All Things Nature describes pioneer species as hardy organisms that tend to be small, simple, and capable of producing their own food. Pioneer species include bacteria, fungi, lichens, algae and many types of herbaceous plants.
Environments that are colonized by pioneer species are too hostile for most large organisms. Events like volcanic eruptions, landslides, avalanches and forest fires create the types of environments where pioneer species can thrive. Over time, pioneer species alter their environment and create conditions that are less stressful for more complex successional species.
Lichens are often the first species to move into a highly disturbed and completely barren site. Lichens are capable of colonizing bare rocks and provide their own food through photosynthesis. Lichens, in conjunction with geological processes like weathering, break rocks down into small particles. Eventually, these particles form soil that supports other pioneer species, including bacteria, fungi, and simple plants like mosses and ferns.
Pioneer species provide a host of ecological benefits, such as protection from the elements, increased water retention and abundant food sources. As the site's conditions continue to improve, the species composition gradually shifts and pioneer species die back. The loss of pioneer species allows primary successional species, including grasses, insects, birds and small mammals, to move into the site.