Answered By: Steven Profit Last Updated: Jul 16, 2018 Views: 750
Here is a definition from the Oxford Dictionary of Environment and Conservation:
Species of plants, lichens, and micro‐organisms that dominate the vegetation community in the early stages of succession, because they can colonize bare sites.
Here is the beginning of the topic review from the Encyclopedia of Ecology:
In early ecological literature, the term was used to describe those plant that initiate community development on bare substrate (primary succession). More recently, usage of the term has included microbial and invertebrate taxa, and describes the first colonists of sites affected by less extreme disturbance which undergo secondary succession.