https://phys.org/news/2018-05-ants-clues-biodiversity-higher-tropics.html

Yet a third points out that the colder ecosystems of earth are younger than their equatorial equivalents. During a period of rapid global cooling called the Eocene-Oligocene boundary, which occurred 34 million years ago, the planet's tropical habitats shrank dramatically toward the equator, while ice sheets grew at the poles. These newer, colder habitats have simply not existed long enough for as many species to accumulate as they have in the tropics, some scientists say.

Attempts to study this puzzle have involved comparing the number of species in several plant and animal groups, such as trees, birds and mammals, across distinct regions of the planet. But tallying species from Borneo to Belgium has yet to deliver concrete answers about why regional differences in diversity exist at all.