When certain exotic species are introduced into a new geographical area, they often become invasive and spread fast. Which of the following explains this?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The invaded land does not have their natural predators.
Predators keep prey populations under control. Without predators, a prey species could reach very high numbers and upset the ecosystem. An exotic species is one brought into a new area from somewhere else, and it leaves its own predators behind in its home range.
Option (1) They become resistant to the pesticides used in the new area. — Incorrect. Pesticide resistance builds up over many generations in a population exposed to pesticides, through natural selection. It does not explain why an exotic species spreads fast soon after it arrives in a new area.
Option (2) The invaded land does not have their natural predators. — Correct. Nothing in the new land eats the exotic species, so its numbers are not kept in check and it spreads fast. The prickly pear cactus spread over millions of hectares in Australia. It was controlled only when a moth that feeds on the cactus was brought from its natural habitat.
Option (3) They form mutualistic associations with the native plants of the area. — Incorrect. Mutualism is an interaction in which both species gain, as in lichens or mycorrhizae. The book does not link the spread of exotic species to such partnerships; the cause it gives is the missing predators.
Option (4) The native predators of the area avoid them because they are distasteful. — Incorrect. Being distasteful is a defence some prey use against predators, as the monarch butterfly does with a chemical in its body. The book gives a different cause for invasion: the predators that kept the species in check at home are absent in the new land.
NCERT Extract:
Population Interactions – “… When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators. The prickly pear cactus introduced into Australia in the early 1920’s caused havoc by spreading rapidly into millions of hectares of rangeland. Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country.”