Chapter 23 P2

Question 1

Worked example: Efficiency of biomass transfer

Answer 1

A region of grassland has a net production of 40g m-2 year*’. A goat grazes an area of 20m x 20m of this grassland. Assume that the goat consumes all of the biomass in this area.
Calculate the total biomass consumed by the goat each year:
Biomass consumed = mass (per metre squared per year)
x area of land
= 40 × (20 × 20)
= 16000g
= 16kg
2 The mass of the goat increases in this time by 2.4 kg. Calculate the efficiency of biomass transfer between the grass and the goat.
biomass available after transfer
Efficiency of transfer =
biomass available before transfer * 100
= 2.4 = 15%

Question 2

Human activities can manipulate biomass through ecosystems p1

Answer 2

• Human civilisation depends on agriculture.
• Agriculture involves manipulating the environment to favour plant species that we can eat (crops) and to rear animals for food or their produce.
• Plants and animals are provided with the abiotic conditions they need to thrive such as adequate watering and warmth (e.g., greenhouse use, stabling of animals).
• Competition from other species is removed (e.g., the use of chemicals such as pesticides) as well as the threat of predators (e.g., by creating barriers such as fences to exclude wild herbivores or predators).
• In a natural ecosystem, humans would occupy the second, third or even fourth trophic level.

Question 3

Human activities can manipulate biomass through ecosystems p2

Answer 3

• at each trophic level there are considerable energy losses, therefore only a tiny proportion of the energy available at the start of the food chain is turned into biomass for consumption at these third and fourth levels.
• Agriculture creates very simple food chains.
• In farming animals or animal produce for human consumption, only three trophic levels are present - producers (animal feed), primary consumers (livestock), and secondary consumers (humans).
• In cultivating plants for human consumption, there are just two trophic levels - producers (crops) and primary consumers (humans).
• This means that the minimum energy is lost since there are fewer trophic levels present than in the natural ecosystem.
• This ensures that as much energy as possible is transferred into biomass that can be eaten by humans.

Question 4

Monitoring biomass during conservation p1

Answer 4

• Sea urchins (Strongylocentrotus spp) are marine invertebrates that feed on kelp (a type of seaweed).
• In regions where sea urchins are abundant, kelp forest ecosystems can be disrupted.
• The urchins eat the kelps’ holdfasts, these are strong structures which anchor the kelp to the sea bed.
• The remainder of the plant floats away resulting in an ecosystem known as an ‘urchin barren’ ecosystem, which contains so little biomass of seaweeds that few species are able to live in this region.
• The presence or absence of kelp beds therefore has a major influence on the structure of the marine community
• In many areas, sea otters (Enhydra lutris) feed on urchins, keeping their levels low and therefore the kelp forests intact.

Question 5

Monitoring biomass during conservation p2

Answer 5

• During the 19th century, this ecological balance was destroyed when populations of sea otters were virtually wiped out by excessive hunting for otter fur.
• As a result, urchin numbers grew rapidly and kelp forests were destroyed.
• This balance has since been restored by the cessation of the hunting of sea otters, allowing them to again control the abundance of the urchins.
• In turn, the productive kelp forests have been able to redevelop.
• As part of a conservation management project, scientists studied sea urchin populations around two of the Aleutian Islands off the coast of Alaska.
• Data on sea urchin size, density, and biomass were recorded per 0.25 m? from samples collected from Amchitka Island (which has sea otters) and Sheyma Island (which has no sea otters] The results are shown in Figure 9.

Question 6

Recycling within ecosystems

Answer 6

• Energy has a linear flow through an ecosystem.
• It enters the ecosystem from the Sun, and is ultimately transferred to the atmosphere as heat.
• As long as the Sun continues to supply Earth with energy, life will continue.
• In contrast, nutrients constantly have to be recycled throughout ecosystems in order for plants and animals to grow.
• This is because they are used up by living organisms and there is no large external source constantly replenishing nutrients in the way the Sun supplies energy.

