Data on the proportion of stock raised under different systems were found for 86 and 97
countries for chicken and pigs, respectively (S1 Fig).
The analyses also required three spatial databases. First, we used the most recent revisions
of the pig and chicken distribution maps from the gridded livestock of the world (GLW 2.0)
published by Robinson et al. [34] and aggregated these data at a spatial resolution of 5 minutes
of arc (0.083333 decimal degrees, or approximately 8 × 8 km at the equator) to obtain the total
number of chickens or pigs per pixel. This data set was also used to estimate the total stocks of
chickens and pigs per province in China. Second, the human population data layer was
obtained from the 2010 version of the LandScan dataset [39] adjusted with country values to
2010 United Nations (UN) estimates [6]. These data were aggregated at the same spatial resolution
of 5 minutes of arc to obtain the total human population per pixel and corrected to
match FAOSTAT 2010 country totals. Finally, in order to remain consistent with the GLW
2.0, the same masking criteria were used to exclude land pixels defined as unsuitable for
chicken and pig production, i.e. with elevations higher than 4,750 m above sea level, at a slope
of gradient higher than 40 percent, urban areas and pixels permanently covered with snow or
ice