Final answer:
Temperate and polar regions have less biodiversity than tropical regions due to the greater age, stability, and complex ecological niches of the tropics, which promote speciation and have more opportunities for species diversification.
Step-by-step explanation:
The factor explaining why in general, temperate and polar regions have less biodiversity than tropical regions is related to the ecological histories of these regions and climate. The temperate regions were largely devoid of life or drastically impoverished during the last ice age, reducing opportunities for speciation, or the evolutionary process of creating new species. In contrast, the tropical regions are older ecosystems that have had more stable climates and more time for species to evolve and fill a multitude of ecological niches. Enhanced solar energy in the tropics compared to the polar regions fuels plant growth and supports more complex food webs.
Tropical ecosystems also tend to have higher levels of primary productivity and heterogeneity, allowing for greater opportunities for coevolution and specialization. This greater complexity of tropical ecosystems can promote speciation and lead to a higher number of species.
The higher stability of tropical climates compared to temperate regions, with less pronounced seasonality, might promote speciation into highly specialized niches, contributing to the high levels of biodiversity observed in the tropics, including a larger number of endemic species and biodiversity "hotspots."