Question 7

Decomposition

Answer 7

• Decomposition is a chemical process in which a compound is broken down into smaller molecules, or its constituent elements.
• Often an essential element, such as nitrogen or carbon, cannot be used directly by an organism in the organic form it is in, in dead or waste matter.
• This organic material must be processed into inorganic elements and compounds, which are a more usable form, and returned to the environment.

Question 8

decomposer

Answer 8

• is an organism that feeds on and breaks down dead plant or animal matter, thus turning organic compounds into inorganic ones (nutrients) available to photosynthetic producers in the ecosystem.
• Decomposers are primarily microscopic fungi and bacteria, but also include larger fungi such as toadstools and bracket fungi.

Question 9

Decomposers are saprotrophs because

Answer 9

• they obtain their energy from dead or waste organic material (saprobiotic nutrition).
• They digest their food externally by secreting enzymes onto dead organisms or organic waste matter.
• The enzymes break down complex organic molecules into simpler soluble molecules - the decomposers then absorb these molecules.
• Through this process, decomposers release stored inorganic compounds and elements back into the environment.

Question 10

Detritivores

Answer 10

• are another class of organism involved in decomposition.
• They help to speed up the decay process by feeding on detritus - dead and decaying material.
• They break it down into smaller pieces of organic material, which increases the surface area for the decomposers to work on.
• Examples of detritivores include woodlice that break down wood, and earthworms that help break down dead leaves.
• Detritivores perform internal digestion.

Question 11

Recycling nitrogen

Answer 11

• Nitrogen is an essential element for making amino acids (and consequently proteins) and nucleic acids in both plants and animals.
• Animals obtain the nitrogen they need from the food they eat, but plants have to take in nitrogen from their environment.
• Nitrogen is abundant in the atmosphere, 78% of air is nitrogen gas (N,).
• However, in this form nitrogen cannot be taken up by plants.
• To be used by living organisms, nitrogen needs to be combined with other elements such as oxygen or hydrogen.
• Bacteria play a very important role in converting nitrogen into a form useable by plants.
• Without bacteria, nitrogen would quickly become a limiting factor in ecosystems.

Question 12

Nitrogen fixation

Answer 12

• Nitrogen-fixing bacteria such as Azotobacter and Rhizobium contain the enzyme nitrogenase, which combines atmospheric nitrogen (N2) with hydrogen (H2) to produce ammonia (NH3) - a form of nitrogen that can be absorbed and used by plants.
• This process is known as nitrogen fixation.

Question 13

Azotobacter

Answer 13

• is an example of a free-living soil bacterium.
• However, many nitrogen-fixing bacteria such as Rhizobium live inside root nodules.
• These are growths on the roots of leguminous plants such as peas, beans, and clover.
• The bacteria have a symbiotic mutualistic relationship with the plant, as both organisms benefit:

Question 14

The bacteria have a symbiotic mutualistic relationship with the plant, as both organisms benefit:

Answer 14

• the plant gains amino acids from Rhizobium, which are produced by fixing nitrogen gas in the air into ammonia in the bacteria
• the bacteria gain carbohydrates produced by the plant during photosynthesis, which they use as an energy source.
Other bacteria then convert the ammonia that is produced by nitrogen fixation into other organic compounds that can be absorbed by plants.

Question 15

Reward and punishment

Answer 15

• Recent work suggests that legumes ‘select’ the Rhizobium colonies which provide them with the most nitrates.
• Careful measurements show that the plants reward the nodules which make lots of nitrates with extra carbohydrates - but the punishment for nodules containing less-productive bacteria is swift and unforgiving.
• The plant cuts off the supply of carbohydrates and starves the nodule to death.
• This is a form of natural selection which maximises the benefit to the plant - and to those bacteria which deliver the goods.

