lid
pot
alcohol
Avoiding Errors When Using a Pitfall Trap

Several pitfall traps should be set up to give reliable results.

The opening of the trap should be disguised by a lid, eg leaves or bark
so that trapped animals are not seen and eaten by predators.

A preservative liquid, eg alcohol is put in the bottom of the trap to
humanely kill and preserve the animals.
It is unlikely that you would be able to count all of the plants and animals in an
ecosystem because this would take too long. Instead, small samples which
represent the whole ecosystem are taken
Plants are often sampled in this way using a quadrat. This is a square of a
known area which is randomly placed on the site being studied. The plants
inside the quadrat are identified and counted. From this information the
estimate of the number of a certain plant can be calculated.
quadrat
sample site
plant
Avoiding errors when using a quadrat

Make the results more reliable by using a bigger number of quadrats.

Place the quadrats at random over the area being studied, don’t
choose where to place it.

Make a rule to decide what to do with plants that fall partly in or out of
the quadrat, eg more than half the plant in the quadrat counts, more
than half out the quadrat doesn’t.
out
in
out
in
in
out
Identification of Organisms
If you are studying the populations and communities in ecosystems you need
to be able to identify the plants and animals that are there. To do this you can
use a paired statement key.
The following shows a simple example of a paired statement key that could be
used to identify some species of moths and butterflies. You should be able to
use and construct paired statement keys.
KEY:
1
Antennae (feelers) feathery
Antennae clubbed (not feathery)
go to 2
go to 6
2
Front pair of wings much larger than rear wings
Both pairs of wings similar in size
go to 3
go to 4
3
Body 2.5 – 3.0 cm long
Body 3.0 – 3.5 cm long
4
Both pairs of wings patterned
Front wings only patterned
5
Wings with large eye spots
Wings without eye spots
6
Dark wings with light coloured edges
Camberwell Beauty Butterfly
Light wings with dark markings
Small White Butterfly
Elephant Hawk Moth
Privet Hawk Moth
go to 5
Alder Moth
Emperor Moth
Gipsy Moth
Measurement of Abiotic Factors
It is useful to measure abiotic factors and various pieces of equipment can be used to
do this. To make sure that your measurements are reliable the equipment must be
used properly.
Abiotic
Factor
Light
Intensity
Equipment
Used
Light Meter
Sources
of
Error
- standing in the way of the
light and casting a shadow
on the light meter
Ways in Which Error is
Minimised
- don’t stand in front of the
sensor.
- light intensity may
change throughout the day.
- take readings at the same
time of day and take
several readings for
reliability.
- probe not left long
enough to monitor the
moisture level.
- leave the probe in the soil
for a few seconds before
taking the reading.
Soil Moisture
Moisture Meter
- moisture left on probe
from a previous
measurement
Temperature
pH
(of soil or
water)
Thermometer
(or
temperature
probe)
pH probe
(or chemical
test using
indicator
solution)
- wipe the probe between
each reading.
- thermometer not left in
position long enough to
monitor the temperature.
- allow the thermometer to
settle before taking the
reading
- temperature may change
throughout the day.
- take readings at the same
time of day and take
several readings for
reliability.
- probe not left long
enough to monitor the
moisture level.
- leave the probe in the
sample for a few seconds
before taking the reading.
- moisture left on probe
from a previous
measurement
- clean the probe between
each reading.
Effect of Abiotic Factors on the Distribution of Organisms.
Organisms are affected by the abiotic factors around them and this may
affect their distribution.
Here are some examples:
Daisies
 The more light available, the more daisy plants will be present.
 This is because daisies need light energy from the sun to make their own
food (photosynthesise).
Grasses
 Grasses are found in the full sunlight of a field rather than in shady woodland.
 This is because grasses need lots of light energy from the sun to
photosynthesise.
Adaptation, Natural Selection and the Evolution of Species
Mutation
Changes to the genetic material are called mutations. Mutations may be a
source of new alleles, (if you can’t remember the meaning of the term allele
look back at your Variation and Inheritance notes in the Multicellular
Organisms unit, or look it up in the glossary on the Biology web pages).
Mutations can be spontaneous (they just happen). They can also happen
because of environmental factors such as:



