Organisation of an Ecosystem (AQA GCSE Biology)

The term abundance is how common a species is within a quadrat. Abundance across a series of quadrats can be used to estimate the size of a population.

The abundance of a species can be measured within a quadrat as follows:

• Recording whether or not a species is present

• Counting the number of individuals of a species

• Estimating percentage cover

Percentage cover is often used to measure the abundance of a species where individual organisms are difficult to tell apart, e.g. grass or barnacles

Placing quadrats randomly avoids bias in an investigation, e.g. a student might choose to place all of their quadrats on bare ground because it would make counting easier, resulting in a biased sample.

Variables that should be kept the same when using quadrats to compare the populations of a species in two areas include:

• Size of quadrat

• Number of quadrats sampled

• Time of day / time of year

• Knowledge of species identification

Challenges of using quadrats to investigate populations include:

• It can be easy to miss organisms when counting, e.g. if they are underneath another organism or if two individuals are counted as one

• Some species might be difficult to identify

True.

Carrying out more quadrats will increase the sample size, meaning that the results are more likely to be a good representation of the area sampled.

Distribution refers to where a species occurs within its habitat.

Quadrats can be used to investigate the effects of a factor on the distributions on a species as follows:

• laying out a transect along a gradient for the factor being investigated

• use quadrats at intervals along the transect and record abundance within each quadrat

• observe any relationship between the factor being studied and the abundance of the specie

Nutrients cycling is important because it allows nutrients to be released from tissues so that they become available again for other organisms to use. Without nutrient cycling ecosystems would run out of essential nutrients.

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

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

• Reversal of the dissolving process by which carbon dioxide dissolves in the oceans

Mass deforestation (cutting down many trees) affects the carbon cycle as follows:

• It reduces the number of trees carrying out photosynthesis so less carbon is removed from the atmosphere

• It is often followed by combustion as trees are burned, increasing the addition of carbon to the atmosphere

False.

While the water cycle does provide fresh water to land plants and animals, this occurs due to continuous evaporation and precipitation, not by passing water between different living organisms.

Water moves from the ocean to the atmosphere by evaporation.

Precipitation occurs when water falls from clouds, e.g. in the form of rain, snow or sleet.

False.

Microorganisms do not return carbon to the soil, but instead release it into the atmosphere when they respire.

Microorganisms do return mineral ions to the soil during nutrient cycling.

Decomposers are organisms that break down dead and waste material, e.g. bacteria and fungi

True.

Decomposers do not fit into any particular trophic level, as they can feed on waste or dead organisms from any part of a food chain.

The rate of decay is influenced by:

• temperature

• oxygen availability

• water availability

pH can also affect the activity of decomposers.

True.

Higher temperatures increase the rate of decay. This is because the enzyme controlled reactions required for decomposition occur faster at higher temperatures.

Oxygen availability impacts the rate of decay because decomposers require oxygen for respiration. If oxygen levels become depleted decomposers will die and decomposition will slow down or stop.

A gardener can use a compost heap to improve growing conditions in their garden as follows:

• set up a compost heap to optimise the temperature, moisture and oxygen levels in order to maximise the rate of decay

• use the resulting compost as a natural fertiliser, providing key minerals, e.g. magnesium and nitrates

Anaerobic decay describes the process of decomposition when it is carried out in the absence of oxygen.

Methane gas is produced during anaerobic decay.

In an investigation into the effect of temperature on the rate of decay of fresh milk the independent variable would be temperature.

The rate of decay of fresh milk is likely to increase as temperature increases from 10°C to 30°C. This is because the enzyme-controlled reactions required for decomposition occur faster at higher temperatures.

In an investigation into the effect of temperature on the rate of decay of fresh milk, a decrease in pH indicates decay because the microorganisms use the sugars in milk to fuel anaerobic respiration, producing lactic acid.

In some versions of this practical lipase may be used to speed up the decay process; in this case the pH change occurs due to the breakdown of fats into fatty acids.

In an investigation into the effect of temperature on the rate of decay of fresh milk, increasing the temperature to 90°C would denature any enzymes involved in decay.

Environmental changes that might affect the distribution of organisms in an ecosystem include changes in:

• temperature

• water availability

• atmospheric gas concentrations

Rising average temperatures can cause the distribution of plant species to:

• spread northwards

• spread to higher altitudes

These changes would provide the species with habitats where average temperatures are lower.

True.

Changes in the availability of water can affect the distribution of living organisms because water is a fundamental requirement for biological processes.

Oxygen is required for aerobic respiration in aquatic animals, and some species can only survive in water with high oxygen concentrations. If water oxygen levels fall, these aerobic organisms may migrate to other locations or die out.

Environmental change can be caused by:

• seasonal change, e.g. winter migration of birds

• geographic change, e.g. changes to sea ice and glaciers

• human activities, e.g. global warming and water pollution