Diversity: A Key Element of Sustainable Agricultural Systems
Over the last five decades, Iowa's agricultural landscape has become markedly less diverse in both crop and non-crop vegetation. In 1950, corn and soybean were planted on 27% and 6%, respectively, of Iowa's farmland, but by 1994, corn and soybean occupied 39% and 27% of the state's farmland (Kanwar and Klonglan, 1998). During this same period the percentage of Iowa farmland planted with oat fell from 19 to 2%; hay acreage dropped from 11 to 5% (Kanwar and Klonglan, 1998). This type of reduction in crop diversity across the landscape is related to low diversity in individual fields over time. More than 90% of the >21 million acres planted with corn and soybean in Iowa in 1991 contained only those two crops in 1989 and 1990 (USDA-ERS, 1992). Intensification of crop production in Iowa through drainage and field enlargement has resulted in a reduction of wetland vegetation and tree cover along streams and former field borders (Schultz et a!., 1997).