Crop species diversity changes in the United States: 1978-2012

Authors: AQUILAR, JONATHAN - Kansas State University; GRAMIG, GRETA - North Dakota State University; Hendrickson, John; Archer, David; Forcella, Frank; Liebig, Mark

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2015
Publication Date: 8/26/2015
Publication URL: http://handle.nal.usda.gov/10113/61321
Citation: Aquilar, J., Gramig, G.G., Hendrickson, J.R., Archer, D.W., Forcella, F., Liebig, M.A. 2015. Crop species diversity changes in the United States: 1978-2012. PLoS One. 10(8):e0136580.

Interpretive Summary: Anecdotal evidence has suggested that crop diversity has declined in the US over time but there have not been any published studies that specifically looked at changes in crop diversity at the national level. We used USDA-National Agricultural Statistics Service, Census of Agriculture data collected every five years at a county level basis to evaluate changes in crop diversity. Besides county level changes, we also looked at changes both nationally and at the level of Farm Resource Regions. There are nine Farm Resource Regions in the US and these are grouping of counties with similar types of farms as well as similar soils, topography and climatic traits. We found that between 1978 and 2012, crop diversity has decreased at the national level; however, regionally and within regions, different patterns often emerged. Other analyses showed that nationally, counties were becoming grouped into clusters with either high diversity or low diversity with more counties shifting into the low diversity cluster. Lower crop diversity and agricultural system homogenization can have far-reaching impacts including lowering agricultural system resilience and the value of ecosystem services.

Technical Abstract: Anecdotal accounts regarding reduced US cropping system diversity have raised concerns about negative impacts of increasingly homogeneous cropping systems. However, formal analyses to document such changes are lacking. Using US Agriculture Census data, which is collected on 5-year intervals, we quantified crop species diversity from 1978 to 2012, for the contiguous US on a county level basis. We then evaluated changes in county level crop diversity both nationally and for each of the eight Farm Resource Regions developed by the National Agriculture Statistics Service. We used Shannon diversity index values expressed as effective number of crop species (ENCS) to determine crop diversity. Over the 34 years, both national and regional ENCS changed. Nationally, crop diversity was lower in 2012 than in 1978. However, the analysis also revealed interesting trends between and within different Resource Regions. The Heartland Resource Region was generally had the lowest crop diversity while the Fruitful Rim and Northern Crescent had the highest. In contrast to the other Resource Regions, the Mississippi Portal had significantly higher crop diversity in 2012 than in 1978. There were also differences within regions with some counties having greater diversity than others within a region. Spatial autocorrelation revealed clustering of low and high ENCS and this trend became stronger over time. This suggests, that nationally, counties are clustering into areas of either low diversity or high diversity. Moreover, a significant trend of more counties shifting to lower rather than to higher crop diversity was detected. This clustering and shifting demonstrates a trend toward crop diversity loss and attendant homogenization of agricultural production systems, which could have far-reaching consequences for provision of ecosystem system services associated with agricultural systems as well as food system sustainability.