Food chains and food webs

Organisms are connected by feeding relationships: when one organism eats another, food and energy are transferred between them.

Producers, usually green plants or algae, use sunlight to make organic material by photosynthesis. Consumers obtain food by eating other organisms. A primary consumer eats a producer; a secondary consumer eats a primary consumer. Higher-level consumers eat other consumers.

Food chains: one feeding pathway

A food chain shows one pathway along which food and energy are transferred. Each arrow points from the organism being eaten towards the organism that eats it.

In a small UK freshwater pond, algae can be eaten by water fleas, which can be eaten by small fish:

The algae are the producer, the water fleas are primary consumers, and the small fish are secondary consumers in this chain.

Food webs: connected feeding pathways

A food web links several food chains to show the feeding relationships within a community. It gives a fuller picture than a single chain because an organism may eat several species and may itself be eaten by more than one species.

In the pond, consumers other than water fleas can feed on algae, and small fish can eat prey other than water fleas. These connections mean that a change in one population can affect both the organisms it eats and the predators that eat it.

For example, if the water flea population falls, less algae may be eaten and small fish may have less food. The effects can extend beyond the algae–water flea–fish chain because other consumers and prey are connected to it in the food web.

Quadrat sampling

A quadrat is a frame enclosing a known area of habitat. Organisms inside the frame are recorded to estimate how common a species is across the habitat. Using several quadrats provides a sample of the habitat without needing to examine every part of it.

Measuring abundance and frequency

Abundance describes how much of a species is present. In each quadrat, it can be measured by counting the individuals of that species. When individuals are difficult to distinguish, abundance can instead be estimated as percentage cover: the percentage of the quadrat area covered by the species.

Frequency describes how often a species occurs in the sampled quadrats. Record whether the species is present or absent in each quadrat, then calculate the proportion in which it is present. For example, if it occurs in 6 out of 10 quadrats, its frequency is $$6/10$$, or $$60%$$. Frequency does not show how many individuals are present in each quadrat.

Random sampling and mean abundance

Place quadrats at random locations, for example by using randomly selected coordinates. This helps avoid choosing only particular parts of the habitat. Take multiple samples because abundance may vary from place to place; more samples generally make the estimate more representative.

To calculate mean abundance per quadrat, add the individual counts from all quadrats and divide by the number of quadrats sampled.

Estimating population size from quadrat counts

When individuals have been counted, the mean count per quadrat can be used to estimate the total population. Divide the habitat area by the quadrat area to find how many quadrat-sized areas would fit into the habitat, then multiply by the mean count:

Use the same units for both areas. For example, if the mean count is 8 individuals per $$0.5\ \mathrm{m^2}$$ quadrat and the habitat covers $$100\ \mathrm{m^2}$$, the estimated population is $$8 \times (100/0.5) = 1600$$ individuals. This is an estimate, not an exact count: its reliability depends on how representative the sampled quadrats are of the whole habitat.

Transect sampling

A transect is a line laid across a habitat to investigate how the distribution or abundance of species changes with position. Distribution describes where a species is found; abundance describes how much of it is present.

Line and belt transects

In a line transect, organisms touching the line are recorded along its length. Their positions show where the species occur across the habitat.

In a belt transect, quadrats are placed along the line to sample an area at each position. The quadrats may be placed continuously, next to one another, or at regular intervals. For each quadrat, the species present or their abundance is recorded.

Comparing positions along a transect

Sampling systematically along a transect allows observations from different positions to be compared. The line can cross an environmental gradient, where a condition such as light intensity changes across the habitat.

If a species becomes more or less abundant along the gradient, its abundance may be associated with that environmental change. The pattern alone does not establish that the changing condition caused the difference: other factors may also vary across the habitat.

Plotting scientific data

A graph represents data visually, making individual values and patterns easier to inspect. The type of graph should match the kind of data collected.

Choosing a graph type

A bar chart compares separate categories, such as different species. Each category has its own bar, and the bars are separated by gaps.

A line graph can show how one variable changes across a continuous scale, such as time. Each pair of values is plotted as a point. A line is then drawn as appropriate to the data, either joining the points or showing the overall trend.

Axes and scales

Place the variable being changed or selected on the horizontal x-axis, and the variable measured on the vertical y-axis. Label each axis with the quantity and its unit where applicable, for example, Time (minutes). A category axis needs category names rather than a numerical unit.

Choose a numerical scale that covers the full range of data and uses a consistent interval: equal distances along the axis must represent equal changes in value. Make effective use of the available plotting area so that the values and patterns are easy to see.

Plotting values accurately

For a bar chart, draw each bar to the height given by its value on the numerical scale. For a line graph, find each value on the x-axis and its corresponding value on the y-axis, then mark the point where they meet. Check positions against the scales before drawing the bars or line.

Predator–prey cycles

A predator is a consumer that kills and eats another animal. The animal eaten is its prey. In a stable community, predator and prey populations can rise and fall in linked cycles: changes in one population affect the other.

When prey become more abundant, more food is available for predators. This can allow more predators to survive and reproduce, so the predator population increases. As predator numbers rise, more prey may be eaten, contributing to a decline in the prey population.

With fewer prey available, predators have less food, so the predator population may then fall. Reduced predator numbers mean less predation, allowing the prey population to recover. As prey become more abundant again, the cycle can repeat.

Reading predator–prey population graphs

A population graph shows how population sizes change over time. To compare the two populations, read their values at the same points in time and identify where each population rises or falls. A peak is a high point before a population falls; a trough is a low point before it rises.

Predator population peaks typically occur after prey population peaks. The predator population takes time to respond to increased prey availability: predators need time to survive and reproduce before their numbers increase. Predator numbers can therefore continue rising even after prey numbers have begun to fall.