Radiation, such as ultra-violet (UV) radiation in sunlight.
High temperatures
Chemicals, such as tar from cigarette smoke.
Mutations may be:
 Neutral, ie have no effect. For example, the protein that a mutated
gene produces may work just as well as the protein from the nonmutated gene.
 Disadvantageous, ie be harmful. For example, haemophilia is an
inherited disorder that stops blood from clotting properly. It is caused
by a mutated gene
 Advantageous, ie be useful. For example, some rats have become
resistant to warfarin (a rat poison). This is an advantage to the rat
since it is no longer killed by warfarin. It is caused by a mutated
gene.
New alleles produced by mutation allow plants and animals to adapt to
their environment. These adaptations can be structural (ie the way in which
their body is made or works) or behavioural (ie the way in which the animal
behaves)
Variation
Although all members of a species are similar to one another, eg a population
of mice all have mice-like features, they are not identical to each other. This is
because variation occurs within a species.
This variation makes it possible for a population to evolve over time in
response to changing environmental conditions. Natural selection is the
process by which evolution occurs.
Natural Selection
Imagine a breeding pair of mice arriving in a habitat where no other mice
exist. In perfect conditions the pair could produce six offspring every two
months. The offspring of mice become mature after only six weeks and so
could go on to produce offspring of their own at the same rate. If all the mice
survived and continued to breed the habitat would become overrun with mice.
This does not happen because there is a struggle to survive and only a few
mice will survive to reproduce.
Some mice will be:
 poorly camouflaged or have slow reactions and be eaten by
predators
 poor at competing for food and so die of starvation
 poor at competing for shelter or have thinner coats and so die of cold
 killed by disease
Only the mice with the best characteristics for the habitat will survive to breed.
This is known as survival of the fittest.
Mice are not genetically identical. They are produced by sexual reproduction
which ensures they possess different combinations of genes from their
parents. Important characteristics such as coat colour and thickness, speed
and reactions, food finding ability, resistance to disease and aggressiveness
will vary from mouse to mouse.
Only those mice with the best combinations of genes for the habitat will
survive. This means that their gene combinations will be passed on to
offspring. Mice with less useful combinations of genes will dies and so these
genes are not passed on.
(The above example uses mice, but the same ideas can be applied to all species of plants
and animals)
This process is known as natural selection and if it operates over millions of
years it is thought that it can give rise to new species of plants and animals.
Speciation
A species is a group of organisms able to interbreed and produce fertile
offspring.
As long as a population has the opportunity to interbreed and exchange
genes, they remain one species. A population of one species can only evolve
into more than one species if groups within the population become isolated
from each other by an isolation barrier that prevent the two groups from inter
breeding. An isolation barrier may be:

Geographical, eg a mountain or a wide river or ocean,

Ecological, eg caused by changes in temperature, humidity, ph, etc.

Reproductive, eg different populations have mating seasons at
different times of the year.
The diagram illustrates what could happen to populations of animals, which
become separated. Once two groups are separated different mutations occur
in each group. If the environments differ, different adaptations are favoured by
natural selection. This leads to different characteristics evolving in each group
as time passes. Eventually the groups become so different that if they come
together again they are unable to interbreed and are now two different
species.
Human Impact on the Environment
The world population is increasing and as it does so more food is needed.
Traditionally if more food was needed more land was planted. But the supply
of suitable land is limited and there are increasing demands for land for uses
other than agriculture, eg building, recreation, etc. In addition some productive
land is being lost because of desertification, influx of sea water and coastal
erosion for example. As a result the land that is available for food production
has to be made more productive. There are many ways of achieving this, but
few come without problems.
Fertilisers
Fertilisers are used to put nutrients, such as nitrate and phosphate back into
the soil so that the soil can be re-used, often for the same crop. Traditionally
farmyard manure (organic fertiliser) was used but this has been replaced
largely by artificially produced inorganic chemical fertilisers. These are
expensive to produce in terms of money and energy.
This is only part of the problem. Some of the fertiliser applied to the soil is
may be leached (washed) out into waterways after application. This can lead
to eutrophication which can kill many living organisms in the aquatic
environment. Eutrophication works like this:
1.
Nitrates and phosphates run off the
land and enrich the water. This
causes rapid growth of algae forming
an algal bloom
2.
The algal bloom blocks light and
causes oxygen producing plants to
die. The dead plants provide food
for decomposing microbes which…..
3.
….. use up more oxygen causing
fish and other organisms to die.
Pesticides
Many traditional methods have been use to protect crops from pests, eg crop
rotation, weeding, use of physical barriers such as netting. However, modern
intensive farming also uses chemical pesticides to control pests.
There are three main types of pesticides:

Herbicides - used to kill weeds which compete with the crop and
remove valuable nutrients from the soil.

