There is greater

Greater biodiversity is found in the tropics and subtropics because of its latitude gradient which is located near to the equator.

Solar light falls more prominently near the equator region, contributing in higher primary productivity (due to photosynthesis).

Moreover, tropical regions have remained undisturbed for millions of years, they are comparatively more seasonal and more predictable then polar regions. All these factors helped the tropical and subtropical regions in niche specialisation.