1. **Land bridges**: during the ice ages, sea levels were lower because so much of the Earth’s water was frozen as ice, and so many places that are islands now were actually connected to continents as peninsulas. When the ice melted, sea levels rose, and turned the peninsulas into islands.
2. **Rafting**: storms tear apart trees and create natural rafts of tangled branches and vines that float down rivers and out to sea. Animals trapped on these rafts get carried by the ocean currents to islands.
3. **Swimming**: Some animals can swim really well.
4. **Humans**: Humans with boats carried pigs (deliberately) and rats (accidentally) with them to islands they settled.
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