Fungicides – used to kill disease causing fungi.

Insecticides – used to kill insects which may eat the crop and/or
spread disease causing organisms.
Many pesticides are biodegradable, that is they can be broken down by
microbes in the soil into harmless substances. However, some of the oldertype of pesticides like DDT are not biodegradable and may stay or persist in
the soil for many years.
DDT is an insecticide but it can kill organisms higher up in the food chain that
it wasn’t intended for (non-target species).
Because DDT isn’t biodegradable it accumulates in the tissues of living
organisms. This build up in living tissue is called bioaccumulation. Once in
an organism, the pesticide can be passed along a food chain and become
concentrated in organisms. Notice in the diagram below that the larger
organisms higher up in the food chain get a massive dose, often large enough
to kill them.
DDT is now banned
for use in the UK and
many other
countries.
Biological Control
Biological control is an alternative to using pesticides. By releasing a
natural predator into the crop growing area, the number of pests can be
reduced. This can have unforeseen consequences as the numbers of different
organisms in the food web are changed. There have been examples of the
predator becoming a more serious pest than the original
problem.
The most successful examples of biological control are
found in the food industry where crops are grown in greenhouses, eg
control of aphids by ladybirds in tomato crops.
Genetically Modified Crops
(Look back at your notes on Genetic Engineering in the Cell Biology unit)
Genetic modification, or GM for short is where certain enzymes are used to
cut pieces of DNA from one organism, and join them into a gap in the DNA of
another organism. This means that the new organism with the inserted genes
has the genetic information for one or more new characteristics. For example,
the organism might produce a useful substance, or be able to carry out a new
function. We say that the organism has been genetically modified.
Genetic modification works in animals, plants and microorganisms. For
example, new genes can be transferred to crop plants to make GM crops.
Some GM crops are resistant to certain herbicides (weed killers) while others
are resistant to insect pests.
There are strong arguments for and against genetic modification of crop
plants. GM crops generally have increased yields, useful for feeding a
growing population. Plants that glow in the dark when they need watering
have even been produced.
However, some people are excited by the almost limitless possibilities of
genetic modification, while others believe the process is unethical and should
be banned. There are concerns about the effect of GM crops on wild flowers
and insects, and whether eating GM food may harm human health.
Indicator Species
Pollution levels can be measured directly but the presence or absence of
certain living organisms can also act as an indicator of the amount of
pollution.
Water pollution
Water pollution is caused by the discharge of harmful substances into rivers,
lakes and seas. Many aquatic invertebrate animals cannot survive in polluted
water, so their presence or absence indicates the extent to which water is
polluted.
Indicator species for levels of water pollution in fresh water
level of water pollution
indicator species in fresh water
clean
mayfly larva
low
freshwater shrimp
high
water louse
very high
rat-tailed maggot, sludgeworm
Air pollution
The most common source of air pollution is the combustion of fossil fuels.
This usually happens in vehicle engines and power stations. This gas
contributes to acid rain. Lichens can be used as air pollution indicators,
especially of the concentration of sulphur dioxide in the atmosphere.
Lichens are plants that grow in exposed places such as rocks or tree bark.
They need to be very good at absorbing water and nutrients to grow there.
Rainwater contains just enough nutrients to keep them alive. Air pollutants
dissolved in rainwater, especially sulphur dioxide, can damage lichens and
prevent them from growing. This makes lichens natural indicators of air
pollution.
For example:



bushy lichens need really clean air
leafy lichens can survive a small amount of air pollution
crusty lichens can survive in more polluted air.
In places where no lichens are growing it is often a sign that the air is
heavily polluted with sulphur dioxide.