Mapping recent temporal changes in crop species diversity for the 48 contiguous US states

Background/Question/Methods

During the mid- to late-twentieth century, agricultural intensification and increased specialization of crop production led to an increase of contiguous acres of crop monocultures and a decline in landscape-level diversity of cropping systems. Intensive cultivation of crop monocultures often eliminates remnant patches of non-cultivated land, which are critical habitat for wildlife. Large unbroken tracts of crop monocultures may be associated with rapid spread of crop diseases and insect pests, leading to increased pesticide usage and possible harmful environmental consequences. Recent trends in agricultural production practices, such as wide-scale shifts toward planting genetically-modified crop species or adoption of conservation tillage, may be further changing the distribution of crop species diversity across agricultural landscapes. Therefore, as a first step toward understanding recent changes in landscape-level patterns of crop diversity, 1982 and 2002 U.S. Census of Agriculture crop harvest and land use data were used to calculate crop diversity indices for every county in each of the contiguous 48 states. Crop diversity was defined as the effective number of species (ENS), calculated as the natural exponent of the Shannon diversity index. Ranges of ENS values were designated as extremely low (0-5 ENS), low (6 to 10 ENS), moderate (11 to 20 ENS), high (21 to 30 ENS), and extremely high (31+ ENS). ArcView was used to map the spatial distribution of ranges of ENS values. Census data were used to map the spatial distribution of U.S. counties characterized by intensive agricultural production (IAP), defined as counties with 25% or greater harvested area as a percentage of total land area. GIS analysis tools were used to overlay ENS distribution maps with IAP area distribution maps and to extract summary statistics describing the change (from 1982 to 2002) in ENS distribution within IAP areas. 

Results/Conclusions

Land areas designated as IAP decreased by 11% from 1982 to 2002. Areas that shifted from IAP to non-IAP were primarily located near the Atlantic coast in Virginia, North Carolina, South Carolina, and Georgia. IAP areas characterized as having extremely low, low, and moderate ENS decreased by 12%, 15%, and 13%, respectively, from 1982 to 2002. IAP areas characterized as having high and extremely high ENS increased by 19% and 25%, respectively, from 1982 to 2002. In general, increased ENS occurred in California, New York, parts of the Upper and Central Great Plains, and the Mississippi Delta, whereas most major decreases in ENS occurred in Iowa, Indiana, Illinois, and Ohio.