. 2014 Nov 10;9(11):e112764. doi: 10.1371/journal.pone.0112764

Table 1. Lists of the 26 environment variables.

Variable	Symbol
Annual mean temperature (°C)^a ^, ^b	Bio1
Mean diurnal range (Mean of monthly (max temp - min temp)) (°C)^a ^, ^b	Bio2
Isothermality (×100)^b	Bio3
Temperature seasonality (standard deviation×100) (°C)^a ^, ^b	Bio4
Max temperature of warmest month (°C)^b	Bio5
Min temperature of coldest month (°C)^b	Bio6
Temperature annual range (°C)^b	Bio7
Mean temperature of wettest quarter (°C)^a ^, ^b	Bio8
Mean temperature of driest quarter (°C)^b	Bio9
Mean temperature of warmest quarter (°C)^b	Bio10
Mean temperature of coldest of quarter (°C)^b	Bio11
Annual precipitation (mm)^a ^, ^b	Bio12
Precipitation of wettest month (mm)^b	Bio13
Precipitation of driest month (mm)^b	Bio14
Precipitation seasonality (coefficient of variation) (mm)^a ^, ^b	Bio15
Precipitation of wettest quarter (mm)^b	Bio16
Precipitation of driest quarter (mm)^b	Bio17
Precipitation of warmest quarter (mm)^b	Bio18
Precipitation of coldest quarter (mm)^a ^, ^b	Bio19
Human footprint^a ^, ^c	HF
Human influence index^c	HII
Human population density in year 2000 (persons/km²)^a ^, ^c	HPD
Soil organic carbon density (kg/m² at 1 m depth)^a ^, ^d	SOC
Soil pH value^a ^, ^d	SPH
Soil moisture index^a ^, ^d	SMI
Altitude (m)^a ^, ^b	ALT

Variables used in modeling.

See http://www.worldclim.org/.

See http://sedac.ciesin.columbia.edu/.

See http://www.sage.wisc.edu/atlas/maps.php.

Human footprint (HF) is based on the premise that the impact of human influence varies by biogeography and HF expresses as a percentage the relative human influence in every biome on the land’s surface.

Human influence index (HII) is a measure showing direct human influence on ecosystems using eight measures of human presence (population density/km², score of railroads, score of major roads, score of navigable rivers, score of coastlines, score of nighttime stable lights values, urban polygons, and land cover categories).

Soil moisture index (SMI) reflects the ability of soil to supply moisture to plants and SMI can identify a quick onset of drought by demonstrating the observed dryness of a soil relative to the plant’s ability to extract water as scaled over the range from field capacity to wilting point.