Biology Chapter 23 (A-Level) - Ecosystems

Question 1

What is an ecosystem?

Answer 1

All the living organisms found in one area, and the non-living aspects of their environment.

Question 2

Why are ecosystems and the size of them known as ‘dynamic’?

Answer 2

Depends on interactions between biotic and abiotic factors.

Question 3

What is a biotic factor? Give examples.

Answer 3

Living features of an ecosystem
e.g. predators and disease

Question 4

What is an abiotic factor? Give examples.

Answer 4

Non-living features of an ecosystem
e.g. Light and temperature

Question 5

What is biomass?

Answer 5

The total weight of living matter in a certain area, transferred up trophic levels through consumption

Question 6

How can we measure biomass?

Answer 6

In terms of mass of carbon, or dry mass of tissue per given area per given time

Question 7

How can the chemical energy stored in dry biomass be estimated?

Answer 7

Calorimetry

Question 8

Give the equation for the efficiency of biomass transfer (ecological efficiency).

Answer 8

(Biomass transferred / biomass intake) x 100

Question 9

How can human activities affect biomass transfer?

Answer 9

• Light, water, and temperature maximised
• Increased nutrients in soil
• Pests and weeds removed
• Growth rates boosted through steroids, selective breeding, etc.

Question 10

Outline the role of organisms in the carbon cycle.

Answer 10

1) Respiring plants and animals release carbon dioxide into the atmosphere
2) Photosynthesis of plants removes carbon dioxide from the atmosphere
3) Decomposers/microorganisms decay plant and animal material into carbon dioxide

Question 11

Define Net Primary Productivity (NPP).

Answer 11

The rate at which energy is transferred into the organic molecules that make up new plant biomass, that is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account

Question 12

Define Gross Primary Productivity (GPP).

Answer 12

The rate at which energy is incorporated into organic molecules in the plants in photosynthesis, that is the chemical energy store in plant biomass, in a given area or volume, in a given time

Question 13

Give the equation to calculate NPP.

Answer 13

NPP = GPP - R

R: Respiratory losses to the environment

Question 14

Give the equation to calculate the net production of consumers (N).

Answer 14

N = I - (F + R)

N: Net production of consumers
I: Chemical energy stored in ingested food
F: Chemical energy lost to the environment in faeces and urine
R: Respiratory losses to the environment

Question 15

Name the 4 organisms involved in the Nitrogen Cycle.

Answer 15

Nitrogen-fixing bacteria:
1) Rhizobium
2) Azotobacter

Nitrifying bacteria:
3) Nitrosomonas (chemoautotrophic bacteria)
4) Nitrobacter

Question 16

Where are nitrogen-fixing bacteria found? Use Rhizobium and Azotobacter as examples.

Answer 16

Rhizobium live in the root nodules of leguminous plants (e.g. pea plants)

Azotobacter live in the soil

Question 17

Why is nitrogen an essential component of metabolism?

Answer 17

Required for the synthesis of protein and nucleic acids

Question 18

What has to happen to nitrogen before it is used by plants?

Answer 18

Since nitrogen cannot be absorbed by plants in its gaseous form, it is ‘fixed’ in either the form of ammonium ions or nitrates

Question 19

Name the 4 stages of the Nitrogen Cycle.

Answer 19

1) Nitrogen fixation
2) Nitrification
3) Denitrification
4) Ammonification

Question 20

Nitrogen Fixation: what is the function of nitrogen-fixing bacteria? Use Rhizobium and Azotobacter as examples.

Answer 20

Produce a nitrogenase enzyme that converts both nitrogen and hydrogen into ammonia (NH3)

Question 21

Outline what happens during nitrification.

Answer 21

Ammonia is oxidised and converted to Nitrites (NO2-) which are then converted into Nitrates (NO3-) which is readily soluble so can be absorbed by plants

Question 22

What type of bacteria converts Ammonia to Nitrites?

Answer 22

Chemoautotrophic Bacteria called Nitrosomonas.

Question 23

What type of bacteria converts Nitrites to Nitrates?

Answer 23

Nitrobacter oxidises nitrites to nitrates in the presence of oxygen.

