Latitudinal region
We recorded the geographic location of each study to create an
additional categorical variable for latitudinal region. Species were
coded as tropical, sub-tropical, or temperate. Tropical regions included
sites between the tropics of Cancer and Capricorn (23.5° north and south
of the equator, respectively), which are characterized by relatively low
variation in daylight and temperature throughout the year, but with
large environmental heterogeneity over short distances. Sub-tropical
regions included latitudes from 23.5° to 35° (north and south). These
regions have climates similar to the tropics, but with more seasonal
fluctuations. Temperate regions included latitudes greater than 35°
north and south. These zones are characterized by a wide range of
temperatures throughout the year, and by clearly marked seasonal
changes.