What changes and what food chains? Your statement is just false. We don't live off the wild. We're long past that point, and the developing world is increasingly past that point.
I see your point. If we can survive on Mars without bringing along a huge amount of biodiversity then we can survive on Earth after turning it into Mars. Awesome. But maybe we can't survive on Mars after all.
They also cause further changes to the ecosystem in weird complex feedback loops, which will also kill us off.
They'll also collapse some food chains which will kill off many humans.