Some conservationists blame the reductions in monarch overwintering numbers in Mexico on the loss of milkweeds in the Midwestern region of the United States. 167 million acres of monarch habitat has been lost since 1996.[131] These conservationists argue that the reduction in milkweed habitat in agricultural regions of North America is a major cause of the declines in the number of monarchs that reach Mexico.[132][133] However, other top researchers doubt this claim, because it is not consistent with data collected by several long-term butterfly monitoring programs in the United States.