1. Biology – Cellular Control, Biotechnologies, Ecosystems and
Responding to the Environment
Module 3 – Ecosystems and Sustainability
1) ECOSYSTEMS
Ecosystem – a unit of living (biotic) and non-living (abiotic) components through
which interactions occur, nutrients cycle and energy flows.
Habitat – a small part of the ecosystem where organisms live.
Microhabitat – a very small part of the ecosystem where organisms live.
Niche – how an organism lives as well as where it lives. A niche is its role within the
ecosystem, including what it feeds on, what it excretes, how it reproduces, etc
Population – organisms of a species that live in a habitat at the same time and
breed together.
Community – populations of different species living in a habitat and interacting with
one another.
Ecosystems are Dynamic
Any small changes in abiotic or biotic features of an environment can have an effect
on other parts of the ecosystem. Therefore, ecosystems are constantly undergoing
change.
Photosynthesis Respiration
Light Energy Biotic Component Heat Energy
Nutrients
Abiotic Component
Energy is not recycled, but it flows through the ecosystem as above. Nutrients,
however, are recycled via the nitrogen cycle, the carbon cycle, etc.

2. Plants, and other photosynthetic organisms, are known as producers, because they
can synthesise their own form of chemical energy, which can be supplied to other
organisms.
Organisms that eat other living organisms for energy are called consumers.
Primary consumers feed on plants, and are herbivores. Secondary consumers
feed on the primary consumers, and are usually carnivores. Secondary consumers
are, in turn, consumed by tertiary consumers.
Other organisms, known as decomposers, consume dead or waste material from
organism.
2) UNDERSTANDING ENERGY TRANSFER
Within an ecosystem, living organisms are usually members of more than one food
chain, and also often feed at different trophic levels in different food chains. Food
webs help us to see how energy is transferred through an entire ecosystem.
At each tropic level, some energy is lost from the food chain, making it unavailable
for the organism at the next tropic level:
- All living organisms require energy to carry out life processes, which they get
from respiring metabolites from either photosynthesis or consuming other
organisms. Respiration produces energy in the form of ATP which the
organism uses for cellular activities.
- When an organism dies, its energy can no longer be released as heat, and
some of it cannot be consumed by predators. Therefore, decomposers
become the only organisms the energy is available to.
Because of the loss of energy, there is less energy at higher trophic levels to sustain
large numbers of organism; therefore, organisms higher up the food chain occur in
lower frequencies than producers, primary consumers, etc. This is expressed in a
pyramid of numbers.
Organisms may have different masses, so counting the population numbers may not
give an accurate representation of how much living tissue there is. A better measure
is to measure the biomass. Dry mass, although destructive to the ecosystem, is a
better measure of mass of tissue because it does not take the water in the organism
into account. It is shown in a pyramid of biomass.
Even measuring biomass has problems with accuracy, as different organisms may
release different units of energy per unit of mass. Therefore, it would be more
accurate to measure how much energy is released from the organism, done by

3. burning the organisms in a calorimeter and working out how much heat energy is
released. It is shown in a pyramid of energy.
Energy is lost through a number of ways; the Sun’s energy is lost because:
- Not all sunlight penetrates the atmosphere
- Only around 3% of sunlight is absorbed by photosystems in plants/autotrophs
- Light reflects off vegetation or passes through, missing photosystems
completely
- Only certain wavelengths of light are absorbed
The amount of sunlight energy actually converted to chemical energy through
photosynthesis is called the gross primary productivity. After the plant has used
some of the chemical energy itself, the net production is taken to the next trophic
level by the primary consumer. As the net production is transferred to the consumer,
some energy is lost to the decomposers, as consumers cannot eat some parts of the
plant if it is not digestible or available.
3) INFLUENCING FOOD CHAINS
Herbicides – the increase the population by removing competition of weeds and
also by removing plants that harbour pests or diseases.
(-) It kills all plants apart from crop, decreasing biodiversity and niches, decreasing
the number of small animals, which impacts higher up the food chain.
Pesticides – they increase production by removing pests.
(-) It removes the lower levels of the food chain which impacts higher up. Also, the
amount of pesticide can accumulate higher up the food chain, creating dangerous
levels.
Fertilisers – nitrogen based fertilisers can help crop to grow taller.
(-) Nitrates enter water, encouraging algae growth, blocking sunlight. The algae
below the water die, causing decomposers to feed on them and respire, using
oxygen. This causes a lack of oxygen in the water, causing fish and other animals to
die, creating a stagnant pool.

