Where did the rats come from and why did they have such a negative effect on palm tree?

In one of the studies of about rats in Easter Island, it was believed that rats were the reason for the decrease of palm trees. 
According to a system framework study, there are relationships between the birth and death of people with the rats and the palm trees. It explains that peoples' population in Easter island has a relationship with rats and palm trees in the area. 
The number of palm trees decreases because of the increased number of rats that eats its fruits. Seed germination became frequently interfered because of this activity. 
However, the slow growth of palm trees was also blamed for the tree cutting off the people.