chapter 23 P1

Question 1

ecosystem

Answer 1

is made up of all the living organisms that interact with one another in a defined area, and also the physical factors present in that region.

Ecosystems can vary dramatically in size - from a tiny bacterial colony to the entire biosphere of Earth.

The boundaries of a particular ecosystem being studied are defined by the person or team carrying out the study.

For example, individual habitats may be studied such as a rock pool or a large oak tree, or small areas of land such as a playing field or a particular stretch of a river.

Question 2

Factors that affect ecosystems

Answer 2

All ecosystems are dynamic, meaning that they are constantly changing. This is a result of the living organisms present and the environmental conditions.

A large number of factors affect an ecosystem. The factors can be divided into two groups:
biotic factors
abiotic factors

Question 3

biotic factors

Answer 3

the living factors. For example, in a forest ecosystem, the presence of shrews and hedgehogs are biotic factors, as is the size of their populations - the competition between these two animal populations for a food source (e.g., insects) is also a biotic factor.

Question 4

abiotic factors

Answer 4

the non-living or physical factors. Within the forest ecosystem, abiotic factors include the amount of rainfall received and the yearly temperature range of the ecosystem.

Question 5

Biotic factors definitions

Answer 5

• Biotic factors often refer to the interactions between organisms that are living, or have once lived.
• These interactions often involve competition, either within a population or between different populations.
• Examples of things for which animals compete include food, space (territory), and breeding partners.

Question 6

examples of Abiotic factors

Answer 6

Light
Temperature
Water availability
Oxygen availability
Edaphic (soil) factors

Question 7

Light

Answer 7

• Most plants are directly affected by light availability as light is required for photosynthesis.
• In general the greater the availability of light, the greater the success of a plant species.
• Plants develop strategies to cope with different light intensities.
• For example, in areas of low light they may have larger leaves.
• They may also develop photosynthetic pigments that require less light, or reproductive systems that operate only when light availability is at an optimum.

Question 8

Temperature

Answer 8

• The greatest effect of temperature is on the enzymes controlling metabolic reactions.
• Plants will develop more rapidly in warmer temperatures, as will ectothermic animals. (Endothermic animals control their internal temperature, and so are less affected by the external environment.)
• Changes in the temperature of an ecosystem, for example, due to the changing seasons, can trigger migration in some animal species, and hibernation in others.
• In plant species it can trigger leaf-fall, dormancy, and flowering.

Question 9

Water availability

Answer 9

• In most plant and animal populations, a lack of water leads to water stress, which, if severe, will lead to death.
• A lack of water will cause most plants to wilt, as water is required to keep cells turgid and so keep the plant upright.
• It is also required for photosynthesis.
• Cacti are an example of xerophytes, plants that have developed successful strategies to cope with water stress.

Question 10

Oxygen availability

Answer 10

• In aquatic ecosystems, it is beneficial to have fast-flowing cold water as it contains high concentrations of oxygen.
• If water becomes too warm, or the flow rate too slow, the resulting drop in oxygen concentration can lead to the suffocation of aquatic organisms.
• In waterlogged soil, the air spaces between the soil particles are filled with water.
• This reduces the oxygen available for plants.

Question 11

Edaphic (soil) factors

Answer 11

Different soil types have different particle sizes. This has an effect on the organisms that are able to survive in them. There are three main soil types:
* clay - this has fine particles, is easily waterlogged, and forms clumps when wet
* loam - this has different-sized particles, it retains water but does not become waterlogged
* sandy - this has coarse, well-separated particles that allow free draining - sandy soil does not retain water and is easily eroded.

Question 12

summary of Biomass transfer through an ecosystem

Answer 12

• All organisms found within an ecosystem require a source of energy to perform the functions needed to survive.
• Ultimately, the Sun is the source of energy for almost all ecosystems on Earth.
• Through the process of photosynthesis, the Sun’s light energy is converted into chemical energy in plants and other photosynthetic organisms.
• This chemical energy is then transferred to other non-photosynthetic organisms as food.

Question 13

food webs

Answer 13

• systems of interlinked food chains
• These are diagrams that scientists use to show the transfer of biomass (mass of living material), and therefore energy, through the organisms in an ecosystem.

Question 14

trophic level.

Answer 14

Each stage in the chain

Question 15

different levels of trophic levels

Answer 15

• The first trophic level is always a producer and the subsequent trophic levels are all consumers
• The second trophic level is occupied by a primary consumer - an animal that eats a producer.
• The following trophic levels are labelled successively - secondary consumer (an animal that eats a primary consumer), tertiary consumer (an animal that eats a secondary consumer), and a quaternary consumer (an animal that eats a tertiary consumer).
• Food chains rarely have more trophic levels than this as there is not sufficient biomass and stored energy left to support any further organisms.
• Decomposers are also important components of food webs - they break down dead organisms releasing nutrients back into the ecosystem.

Question 16

producer

Answer 16

• an organism that converts light energy into chemical energy by the process of photosynthesis.

