Energy & Feeding Relationships (Cambridge (CIE) IGCSE Biology)

False.

Herbivores are primary consumers. They are animals that gain their energy by eating plants.

Carnivores are animals that gain their energy by eating other animals.

Tertiary consumers are organisms that feed on secondary consumers. They are carnivores and are present at the fourth trophic level of food chains.

Decomposers are organisms that gain energy from dead or waste organic material.

Overfishing might affect a marine food web as follows:

• there might be an increase in population size of the organisms normally consumed by the fish

• there might be an increase in population size of any organisms that usually compete with fish for food

• there might be a decrease in population size of any organisms that depend on the fish for food

A pyramid of numbers shows how many organisms are present at each level of a food chain.

False.

Pyramids of numbers are not always pyramid-shaped. This is because large producers, e.g. trees may be present in small numbers at the base of a food chain.

Pyramid of biomass show the mass of living material present at each stage of a food chain.

True.

Pyramids of biomass are always pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like.

Rules that should be followed when drawing pyramids of numbers or biomass include:

• The producers should always be at the base, with each subsequent trophic level in order above this

• Each box should be labelled with its trophic level and species

• If graph paper is provided then boxes should be drawn to scale

The advantage of a pyramid of biomass over a pyramid of numbers is that it gives a clearer representation of the mass of living material present at each trophic level.

A trophic level is the position of an organism in a food chain, food web or ecological pyramid.

The first trophic level contains producers. These are mainly plants that produce their own carbon compounds through photosynthesis.

False.

The second trophic level contains primary consumers, which are herbivores that feed on producers. Secondary consumers are found at the third trophic level.

The third trophic level contains secondary consumers, which are carnivores or omnivores that feed on primary consumers.

A pyramid of energy illustrates the stored chemical energy contained within the biomass of organisms at each trophic levels of a food chain.

True.

Energy losses at each trophic level mean that each subsequent trophic level will contain less stored energy in the form of biomass.

The advantage of using a pyramid of energy to represent a food chain rather than a pyramid of numbers or biomass is that it shows:

• the energy available at each trophic level

• the efficiency of energy transfer at each trophic level

False.

While some of the chemical energy stored in the producer is transferred to the biomass of the primary consumer when it is eaten, much of it is transferred elsewhere, e.g. to the environment as heat or in waste products

Energy is lost at each trophic level because:

• organisms rarely eat every part of the consumed organism

• some tissues cannot be fully digested by consumer so any undigested material is egested in faeces

• energy released from consumed tissues may be used for, e.g. movement, meaning that it is eventually transferred to the environment as heat

• energy is lost in metabolic waste, e.g. urea in urine

Food chains are rarely made up of more than five trophic levels because of the energy losses at each trophic level; the total energy available eventually becomes too small to support another trophic level.

It is more energy efficient for humans to eat plants than meat because the resulting food chain has fewer trophic levels, e.g:

crophuman

cropcowhuman

In a shorter food chain there are fewer energy transfers, and so less energy is lost to the environment.

Carbon is taken out of the atmosphere in the form of carbon dioxide by plants during photosynthesis.

Carbon is transferred from one organism to another by the process of feeding.

Carbon is returned to the atmosphere when it is released as carbon dioxide by the following processes:

• respiration in plants and animals

• respiration of decomposers, e.g. bacteria

• combustion of fossil fuels or other organic matter, e.g. wood

Combustion releases carbon dioxide into the atmosphere. This occurs during the combustion of fossil fuels, or of other organic matter such as wood.

True.

Decomposers break down proteins in the dead and waste material, converting the proteins into ammonium ions.

Nitrifying bacteria convert ammonium compounds to nitrites and then to nitrates, which can then be absorbed by plants.

False.

Neither plants nor animals can absorb nitrogen gas (N₂) from the air. N₂ needs to be fixed to produce nitrates before nitrogen can be absorbed by plants and passed on to animals.

Nitrogen gas can be converted into a usable form by:

• Nitrogen-fixing bacteria

• Lightning

Plants absorb nitrogen from the soil in the form of nitrates.

Nitrogen is required by living organisms to make amino acids and proteins.

Animals get the nitrogen they need by eating other organisms and digesting the proteins in their tissues.

Deamination is the removal of the nitrogen-containing part of an amino acid, resulting in the formation of ammonia. This ammonia can then be converted into nitrates by nitrifying bacteria.

False.

Denitrifying bacteria take nitrates out of the soil and convert them back into nitrogen gas, reducing soil fertility and plant growth.

The nitrogen cycle processes that involve microorganisms are:

• decomposition

• nitrification

• nitrogen fixation

• denitrification

A population is a group of organisms of the same species living in the same place at the same time.

A community includes all of the populations of different species living in an ecosystem.

An ecosystem the community of organisms that live in an area, and their interactions with the living (biotic) and non-living (abiotic) factors in their environment.

Factors that affect the rate of population growth include:

• food supply

• competition

• predation

• disease

The phases of the growth curve are:

• A = lag phase

• B = exponential / log phase

• C = stationary phase

• D = death phase

False.

During the lag phase organisms are reproducing slowly as they adjust to their environment.

The exponential part of a population growth curve occurs because resources are plentiful, meaning that:

• birth rate is high

• death rate is low

The stationary phase of growth curves occurs because once the population reaches a certain size resources in the environment become limited; the result of this is that death rates increase until death rate equals birth rate.

The death phase occurs when death rates exceed birth rates, e.g. due to:

• limited resources, such as food

• production of harmful waste products