Energy flow and food webs

Animals and plants do not live in complete isolation. They are affected by their environment.
Their environment is also affected by them. The study of the interaction between living
organisms and their environment is called ecology.
● The area where an organism lives is called its habitat. Each species of organism has
adaptive features that enable it to live in its specific habitat.
● Biodiversity is the number of different species that live in an area.
● A population is a group of organisms of the same species, living in the same area at the
same time.
● A community is all the organisms, of all the different species, living in the same habitat.
● An ecosystem consists of the interactions between the organisms in a community and
their environment.
● The way in which an organism lives its life in an ecosystem is called its niche.

Energy flow
● All living organisms need energy, which originates from the Sun. Some of the energy in
sunlight is captured by plants, mostly used to make organic nutrients (glucose, starch, fats,
proteins). When a plant cell needs energy, it breaks down some of this food by respiration.
● Animals get their food, and therefore their energy, by ingesting plants, or by eating animals
which have eaten plants. When a cell needs energy to carry out a particular process (muscle
contraction or active transport) the energy is released by respiration.
● Food chain → sequence by which energy, in the form of chemical energy in food, passes from
a plant to an animal and then to other animals.
● different food chains link to form a food web

Producers and consumers

● Every food chain begins with green plants because only they can capture the energy from
sunlight. They are called producers because they produce the energy-containing organic
nutrients that all the other organisms in the food chain need.
● Animals and fungi are consumers. Fungi get their energy by feeding on dead and decaying
bodies and waste organic material from plants and animals (decomposers).
1. primary consumer → first consumer in a food chain (herbivore)
2. secondary consumer → animal that eats that animal (carnivore)
3. tertiary consumer → animal that eats a secondary consumer (carnivore)
4. quaternary consumer → (rarely) animal that eats a tertiary consumer (carnivore)

Energy losses
As energy is passed along a food chain, some of it is lost to the environment
● When an organism uses glucose and other organic compounds for respiration, some of
the energy released from the glucose is lost as heat energy to the environment.
● When one organism eats another, it rarely eats absolutely all of it.
● When an animal eats another organism, enzymes in its digestive system break down
most of the large nutrient molecules, so that they can be absorbed. However, some just
pass through the alimentary canal and are eventually lost from the body in the feaces.
These contain energy that is lost from this food chain.
This means, the further you go along a food chain, the less energy is available for each
successive group of organisms.
Pyramid of numbers
There are more plants than animals, and more herbivores than carnivores. The area of each
block represents the number of organisms at that step in the food chain. Each level in the
pyramid is called a trophic level.
Many organisms feed at more than one trophic level (humans)

● There is less energy available as you go up the trophic levels → fewer organisms at each level
● There is a lot of energy for the plants in this ecosystem. The consumers that eat the plants do
not get all of this energy, because most is lost to the environment. This means that there
cannot be as many grasshoppers as there are plants (not enough energy to support them).
Only enough energy reaches the flycatchers to support a small population of them.

Pyramids of biomass

The pyramid is this shape because of the masses (biomass) of the organisms in the food chain.
Although there is only a single tree, it is huge compared with the caterpillars which feed on it.
These are better than pyramids of number, because they take the size of each organism into
account. They give us a much better idea of the quantity of energy at each level than a pyramid
of numbers does.

Pyramids of energy

What determines the numbers or biomass of organisms at each trophic level is the quantity of
energy that is available to them.
How much energy passes through each trophic level in one square metre of the grassland in one
year. It is a good way of measuring the energy flow through the ecosystem.
Very difficult to produce

Efficiency of energy transfer

The nearer to the beginning of the food chain we feed the more energy there is available for us.
When we eat meat, eggs or cheese or drink milk, we are feeding further along the food chain.
There is less energy available for us from the original energy provided by the Sun.

Nutrient cycles
● Decomposers (fungi and bacteria)
● Feed on organic waste material from animals and plants, and on their dead bodies
● They help to release substances from dead organisms. The released substances (carbon and
nitrogen) can then be used by other living organisms.

