Populations in Ecosystems

This topic is about how populations fit into ecosystems, and how ecologists measure and manage them.

Populations, Communities and Ecosystems

Here are some definitions to know…

• Population: A group of organisms of the same species, in a particular space, at a particular time, that can potentially interbreed (see Populations)
• Community: All the populations of different species living in the same place at the same time
• Ecosystem: A community and the non-living (abiotic) components of its environment. Ecosystems can be very small (e.g. a pond) or very large (e.g. a forest)

Habitat and niche

• Habitat: The place where an organism lives
• Niche: The role of a species within its habitat, e.g. what it eats and when it’s active. No two species can occupy exactly the same niche in a habitat. A species’ niche depends on its adaptations to both:
  • Abiotic conditions: the non-living factors, e.g. temperature, light, water and pH
  • Biotic conditions: the living factors, e.g. food, predators and competitors

Carrying Capacity and Population Size

The carrying capacity is the population size of a species that an ecosystem can support in the long term, given the resources available. A population usually grows until it reaches the carrying capacity. It can then briefly overshoot, but there aren’t enough resources (e.g. food, water, shelter) to support that many individuals, so it falls back. This is why the population fluctuates around the carrying capacity.

Population size can change because of:

• Abiotic factors, e.g. if the temperature moves away from a species’ optimum (too high or too low) fewer individuals can survive and reproduce
• Biotic factors
  • Interspecific competition: Competition between individuals of different species for the same resources (e.g. for food, light or shelter). The population of the less competitive species usually decreases
  • Intraspecific competition: Competition between individuals of the same species. As the population grows, each individual gets fewer resources, so fewer survive and reproduce and the population falls. With less competition it rises again, so it fluctuates around the carrying capacity
  • Predation: The sizes of predator and prey populations have an impact on each other (as per graph below)

Predator–prey cycles

If you look at the graph above, you will notice a common pattern: the prey peak comes before the predator peak.

1. When there are lots of prey, there’s more food for the predators, so the predator population increases
2. More predators eat more prey, so the prey population decreases
3. With less food, the predator population then decreases
4. With fewer predators, the prey population increases again, and the cycle repeats

Note: In the wild, other factors (such as disease and climate) also affect both populations, so real cycles are less regular than this.

Growth of microorganisms

A population of microorganisms in a broth culture follows the same pattern as any other population. It grows slowly at first, then rapidly, then levels off when nutrients run out and waste products (such as ethanol from yeast) build up.

You can follow the growth by measuring the turbidity (cloudiness) of the broth with a colorimeter. The more cells there are, the cloudier the broth, so the less light passes through.

Why use a log scale?

• In ideal conditions each cell divides, so the number of cells doubles each generation (e.g. 100, 200, 400, 800…) and quickly reaches millions
• On a normal scale, the early numbers are so small compared with the later ones that they look like zero
• On a log scale, each step up the axis is ×10 (100, 1000, 10 000…). The whole large range of numbers fits on one graph, and steady exponential growth shows as a straight line

How to read a log scale?

• Each labelled step is 10 times bigger than the one below (100, 1000, 10 000…).
• Sometimes the axis shows log values (2, 3, 4…) instead of the numbers.
  • A log value of 3 means 10³ = 1000 cells, and 4 means 10 000 cells.
  • Each +1 on the log value is ×10 cells
• The space between two labels isn’t evenly split.
  • Halfway between 1000 and 10 000 is about 3000, not 5500.
  • To get an exact value from a log value, convert it with 10^x

What the shape tells you?

• At first the line is nearly flat (slow start) because the cells are adjusting to their new conditions (e.g. making enzymes) before they start dividing quickly. This is the lag phase
• Exponential growth means the population grows by the same factor in each time period, e.g. it doubles every 20 minutes. Every new cell divides too, so the bigger the population gets, the faster it grows
• On a log scale, growing by the same factor each time gives a straight line. A steeper line means faster growth (a shorter doubling time)
• When the line levels off, the population has stopped growing, e.g. because nutrients have run out
• If the line falls, cells are dying faster than new ones are being made

Estimating Population Size

Counting every individual is usually impossible, so ecologists count a sample and use it to estimate the whole population. The method depends on whether the organisms move:

• Non-motile (i.e. don’t move) or slow-moving organisms (e.g. plants): randomly placed quadrats or quadrats along belt transect
• Motile (moving) organisms (e.g. insects, fish, mammals): mark-release-recapture

Non motile organisms

Measuring abundance in a quadrat

You can measure the abundance of a species in a quadrat in different ways:

