Practice question

The facilitation model proposes that early colonists make an initially harsh environment more suitable for species that arrive later. Pioneers may stabilize substrate, weather rock, add organic matter or nitrogen, retain water, moderate temperature, reduce wind, or provide shade. These changes allow successors to germinate and survive where they previously could not. Later species may then outcompete, overtop, or otherwise replace the pioneers, so helping successors does not imply that early species persist indefinitely. This differs from tolerance, in which later species can establish without environmental improvement by residents and eventually dominate because of superior competitive or longevity traits. It also differs from inhibition, in which current occupants suppress newcomers through shading, allelopathy, territorial occupation, or resource capture until damage or death opens space. Equal colonizing ability is not required by facilitation. The central tested mechanism is a positive effect of one successional stage on establishment of the next. Classic examples include nitrogen-fixing shrubs enriching young substrates and nurse plants protecting seedlings from heat and drought.