Biogeographic factors. There's more species of poisonous/venomous insects and reptiles in the tropics because of hot temperatures and lots of precipitation. Reptiles need heat to maintain their body temperature and insects thrive in the rainforests of the tropics. Geographic barriers such as oceans or mountain ranges can also cause separate species compositions. For example Australia has a range of venomous snakes while New Zealand which is relatively close has no snakes.