<P> In Mesoamerica, the Aztecs were active farmers and had an agriculturally focused economy . The land around Lake Texcoco was fertile, but not large enough to produce the amount of food needed for the population of their expanding empire . The Aztecs developed irrigation systems, formed terraced hillsides, fertilized their soil, and developed chinampas or artificial islands, also known as "floating gardens". The Mayas between 400 BC to 900 AD used extensive canal and raised field systems to farm swampland on the Yucatán Peninsula . </P> <P> In the Andes region of South America, with civilizations including the Inca, the major crop was the potato, domesticated approximately 7,000--10,000 years ago . Coca, still a major crop to this day, was domesticated in the Andes, as were the peanut, tomato, tobacco, and pineapple . Cotton was domesticated in Peru by 3,600 BC . Animals were also domesticated, including llamas, alpacas, and guinea pigs . </P> <P> The indigenous people of the Eastern U.S.A. domesticated numerous crops . Sunflowers, tobacco, varieties of squash and Chenopodium, as well as crops no longer grown, including marsh elder and little barley, were domesticated . Wild foods including wild rice and maple sugar were harvested . The most common varieties of strawberry were domesticated from Eastern North America . Two major crops, pecans and Concord grapes, were utilized extensively in prehistoric times but do not appear to have been domesticated until the 19th century . </P> <P> The indigenous people in what is now California and the Pacific Northwest practiced various forms of forest gardening and fire - stick farming in the forests, grasslands, mixed woodlands, and wetlands, ensuring that desired food and medicine plants continued to be available . The natives controlled fire on a regional scale to create a low - intensity fire ecology which prevented larger, catastrophic fires and sustained a low - density agriculture in loose rotation; a sort of "wild" permaculture . </P>

What factor best explains the development of different native american communities in the new world