14.1 Essential ideas

14.1.5 Population ecology (HL)

Populations grow according to two patterns:

1. Exponential growth

Exponential growth occurs when resources are unlimited and conditions are ideal. In nature, this situation is rare, except in cases of alien, invasive species. The exponential growth curve has a characteristic J-shape.

growth curve J-shapeFigure 14.1.5a – Exponential growth in human populations

2. Sigmoid growth

Sigmoid growth occurs when resources are limited. Growth occurs in stages, which can be explained by relative rates of natality (N), mortality (M), immigration (I) and emigration (E). Populations grow rapidly and then stabilise at the carrying capacity of the environment. The sigmoid growth curve has a characteristic S-shape.

Figure 14.1.5b – Sigmoid growth

Activity 1

  • Determine on the sigmoid growth curve, where the following conditions occur:
    • N + I > M + E
    • N + I = M + E
  • Suggest reasons why the rates of N, M, E, and I change during the stages of population growth. Scroll over Figure 14.1.5b for hints.

Sampling populations using the Lincoln index

Sampling is necessary to estimate population sizes. Quadrats are a useful sampling method for plants, but they are not very useful for estimating populations of more mobile species. We can use a Lincoln index in these cases. This is also known as the capture-mark-release-recapture method. 

Step 1: Use an appropriate capture technique to sample specimens in your test area. Mark them in a way that doesn’t harm them, and release them to the environment.

Step 2: Come back on a different day and capture specimens in the same test area. Count how many are marked and unmarked.

Step 3: Use the following formula, called the Lincoln index, to estimate the population size:

 N equals fraction numerator n subscript 1 cross times end subscript n subscript 2 over denominator n subscript 3 end fraction

N is the estimated population size, n1 is the number marked, n2 is the number recaptured, and n3 is the number marked and recaptured.

Top-down or bottom up?

Some factors limit populations at top trophic levels by affecting production at the lowest trophic level. These are called bottom up limiting factors. Other factors limit populations through predation or herbivory. These are called top down limiting factors

  • Bottom up limiting factors affect populations through the availability of nutrients. For example, growth of marine algae is normally limited by a lack of nutrients in shallow coral reefs.
  • Top down limiting factors affect populations through feeding relationships, either herbivory or predation. When algal blooms occur, herbivorous fish in the reef control populations of algae from the top down.

coral reef fish and algal bloomFigure 14.1.5c – Left: a school of blue tang fish, Paracanthurus hepatus, grazing on coral algae. Right: algal bloom covering a reef. Herbivory limits populations from the top down.

Essential idea

Dynamic biological processes have an impact on population density and population growth.

Nature of science

Avoiding bias: a random number generator helps ensure a population sample is truly random.

International mindedness

Human populations grow exponentially. So far, resources have not been exhausted and the carrying capacity has not been reached. However, population growth, and the subsequent strain on natural resources, are issues of global concern. Some nations have adopted anti-natalist policies. Others are advocating pro-natalist policies. Is population control in some parts of the world and population growth in others ethically justifiable?

Activity 2

Draw a sigmoid growth curve and annotate it by showing how rates of mortality, natality, emigration and immigration change at each stage.

In the lab

In order to sample populations using a Lincoln index, you need to capture animals. How can you ensure ethical treatment of animals? How do you ensure the mark does not hinder the animal? For which organisms would these methods of capture be most appropriate?

  • Pitfall traps
  • Areal nets (butterfly nets)

how to make a pitfall trapFigure 14.1.5d – How to make a pitfall trap

Activity 3

Practise using the Lincoln index to estimate the number of sweets in a jar! Then check how the size of the sample improves the accuracy of your calculation.

Concept help

Removing an organism from a food chain results in effects through all of the trophic levels. These effects are known collectively as ‘trophic cascades’. They can also be classified as either bottom up or top down. Removing a keystone species results in amplified trophic cascades.

Science and social responsibility (Aim 8)

Eutrophication means nutrient enrichment. Fertilisers used in agriculture enrich soils with nitrogen and phosphorus in order to increase productivity but they run off into lakes and rivers, where they encourage deadly algal blooms. When algae cover a body of water, they block sunlight, causing deaths in the ecosystem. How should fertilisers be used more responsibly to limit damage caused by eutrophication?

algal bloomFigure 14.1.5e – Algal bloom