The carbon cycle

When plants photosynthesise, carbon atoms from carbon dioxide become part of glucose and
starch molecules in the plant. Some of the glucose is then broken down by the plant in
respiration. The carbon in the glucose becomes part of a carbon dioxide molecule again, and is
released back into the air. Some of the carbon-containing compounds in the plant will be eaten
by animals. The animals respire, releasing some of it back into the air as carbon dioxide.
When the plant or animal dies, decomposers will feed on them. The carbon becomes part of the
carbon-containing compounds in the decomposers' bodies. When the decomposers respire,
they release carbon dioxide into the air again.
Fossil fuels → dead bodies and waste materials that are not broken down by decomposers that
are gradually changed into coal, oil or natural gas
When these fossil fuels are burnt, the carbon in them combines with oxygen from the air, in a
process called combustion. Wood can also undergo combustion. The carbon and oxygen
combine to form carbon dioxide, which is released into the air again.

The nitrogen cycle → make proteins and DNA

Although the air is full of nitrogen, it is in such an unreactive form that plants and animals
cannot use it at all. It must first be changed into a more reactive form, such as ammonia (NH 3) or
nitrates (NO3-).
Changing nitrogen gas into a more reactive form is called nitrogen fixation. There are several
ways that nitrogen fixation can happen:

● Lightning
Lightning makes some of the nitrogen gas in the air combine with oxygen, forming nitrogen
oxides. They dissolve in rain, and are washed into the soil, where they form nitrates.
● Artificial fertilisers
Nitrogen and hydrogen can be made to react in an industrial chemical process, forming
ammonia. The ammonia is used to make ammonium compounds and nitrates, which are sold as
fertilisers.
● Nitrogen-fixing bacteria
They live in the soil, or in root nodules (small swellings) on plants. They use nitrogen gas from
the air spaces in the soil and combine it with other substances to make ammonium ions and
other compounds.
Once the nitrogen has been fixed, plants can use it to make amino acids.
When an animal or plant dies, bacteria and fungi decompose the body. The protein, containing
nitrogen, is broken down to ammonium ions and this is released. Another group of bacteria,
called nitrifying bacteria, turn these ions into nitrates, which plants can use again. Changing
ammonium ions to nitrates is called nitrification.
Nitrogen is also returned to the soil when animals excrete nitrogenous waste material, which
they have produced by deamination of excess amino acids. The nitrogen may be in the form of
ammonia or urea. Again, nitrifying bacteria will convert it to nitrates.
A third group of bacteria complete the nitrogen cycle. They are called denitrifying bacteria,
because they undo the work done by nitrifying bacteria. They turn nitrates and ammonia in the
soil into nitrogen gas, which goes into the atmosphere. This process is called denitrification.

Populations
Most populations tend to stay roughly the same size over a period of time. They may go up and
down (fluctuate), but they'll get back to normal.

Birth rate and death rate

● The size of a population depends on how many individuals leave the population, and how
many enter it.
● Individuals leave a population when they die or when they migrate to another population.
Individuals enter a population when they are born or when they migrate into the population
from elsewhere.
● A population increases if new individuals are born faster than the old ones die - that is, when
the birth rate is more than death rate. If birth rate is less than death rate, the population will
decrease. If birth rate and death rate are equal, the population will stay the same size.

Factors affecting population growth

● Yeast cells are put into a container of nutrient broth. The cells feed on the broth, grow and
reproduce. For the first 12 hours, the population grows only quite slowly, because there are
not many cells there to reproduce. They also need time to adjust to the new conditions. This
is called the lag phase. Once they get going, growth is very rapid. Each cell divides to form 2,
then 4, then 8, then 16. This is called the log/exponential phase.
● As the population gets larger, the individual cells can no longer reproduce as fast, and begin
to die off more rapidly. This may be because there is not enough food left for them all, so
there is competition for food. It could also be because they have made so much ethanol that
they are poisoning themselves. This is called the stationary phase.
● Eventually, the death rate exceeds the birth rate, so the number of living yeast cells in the
population starts to fall. This is called the death phase.

Limiting factors and population growth

Some environmental factor comes into play, which slows down the population growth rate. This
is a limiting factor - the factor that limits the rate of population growth even if everything else
would allow it to increase.

