Chapter 23 P2 Flashcards
1
Q
Worked example: Efficiency of biomass transfer
A
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%
2
Q
Human activities can manipulate biomass through ecosystems p1
A
- 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.
3
Q
Human activities can manipulate biomass through ecosystems p2
A
- 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.
4
Q
Monitoring biomass during conservation p1
A
- 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.
5
Q
Monitoring biomass during conservation p2
A
- 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.
6
Q
Recycling within ecosystems
A
- 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.
7
Q
Decomposition
A
- 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.
8
Q
decomposer
A
- 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.
9
Q
Decomposers are saprotrophs because
A
- 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.
10
Q
Detritivores
A
- 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.
11
Q
Recycling nitrogen
A
- 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.
12
Q
Nitrogen fixation
A
- 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.
13
Q
Azotobacter
A
- 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:
14
Q
The bacteria have a symbiotic mutualistic relationship with the plant, as both organisms benefit:
A
• 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.
15
Q
Reward and punishment
A
- 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.
20
Q
Nitrogen cycle
A
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.
22
Q
Diagram of the carbon cycle
A
