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
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 difficult task!
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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
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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 fit 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
Figure 1 European wasps are a threat to many Australian ecosystems.
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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 significant predators of small invertebrates and abundant prey for larger predators.
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