What is the role of a pioneer species in primary succession

What role do pioneer species play in succession?

Pioneer species are species which colonize previously uncolonized land, usually leading to ecological succession. They are the first organisms to start the chain of events leading to a livable biosphere or ecosystem. Since uncolonized land may have thin, poor quality soils with few nutrients, pioneer species are often hearty plants with adaptations such as long roots, root nodes containing nitrogen-fixing bacteria, and leaves that employ transpiration. Pioneer species will die creating plant litter, and break down as 'leaf mold' after some time, making new soil for secondary succession (see below), and nutrients for small fish and aquatic plants in adjacent bodies of water. 
Pioneer species can also be found in secondary succession, such as an established ecosystem being reduced by an event such as: a forest fire, deforestation, or clearing; quickly colonizing open spaces which previously supported vegetation