Table 1.
Independent variables used in stepwise regression analyses of species richness
| Independent variables | Mnemonic | Hypothesis |
|---|---|---|
| Precipitation (mm/yr−1) | PREC | A,B |
| Wet-day frequency (days) | WET | A,B |
| Mean annual temperature (C°) | Tmean | A,B |
| Mean daily maximum temperature (C°) | Tmax | A,B |
| Mean daily minimum temperature (C°) | Tmin | A,B |
| Mean daily temperature range (C°) | Trange | A,B |
| Mean vapor pressure (hPa) | VAPOR | A,B |
| Radiation (W/m2) | RAD | A,B |
| Cloud cover (Percent) | CLOUD | A,B |
| Frost frequency (days) | FROST | A,B |
| Mean wind speed (m/second) | WIND | A,B |
| Latitude | LAT | A,B,C |
| Quadrat area (km2) | AREA | A,B,C,D |
| Ecosystem diversity (number of ecosystems in quadrat) | ECO | A,C |
| Topographic relief (elevational range, m) | TOPO | A,D |
| Latitude × topographic relief | L × T | A,D |
Letters under hypothesis refer to regression models to which variables have been assigned: (A) ad hoc model; (B) contemporary climate model (4–6, 13, 28–30); (C) ecosystem diversity model (3, 5); and (D) topography × latitude model (14–16, 31). All variables are correlated significantly with avian species richness (P < 0.001, Bonferroni-adjusted for simultaneously testing) at the 1° × 1° latitude-longitude resolution (n = 1,679 quadrats).