6.5 Introducing a new species may disrupt a food web

6.5

Introducing a new species may disrupt a food web

Learning intentions

By the end of this topic, you will be able to: • define introduced species • explain how an introduced species can impact an ecosystem.

Key ideas

• Introduced species can change the amount of food for other organisms. • New species can cause a new equilibrium in an ecosystem. • Biological control is the deliberate introduction of a new species to control a non-native plant or animal.

introduced species an organism that has been brought to and has established itself in an area it is not native to

Figure 1 Cane toads were introduced to Australia to control scarab beetles in 1935. Cane toads were Balanced populations 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, which will be balanced by a decrease in the animals who eat the grass. This increase–decrease population cycle is a balance that can be disrupted by introduced species or the removal of predators. Cane toads Scarab beetles are a family of beetles that can range in size from 2–70 millimeters in length.

One species from this family, Dermolepida albohirtum can cause a lot of damage to the sugar cane crops in Queensland. The female of this species lay eggs in the soil of the sugarcane. When the larvae hatch, they eat the roots of the vane plant, causing it to die. Cane toads ( Rhinella marina ) were introduced to Queensland in 1935 in an attempt to control the beetle population (Figure 1). While the cane toads did eat the scarab beetles, they preferred other insects. Cane toads also lacked many natural predators in Australia. When larger animals such as quolls tried to eat the toad, they were killed by the poisonous toxin on the toads’ backs. This meant the cane Kites (birds of prey) Crocodiles CrowsKeelback snakes DRAFT

Beetles Small mammals

Cane toad Small birds

Ants Grasshoppers Termites Small spiders Small toads/frogs Small toads/frogs Snails

Figure 2 The introduction of the cane toad into this food web has affected many other animals. Fruit Sugar cane Wild plants

toad population (with a lot of food and few predators) could continue to increase.

The introduction of cane toads has also had negative social effects on First Nations peoples who rely on the local ecosystem for food. The yellow-spotted monitor goanna is an important meat source for some First Nations peoples that live in remote communities. Many of these goannas are killed when they try to eat cane toads. This decreases the availability of meat in areas where there is already limited supply of meat and protein.

The Yugul Mangi Aboriginal Ranger group work with government departments to help manage the population and spread of cane toads in Queensland. Gamba grass In the 1930s, farmers concerned about the lack of grass in the dry parts of Australia brought gamba grass from the tropical savannas of Africa (Figure 3). This grass grows very quickly, taking over land that would usually grow native Australian grasses. Because gamba grass grows so quickly, it produces a lot of vegetation that burns in a bush re.

It also has the potential to prevent small tree seedlings from growing, changing woodlands to a grassland. While cattle farmers might like the increase in grass for their cattle to eat, it has an environmental impact on the plant and animal life in Kakadu National Park. This means it also has an impact on important food sources and medicines for First Nations peoples who live in that area. Macquarie Island rabbits Not all relationships in a food web are easily predicted. In 1985, scientists on Macquarie Island (halfway between Australia and Antarctica) devised a plan to eradicate the cats that had been introduced to the island since the early nineteenth century. The scientists thought that if the cat population decreased, there would be an increase in the native burrowing bird populations on the island. However, the cats were also predators of rabbits. When the cats were gone, the rabbits were no longer hunted. This allowed the rabbits to survive, destroying native plants and affecting many other organisms that were native to the island. Scientists needed to nd a way to control the rabbits. DRAFT

Figure 3 Gamba grass

a

biological control a method of controlling a population by releasing a living organism into an ecosystem

disease a disorder or condition that interrupts the normal functioning of an organism

immune able to ght an infection as a result of prior exposure Figure 4 a Before: This slope on Macquarie Island had vegetation as recently as 2007. b After: The same slope a few years later – it has been ravaged by rabbits since cats were eradicated. Biological control All efforts to control the rabbit population by physical means were unsuccessful. In 1938, CSIRO scientists studied a way to control the disease using a living organism (biological control). They tested a virus called Myxoma for its ability to cause disease in rabbits. This virus causes a disease with symptoms including fever and swelling around the head of the rabbit. Death occurs within 14 days. Myxoma was eventually released in the wild Before a species is introduced as part of and quickly killed almost all the rabbits biological control of pests, scientists must that caught the infection. This increased model the possible effects on populations Australia’s wool and meat production within that compete for the same food source or the two years. A small percentage of rabbits were predators that may prey on them. Scientist unaffected by the disease. These rabbits must effectively map the food web of the survived and bred a new population of rabbits ecosystem and how the balance between all that were immune to the disease. New organisms in the community will be affected viruses, such as the rabbit calicivirus, have by the introduction of the biological control been tried, with similar results. organism.

b DRAFT

Loss of organisms

Echidnas Dingoes Kookaburras

Termites Eucalyptus trees Wattle trees Grasses

Kangaroos

Wombats Rabbits Frill-necked lizards Grasshoppers Figure 5 Rabbits compete with grasshoppers, wombats and kangaroos for grass. Retrieve 1 Identify two reasons why the cane toad population was able to increase so quickly when introduced to Australia. 2 De ne the term ‘biological control’. Comprehend 3 Describe why cane toads are referred to as an introduced animal in Australia. 4 Use the food web in Figure 5 to suggest two populations that would increase as a result of the introduction of rabbits. 5 Explain why some cattle farmers would have supported the introduction of gamba grass to Australia. 6 Explain why local First Nations peoples should have been consulted before the gamba grass was introduced.

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. 6.5 Check your learning Apply 7 Evaluate the ethics of using biological control of rabbits, by: a describing how Myxoma infection affects the health of a rabbit b describing the effect a large rabbit population has on a native environment, including the native plants and animals c describing how a large rabbit population affects our ability to grow food d deciding whether the life (and Myxoma- related death) of a rabbit is more or less important than the effects you described above. (Does the end justify the means?)

DRAFT indicator species an organism that can be used to measure the environmental condition of an area

Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level.