Crop Species Diversity Changes in the United States: 1978-2012

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 are collected every five years, we quantified crop species diversity from 1978 to 2012, for the contiguous US on a county level basis. We used Shannon diversity indices expressed as effective number of crop species (ENCS) to quantify crop diversity. 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. During the 34 years we considered in our analyses, both national and regional ENCS changed. Nationally, crop diversity was lower in 2012 than in 1978. However, our analyses also revealed interesting trends between and within different Resource Regions. Overall, the Heartland Resource Region had the lowest crop diversity whereas 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. Also, within regions there were differences between counties in crop diversity. Spatial autocorrelation revealed clustering of low and high ENCS and this trend became stronger over time. These results show that, nationally counties have been 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. The 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.

Fig 1. Farm Resource Regions of the US.

Adapted from USDA-ERS [11].

Fig 2. Mean effective number of crop species (ENCS) at the county level for each ERS Farm Resource Region (Eastern Uplands, Heartland, Southern Seaboard, Northern Crescent, Fruitful Rim, Northern Great Plains, Basin and Range, Prairie Gateway, and Mississippi Portal) and on a national basis (National).

The ENCS was calculated from US Census of Agriculture data which was collected every five years from 1978 to 2012. Mean ENCS values are denoted by solid black circles, with error bars representing standard errors of the mean. Within each individual ERS Farm Resource Region and also at the national level, mean ENCS values for each census year labeled with different lowercase letters were significantly different according to Tukey’s HSD test (P<0.05).

Fig 3. Crop species diversity as effective number of species in 1978, 1987, 1997 and 2012 on a county level basis for the contiguous US.

The hotter colors (red hues) indicate lower ENCS values (low crop diversity) while colder colors (blue hues) indicate higher ENCS values (high crop diversity). Maps showing crop diversity for all Census years are available in S1 Fig.

Fig 4. Measures of geospatial clustering, Moran’s I and High-Low Cluster Ratio over the period of analysis.

Moran’s I value of 1.0 indicates very strong clustering of low or high ENCS values (Moran’s I = -1.0 indicates random occurrence). HL Cluster Ratio, derived from H/L Clustering analysis, indicates the number of clustered counties with high ENCS in relation to the number of clustered counties with low ENCS values. An HL cluster ratio of 1.0 indicates an equal number of clustered counties with high and low ENCS values, and a ratio approaching zero indicates a relatively large number of clustered counties with low ENCS than those with high ENCS.

Fig 5. Crop species diversity hot spots based on the geostatistical analysis of the ENCS values for 1978, 1987, 1997 and 2012.

Hot (red hues) spots are areas with significant clustering of counties with low ENCS values (low cropping diversity) and cold (blue hues) spots are clustering of counties with high ENCS values (high cropping diversity). Maps showing hot spot analysis for all Census years are available in S2 Fig.

Fig 6. Crop Species Diversity and Dominant Crops in North Dakota.

Solid colors indicate crop species diversity and federal lands while the overlayed pattern indicate the dominant crop for that county. The hotter colors (red hues) indicated lower ENCS values (low crop diversity) while colder colors (blue hues) indicate higher ENCS values (high crop diversity).