Final answer:
Species richness is highest in the equatorial region due to a combination of the geographic area hypothesis, productivity hypothesis, and the mid-domain effect hypothesis, all contributing to increased biodiversity and speciation in the tropics.
Step-by-step explanation:
Species richness is predicted to be highest in the equatorial region by d. all of the above, which includes the geographic area hypothesis, productivity hypothesis, and the mid-domain effect hypothesis. Multiple factors likely contribute to the increase in biodiversity closer to the equator. These include the greater age of ecosystems in the tropics, which allows for more time for speciation; the increased energy the tropics receive from the sun, which may lead to greater ecosystem complexity and habitat heterogeneity; and the relative stability of the tropical climate, which may promote speciation. Overall, all levels of biodiversity are greatest in the tropics due to these and potentially other yet undiscovered reasons.