Practice question

Primary succession on bare rock begins where soil, organic matter, and an established biological community are absent. Lichens are effective pioneers because the partnership between a fungus and a photosynthetic alga or cyanobacterium can survive desiccation, temperature extremes, and low nutrient supply. The fungal component anchors to rock, retains water, and releases organic acids, while the photosynthetic partner supplies carbohydrates; cyanobacterial partners may also fix nitrogen. Physical and chemical weathering loosen mineral particles, and dead lichen material contributes organic matter. Over time, a thin soil develops that can retain more water and nutrients, permitting mosses, herbs, shrubs, and eventually trees where climate allows. Lichens are not always the very first microorganisms present, and succession does not follow one fixed sequence, but they are the standard visible pioneer group in a lithosere. Herbs, bryophytes, and shrubs generally need more developed substrate. Their later establishment depends on the habitat modification initiated by weathering, dust capture, and organic accumulation on the previously bare surface.