14.2 Applications and skills

14.2.3 Case studies: Human impacts on ecosystems

We have already encountered numerous examples of how humans have an impact on ecosystems. Case studies are often used in ecology to study general principles because controlled experiments are not always possible in the field. As you go through this page, consider:

  • how the causes and consequences of these examples illustrate basic ecological concepts
  • ways that conservation/control programmes can be evaluated.

Invasive species: Cane toads in Australia

The poisonous cane toad, Bufo marinus, is native to central and south America and was introduced to Australia in 1935 in order to control a common sugar cane pest, the cane beetle, Dermolepida albohirtum (Figure 14.2.3d).

  • Cane toads have no natural predators in Australia, and are poisonous during all stages of their life. Any small predator that attempts to eat a cane toad is almost always lethally poisoned, although some species are not susceptible to the toxin.
  • The Australian government suggests that the toads are advancing at a rate of about 45 to 60km per year westward from their current distribution.

distribution of cane toads in AustraliaFigure 14.2.3a – Distribution of cane toads in Australia
Source: Kearney et al., WP 2008, 'Modelling species distributions without using species distributions: the cane toad in Australia under current and future climates’, Ecography, vol. 31, pp. 423-434

Scientists and policymakers are uncertain how to proceed in dealing with cane toads. When evaluating what type of biological control is necessary, scientists must consider:

  • The life cycle of the invasive species. Cane toads disperse quickly because cannibalism is common. A tadpole has a better chance of survival if it is far away from its more mature (and hungry) cousins.
  • The effect on local flora and fauna. Setting up traps has been unsuccessful since other amphibians often get caught. Egg and tadpole collection is difficult because cane toads look very much like native amphibian species during these stages.

Biomagnification: Mercury in fish

Mercury released by burning coal and incinerating wastes eventually settles in freshwater ecosystems.

total mercury wet deposition, 2008

Figure 14.2.3b – Location of mercury deposition in the United States (1mg = 10-6g)

Once in the water supply, elemental mercury is converted by specific bacteria into methylmercury, a persistent organic neurotoxin that accumulates in fatty tissues and is biologically magnified through aquatic food chains.

methylmercury concentration

Figure 14.2.3c – Methylmercury concentrations in organisms collected from Lake Washington. 


Using the information from Figures 14.2.3b and 14.2.3c and your own knowledge:

  • Explain the causes of mercury biomagnification.
  • Draw a food web for Lake Washington species. Name species that occupy more than one trophic level.
  • Categorise the species as having a high (>100mm/kg), medium (20–100mm/kg) or low (<20mm/kg) concentration of methylmercury. Explain the categories.
  • Estimate the concentration of methylmercury in a smallmouth bass from Lake Norman. Justify your prediction.

Course link

The cane toad on discussion here, Bufo marinus, is the same species that we encountered in Page 5.2.2. Red-bellied black snakes in Australia have evolved mechanisms to avoid the poisonous toad. Often, the effect of a disturbance on an ecosystem is impossible to predict!

cane beetle - ©Alexander Dudley 2005Figure 14.2.3d – The cane beetle, Dermolepida albohirtum
Cane toads were originally brought to Australia to eat the beetles’ eggs during the breeding season. The effectiveness of this programme remains in question today. (Photo credits: ©Alexander Dudley 2005)

cane toad cannibalFigure 14.2.3e – Cannibal cane toad
This cane toad was found with these immature cane toads in its stomach!

Did you know?

If you don’t know the name of something, it helps to describe it as accurately as possible, using scientifically appropriate language, on an internet search. It’s like a reverse glossary search!

Lake WashingtonFigure 14.2.3f – Lake Washington, Washington State, USA 
Lake NormanFigure 14.2.3g – Lake Norman, North Carolina, USA