What is one way in which a field investigation can differ from a classroom or laboratory experiment? In field investigations, you are always working with microscopic organisms.
In field investigations, it is much easier to separate your control and experimental groups.
In field investigations, you are usually working with much larger animals than in the lab.
In field investigations, it is much more difficult to separate your control and experimental groups.

lichens that colonize bare rock secrete acids that break down the rock and start the soil-production process. so they expose rocks

the right option is; decreased genetic variation  

decreased genetic variation describes the effect of this environmental change.

genetic variation is the naturally occurring genetic diversities or differences between organisms of the same species in a population. genetic variation allows the survival and adaptability of a species when they encounter changing environmental conditions. when an endangered species reduces, it loses genetic variation which may not be restored even if the species rebound and will only be recovered through the gradual collection of mutations over many years.