Table 1. Summary of factors hypothesized to determine regional patterns of species richness. Although all of these factors were examined in this study, the focus was on the two that provide notably powerful statistical results: land cover variability and potential evapotranspiration.

Factor
Basis

Test variables (resolution)

Energy
(heat)
Warm climatic conditions are more easily tolerated physiologically
Potential evapotranspiration (30 min)

Maximum and minimum annual temperature (30 min)
Energy
(resources)
Greater resource availability increases number of species that may coexist
Net primary productivity (30 min)
Annual
climatic
stability
Reduced seasonality permits specialization
Difference between minimum and maximum annual temperature
(30 min)
Habitat
heterogeneity
Spatial heterogeneity in physical or climatic conditions provides more niches
Number of different Holdridge1 or Olson2 biomes per quadrat (30 min)

Relief or topographic variation
(5 min)

Land cover variability
(8-km AVHRR)
1R. Leemans. Global Data Sets Collected and Compiled by the Biosphere Project: Working Paper. (Laxenburg: International Institute for Applied Systems Analysis, 1990).
2J. S. Olson, J. Watts, and L. Allison. Carbon in Live Vegetation of Major World Ecosystems. (Oak Ridge: Oak Ridge National Laboratory, 1983).