The species diversity decreases from lower to higher altitudes on a mountain. This is due to:

Biodiversity varies with a change in altitude or latitude. The diversity increases as we move from high to low altitudes (i.e., from poles to equator). In the temperate region, the climate is severe with the short growing period for plants while in tropical rain forests the conditions are favourable for growth throughout the year. This makes it possible for a large number of species to occur and grow. There is a decrease in species diversity from lower to higher altitudes on a mountain. Drop in temperature and greater seasonal variability at higher altitudes are a major factor that reduces diversity. So, the correct option is D.