Human pressures on ecosystems

Our activities are having negative effects on ecosystems all over the world.

Food production
Most of the world's supply of food is produced by growing crops or by keeping animals -
although we do still harvest fish from the wild in large quantities.
● Agricultural machinery
Machinery (tractors and combine harvesters) → a farmer can cultivate a much greater area of
land in a much shorter time than using manual labour and animals to pull machinery
● Agricultural chemicals
Farmers are also using agricultural chemicals to help to improve the growth of their crops, so
that they can get more yield from the same area of ground.
- Chemical fertilisers add more mineral ions (nitrate ions) to soils that do not contain
enough of them.
- Insecticides are sprayed onto crops to kill insect pests that might reduce yields or
make the crop look less appealing to buyers.
 - Herbicides are sprayed to kill weeds, which would compete with the crop plants and
reduce their growth.
● Selective breeding
Careful selection of plants and animals to produce new varieties has played an important part in
increasing world food production (to produce new and improved varieties of animals and
plants).

Age pyramids
When scientists begin to study a population, they want to know whether the population is
growing or shrinking. This can be done by counting the population over many years, or by
measuring its birth rate and death rate. But often it is much easier just to count the numbers of
individuals in various age groups at one point in time, and to draw an age pyramid.

Figure A is a bottom-heavy pyramid. There are more young individuals than old ones → birth
rate is greater than death rate, so this population is increasing.
Figure B shows that birth rate and death rate are probably the same → same size population

Monocultures → Crop plants that are usually grown as large areas of a single variety
Advantages
● using machinery to cultivate a large, uninterrupted area of land can be done more quickly
and efficiently
● a uniform crop is likely to grow to the same height, and ripen at the same time, so it can
all be harvested at once using specialised machinery
● a large area of the same crop can all be treated with the same herbicides or pesticides in
the same way at the same time, increasing efficiency
● a monoculture will probably produce large quantities of seeds, fruit, or other harvestable
parts that are all uniform in size and quality, making it easier for the farmer to market
and more profitable to sell.
Disadvantages
● Reduction in biodiversity
In a monoculture, biodiversity is low. Only a few species can live where the crop is growing.
● Increase in pests
They can lead to an increase in the populations of organisms that are pests of the crop. This can
end up reducing yields and transferring plant viruses to the crop, which cause serious diseases in
the plants.
Farmers can try to reduce the quantities of these pests by spraying insecticides onto the crop.
However, this also kills other harmless insects - including predatory insects and spiders that
would help to reduce the population of organisms that are pests. It is also very expensive.
Another approach is to use mixed cropping, where only fairly small areas of ground are covered
with the same crop at the same time of year. This makes it more difficult for insect pests to
spread from one rice field to another.
● Reduction in soil fertility
The crop plants need the same mineral ions, and over time these minerals are removed from
the soil. The farmer has to keep adding fertilisers to replace these minerals or the yield from the
crop will steadily reduce year by year. Growing a mixture of different crops and changing the
crop that is grown on a particular piece of ground from one year to the next, helps to maintain a
better balance of minerals in the soil.

Intensive livestock production

Livestock are often farmed intensively → large numbers of livestock are kept in an area that
would not normally be able to support more than a very small number of animals.
Farmers use high-energy foods to feed the animals and give them regular medication to stop the
development of disease. The animals may be kept in temperature-controlled buildings to
maximise their growth rates.
Advantages
● Provide more food for people (sometimes more cheaply)
● Takes up less land than excessive farming (more land left in its natural state for habitats)
Disadvantages
● Welfare issues for the livestock, which may suffer in the crowded conditions in which
they are kept.
● Disease can spread easily among them. This is dealt with by giving the animals regular
doses of antibiotics - which increases the risk that bacteria will develop resistance to the
antibiotics.
● The waste from the intensive farming unit can pollute land and waterways nearby.
● Food for the animals is often produced from food that we could eat ourselves. This is
wasteful because we would get more energy from the beans themselves.
● Energy is used to transport feed to the livestock, often over very long distances. If the
animals are kept inside buildings, energy may also be used to maintain the temperature
of the buildings in which they are kept, either by heating it or using air conditioning to
keep it cool.
● Large quantities of water may need to be provided if large numbers of animals are kept in
one place. Sometimes, this takes badly needed water from the surrounding environment,
so that other animals and plants cannot live there.