4. 4) SUCCESSION
Ecosystems can change due to communities of organisms and communities of
organisms can change the ecosystem. Because of this, sometimes there is bare
ground in an ecosystem. Pioneer plants are the first plants to colonise this bare
ground.
Pioneer plants change the environmental conditions; for example, they can provide
shelter for germinating seeds, places for insects to hide, dead leaves, increased
humus which improves soil quality.
Eventually conditions become suitable enough for a wider range of species, so the
biodiversity of the ecosystem increases. Primary succession is the succession of
species onto bare ground. Succession of species after the pioneer plants onto this
soil is called secondary succession and the community produced by this succession
is the climax community.
5) DECOMPOSERS AND RECYCLING
Bacteria and fungi involved in decomposition feed in a different way from animals.
They feed saphrotrophically so they are described as saphrotrophs. These
organisms secrete enzymes onto dead waste and material, which digest the material
into small molecules before absorbing the molecules into the organism’s body.
From these molecules, the microbes have enough energy for respiration, allowing
them to carry out processes that aid in the cycling of nitrogen and carbon in the
ecosystem.
Living things require nitrogen as part of their nutrition to make proteins, nucleic acids,
etc. They get nitrogen through ammonification, nitrogen fixation, nitrification and
denitrification.
It is impossible for plants to use nitrogen directly, so they need a “fixed” supply of
nitrogen, i.e. ammonium ions (NH4+) or nitrate ions (NO3-). Nitrogen fixation can
occur when lightning strikes or through the Haber process. However, these
processes only account for about 10% of nitrogen fixation.
Nitrogen-fixing bacteria, such as Rhizobium, lives inside root nodules of bean plants:
- They have a mutualistic relationship with the plant, receiving carbon
compounds from the plant in return for providing fixed nitrogen.

5. Nitrification occurs when chemoautotrophic bacteria in the soil absorb ammonium
ions:
- Ammonium ions are released by bacteria involved in putrefaction of proteins
found in dead/waste organic matter.
- Chemoautotrophs obtain their energy by oxidising ammonium ions to nitrites
(Nitrosomonas bacteria) or by oxidising nitrates to nitrates (Nitrobacter
bacteria).
- Because this oxidation requires oxygen, these reactions only occur in well-
aerated soils.
- Nitrates can be absorbed from the soil by plants and used to make nucleotide
bases and amino acids.
Denitrification occurs when other bacteria convert nitrates to nitrogen gas. When
these bacteria are under anaerobic conditions, they use nitrates as a source of
oxygen for respiration, producing nitrogen bas and nitrous oxide.

6. 6) WHAT AFFECTS POPULATION SIZE?
- At first (the lag phase), there are only a few individuals who are still
acclimatising to their habitat. The rate of reproduction is slow, meaning
population size grows slowly.
- After this (the log phase), resources are plentiful and conditions are good, i.e.
limiting factors are at a minimum. The rate of reproduction greatly exceeds the
mortality rate, meaning population size grows rapidly.
- Finally (the stationary phase), the population size has levelled out to its
carrying capacity – the habitat itself can’t support a larger population of the
species. In this phase, the reproduction rate is approximate to the mortality
rate. The population size fluctuates very slightly during different seasons.
When the population size reaches the stationary phase, the habitat is unable to
support a larger population because of limiting factors, i.e. the availability of food,
water, light, oxygen, nesting sites, shelter, etc. Predation and competition between or
within species can also affect the population size as a limiting factor.
7) COMPETITION
Competition happens when resources (like food or water) are not present in
adequate amounts to satisfy the needs of all the individuals who depend on those
resources. As the intensity of competition increases, the rate of reproduction
decreases.
Intraspecific competition happens between members of the same species. As
factors of the ecosystem become limiting, individuals must compete for them.
Individuals who are well adapted and survive the competition breed and pass on

7. their alleles to the next generation. This slows down population growth and the
population enters the stationary phase.
The fluctuations in the stationary phase can be explained by intraspecific
competition:
- If the population size drops, competition reduces, and the population size then
increases.
- If the population size increases, competition increases, and the population
size then drops.
Interspecific competition happens between individuals of different species, and
can affect both the population size of a species and the distribution of a species in an
ecosystem.
It was concluded by Russian scientist Georgyi Frantsevitch Gause that the more
overlap between two species’ niches would result in more intense competition. If two
species have exactly the same niche, one would be outcompeted by the other and
would die out/become extinct in that habitat – competitive exclusion principle.
Sometimes, however, interspecific competition can result in one species being much
smaller than the other without actually dying out, thus resulting in relatively constant
population sizes. As well as this, a laboratory study may not take into account other
variables that are present in the world that also affect population size.
8) SUSTAINABLE MANAGEMENT
As the human population is increasing exponentially, the resources we require is
also increasing at this rate. Because of this, we need to exploit our environments
more intensively, often resulting in destroying ecosystems, reducing biodiversity, etc.
One such example is the harvesting of timber from woodland, however, there are
many sustainable methods to gain the resources required.
Coppicing involves cutting a tree trunk close to the ground to encourage new
growth, and harvesting the timber cut. Once cut, several new shoots grow from the
cut surface and eventually mature into stems of quite narrow diameter.
To provide a continuous supply of wood, the area is divided into sections, and one
section is cut at a time, giving time for the woodland to recover from the coppicing.
This is known as rotational coppicing. Although coppicing is good for biodiversity, it
does not provide a large amount of timber, so it a relatively small-scale method of
management.

8. Large-scale production of timber can often involve felling trees in an entire area,
known as clear-felling. It can destroy habitats on a large scale, and thus it is rarely
ever practised in the UK. Soil erosion can occur which allows soil to run off into
waterways, resulting in desertification.
Leaving each area of woodland for 50-100 years to mature before felling allows
biodiversity to increase. This is not cost-effective, so most modern foresters:
- Plant another tree for each that is harvested.
- Maintain the woodland’s ecological function regarding biodiversity, climate,
mineral and water cycles.
- Allow local people to derive benefit from the forest.