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Article: Today, many species of animals and plants are endangered. This means they are in danger of becoming extinct and living on only in the pages of history books. The famous dodo is a classic example of a creature that became extinct. A flightless bird that lived on the island of Mauritius, it was discovered by sailors in 1598 but was hunted to extinction by 1681. Hunting has caused the Bengal tiger and the African elephant to be endangered today but habitat destruction can also lead to extinction. This is equally true for plants. Animals and plants disappear for other reasons too, but the main cause is often a disruption(,) in the natural food chain, whether due to hunting, habitat destruction, or even the introduction of alien species. The natural food chain is the cycle that governs the existence of all life on this planet. It is a carefully balanced cycle and any imbalance that occurs can cause knock-on effects that have serious consequences. At the beginning of the natural food chain are plants which turn sunlight into energy and draw nutrients from the earth. Plants are called producers. After the producers come the consumers. There are three tiers of consumers. First are creatures such as plant-eating animals, fish and insects which feed off the producers. These animals that only eat plants are called herbivores. The second tier of consumers are carnivores - animals that live off other animals. The third tier of consumers eats both other animals and plants. These consumers, including most humans, are called omnivores. After animals and plants die, they become food for other smaller creatures, such as bacteria and some plants, such as fungi. As they feed, these creatures turn the dead bodies back into gases and minerals which are again food for the producers at the beginning of the food chain. And so the cycle continues. All of nature is connected and governed by hundreds of these delicate food chains and if a single plant in the chain cannot survive, then the insects that live off the plant start to die and the animals that eat the insects also start to die. When a food chain is disrupted, the consequences can be extremely serious. One estimate suggests that for each plant species that is lost, up to 30 animals and insects may also die out. One wonders how many species were affected by the extinction of the dodo? Humans can have disastrous effects on food chains. We've already mentioned hunting but now let's look at travel. When people first started to explore the world they took plant and animal species from their home countries and introduced them wherever they went. They didn't realize that by introducing alien species they were disrupting the natural food chains of the areas they discovered. Although there are strict rules in place today controlling the import and export of alien species, some places are still fighting the effects of aliens introduced hundreds of years ago. For example, Gough Island in the South Atlantic Ocean is a breeding ground for albatrosses that have been nesting there for centuries. But in the 19th century, mice from passing ships were brought to the island. Being a species alien to the island, they had no natural predators and have now grown to such a size that they are attacking and killing albatross chicks. If they are allowed to continue, they will wipe out the albatross population. With import laws and people's rising awareness of how humans affect the natural environment, hopefully we can learn to fit better into the natural food chains that govern our world. Otherwise we need to accept that the loss of any more plants and animals could eventually mean our own extinction.
Answer: Animals and plants become extinct because alien species are imported.
Which of the following sentence is TRUE according to the passage?