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

The best example of a pioneer species that would be found where a glacier has retreated is moss or lichen. These species can survive in harsh, barren environments and help to break down rock and establish soil, paving the way for other plants to colonize the area. Their ability to grow in nutrient-poor soils and harsh conditions makes them well-suited for colonization after the retreat of a glacier.