Question 17

consumers

Answer 17

• organisms that obtain their energy by feeding on other organisms.

Question 18

diagram of trophic levels

Answer 18

• Food chains can be presented diagrammatically as a pyramid of numbers, with each level representing the number of organisms at each trophic level (Figure 2).
• In a pyramid the producers are always placed at the bottom of the diagram with subsequent trophic levels added above.

Question 19

Measuring biomass

Answer 19

the mass of living material present in a particular place or in particular organisms. It is an important measure in the study of food chains and food webs as it can be equated to energy content.

Question 20

biomass at each trophic level formula

Answer 20

• biomass present in each organism * the total number of organisms in that trophic level.
• This information can be presented diagrammatically as a pyramid of biomass
• This represents the biomass present at a particular moment in time - it does not take into account seasonal changes.

Question 21

easiest way to measure biomass

Answer 21

• measure the mass of fresh material present.
• However, water content must be discounted and the presence of varying amounts of water in different organisms makes this technique unreliable unless very large samples are used.
• Scientists therefore usually calculate the ‘dry mass’ of organisms present.
• This is not without problems.
• Organisms have to be killed in order to be dried.
• The organisms are placed in an oven at 80°C until all water has evaporated - this point is indicated by at least two identical mass readings.
• To minimise the destruction of organisms (particularly animals) only a small sample is taken.
• However, this sample may not be representative of the population as a whole.
• Biomass is measured in grams per square metre (gm-2) for areas of land, or grams per cubic metre (gm}) for areas of water.

Question 22

Efficiency of biomass and energy transfer between trophic levels

Answer 22

• The biomass in each trophic level is nearly always less than the trophic level below.
• This is because biomass consists of all the cells and tissues of the organisms present, including the carbohydrates and other carbon compounds the organisms contain.
• As carbon compounds are a store of energy, biomass can be equated to energy content.
• When animals eat, only a small proportion of the food they ingest is converted into new tissue.
• It is only this part of the biomass (and hence energy) which is available for the next trophic level to eat.

Question 23

The energy available at each trophic level is measured in

Answer 23

kilojoules per metre squared per year (kJm-2yr-‘), to allow for changes in photosynthetic production and consumer feeding patterns throughout the year.

Question 24

ecological efficiency.

Answer 24

• The efficiency with which biomass or energy is transferred from one trophic level to the next
• The amount of biomass or energy converted to new biomass by each trophic level in a food chain can be represented by a pyramid of energy (Figure 4).

Question 25

Producers only convert 1-3% of the sunlight (solar or light energy) they receive into chemical energy and hence biomass. This is because:

Answer 25

• not all of the solar energy available is used for photosynthesis - approximately 90% is reflected, some is transmitted through the leaf, and some is of unusable wavelength
• other factors may limit photosynthesis, such as water availability
• a proportion of the energy is ‘lost’, as it is used for photosynthetic reactions.

Question 26

The gross production

Answer 26

The total solar energy that plants convert to organic matter

However, plants use 20-50% of this energy in respiration. The remaining energy is converted into biomass. This is the energy available to the next trophic level and is known as the net production.

Question 27

The energy available to the next trophic level can be calculated using the following formula:

Answer 27

Net production = gross production - respiratory losses
Note that this calculation can be applied equally to the biomass or energy production within an organism. The generation of biomass in a producer is referred to as primary production - in a consumer, it is known as secondary production.

Question 28

Worked example 1: Calculating net production (biomass)

Answer 28

A group of scientists measured the gross production of a grassland area as 60 gm 2 year’. If the respiration loss was
20gm year!, calculate the annual net production of this area of grassland.
Net production = gross production -
respiratory losses
= 60 - 20
= 40 gm 2 year’
This is an example of primary production.

Question 29

Worked example 2: Calculating net production (energy)

Answer 29

Sheep in a grassland digest and absorb 15000 kJm 2yr ‘ from the biomass they take in. Of this, 8000 kJm 2 yr ‘ is used in respiration.
How much energy is available to humans, the next organism in the food chain?
Net production = gross production -
respiratory losses
= 15000 - 8000|
= 7000 kJ m~”yr”
This is an example of secondary production.

Question 30

Efficiency at consumer levels

Consumers at each trophic level convert at most 10% of the biomass in their food to their own organic tissue. This is because:

Answer 30

• not all of the biomass of an organism is eaten, for example, plant roots or animal bones may not be consumed
• some energy is transferred to the environment as metabolic heat, as a result of movement and respiration
• some parts of an organism are eaten but are indigestible - these parts (and their energy content) are egested as faeces
• some energy is lost from the animal in excretory materials such as urine.
  Only around 0.001% of the total energy originally present in the incident sunlight is finally embodied as biomass in a tertiary consumer.

Question 31

You can use the following formula to calculate the efficiency of the energy transfer (approximately equivalent to biomass transfer) between each trophic level of a food chain:

Answer 31

Question 32

Energy flow through organisms

Answer 32

Question 33

energy available at each trophic level in a food chain found in Cayuga Lake, New York State, USA

Answer 33