C.2 Communities and ecosystems

Question 1

What is a trophic level?

Answer 1

A trophic level is the position an organism occupies within a feeding sequence.
- Producers always occupy the first trophic level in a feeding sequence
- Primary consumers feed on producers and hence occupy the second trophic level
- Further consumers (e.g. secondary, tertiary, etc.) may occupy subsequent trophic levels

Question 2

What is a food chain and what does it show?

Answer 2

A food chain shows the linear feeding relationships between species in a community.
- Arrows represent the transfer of energy and matter as one organism is eaten by another (arrows point in direction of energy flow)
- The first organism in a food chain is always a producer, followed by consumers (primary, secondary, etc.)
- Most species feeding requirements will consist of multiple food chains, with organisms occupying different trophic levels

Question 3

What is a food web?

Answer 3

A food web is a diagram that shows how food chains are linked together into more complex feeding relationships within a community. It shows all the possible food chains in a community.
- There can be more than one producer in a food web, and consumers can occupy multiple trophic levels

Question 4

What is ecological production (or producitivity)?

and what is biomass?

Answer 4

Ecological production (or productivity) refers to the rate of generation of biomass in an ecosystem
- It is usually expressed in units of mass per area per time (e.g. kg m
^–2 day^–1)

Biomass is the total dry weight of organic matter in organisms or ecosystems
- Biomass is essentially the entirety of all biologically produced organic matter (e.g. proteins, carbohydrates, etc.)

Question 5

The percentage of energy that is converted into biomass depends on what factors?

Answer 5

• Energy is lost as heat from cellular respiration (higher respiration rate results in more heat lost)
• Energy is lost as inedible materials – such as bones, teeth and hair
• Energy is lost via excretion of undigested and unabsorbed materials

Question 6

Primary production

The production of chemical energy in organic compounds by producers

Answer 6

Primary production may be categorised as one of two types:
- Gross primary production (GPP) is the amount of chemical energy as biomass that a producer creates in a given length of time. This is what goes in the bottom tier of a pyramid of energy
- Net primary production (NPP) is the amount of chemical energy that is not consumed by respiration (NPP = GPP – respiration)

Question 7

Secondary production

The generation of biomass by heterotrophic organisms (consumers)

Answer 7

• This biomass generation is driven by the transfer of organic compounds between trophic levels via feeding
• Secondary production may also be categorised according to gross (total) and net (usable) amounts of biomass

Question 8

How to calculate the energy lost by respiration

Answer 8

respiration = gross production - net production

Question 9

What are conversion ratios in sustainable food practices and what are they for?

Answer 9

In commercial (animal) food production, farmers measure the food conversion ratio
(FCR).
Feed conversion ratios measure the efficiency of an animal in converting the food provided (feed mass) into a desired output. For dairy cows, for example, the output is milk, whereas animals raised for meat, for example, pigs the output is the mass gained by the animal.

Feed Conversion Ratio (FCR) = mass of food eaten ÷ (increase in) desired output
- The lower the FCR, the more efficient the method of food production.

Question 10

How to obtain a good (low) FCR

Answer 10

By minimising the losses of energy by respiration, for example:
- Restricting animal movement
- Slaughtering the animal at a young age (older animals have higher FCRs as they grow more slowly)
- Optimising feed so it is efficiently digested

Note: while more efficient food production practices lower costs, there are a number of potential ethical issues that may be involved

Question 11

Open vs closed ecosystems

Answer 11

Ecosystems can be described as closed or open according to whether matter moves into and out of the system.
- A closed ecosystem exchanges energy but not matter with its surroundings – it is self-contained (e.g. mesocosm)
- An open ecosystem exchanges both energy and matter with surrounding environments (e.g. a natural ecosystem)

Question 12

What is a biome?

Answer 12

A biome is a geographical area that has a particular climate and sustains a specific
community of plants and animals (it’s a a type of ecosystem). Because of our knowledge on biomes, the type of stable ecosystem that will emerge in an area is predictable based on climate
- The main factors affecting the distribution of biomes is temperature and rainfall
- Temperature is influential because it affects the rate of metabolism.
- These factors will vary according to lattitude and longitude, as well as altitude and proximity to the ocean

Question 13

Types of biomes

Answer 13

• Tropical rainforests – hot and humid environments near the equator with dense vegetation and high biodiversity
• Taiga – coniferous forests near the poles that have cold temperatures and little precipitation (moisture trapped as snow / ice)
• Deserts – dry and arid environments that display extreme temperature conditions (hot and cold)

Question 14

Why do pyramids of energy differ between ecosystems?

Answer 14

Because of the effect of climate on primary productivity.
- Warmer temperatures will speed up enzyme reactions required for photosynthesis (i.e. light independent reactions)
- High precipitation will also increase photosynthesis as the photolysis of water is essential for non-cyclic photophosphorylation

Consequently, tropical rainforests have a high net primary productivity (NPP), whereas deserts have a low NPP

• Ecosystems with higher productivity will be able to supply more energy to consumers and hence support more trophic levels
• Therefore a pyramid of energy for a tropical rainforest will display a wider base and more levels than a desert

Question 15

Gersmehl diagrams show the inter-relationships between nutrient stores and flows between taiga, desert and tropical rainforest. They show how nutrients are stored the three nutrient sinks…

Answer 15

• Biomass is the total mass of living organisms (mainly plant tissue) in a given area
• Litter is any organic matter in and on the soil – it includes humus and leaf litter (defoliation)
• Soil is the top layer of the earth that is composed of disintegrated rock particles

