Pioneer species

The term pioneer is used to describe the species that first colonize new habitats created by disturbance. Although the term is usually applied to plants, microbial and invertebrate pioneer species are also sometimes recognized. For terrestrial habitats two groups of pioneers can be distinguished: those that colonize sites lacking developed organic soil and initiate primary succession and those that initiate secondary succession, often via recruitment from propagules in the soil. Pioneers of primary succession must cope with unfavorable conditions for establishment, and their growth and distribution is often restricted by available nutrient supply as a consequence of limited soil development. In contrast, pioneers of secondary successions often encounter resource-rich environments where competition with existing vegetation is reduced. Some of the fastest-growing plant species are pioneer herbs and shrubs, many of which have become important weeds in agricultural systems. Tropical forests also support pioneer tree species that regenerate in canopy gaps created by treefalls. Some of these pioneers are capable of growing to forest canopy within a decade and several are important timber species. The distinction between pioneer and nonpioneer life histories is difficult to delineate as traits such as growth or mortality rate, or shade-tolerance vary continuously among species. Nonetheless, a life-history tradeoff determined by growth in high light and survival in the shade limits the habitat range of pioneers of secondary successions and prevents populations from persisting long after the disturbance has passed.