19: Organisms and Their Environment

Keywords:
Ecology- the study of an organisms in their environment

Habitat-the place where an organism lives

Community- all of the populations of all the different species in an ecosystem

Ecosystem- a unit containing all of the organisms in a community and their environment,
interacting together

Niche- the role of an organism in its natural environment; the way in which it interacts with
other organisms and with the non-living parts of the environment

Food chain- a diagram showing the flow of energy from one organism to the next,
beginning with a producer

Food web- a network of interconnected food chains

Producer-an organism that makes its own organic nutrients, usually using energy from
sunlight, through photosynthesis

Consumer-an organism that gets its energy by feeding on other organisms

Herbivore- an animal that gets its energy by eating plants

Carnivore- an animal that gets its energy by eating other animals

Decomposer- an organism that gets its energy from dead or waste organic material

Pyramid of numbers- a diagram in which the area of the bar at each trophic level shows the
relative number of organisms at that level in the food chain

Trophic level- the position of an organism in a food chain, food web or pyramid of biomass
or numbers

Pyramid of biomass- a graph showing the relative quantity of biomass at each trophic level

Pyramid of energy- a graph showing the relative quantity of energy at each trophic level

Nitrogen fixation- convert inert nitrogen gas into a more reactive form, such as nitrate ions
or ammonia

Nitrification- converting ammonium ions to nitrate ions

Denitrification- converting nitrate ions to nitrogen gas

Lag phase- the stage at the start of a population growth curve where the population
remains small and grows only very slowly

Log phase or exponential phase- the stage in a population growth curve where the
population grows at its maximum rate; birth rate exceeds death rate

Stationary phase- the stage in a population growth curve where the population remains
roughly constant; birth rate equals death rate

Death phase- the final stage in a population growth curve where the population falls; death
rate exceeds birth rate

Sigmoid growth curve- an S-shaped curve showing the change in the size of the population
through all the phases in population growth

Age pyramid- a diagram showing the relative numbers of individuals of different ages in a
population
 Food chains

Energy is introduced into biological systems as light energy which is absorbed by plants, where
the Sun is the principal source of energy. This energy is then transferred to chemical energy
and can pass to other organisms through feeding. When these animals die, they are broken
down by decomposers which return their nutrients to the soil.

Figure : A food chain.

A food chain shows the order of energy transfer between organisms. Food chains start with
producers, which are plants which convert light energy from the Sun to chemical energy.
Organisms which feed on plants, or other animals, are known as consumers. Plants are eaten
by herbivores, which gain nutrients from the plants that are used for growth. This consumer is
then eaten by another animal which gains the nutrients from the first. Each of these organisms
has its own trophic level. Consumers are classed as primary, secondary, tertiary or
quaternary, depending on their position in the food chain.
Many different food chains link to form a food web. Figure shows an example of a food web.

Figure : A food web.

 Energy is passed between trophic levels, although this process is highly inefficient due to a
variety of reasons:
● Not all animal and plant material can be digested to gain energy from, e.g. fur and
bones.
● Energy is lost through excretion and decay.
● Energy is used in other processes, e.g. movement and keeping warm.

This means that organisms later on in the food chain gain less energy from their food than
organisms earlier on, as energy is lost at each level. Consequently, organisms later in the food
chain must eat a larger amount to gain enough energy for survival. For this reason, there are
usually not more than five trophic levels as too much energy would be lost to sustain
another. This is also why it is more efficient for humans to eat plants rather than animals, as
there are more stages in the food chain if animals are present.

A pyramid of numbers can be used to show the number of organisms in each trophic level. This
is similar to a pyramid of biomass, which measures the total biomass of all the organisms at
each level. A pyramid of biomass tends to have a true pyramid shape as biomass is lost in
each level which corresponds to the energy lost. A pyramid of biomass therefore is more
useful as it gives an indication of the amount of energy being passed on at each stage of the
food chain.
Advantages of using a pyramid of biomass rather than a pyramid of numbers to represent a food

chain:

in a pyramid of numbers one large individual is shown in the same way as one very tiny

individual ;

biomass indicates how much food there is, available / left ;

biomass is an indicator of the energy available ;

pyramid of biomass is pyramid shaped whereas a pyramid of numbers is not always

Advantages of using a pyramid of energy rather than pyramids of numbers or biomass to represent a food
chain:
it shows how much energy is available

it shows how much energy is passed through per meter square

other pyramids are not very informative, because we cannot be sure that one gram of biomass
for one species contains the same quantity of energy as one gram of biomass of a different species.
 Q. Explain why the fourth trophic level has the least biomass in any food chain
energy is lost between the trophic levels / energy decreases up the trophic levels ;
not all of the organism is, eaten / digested / absorbed ;
energy is lost, as heat / in respiration / in metabolic processes / named metabolic
process / movement ;
energy lost in, excretion / faeces / urine ;
(so) less energy to support the next trophic level ;
Human impact on food chains:
Humans alter food webs through over-harvesting food species and introducing foreign species
to habitats. Over-harvesting will damage food chains as other organisms which consume these
organisms will not have enough food to survive, meaning that many of them will die.
Introducing foreign species may have the same effect as there is now competition for
resources, which could damage existing species by interfering with the food chain.
Q. Why it is more energy efficient for humans to eat crop plants than to eat
livestock that have been fed on crop plants
as fewer trophic levels / plants are at a lower trophic level (than salmon / livestock) ; ora idea that
energy transfer along food chain is inefficient ; (energy lost in) respiration / heat / (named) metabolic
process / movement / muscle contraction ; ref to (more) material that is lost as, faeces / egested /
inedible / not digestible (in longer food chains) ; (energy lost in) excretion / urine ;

