Welcome to the Homogenocene, where ecosystems across the world are irreversibly mixed. All those species that depended on isolation? Sorry Charlie.
I was watching Star Wars recently and a little voice in the back of my head kept freaking out about the lack of biosecurity -- "sure let's just tromp directly from this planet to another one without even changing our shoes!"
I'm guessing by the time of these stories the ecosystems have already been hopelessly corrupted. This could be especially catastrophic when some alien biochemistry that uses (say) dioxins as building blocks gets introduced. It makes all the "let's blow up planets" more understandable, I suppose.