|Ecologists and biogeographers have devised many schemes that help us understand the general, broad-scale patterns of animal and plant distributions. These schemes include life zones, ecoregions, and biotic provinces.
Perhaps the most useful scheme for describing butterfly distributions is that of the life zones, devised by C. H. Merriam. The seven life zones are based on elevational and latitudinal changes in plant communities that reflect the physiological tolerance of different plants to changes in seasonal temperatures and length of growing season.
In the tropics, temperatures almost never fall below freezing, and there is a year-round growing season. As one travels further north or higher in elevation the average temperature decreases and the growing season, the number of days between the last killing spring frost and the first fall or winter killing frost shortens.