An example of this is cane toads, which were introduced to Australia to eat pests. Due to their
toxic skin, they have destroyed many native species and damaged habitats, especially water
habitats, where the biodiversity has been reduced. This has had a negative effect on the bird
population which preyed on animals living in these areas. This shows that when one trophic
level is damaged, all that follow are also impacted as the amount of food for them decreases

Nutrient cycles
The carbon cycle:
Carbon is an essential element which makes up the majority of molecules in living organisms.
The carbon cycle is used to show how carbon atoms move between the atmosphere and living
organisms:

1. Photosynthesis: Plants take in carbon dioxide (CO₂) from the air and use sunlight to make
their food. This process is called photosynthesis. During photosynthesis, plants convert
carbon dioxide and water into glucose (a type of sugar) and release oxygen.
2. Respiration: Both plants and animals perform respiration. This means they take in oxygen
and break down glucose (from food) to release energy. In the process, they produce
carbon dioxide (CO₂), which is then released back into the air.
3. Feeding: Animals eat plants (or other animals) to get energy and carbon. When they eat
plants, they take in the carbon stored in those plants. This carbon is then used for energy
or stored in the animal’s body as part of its own tissues.
 4. Decomposition: When plants and animals die, decomposers like bacteria and fungi break
down their remains. As they break down dead organisms, carbon is released back into the
soil or the air as carbon dioxide (CO₂).
5. Formation of Fossil Fuels: Over millions of years, the remains of dead plants and animals
can be buried under layers of earth. Under heat and pressure, these remains slowly turn
into fossil fuels like coal, oil, and natural gas. These fuels store carbon, which has been
locked away for a long time.
6. Combustion: When fossil fuels (like coal, oil, or gas) are burned for energy (in cars,
factories, or power plants), carbon dioxide (CO₂) is released into the atmosphere. This is
called combustion, and it returns carbon to the air.

Carbon can be trapped in dead organisms when decomposition does not occur. These
organisms become fossilised over thousands of years to form fossil fuel. When this fossil fuel is
burned, large amounts of carbon dioxide is released back into the atmosphere. This, along with
the effect of deforestation, leads to the amount of carbon dioxide in the atmosphere
increasing, which causes global warming

Figure: A Carbon cycle

The nitrogen cycle:
Nitrogen is another element which is key to biological lifeforms. Nitrogen is used to make
amino acids, DNA and ATP, which is a molecule that releases energy. Microorganisms are key
to the nitrogen cycle as they help to convert nitrogen to different forms so that it can be
used.

1. Decomposition of plant and animal protein to ammonium ions: When plants and animals
die or produce waste, bacteria and fungi break down their proteins (which contain
nitrogen) into ammonium ions (NH₄⁺). This process is called decomposition.
2. Nitrification: Some bacteria in the soil change ammonium ions into nitrites (NO₂⁻), and
then other bacteria convert the nitrites into nitrates (NO₃⁻). Nitrates are a form of
nitrogen that plants can use. This process is called nitrification.
3. Nitrogen fixation by lightning and bacteria:
o Bacteria: Some bacteria in the soil and on plant roots (especially legumes like peas)
change nitrogen gas (N₂) from the air into a form plants can use, such as ammonia.
This process is called nitrogen fixation.
o Lightning: Lightning can also split nitrogen gas in the air (N₂) and turn it into
nitrogen oxides, which dissolve in rain to form nitrates that can be absorbed by
plants.
4. Absorption of nitrate ions by plants: Plants take in nitrate ions (NO₃⁻) from the soil
through their roots. They use these nitrates to help make proteins and other important
molecules.
5. Production of amino acids and proteins: Inside the plants, nitrates are used to make
amino acids, which are the building blocks of proteins. When animals eat the plants, they
get the amino acids and proteins too.
6. Feeding and digestion of proteins: Animals eat plants or other animals to obtain proteins.
The proteins are digested in the stomach and broken down into amino acids.
7. Deamination: When animals use amino acids, the nitrogen in those amino acids must be
removed in a process called deamination. This releases ammonia (NH₃), which is then
converted into less toxic forms (like urea in mammals) and excreted as waste.
8. Denitrification: Finally, some bacteria in the soil convert nitrates (NO₃⁻) back into nitrogen
gas (N₂), which is released into the atmosphere. This process is called denitrification, and it
completes the nitrogen cycle, returning nitrogen back to the air.

Questions:
 Population size
A population is a group of organisms of one species, living in the same area, at the same
time.
A community is all of the populations of different species in an ecosystem.
An ecosystem as a unit containing the community of organisms and their environment,
interacting together.

Factors affecting rate of population growth:

● Food supply - if there is a large amount of food, organisms can breed more successfully.
If there is a food shortage, there is a higher death rate which results in a slow or
negative population growth.
● Predation - organisms which have lots of predators will have a slower rate of population
growth as more will be killed by predators.
● Disease - disease can reduce the population by killing organisms. In densely populated
areas, disease can spread quickly, thus a large proportion of the population may be
wiped out.
 Competition-competition among organisms lead the better adapted ones to survive and
reproduce.
 Figure : The growth of a population of yeast

Human population growth:

Over the last 250 years, the human population has risen from just over 1 billion to 7.6 billion.
There are many social and economic implications of this. Due to the massive demand for
resources and space, deforestation occurs, and a high amount of fossil fuels are burnt. This
leads to global warming and also damages habitats. Rapid population growth also puts a strain
on services such as healthcare and education, meaning that many people cannot access these
services, which lowers quality of life