Habitat destruction
If humans destroy a species’ habitat, then it is difficult for them to survive in the new
environment that results.
Habitats are destroyed when we use land for our own purposes:
● We cut down native vegetation to make land available for growing crops, for farming
livestock, for building houses and factories, and for building roads.
● We damage habitats when we mine for natural resources and building materials. The
mine itself destroys the habitats by removing the soil and vegetation, and toxic run-off
(water that flows away from the mine) can enter rivers or the soil in the surrounding
area.
● We add pollutants to land and water, which can kill the plants that normally live there.
Habitats can also be damaged if we remove key species from them. By damaging the food webs
in a habitat, we can affect the habitats of all the plants and animals that live there.

Deforestation → cutting down of large numbers of trees

● Concern about deforestation has been about the loss of tropical rainforests → full of many
different species of plants and animals (high biodiversity)
● When an area of rainforest is cut down, the soil under the trees is exposed to the rain. It is
quickly washed away once it loses its protective cover of plants, and the roots that help to
bind it together. This soil erosion may make it very difficult for the forest to grow back again.
The soil can also be washed into rivers, which end up flooding.

Loss of a habitat for many different species of animals

The loss of so many trees can also affect the water cycle, and the amount of rain that falls
nearby. While rain falls, a lot of it is taken up by the trees, and transported into their leaves. It
then evaporates, and goes back into the atmosphere in the process of transpiration. If the trees
have gone, then the rain simply runs off the soil and into rivers. Much less goes back into the air
as water vapour. The air becomes drier, and less rain falls. This can make it much more difficult
for people to grow crops and keep livestock.

Pollution by greenhouse gases → addition to the environment of something that harms it

● The Earth's atmosphere contains several different gases that act like a blanket, keeping the
Earth warm. They are sometimes called greenhouse gases (carbon dioxide and methane).
● Greenhouse effect: carbon dioxide allows shortwave radiation from the Sun to pass through
it. The sunlight passes freely through the atmosphere, and reaches the ground. It is warmed
by the radiation, and emits longer wavelength, infrared radiation. Carbon dioxide does not
let all of this infrared radiation pass through. Much of it is kept in the atmosphere, making
the atmosphere warmer. The glass around the greenhouse behaves like the carbon dioxide in
the atmosphere. It lets shortwave radiation in, but does not let out the longwave radiation.
It’s trapped inside the greenhouse, making the air inside it warmer.
● We need the greenhouse effect. If it did not happen, then the Earth would be frozen and
lifeless. Enhanced greenhouse effect → the amount of greenhouse gases in the atmosphere is
increasing. This traps more infrared radiation, and makes the atmosphere warmer.
Sources of greenhouse gases
Since the Industrial Revolution, the quantity of fossil fuels which have been burnt by industry,
and in engines of vehicles has increased greatly. This releases carbon dioxide.
Deforestation can also result in an increased amount of carbon dioxide in the atmosphere.
Cutting down rainforests leaves fewer trees to photosynthesise and remove carbon dioxide
from the air. Moreover, if the tree is burnt or left to rot when it is chopped down, then carbon
dioxide will be released from it by combustion, or by respiration of decomposers.
Other gases that contribute to the greenhouse effect have also been released by human
activities. The most important of these is methane. It’s produced by farming activities and
released by bacteria which break down organic matter in the mud in paddy fields, in the
stomachs of animals which chew the cud, and in some insects. It’s also produced by
decomposers acting on decaying rubbish in landfill sites. We can reduce this problem by
decreasing the amount of rubbish and by preventing the methane escaping into the
atmosphere. The methane can be collected and used as fuel. Although burning methane for
fuel does release carbon dioxide, this carbon dioxide does not trap so much infrared radiation
as the methane.
As the concentration of carbon dioxide and methane in the atmosphere increases, the mean
temperature on Earth also increases.