Question 16

Nitrification

Answer 16

• ammonium compounds in the soil are converted into nitrogen-containing molecules that can be used by plants.
• Free-living bacteria in the soil called nitrifying bacteria are involved.
• Nitrification is an oxidation reaction, and so only occurs in well-aerated soil. It takes place in two steps:

Question 17

Nitrification is an oxidation reaction, and so only occurs in well-aerated soil. It takes place in two steps:

Answer 17

1 Nitrifying bacteria (such as Nitrosomonas) oxidise ammonium compounds into nitrites (NO,*).
2 Nitrobacter (another genus of nitrifying bacteria) oxidise nitrites into nitrates (NO,).
Nitrate ions are highly soluble, and are therefore the form in which most nitrogen enters a plant.

Question 18

Denitrification

Answer 18

• In the absence of oxygen, for example, in waterlogged soils, denitrifying bacteria convert nitrates in the soil back to nitrogen gas.
• This process is known as denitrification - it only happens under anaerobic conditions.
• The bacteria use the nitrates as a source of energy for respiration and nitrogen gas is released.

Question 19

Ammonification

Answer 19

Ammonification is the name given to the process by which decomposers convert nitrogen-containing molecules in dead organisms, faeces, and urine into ammonium compounds.

Question 20

Nitrogen cycle

Answer 20

The processes of nitrogen fixation, nitrification, denitrification, and ammonification all form part of the nitrogen cycle. Their place in the cycle can be seen in Figure 5.

Question 21

Recycling carbon

Answer 21

• Carbon is a component of all the major organic molecules present in living organisms such as fats, carbohydrates, and proteins.
• The main source of carbon for land-living organisms is the atmosphere.
• Although carbon dioxide (CO,) only makes up 0.04% of the atmosphere, there is a constant cycling of carbon between the atmosphere, the land, and living organisms.
• Figure 6 summarises the key points of the carbon cycle.

Question 22

Diagram of the carbon cycle

Answer 22

Question 23

Fluctuations in atmospheric carbon dioxide

Answer 23

• Carbon dioxide levels fluctuate throughout the day.
• Photosynthesis only takes place in the light, and so during the day photosynthesis removes carbon dioxide from the atmosphere.
• Respiration, however, is carried out by all living organisms throughout the day and night, releasing carbon dioxide at a relatively constant rate into the atmosphere.
• Therefore, atmospheric carbon dioxide levels are higher at night than during the day.
• Localised carbon dioxide levels also fluctuate seasonally.
• Carbon dioxide levels are lower on a summer’s day than a winter’s day, as photosynthesis rates are higher.

Question 24

Over the past 200 years, global atmospheric carbon dioxide levels have increased significantly. This is mainly due to:

Answer 24

• the combustion of fossil fuels - which has released carbon dioxide back into the atmosphere from carbon that had previously been trapped for millions of years below the Earth’s surface
• deforestation - which has removed significant quantities of photosynthesising biomass from Earth. As a result, less carbon dioxide is removed from the atmosphere. In many cases the cleared forest is burnt, therefore releasing more carbon dioxide into the atmosphere.

Question 25

What do Increased levels of atmospheric carbon dioxide do

Answer 25

- Increased levels of atmospheric carbon dioxide trap more thermal energy (heat) in the atmosphere - it is called a greenhouse gas for this reason. - Its production through human activities is contributing to global warming. - The amount of carbon dioxide dissolved in seas and oceans is affected by the temperature of the water (the higher the temperature, the less gas is dissolved). - Therefore, global warming reduces the carbon bank in the oceans and releases more carbon dioxide into the atmosphere - further contributing to the process in a positive feedback loop.

Question 26

Atmospheric carbon dioxide levels have

Answer 26

- varied significantly over million-year timescales. - To gain information about how the atmosphere has changed over time, samples are taken from deep within a glacier. - For example, at a depth of 3.6 km in the Antarctic glacier the ice is 420000 years old. - When the ice formed air bubbles were trapped within the ice - these bubbles reflect the composition of the atmosphere at this point in time. - Analysis of the gases present within these bubbles therefore reveals the composition of the atmosphere at this point in history. - The graphs in Figure 9 show the variations in carbon dioxide levels which have occurred over time. - The temperature of the atmosphere is directly related to the level of carbon dioxide present.