Population density Lowest limit of tolerance Highest limit

Population density Lowest limit of tolerance Highest limit of tolerance Physiological optimum Inability to survive Low tolerance Inability to survive Species absent Low population Species absent Physical gradient (e. g. , p. H)

Carbon dioxide concentration (ppm) 380 360 340 320 Mauna Loa Observatory, Hawaii

75° 60° 7 Latitude 30° 13 15° 4 17 6 16 3 5 45° 1 19 8 4 Equator 18 9 2 45° 30° 12 11 10 5 30° 60° 15 45° 0° 75° 14 3 20 21 15° 0° 15° 30° 45° 60°

Cold dry air falls Polar cell Moist air rises – rain Polar cap Arctic tundra Polar front Evergreen Ferrell coniferous forest Midlatitude Temperate deciduous westerlies forest and grassland Horse latitudes Subtropical high Desert pressure belt Northeast trades Tropical deciduous forest Doldrums Southeast trades ITCZ 0 Equator Horse latitudes Tropical rain forest Hadley cell Tropical deciduous forest 30 Subtropical high pressure belt Desert Midlatitude westerlies Temperate deciduous forest and grassland 60 Polar front Polar cap Moist air rise – rain Cold dry air falls Polar cell

40 Clouds and rainfall keep temperature uniform across the tropics Mean annual temperature °C 30 20 10 0 -10 -20 0 10 20 30 40 Degrees latitude 50 60 70 80

25 Number of buffalo per km 2 20 15 10 5 500 1000 Rainfall (mm) 1500 2000

10 20 0 0 30 40 40 10 40 30 60 30 80 30 100 20 140 160 180

Species abundance (1) (2) (3) (4) Environmental gradient

Change from 1940 temperature (°C) 0. 4 0. 2 0. 0 -0. 2 -0. 4 -0. 6 1870 1890 1910 1930 Year 1950 1970 1990