It is this last cold period that sets up the present day ecosystems that we are familiar with. We can look at the biomes of the world in the present day and see them as the product, either on a global scale or on a continental scale, of a long series of antecedent processes. They are the products of historical accident. Every one of these antecedent processes ultimately tracks its way back to plants - their evolution and their response to climatic change. Plants are the things that we define biomes by.
Plants are what animals live with.
If we look at the total history of land plants from the Silurian and Devonian to the present day, we find that we live in the most diverse flora that has ever existed. It has been assembled over a long period of time, through successive experiments. If we look at tetrapods on the one hand and insects on the other in terms of the history of their diversity we see that the present day is also the most diverse. Through time, ecosystems have been becoming increasingly more complex . All of this has occasionally been overridden by extinction.