Which process is likely guiding primary succession in an ecosystem where,over a period of years,bare rock was colonized by lichens that gave way to moss that in turn gave way to ferns? 

A)  tolerance 
B)  facilitation 
C)  inhibition 
D)  interspecific competition

The process guiding primary succession in this ecosystem is likely facilitation, where the presence of one pioneer species (in this case lichens) creates conditions that make it easier for the next stage of species (moss) to establish and thrive, and so on. The lichens probably contributed to soil formation and made the bare rock surface less inhospitable for moss growth, which in turn facilitated fern establishment. Tolerance would refer to species that are able to establish and persist in a given habitat due to the absence of other more competitive species, which is not the case in this scenario. Inhibition and interspecific competition are not likely to be the main drivers of this succession, as neither the lichens, mosses, nor ferns are actively preventing the establishment of other species.