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.