Ecological succession describes the natural process of change in an ecosystem’s species and habitat over time. This gradual replacement of communities continues until a relatively stable state, or climax community, is reached, or until a disturbance occurs. Lichens and mosses are integral to this process, playing a foundational role in preparing environments for more complex life forms.
Lichens: The Initial Colonizers
Lichens are typically the first organisms to colonize environments previously devoid of life, a process known as primary succession. This occurs on newly exposed surfaces like bare rock, volcanic ash, or land uncovered by retreating glaciers. Lichens are a symbiotic partnership between a fungus and an alga or cyanobacterium, allowing them to thrive in harsh conditions where other species cannot. Lichens initiate soil formation by secreting organic acids that chemically break down rock surfaces. As lichens grow and eventually die, their decaying organic matter mixes with these rock fragments, forming a rudimentary layer of soil. This initial substrate provides the foundation for subsequent plant colonization.
Mosses: Developing the Soil and Environment
Mosses often appear in successional sequences after lichens have established a thin layer of soil, or they can be early colonizers in secondary succession where some soil already exists. They contribute significantly to the development of soil structure and environmental conditions. Mosses enhance the accumulation of organic matter, which increases the depth and fertility of the developing soil. Mosses can retain large amounts of water, sometimes holding 20 to 30 times their dry weight. This water-holding ability helps create a more humid and stable microclimate at the ground level, beneficial for the establishment of other plant life. Their dense mats also trap windblown dust, debris, and sediment, further building up the substrate. This continuous addition of organic material and fine particles, along with moisture regulation, transforms the nascent soil into a more hospitable environment for future plant communities.
The Cumulative Impact on Ecosystem Development
Lichens and mosses are important pioneer species, collectively paving the way for later successional stages. Lichens begin the process by breaking down barren rock and initiating soil particles, creating a minimal substrate. This initial modification allows mosses to establish, further developing the soil by adding organic matter and significantly improving moisture retention. These combined actions transform an inhospitable, bare surface into an environment capable of supporting more demanding plant communities. The enhanced soil depth and nutrient content, along with the moderated microclimate, enable the establishment of small herbaceous plants, followed by grasses and ferns. Over time, shrubs and eventually trees can colonize the area. Lichens and mosses act as stepping stones, modifying the physical and chemical properties of the environment, thereby facilitating the progression from a barren landscape to a complex, diverse ecosystem.
Observing Lichens and Mosses in Succession
The roles of lichens and mosses in ecological succession can be observed in various natural settings. Newly exposed rock faces, such as those found after a landslide or a retreating glacier, and lava flows, often display the initial stages of primary succession, with lichens appearing as colorful patches or colonizing fresh, barren rock. Areas recovering from disturbances, such as clear-cut forests or sites affected by wildfires, may exhibit secondary succession where mosses rapidly colonize the disturbed ground. Even in urban environments, one might observe lichens and mosses growing on old sidewalks, walls, or roofs, slowly breaking down surfaces and accumulating organic matter. Recognizing these small, resilient organisms helps appreciate their profound impact on transforming landscapes and fostering ecological development.