The natural world often presents a slow, observable transition in plant communities, moving from simple organisms like moss and lichen to more complex forms such as grasses. This ecological change is frequently seen on bare surfaces, where a seemingly stable patch of lichens or moss gradually gives way to a dense carpet of grass. The phenomenon is not a sudden displacement but a progressive environmental modification driven by the initial colonists themselves. Understanding why grasses eventually dominate these pioneer species involves examining the unique survival tactics of the early organisms and the superior adaptations of the grasses that follow.
The Pioneer Stage: Lichen and Moss
Lichens and mosses are the first organisms to colonize harsh, sterile environments like bare rock or freshly exposed soil due to their unique physiological tolerances. These organisms are adapted to survive extreme fluctuations in environmental conditions, notably prolonged periods of desiccation. Lichens, a symbiotic association of a fungus and an alga or cyanobacterium, can dry out completely and rapidly resume metabolic activity upon rehydration, a trait known as reviviscence.
Mosses are non-vascular plants that also possess a high tolerance for drying out and do not require established soil. They attach to surfaces using simple, root-like structures called rhizoids, which function primarily in attachment rather than nutrient and water absorption. Both pioneers thrive on minimal nutrient input, drawing sustenance from rainwater, atmospheric dust, and the substrate itself. Their ability to exist where no other plant life can makes them the necessary starting point for future ecosystem development.
Substrate Transformation and Soil Development
The existence of lichens and mosses initiates a long-term process of substrate transformation, fundamentally altering the environment they inhabit. Lichens begin breaking down rock through both chemical and physical means. Chemically, they secrete organic acids, such as oxalic acid, which dissolve minerals and chelate metallic cations from the rock surface.
Physically, the expansion and contraction of their tissues with changes in moisture and temperature create mechanical stress that fractures the rock. As lichens and mosses live and die, their organic matter accumulates, mixing with weathered rock fragments to form a primitive, nutrient-poor soil layer called humus. Mosses enhance this process by trapping windblown particles and retaining moisture, significantly increasing the water-holding capacity of the developing substrate. This newly formed layer provides the necessary foundation for the next stage of colonization.
The Competitive Advantage of Grasses
The rudimentary soil created by the pioneers ultimately becomes a liability for them, as it allows for the establishment of grasses, which possess superior competitive adaptations. Grasses are vascular plants with specialized tissues for transporting water and nutrients, allowing for much faster growth rates compared to the slow-growing lichens. They develop deep, dense, fibrous root systems that efficiently anchor the plant and access water and nutrients stored in the newly developed soil layer.
These true roots are structurally and functionally different from the mosses’ simple rhizoids, giving grasses a significant advantage in nutrient and water uptake. Furthermore, the greater height and faster growth of grasses allow them to form a canopy that shades out the smaller, light-dependent mosses and lichens. This shading reduces the pioneers’ ability to photosynthesize, effectively starving them of the energy needed for growth and survival, leading to their eventual displacement from the transformed substrate.
Ecological Succession: The Broader Context
The transition from lichens and mosses to grasses is a predictable step within the larger framework of primary ecological succession. This process describes the sequence of communities that colonize and change a newly formed habitat that initially lacks soil. The shift occurs because each community modifies the environment, making it more suitable for the next, more complex group of organisms.
The initial period of lichen and moss colonization is slow, often taking hundreds or even thousands of years to create enough soil to support larger plants. Once grasses and other herbaceous plants become established, they represent the herbaceous stage of succession, stabilizing the soil and adding significant amounts of organic matter. This sequence does not stop with grasses; they are ultimately followed by small shrubs and then trees, demonstrating the continuous, dynamic nature of ecosystem change toward a stable, mature community.