7.2 Organisation of an Ecosystem

7.2.1 Levels of organisation

- Photosynthetic organisms (organisms that photosynthesise) are the producers of biomass for life on Earth.
- A producer is an organism that is capable of making its own food, and they form the base of a food chain.
- Producers are typically green plants or algae, as they carry out photosynthesis to convert light energy from the Sun into chemical energy in the form of glucose.
Here are key characteristics of producers:

• Autotrophic: They create their own food using sunlight (through photosynthesis) or, in some cases, chemical energy (chemosynthesis).
• Source of Energy: They serve as the primary source of energy for all other organisms in the food chain (consumers and decomposers).
• Contain Chlorophyll: Producers, like plants and algae, contain chlorophyll to absorb sunlight during photosynthesis.
• Oxygen Production: During photosynthesis, producers release oxygen as a by-product, which is crucial for the survival of aerobic organisms.

Food Chains

Feeding relationships within a community can be represented by food chains.
All food chains begin with a producer which synthesises molecules.
This is usually a green plant or alga which makes glucose by photosynthesis.
Here's a food web I stole from wikipedia: _{Kestin Schulz, Mariya W. Smit, Lydie Herfort and Holly M. Simon. The image was provided by the Missouri Department of Conservation., CC BY-SA 4.0, via Wikimedia Commons}
In this example, the cattail, algae, caddisfly larva, and coontail are producers.
The daphina, muskrat, and pond snail are primary consumers. This means that they eat producers.
The green sunfish and glizzard shad are secondary consumers. This means that they eat primary consumers.
The great blue heron is a tertiary consumer. This means that it eats secondary consumers.
Consumers that kill and eat other animals are predators, and those eaten are prey. In a stable community the numbers of predators and prey rise and fall in cycles.

Investigating Ecosystems (Transects and Quadrats)

- Ecologists are biologists that study distribution and abundance of species.
- They do this using quadrats and transects.

Quadrats

- A quadrat is a frame made of plastic, wood, or wire with a grid that is used to measure the amount of organisms in an area.
- Because they're stationary and it takes time to collect data, quadrats are used to measure plants or slow-moving organisms.
- They are usually 0.25m² but can be smaller or larger. _{Yohan euan o4, CC BY-SA 3.0, via Wikimedia Commons}

How to Use a Quadrat to Measure a Large Area

1. Divide the Large Area into Sections

The total area you want to study might be too big to count every organism. Instead, you take several small samples (quadrats) from different parts of the area.

2. Placing the Quadrat

• Randomly: To avoid bias, place the quadrat at random points throughout the area. This can be done by using a grid and selecting random coordinates.
• Systematically: Alternatively, you might place quadrats at regular intervals (for example, along a transect) if you want to study how the population changes in different parts of the area.

3. Counting Within the Quadrat

Carefully count all the individuals (or record the percentage cover) of the target species within the quadrat. Record your data for each quadrat.

4. Estimating the Total Population

Calculate the average number (arithmetic mean) of individuals per quadrat.

Multiply this average by the total number of quadrats that would cover the entire area.

Example: If each 1m² quadrat has an average of 5 plants and your total area is 100m², then the estimated number of plants is 5 × 100 = 500.

5. Repeat for Accuracy

Using several quadrats improves the reliability of your estimate because it reduces the effect of unusual clumps or gaps in the distribution.

Why Use a Quadrat?

• It makes sampling a large area more practical.
• It gives a representative sample of the whole area.
• It reduces the time and effort needed compared to counting every individual organism.

This method is widely used in ecological studies because it provides a good balance between accuracy and practicality when measuring population size or species distribution.

Transects

A transect is a straight line along which samples are taken at regular intervals to study changes in organisms across a specific area.

How to Use a Transect

1. Setting Up the Transect

• Choose a straight line to study (e.g., from a riverbank to a forest edge).
• Mark out the transect using a tape measure or rope.

2. Sampling Along the Transect

• Belt Transect: Place quadrats at regular intervals along the transect and count the organisms within them.
• Line Transect: Record any organisms touching the line at set points.

3. Recording and Analyzing Data

Note the species found and their abundance at each point. This helps identify patterns, such as how environmental factors affect distribution.

Why Use a Transect?

• It helps study how populations change across different environments.
• It provides systematic and reliable data collection.
• It is useful for observing gradual environmental changes, like soil moisture or light levels.

Transects are commonly used in ecological fieldwork to study species distribution and environmental gradients.

7.2.2 Material Cycling

FSL: GCSE Biology Revision "The Carbon Cycle", GCSE Biology Revision "The Water Cycle"

All materials in the living world are recycled to provide the building blocks for future organisms.

Carbon Cycle

- The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide to be used by plants in photosynthesis:

1. Plants absorb carbon dioxide from the atmosphere.
2. Plants use the carbon dioxide to make sugars.
3. Said sugars are eaten by animals.
4. When plants, animals, and microorganisms respire, they release carbon dioxide into the atmosphere.
5. Carbon dioxide is also released during decomposition (more on that later).
6. If animals and plants die in areas where microorganisms are not present, the carbon in their bodies is converted, over millions of years, into fossil fuels.
7. In combustion, carbon dioxide is also released into the atmosphere.

Water Cycle

- The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated.

1. Water evaporates from bodies of water such as lakes, rivers, and oceans and transpiration from plants releases water vapour into the air.
2. The warmer air of the lower atmosphere rises, causing the water vapour to rise too. The moist air cools down as it rises and water vapour condenses into liquid water, which forms clouds.
3. As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and sleet (precipitation).

7.2.3 Decomposition

Keywords: Decompose, Detritivore FSL: GCSE Biology Revision "Decomposition"

Decomposer: A decomposer is an organism that breaks down other organisms. Examples include fungi and bacteria.
Detritivore: Detritivores are decomposers that feed on dead organisms. Examples include worms and insects.

Factors Affecting Decomposition

Moisture: Moisture is required for feeding and reproduction.
Oxygen: Oxygen is required for aerobic respiration.
pH/Temperature: The correct pH and temperature is required for enzymes to work at their optimum.

Decay of Milk

- Milk is mixed with an indicator (turns pink).
- When lipase is added, this breaks down the fats in the milk into fatty acids and glycerol.
- The indicator turns yellow to indicate the end of the reaction.
- Repeat until concordant results are obtained.

Uses of Decomposition

Compost

- Gardeners and farmers need optimum conditions so their waste material decays more quickly.
- Compost is a mixture of decomposers and organic matter that is used to improve soil fertility, and is made of this waste.
- This means that recycling of organic waste is done, as new plants can draw minerals from the compost.

Biogas Generators

- Some decomposers can break down material without oxygen (anaerobically).
- This produces methane gas, which is (pretty much) the same thing as natural gas. Natural gas produced in this way is also known as biogas.
- The methane can then either be burned to produce electricity, or used in the natural gas industry.

7.2.4 Impact of environmental change

FSL: GCSE Biology Revision "Environmental change"

- Environmental changes affect the distribution of species in an ecosystem. These changes include:

• temperature
• availability of water
• composition of atmospheric gases

Effect on distribution