Producers make biomass
Definition
Producer
An organism, usually a green plant or alga, that makes its own food by photosynthesis and provides the biomass for a food chain.
- Producers are the organisms that make the biomass for almost all life on Earth.
- A producer is usually a green plant or an alga that makes glucose by photosynthesis.
- The glucose is stored as biomass, the mass of living material, which passes to the animals that eat the producer.
Key Idea
Every food chain starts with a producer, because only producers can capture energy from the Sun.
Food chains
Definition
Food chain
A diagram that shows the feeding relationships between organisms, with arrows pointing from each organism to the one that eats it.
- A food chain shows the feeding relationships in a community, from one organism to the next.
- The arrows point from the organism being eaten to the one that eats it, showing the direction that biomass and energy are transferred.
- Producers are eaten by primary consumers, which are eaten by secondary consumers, which may be eaten by tertiary consumers.
- In the chain grass to rabbit to fox, the grass is the producer, the rabbit a primary consumer and the fox a secondary consumer.
Common Mistake
- The arrow does not mean is eaten by in the way you might expect.
- It points towards the eater, following the flow of biomass and energy along the chain.
Predators and prey
Definition
Prey
An animal that is hunted, caught and eaten by a predator.
- A predator is a consumer that hunts, kills and eats other animals, and the animals it eats are its prey.
- In a stable community, the numbers of predators and prey rise and fall in linked cycles.
- When prey are plentiful the predators have more food, so predator numbers rise.
- The extra predators eat more prey, so prey numbers fall, which then makes predator numbers fall, and the cycle repeats with the predator peak just after the prey peak.
Sampling with quadrats and transects
Definition
Quadrat
A square frame of known area placed on the ground to sample the number or percentage cover of a species.
Definition
Transect
A line laid across a habitat along which samples are taken to show how the distribution of a species changes.
- Ecologists sample an area to estimate how many organisms live there and where they are found, instead of counting every one.
- A quadrat measures the abundance of a species, meaning how many there are.
- A transect measures the distribution of a species, meaning where it is found across a habitat.
Practical
Investigating the population size of a common species
- Aim: to measure the abundance and distribution of a species, such as daisies on a school field.
- A quadrat is a square frame, often 0.25 square metres, that is placed on the ground to sample a small patch.
- To measure abundance, place the quadrats at random positions chosen with a random number generator, so the sample is not biased.
- In each quadrat count the number of the chosen species, or estimate its percentage cover when the plants are too many to count.
- Record several quadrats and work out the mean, so one unusual patch does not distort the result.
- To measure distribution, lay a tape measure in a straight line, the transect, running from a shaded hedge out into open ground.
- Place quadrats at equal intervals along the transect and record the species, and an abiotic factor such as light intensity, at each point.
- The results show how the species changes along the gradient, for example fewer shade plants as the light increases.
- Keep the quadrat size and sampling time the same, and take enough samples to make the results repeatable.
Estimating population size
- First work out the mean number of the species per quadrat from your samples.
- Then scale up to the whole area using estimated population=mean per quadrat×total areaarea of one quadrat\text{estimated population} = \text{mean per quadrat} \times \dfrac{\text{total area}}{\text{area of one quadrat}}estimated population=mean per quadrat×area of one quadrattotal area.
- For a mean of 6 daisies in a 0.25 square metre quadrat on a 200 square metre field, the estimate is 6×2000.25=48006 \times \dfrac{200}{0.25} = 48006×0.25200=4800 daisies.
Self review
- What is a producer, and how does it make biomass?
- In a food chain, what do the arrows show?
- Why does the predator peak come just after the prey peak?
- Would you use a quadrat or a transect to measure how a species is distributed across a habitat?
- A mean of 4 buttercups is found in a 0.25 square metre quadrat on a 100 square metre field; estimate the population.