The toad that marched across a continent
383 words over 35 numbered lines, then 6 parts worth 12 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the surveys, people and figures are invented or rounded for practice.
The toad that marched across a continent
In 1935, 102 cane toads from South America were released on sugar cane
farms in Queensland, in north-eastern Australia. Farmers hoped that
the toads would eat the beetles whose larvae were destroying the roots
of their crops. It was one of the most famous mistakes in the history
of biological control.
The toads did not control the beetles. The adult beetles spend most of
their time high up on the cane plants, where the toads cannot reach
them, and the larvae live underground. Instead, the toads ate almost
anything else they could catch, and they bred rapidly. A female cane
toad can lay more than 20 000 eggs at a time, and in the warm, wet
north of Australia there were few predators, parasites or competitors
to stop the population growing. The tadpoles develop quickly in
shallow, temporary pools.
There are now more than 200 million cane toads in Australia. By 2025
the front of the invasion had moved about 3 000 km from the place
where they were first released. Biologists measuring toads at the
front found something surprising. Toads at the front have longer legs
than toads in the areas that were invaded first, and they travel
further each night. Toads at the front also tend to have more stamina.
Because toads with these features arrive in new areas first and breed
with each other there, the invasion has been speeding up. In the 1940s
the front moved about 10 km each year.
Cane toads are poisonous. Glands behind the head produce a toxin that
affects the heart. Australian predators did not evolve with the toads,
so many of them eat toads and die. Numbers of a small carnivorous
marsupial called the northern quoll, and of some large lizards, have
fallen sharply in areas the toads have reached. In some places, almost
every quoll disappeared within a year of the toads arriving. Some
snakes, birds and native rats have learned to eat only the safer parts
of a toad.
Scientists are now trying to teach predators to avoid toads. In an
illustrative trial, young quolls were fed small pieces of toad meat
containing a harmless chemical that makes them feel sick. Quolls that
had been trained this way were much less likely to attack a live toad
when they were released.
Read the article. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term biological control.
(1)Give two reasons why the cane toads did not control the beetles.
(2)Calculate the mean distance the invasion front travelled each year between 1935 and 2025. Give your answer to 2 significant figures.
(2)Explain how natural selection could have caused toads at the front of the invasion to have longer legs.
(3)Explain why the numbers of northern quolls have fallen in areas the toads have reached.
(2)Suggest why training young quolls in this way could help to protect the quoll population.
(2)Total for this question: 12 marks
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