Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions. Scientists have identified two ways in which species disappear. The first is through ordinary or "background" extinctions, where species that fail to adapt are slowly replaced by more adaptable life forms. The second is when large numbers of species go to the wall in relatively short periods of biological time. There have been five such extinctions, ...

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Scientists have identified two ways in which species disappear. The first is through ordinary or "background" extinctions, where species that fail to adapt are slowly replaced by more adaptable life forms. The second is when large numbers of species go to the wall in relatively short periods of biological time. There have been five such extinctions, each provoked by cataclysmic evolutionary events caused by some geological eruption, climate shift, or space junk slamming into the Earth. Scientists now believe that another mass extinction of species is currently under way - and this time human fingerprints are on the trigger.

How are we doing it? Simply by demanding more and more space for ourselves. In our assault on the ecosystems around us we have used a number of tools, from spear and gun to bulldozer and chainsaw. Certain especially rich ecosystems have proved the most vulnerable. In Hawaii more than half of the native birds are now gone - about 50 species. Such carnage has taken place all across the island communities of the Pacific and Indian oceans. While many species were hunted to extinction, others simply succumbed to the introduced predators" that humans brought with them: the cat, the dog, the pig, and the rat.

Today the tempo of extinction is picking up speed. Hunting is no longer the major culprit, although rare birds and animals continue to be butchered for their skin, feathers, tusks, and internal organs, or taken as savage pets. Today the main threat comes from the destruction of the habitat of wild plants, animals, and insects need to survive. The draining and damming of wetland and river courses threatens the aquatic food chain and our own seafood industry. Overfishing and the destruction of fragile coral reefs destroy ocean biodiversity. Deforestation is taking a staggering toll, particularly in the tropics where the most global biodiversity is at risk. The shrinking rainforest cover of the Congo and Amazon River basins and such place as Borneo and Madagascar have a wealth of species per hectare existing nowhere else. As those precious hectares are drowned or turned into arid pasture and cropland, such species disappear forever.

Source: Final Countdown Practice Tests by D.F Piniaris, Heinle Cengage Learning, 2010

The word "them" in paragraph 2 refers to _______.