Identify and describe Biomass and Producers, Food Webs and Feeding Relationships and Sampling Techniques: Quadrats in the context of levels of organisation.
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GCSE Biology Revision
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GCSE Biology revision
Levels of organisation
Organisation of an ecosystem
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Explain the relationships between Biomass and Producers, Food Webs and Feeding Relationships and Sampling Techniques: Quadrats and how they contribute to levels of organisation.
Apply and analyse knowledge of Biomass and Producers, Food Webs and Feeding Relationships and Sampling Techniques: Quadrats to interpret unfamiliar information about levels of organisation.
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Key knowledge
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Biomass and Producers
- Biomass refers to energy obtained from biological matter, produced by organisms such as green plants and algae through photosynthesis.
- Producers convert sunlight into glucose via photosynthesis, forming the foundation of all food chains and food webs.
- Producers are always found at the base of a food chain and have no prey — they are only eaten by primary consumers.
Food Webs and Feeding Relationships
- A food web tracks the feeding relationships between multiple organisms within an ecosystem.
- Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
- Predators kill and eat other organisms, whilst prey are the organisms that are killed and eaten.
Sampling Techniques: Quadrats
- Sampling is a technique used to study a small proportion of a population in order to draw conclusions about the whole population.
- A quadrat is a square frame (commonly 0.25 m²) placed over an area to count organisms, estimate percentage cover, or assess species diversity.
- To ensure valid, unbiased results, quadrats must be placed randomly, ideally using two measuring tapes as axes and a random number generator to produce coordinates.
- Sampling should be repeated at least three times to improve the reliability of results.
- Data from quadrats can be extrapolated to estimate population sizes across larger areas — for example, 6 buttercups in a 1 m² quadrat suggests 18 buttercups in a 3 m² field.
Sampling Techniques: Transects
- A transect is a line, typically made using a measuring tape, along which quadrats are placed at regular intervals to monitor changes in species distribution.
- Transects are particularly useful for studying how species diversity changes across different habitats, such as along a rocky shoreline.
- By placing quadrats at multiple points along a transect, you can observe the effect of different abiotic and biotic factors on species distribution and density.
Kite Diagrams
- Kite diagrams are used to display the number or percentage cover of species against distance along a transect.
- Distance is plotted on the x-axis and each species is represented along the y-axis, with data points plotted on either side of a horizontal midline.
- The width of the 'kite' shape at any given point represents the abundance or density of that species at that location along the transect.
- Kite diagrams allow you to compare how the distribution of multiple species changes across a habitat simultaneously.
Calculating the Mean
- The mean is calculated by adding all values together and dividing by the total number of values: Mean = (sum of values) / (number of values) When the mean represents a count of organisms, it should be rounded to a whole number as you cannot have a fraction of an organism.
Calculating the Median and Mode
- The median is found by arranging all values in ascending order and identifying the middle value.
- If there is an even number of values, the median is calculated by finding the mean of the two middle values: Median = (value 1 + value 2) / 2 The mode is simply the most frequently occurring value in a data set.
The Predator–Prey Cycle
- The predator–prey cycle describes the cyclical fluctuations in the population sizes of a predator and its prey over time.
- As prey population increases, predators have more food available, so the predator population also increases.
- As the predator population rises, more prey are eaten, causing the prey population to decrease.
- As prey becomes scarce, the predator population then falls due to lack of food, allowing the prey population to recover and restart the cycle.
- The lynx and snowshoe hare are a classic example used to illustrate the predator–prey cycle.
Random vs. Systematic Sampling
- Random sampling avoids bias by ensuring every part of the study area has an equal chance of being selected, producing more valid results.
- Using a random number generator to produce coordinates along two measuring tape axes is the most reliable method of random sampling.
- Belt transects involve placing quadrats at regular intervals along a line and are used to carry out systematic sampling of species across a habitat.