Nutrients can be transferred between nutrient sinks and may also be cycled via environmental inputs and outputs
- Nutrients can be transferred from biomass to litter (fallout), litter to soil (decay) or soil to biomass (uptake)
- Litter can additionally gain nutrients via precipitation (rainfall) and lose nutrients in surface runoff
- Soil can gain nutrients from the erosion of rocks via weathering, but will lose nutrients via leaching

Question 16

Nutrient inputs into the ecosystem

Gersmehl diagrams

Answer 16

• Nutrients dissolved in raindrops - litter gains
• Nutrients from weathered rock - soil gains

Question 17

Nutrient losses from the ecosystem

Gersmehl diagrams

Answer 17

• Nutrients lost through surface runoff - litter loses
• Nutrients lost through leaching - soil loses

Question 18

Using Gersmehl diagrams to analyse a particular ecosystem

Answer 18

• Diameter of sinks are proportional to the mass of nutrients stored in each sink
• The thickness of the arrows are proportional to the rate of nutrient flow.

Question 19

Gersmehl diagram for a taiga

Answer 19

• Large diameter for litter (pine needles)
• Slow rate of nutrient transfer between stores (low temperatures slow decomposition which delays nutrient transfer to soil and biomass)

Question 20

Gersmehl diagram for a desert

Answer 20

• Soil is the main store (few plants exist to store nutrients as biomass or produce litter)
• Slow rate of nutrient transfer except for the transfer from biomass to litter

Question 21

Gersmehl diagram for tropical rain forest

Answer 21

• Biomas is the main store (litter is rapidly decomposed and vast roots quickly draw nutrients from soil)
• Fast rate of nutrient transfer between stores (hot and wet conditions promote precipitation, runoff, weathering and leaching)

Question 22

What is ecological succession?

Answer 22

Ecological succession describes the process by which a sequence of increasingly complex communities develop over time
- The climax community is reached when succession has ended and the community has all of its characteristic

Question 23

What is primary succession?

Answer 23

Primary succession occurs when communities develop on entirely new land without any established soil
- This may occur at river deltas, glaciers, sand dunes or on exposed rock

The organisms which first colonise the region are called pioneer species and typically consist of lichen or moss
- As the lichen and moss die, they decompose, which creates the first organic soil capable of sustaining plant growth

As plant species colonise the area, the litter produced by their growth and their decomposing remains will cause changes:
- Soil depth will increase (as plants add humus to the soil) and soil pH will become altered
- Soil mineral content will increase and rocks will begin to be broken down by the action of roots
- The soil will become aerated and water retention is increased (drainage is reduced)

These changes will allow for the growth of larger plants, which will reduce erosion through the binding action of their roots.

Question 24

Overview of primary succession

Answer 24

1. Bare rock
2. Colonised by pioneer species (e.g. moss)
3. Decomposition creates a layer of topsoil
4. Grasses grow and displace the pioneer species
5. More nutrients in soil allows for shrub growth
6. Increase in soil depth allows for growth of trees

Question 25

Primary Succession Data analysis

Answer 25

When analysing primary succession data remember that regions closer to the glacier are expected to contain plant life common to the earlier stages of primary succession

Question 26

What is secondary succession?

Answer 26

Secondary succession occurs when succession starts on existing soil following the upheaval of a pre-existing ecosystem. Therefore, an environmental disturbance may give rise to secondary succession.
Because the soil is already developed, dominance is usually achieved by the fastest growing plants.

Question 27

Overview of secondary succession

Answer 27

1. An environmental disturbance, such as a bushfire or earthquake, destroys the pre-existing climax community
2. Grasses and herbaceous plants are the first to grow back as the soil is already present (no pioneer species required)
3. Fast growing trees will develop to their fullest, while shade tolerant trees will develop in the understory
4. Eventually the fast-growing trees may be overtaken by larger, slower-growing trees as the ecosystem reverts to its prior state

Question 28

How do humans interfere with nutrient cycling?

Deforestation

Answer 28

Deforestation is the permanent destruction of a forest via the removal or clearance of trees
- This human-induced degradation of forests is driven primarily by a need for timber and cleared land for agricultural purposes.

Deforestation disturbs the normal nutrient cycling within the region in a number of ways:
- Fewer trees means less litter (due to less defoliation), which reduces the production of humus (so less nutrients in soil)
- The soil will become acidic and release iron and aluminium to form an infertile ferrilitic soil (nutrient poor)
- The infertile soil will prevent vegetative growth, reducing biodiversity and further nutrient cycling

Question 29

Investigating the effect of an environmental disturbance on an ecosystem

Answer 29

Environmental disturbances are caused by natural or artificial disruptions to a normal ecosystem, including:
- Fire breaks in bush lands or regions damaged by bushfires
- Outer boundaries of population settlements or regions bordering roads
- Dams and artificial rivers and creeks (e.g. irrigation sites)

The effect of an environmental disturbance on an ecosystem can be measured in a number of ways:
- Population density (using the Lincoln index via the capture-mark-recapture technique)
- Species diversity and richness (using the Simpson’s reciprocal index)
- The presence and distribution of indicator species (to measure levels of pollution)
- Biomass (via the average width of tree stems at a specified height)
- Edaphic factors such as soil erosion (via depth), water retention (via drainage), pH and nutrient content

Measurements taken from a disturbed area need to be compared against measurements taken from an undisturbed control
- This enables the investigator to statistically calculate both the effect and magnitude of the environmental disturbance