• Frequency (counting):
  • Count how many individuals of the species are in the quadrat, e.g. 6 daisies.
  • Counting individuals is what you use to estimate population size with random quadrats, because you can find the mean and scale up
• Percentage cover:
  • Estimate how much of the quadrat’s area the species covers, e.g. grass covers about half the quadrat, so 50%.
  • Use it for plants where you can’t tell individuals apart, such as grasses and mosses

Random quadrats

1. Divide the area into a grid, e.g. using two tape measures laid at right angles
2. Use a random number generator to pick coordinates (or a way to generate a random number so there isn’t any bias in selection)
3. Place a quadrat at each coordinate and count the number of individuals of the species
4. Repeat for a large sample (e.g. 10 or more quadrats) and calculate the mean number per quadrat
5. Estimate the total:
   • First work out how many quadrats would fit in the whole site: area of the whole site ÷ area of one quadrat
   • Then total = mean number per quadrat × number of quadrats that would fit

Placing quadrats at random avoids bias, e.g. choosing the spots where the plants look most common.

Belt transects

Use a belt transect when conditions change across an area, e.g. from the edge of a pond to dry land, or from open grass into woodland.

1. Lay a tape measure (the transect line) across the area
2. Place quadrats at regular intervals along it (e.g. every 2 m but decide distance based on what size of area you want to investigate)
3. Record the abundance of the species in each quadrat, and measure the abiotic factor you’re interested in at each point (e.g. light intensity)

This shows how the distribution of a species changes along the transect.

Motile organisms

Mark-release-recapture

1. Capture a sample of the population, count them and mark them (e.g. with a spot of paint, as in the picture below)
2. Release them back where they were caught
3. Leave enough time for them to mix back into the population
4. Capture a second sample. Count the total number caught, and how many of these are marked
5. Estimate the population size:

Population size = (number in first sample × number in second sample) ÷ number marked in second sample

Assumptions for mark recapture release

The formula assumes the proportion of marked individuals in the second sample is the same as in the whole population. This relies on these assumptions being true:

• The marked individuals mix evenly back into the population
• The mark doesn’t affect survival, e.g. it isn’t toxic and doesn’t make the animal easier for predators to see
• The mark isn’t lost (e.g. doesn’t rub or wash off) between the samples
• There are no births, deaths, immigration or emigration between the two samples

Succession

Ecosystems are dynamic, which means they change over time (often hundreds of years).

1. Pioneer species colonise the bare ground (e.g. lichens on rock). They are adapted to the hostile conditions, such as little water and few nutrients
2. The pioneer species change the abiotic environment, e.g. when they die and decompose, they form a thin layer of soil that holds water and nutrients
3. The environment becomes less hostile, so new species with different adaptations can colonise (e.g. mosses and grasses, then shrubs)
4. The new species may change the environment so that it becomes less suitable for the previous species, which are outcompeted
5. As the environment becomes less hostile (e.g. soil, grasses, shrub), biodiversity increases
6. Eventually a stable climax community forms (e.g. woodland)

Secondary succession happens where soil is already present, e.g. after a forest fire or when farmland is abandoned. It’s faster, because it doesn’t have to start from bare ground.

Conservation and Managing Succession

Some habitats, such as heathland and grassland, are not climax communities. If succession were left to carry on, they would turn into woodland and certain species that depend on heathland and grassland would be lost. So conservation often means managing succession to stop it reaching the climax community.

Examples of conservation activities:

• Grazing: animals such as sheep eat the young shoots of shrubs and trees, so they can’t take over
• Managed burning: clears woody plants, and the habitat plants (e.g. heather) grow back first
• Mowing or cutting: stops shrubs and trees establishing

There is often a conflict between human needs and conservation, e.g. land is needed for farming, housing and timber, but this destroys habitats. Sustainable approaches try to meet human needs without using up natural resources, e.g. replanting trees after felling, or fishing quotas.

In the exam, you may be asked to evaluate data about a conservation issue. Use the data, consider evidence on both sides, and look at how reliable it is (e.g. sample size, and whether a correlation really shows a cause).

Required Practical 12: Distribution of a Species

Example: the effect of light intensity on the distribution of a plant, from open grass into woodland

1. Lay a belt transect from the open grass into the woodland
2. Place a quadrat at regular intervals along the transect (e.g. every 2 m)
3. In each quadrat, record the abundance of the plant as percentage cover or frequency
4. At each quadrat, measure the light intensity with a light meter
5. Keep other variables as similar as possible, e.g. take all readings at the same time of day, and have the same person estimate percentage cover each time
6. Plot abundance against light intensity, and use a statistical test (e.g. a correlation coefficient) to see if there’s a significant relationship

Exam Question Practice

Past paper questions on this topic. Try each one before opening the answer.