Species diversity decreases as we move towards the poles, because:
(i) Temperature decreases and conditions become harsh.
(ii) Both the amount and intensity of solar radiation decreases.
(iii) Vegetation decreases.
(iv) Less resources available to support species.
Speciation is generally a function of time and environmental stability, so if conditions are too harsh, it is difficult for the species to survive and adapt. This results in decrease in biodiversity towards the poles.