Pioneer species are the first species to colonize a particular barren land that has been existing before but distrupted by activity such as fire. Example are lichens and lime grass.
When pioneer species are introduced into a new niche it takes time for them to get adapted to their new environment. During those period of them trying to fit into the new niche some of this plants may be lost with time reducing the population. Also, pioneer plants are mostly photosynthetic when some factors that favors photosynthesis such as temperature, light, Co2 concentration are not available there would be reduction in the pioneer species.
Competition between the old and new species may reduce it population if the old plant out compete them.
