8.3 Food webs can be disrupted

8.3

Food webs can be disrupted

In this topic, you will learn that:

• most food webs have a balance between producers and consumers • introduced organisms can disrupt the balance between organisms in an environment

• the loss of organisms can affect the survival of other species in an environment.

Video 8.3 Food webs can be disrupted

Figure 1 European wasps are a threat to many Australian ecosystems. There is a balance between all organisms in a food web. If more grass grows, the number of animals that eat the grass will also grow. In time, the amount of grass available will decrease. This balance can be disrupted by the introduction of new organisms or the removal of predators.

Introduced organisms

Introduced organisms may become pests or weeds. An example of the accidental introduction of a pest into Victoria is the European wasp, otherwise known as a ‘picnicker’s nightmare’. The fi rst European wasp in Australia was recorded in Tasmania in 1959, and the wasps are now common there. For the mainland, the European wasp was reported in Melbourne in 1977 and in Sydney in 1978. The wasps may have originally arrived in wood shipments; however, with few predators, no diseases and no competition for nest sites, their numbers have increased quickly. So, how does this affect a food web? If it had no predators and no competition for nest sites, the European wasp may have been able to fi t in without affecting other organisms in an ecosystem. However, every living thing consumes resources to live, and the European wasp is no exception.

By the end of an Australian summer, each European wasp nest may contain several thousand individuals. The larvae complete their development after being fed a diet that consists mainly of other insects that the workers catch and kill. This means that each European wasp nest has the potential to remove several thousand native insects – often the caterpillars of moths and butterfl ies – from the environment. This can have a devastating effect on local animal populations. Unfortunately, European wasps do not just consider other insects as food – they also attempt to steal food from picnics and barbecues. This, along with a very painful sting, can make outdoor eating in summer a very diffi cult task!

Loss of organisms

The removal or loss of organisms from an ecosystem can have dramatic effects. Amphibians, such as frogs, are an important part of the biosphere: they may be warning us of unsafe environmental conditions that could eventually seriously affect our health. The thin skin of amphibians helps them drink and breathe, but it also makes them vulnerable to environmental contaminants, especially agricultural, industrial and pharmaceutical chemicals. Consequently, they are commonly referred to as indicator species – indicators of environmental health, as well as protectors of human health.

Amphibians watched the dinosaurs come and go, but today almost one-third of them, representing 1896 species, are threatened with extinction. As many as 165 amphibian species may already be extinct and the population numbers of at least 43 per cent of all species are declining. This means that there will be even fewer frogs and other amphibians in the future. Amphibians feed mainly on insects and other invertebrates. It has been estimated that a single population of approximately 1000 frogs could consume almost 5 million invertebrates in 1 year. Amphibians are signifi cant predators of small invertebrates and abundant prey for larger predators.

Figure 2 The corroboree frog is one of Australia’s many endangered species.

In areas of the world where numbers of amphibians have declined, there has been an increase in invertebrate pests that damage crops and carry human diseases.

Fish numbers in the oceans are dwindling as the demand for seafood continues to increase across the globe. Larger predatory fish, such as tuna, marlin and salmon, are often the most prized, but their numbers are declining, allowing smaller species to thrive in their absence.

Commercial fishing boats with large nets (Figure 4) are removing large numbers of many species, including ‘by-catch’ – fish and other marine species that are not wanted but are left to die before being thrown back.

Figure 3 Predatory fish, such as tuna, are in decline.

Figure 4 Commercial fishing boats using nets catch many species, including some that are not wanted (by-catch).

8.3 Check your learning

Remember and understand

1 Identify an example of an introduced animal.

2 Define the term ‘indicator species’.

Apply and analyse

3 Explain why some animals and plants can become pests when introduced into

Australia.

4 Organism numbers can decrease for a number of reasons, including loss of habitat, climate change and direct removal by humans. Describe an example of each.

Evaluate and create

5 One way to control an introduced organism is to introduce another organism that eats it. This is called biological control. When it works, it can be very effective. However, sometimes things can go wrong and the organism introduced to help the problem becomes a much bigger problem.

Research the introduction of the prickly pear moth or caterpillar, and the cane toad. Analyse the effectiveness of this introduction (by identifying the organisms involved, how they were affected by the introduction and deciding whether the advantages of introducing the prickly pear moth and cane toad outweighed the disadvantages). 6 In winter and spring after it has rained, the most common animal to find outside is the humble garden snail. Do not be fooled by this animal – it is probably the most successful of all introduced animals and causes millions of dollars of damage to food production each year.

Research the introduction of the garden snail. Analyse the effectiveness of this introduction.