Key points are not available for this paper at this time.
The marked gradient in species diversity between the equator (where it is highest) and the poles (where it is lowest) suggests a simple environmental explanation for this phenomenon. However, as Willis and Whittaker point out in their Perspective, the environmental variables driving species diversity are different depending on the size and time scale over which species diversity is measured.
Willis et al. (Fri,) studied this question.