Sea level rise

The melting ice releases a lot of extra water into the oceans, causing sea levels to rise. Sea level
also rises because the volume of a liquid increases as its temperature increases. This is called
thermal expansion.

More extreme weather events

Higher temperatures mean that there is more energy in the atmosphere, which can lead to
different wind patterns and the development of different weather systems.
● Hurricanes and tropical cyclones
 ● Droughts → more forest fires, which affect wildlife
● Rainfall → severe flooding

Eutrophication
When farmers put fertilisers on the soil, they will be absorbed and washed out in solution when
it rains. This is called leaching. Then, the leached nitrate ions are transported to a lake by
underground water.
Algae and green plants in the water grow faster with these. They end up blocking out the light
for plants growing beneath them, so they die. When they do, their remains are a good source of
food for bacteria, which are decomposers.
As they decompose the remains of the plants and algae, the large population of bacteria
respires aerobically, using up oxygen from the water. Soon, there is very little oxygen left for
other living things, like fish. As a consequence, they either die or have to leave the area.
Untreated sewage can also cause eutrophication. Sewage is waste liquid that flows from human
toilets, kitchens and bathrooms. Even though it does not usually increase the growth of algae, it
does provide a good food source for many kinds of aerobic bacteria.
Reducing eutrophication
● treating all sewage before it is released into the environment
● using organic fertilisers as they do not contain many nitrates that can easily be leached out
of the soil

Non-biodegradable plastics
Most plastics are non-biodegradable → decomposers cannot break them down
Discarded plastic objects do not rot, just accumulate. They are dangerous to other organisms.

Conservation
Conservation is the process of looking after the natural environment.
Aims to maintain or increase the biodiversity of an area. One of the greatest threats to
biodiversity is the loss of habitats. Each species of living organism is adapted to live in a
particular habitat. If this habitat is destroyed, then the species may have nowhere else to live,
and will become extinct.

Conserving forests
Tropical rainforests have a very high biodiversity compared with almost anywhere else in the
world.
Forests provide useful resources for humans. They can provide a sustainable resource - a
resource that will not run out, even if we keep on using it.
It is possible to use forests sustainably:
● Governments can refuse to grant licences to companies who want to cut down valuable
forests.
● Instead of cutting down all the trees in a forest (clear-felling), just a small proportion of
the trees are cut down. This is called selective felling. The remaining trees will hold the
soil in place, and will continue to provide habitats for animals. In practice, however,
selective felling often does a lot of damage to the forest because of:
- the roads that are built to allow access
- the large machinery that is used to drag the timber out
- the disturbance caused by the people working in the forest
 ● Trees can be cut down to about one metre or less, and then left to regrow. This is called
coppicing.
● Where large numbers of trees are cut down, new ones should be planted to replace them
This is what happens with most of the trees used to make paper. However, planting new
trees cannot replace primary forest (forest that has never been cut down)
● Education can help to make sure that people understand how important it is to conserve
forests.
● Some parts of a forest can be completely protected by law.

Conserving fish stocks

Humans have probably always used fish as a source of high-protein food. However, there is
increasing concern about the threat to fish populations from the large numbers of fish that are
being caught.
If we do not reduce catches, the populations will get so small that there will not be enough adult
fish left to breed and sustain the populations.
Reducing the number of fish that are caught is not easy. There are a few ways that the number
of fish being caught can be controlled:

● Quotas are agreed limits that allow countries, regions or fishermen to catch only a
certain quantity of fish.
● Some areas of the sea can be closed for fishing at certain times of year. These occur when
fish are breeding and are called closed seasons.
● Some areas of the sea can be completely closed to fishing all the time. These are called
protected areas. They act as refuges for fish, which can live and breed safely.
● Restrictions can be placed on the type of nets and the size of the mesh of the nets. If a
large mesh size is used, then only the biggest fish are caught. The smaller ones can grow
and reproduce, adding new fish to the population.
● All of these rules are enforced by inspectors. Fishing vessels can also be required to fit
electronic position sensors, which continually transmit information about where the
boats are. Inspectors also check the catches that are brought to land.