Question 24

Why is Ammonia ultimately converted into nitrates?

Answer 24

Nitrates are readily soluble by plants from the soil for nucleotide synthesis.

Question 25

What happens during denitrification?

Answer 25

- This occurs in anaerobic conditions in the absence of oxygen. - Denitrifying bacteria converts nitrates back to oxygen gas for respiration. - Nitrogen gas and nitrous oxide are produced in the process.

Question 26

What is the denitrifying bacteria involved in denitrification?

Answer 26

Pseudomonas Denitrificans

Question 27

What happens during Ammonification?

Answer 27

- Organic nitrogen is converted into Ammonia - Nitrogen is found in urine, waste, dead matter

Question 28

Outline the roles of microorganisms in the Nitrogen Cycle.

Answer 28

- Nitrogen-fixing bacteria convert gaseous nitrogen into ammonia --> Rhizobium in roots and Azotobacter in soil - Nitrifying bacteria convert ammonium compounds into nitrites which then converts into nitrates. --> Nitrosomonas converts ammonium compounds into nitrites --> Nitrobacter converts nitrites into nitrates.

Question 29

What is meant by 'Primary succession' ?

Answer 29

Where a newly formed area (previously devoid of life) is colonised by a community of organisms.

Question 30

Summarise the process of primary succession.

Answer 30

1)Seeds and spores are carried by the wind 2) They land on exposed rocks and begin to grow 3) Species such as lichens and moss ('Pioneers') colonise the new land 4) They die and decompose 5) Humus (dead organic matter) forms a basic soil 6) Seeds of small plants and grasses are also carried by the wind (or in faeces) and land in this soil 7) They grow in this soil as these small plants are adapted to survive in shallow, relatively nutrient-poor soils 8) The small plants and shrubs die and decompose 9) The soil becomes more rich in nutrients and deeper 10) The roots of these small plants and shrubs also form a network in the soil which keeps it intact and prevents it from being washed away 11) Now, larger plants and small trees can grow on this land due to the availability of deeper, nutrient-rich soil 12) Over time, the soil becomes deep enough and is nutrient-rich to accommodate more complex organisms and larger plants. --> The final species to colonise this land are known as the dominant species of the now relatively complex ecosystem.

Question 31

What is the climax community?

Answer 31

The final stage of succession

Question 32

How is the climax community established?

Answer 32

This is where the ecosystem of organisms is balanced stable, and self-sustaining. The stage is reached when the environment is no longer changing and the soil is rich enough in minerals to support large plants such as trees/shrubs.

Question 33

How does secondary succession occur?

Answer 33

This occurs in a previously colonised area in which an existing community has been cleared. This type of succession usually occurs after an event such as forest fires. Since the soil layer is already been present, succession occurs at a later stage.

Question 34

How is humus produced?

Answer 34

From weathering of rock

Question 35

What is deflected succession?

Answer 35

When succession is interrupted, usually by human interference. Also, known as 'plagioclimax community'

Question 36

Give several ways that succession can be prevented.

Answer 36

1) Temporarily introducing 'grazing animals' 2) Managed burning when controlled fires are deliberately lit and are allowed to burn away the shrubs and trees

Question 37

Give an example of a way that primary succession is established.

Answer 37

The eruption of a volcano leads to the formation of a rock surface.

Question 38

What is sampling?

Answer 38

Selecting a group of individuals that will represent the whole target population.

Question 39

What is the purpose of sampling?

Answer 39

Allows us to measure the distribution and abundance of organisms.

Question 40

What factors affect distribution of a species?

Answer 40

1) Abiotic factors: - Water availability - Light/UV Radiation - Minerals 2) Biotic factors: - Competition between species - Competition within species - Herbivory

Question 41

What is the formula for abundance (m^-2)

Answer 41

Number of individuals in a sample / Area of sample (m^2)

Question 42

What is the formula for 'The Lincoln Index' ? This gives you the estimated population size.

Answer 42

(No. of individuals in 1st sample x No. of individuals in the 2nd sample) / No. of recaptured marked individuals