It was isolated from the Palaearctic by the Himalayan Mountains. The Indomalayan realm was previously separated by a sea lane that has subsequently been closed by continental drift. The Antarctic realm exhibits a diverse set of ecosystems from temperate forest and grassland in New Zealand to tundra and ice sheets in Antarctica. Many of New Zealand's mammals are like those frequenting Antarctic shores. Finally the Oceanian realm is dominated by tropical forests.
Physical barrier are often the most imposing barriers to diffusion.
Impassable topography, large water bodies, and unsuitable climates are all impediments for the dispersal of plant and and species. The effectiveness of an organism to occupy a habitat depends in part on its means of transportation. Animals must use their own locomotion, while plants disperse by wind, running water, ocean currents, and animals. Thus, climate and topographic barriers are more of an impediment to animals than plants. For either, continental drift poses a significant barrier to diffusion.