What are the main causes of low human population density in many Polynesian Islands?
Polynesian islands varied in a number of factors that supported the development of dense populations. Some islands had more rainfall, more sources of water, more access to shellfish and other seafood. Some had more stones for making tools, or more forests for supplying wood. Others, such as the Chathams and most of New Zealand, were cold, and not suited to traditional Polynesian agriculture. Still others had poor soils. Some of the islands amenable to extensive agriculture developed quite dense populations. On Kauai, Oahu, and Molokai, for example, enormous taro fields enabled a dense, complex society that also domesticated pigs. Even smaller islands like Easter, Anuta and Tonga could support dense populations due to natural advantages.
Source: Diamond, Guns, Germs and Steel, 59-61.