This is because New Zealand was not as self-sufficient as the other islands, especially the northern North Island. Kumara (or sweet potato) was the only crop from the Polynesian agricultural package that could grow in the cold New Zealand climate (from the top of the North Island to Banks Peninsula), though the cultivation of other crops was also attempted. The prehistoric distribution of the various species of moa, an important food in pre-Classical Maori times, was from the bottom of the South Island to around Wellington Region. Thus, people living north of Wellington would probably want food, as well as possibly tools in exchange for greenstone and obsidian. However, this also predicates on good food storage and/or preservation, which may or may not have occurred.