Endangered species
A species that no longer has any living individuals on Earth is said to be extinct. Once a species
has become extinct, it is gone forever.
A species whose numbers have fallen so low that it is at risk of becoming extinct is endangered.
Paleontologists have identified several periods in the past when huge numbers of species seem
to have become extinct. These are called mass extinction events (caused by a major change in
the conditions on Earth).
Another major extinction event is happening now → we’re the cause
Reasons why species can become extinct:
● climate change, habitat destruction and pollution
● hunting by humans - either for food or sport
● overharvesting - such as a species of fish or a species of tree
● introduction of new species to an ecosystem.
As temperatures rise on Earth, organisms with adaptations that allow them to live in a particular
environment may no longer be so well adapted.
The introduction of new species can disrupt food webs and cause changes in ecosystems that
put native species (species that naturally live in that area) at risk.
Seeds are often able to survive for many years in a dormant state, and then germinate when
conditions are right. We can make use of that by collecting and storing seeds of as many
different plant species as possible.

Conserving endangered species

Tactics that can be used to save endangered species:
● monitoring and protecting the species in its natural habitat
● using captive breeding programmes
● educating local people about the importance of conservation, and what they can do to
help
Many zoos are involved in captive breeding programmes for endangered species of animals.
Zoos keep records of the pedigrees of their individual animals, and animals may be moved from
one zoo to another so that males and females that are genetically different from one another
can mate and produce offspring. This helps to maintain genetic diversity in the captive
population. The aim is to be able to reintroduce some of the animals to the wild eventually, so
that the wild population is increased. This can only work if the habitat for the animals is
protected.
Animals in captivity do not always breed readily. Their unusual surroundings may not provide
the correct environment for them to become ready to mate, or males and females often show
no interest in mating. In those cases, the zoo can help by using techniques called assisted
reproduction:
● Artificial insemination (AI) → taking semen from the male and inserting it into the vagina
of the female. Some of the sperm in the semen will find their way to an egg and fertilise
it. The zygote will then develop into an embryo and implant into the uterus in the normal
way. Before AI is carried out, the female must be at the correct stage of her reproductive
cycle, and there must be a good chance that there is an egg in her oviducts.
● In vitro fertilisation (IVF) → collecting eggs from the female, placing them in a sterile
liquid in a sterile container (Petri dish) and then adding semen from the male.
Fertilisation happens in the dish and several zygotes are formed. Some then divide to
form tiny embryos. One or more of the embryos is then placed in the female's uterus. It is
necessary to wait until a female ovulates naturally before eggs can be harvested. She also
needs to be in the correct stage of her reproductive cycle before the embryos produced
in vitro can be implanted. But sometimes the female or male - or both - can be treated
with hormones that cause them to produce gametes.

Maintaining genetic diversity

● When the numbers of a species drop to very low levels much of this genetic variation is lost.
● If there is variation between individuals, then the population as a whole has a better chance
of surviving if they are threatened by a pathogen, or if their habitat changes.
● This makes the species more likely to become extinct, especially if environmental
conditions change.
● Low levels of genetic variation also make it more likely that, when two individuals breed
together, they may each carry the same harmful recessive allele of a gene. Some of their
offspring may therefore inherit this harmful allele from each parent, and consequently have
 a homozygous recessive genotype which produces a phenotype making them less likely to
survive.
● Scientists at seed banks also work to maintain genetic diversity among the seeds that are
stored.

Reasons for conservation programmes

● We share the Earth with many different species and we have a responsibility to make
sure that they can live successfully in their habitats. We are keeping them safe so that
future generations can enjoy them.
● If we damage ecosystems, we can be harming ourselves.
● The loss of one species may have harmful effects on other species that are part of the
same food web. By reducing their food supplies, conserving ecosystems can also help to
maintain efficient recycling or nutrients such as nitrogen, through the nitrogen cycle.
● Many plant species contain chemicals that can be used as drugs. If we lose plant species,
we may be losing potential new medicines.
● Wild relatives of our crop plants contain different alleles of genes that could be useful in
future breeding programmes. Conserving wild plants, as well as all the different varieties
of crop plants, is important if we are not to lose potentially useful alleles.