Which is the best example of a pioneer species that would be found where a glacier has retreated?

Algae is a prime example of a pioneer species that would be found where a glacier has retreated, because the glacial retreat allows for its growth. This process is called succession and usually starts with lichen, an relative of algae. Lichen grows on bare rock which is left behind. Afterwards, acids produced in this environment cause dirt and soil to be created.