The introduced placental mammals outcompeted native marsupials, some of which which became extinct. On islands, such as in Hawaii, we can get a clue as to why island species are so vulnerable to extinction by competition from non-native species . Many island plants have lost competitiveness. The Hawaiian flora has very few poisonous plants. When animals such as goats and pigs are introduced by humans, the they found the plants to be very tasty and plant populations were decimated.
Goats are a problem on many Caribbean islands, where they eat native plants.
The human species has overexploited resources . Overexplotation is not limited to modern civilization. Humans arrived 20,000 years ago in North America and 30,000 years ago in Australia. Polynesians arrived in Hawaii and caused the extinction of at least 39 species of birds. Colonization of New Zealand by Polynesians 1000 years ago caused the extinction of large flightless birds such as Moas.