Ecological succession describes the natural process of change that occurs within an ecosystem over time. It involves the sequential replacement of one community of living organisms by another, following either the creation of new land or a disturbance to an existing area.
The Two Paths of Ecological Change
Ecological change often follows one of two distinct paths: primary succession or secondary succession. Primary succession begins in environments completely devoid of life and soil, such as newly formed volcanic rock, areas exposed by receding glaciers, or bare sand dunes. In these settings, the ground is essentially “virgin” or “bare,” requiring the slow establishment of basic life forms to initiate soil development. This process can span hundreds to thousands of years.
Secondary succession, in contrast, occurs in areas where a pre-existing community has been disrupted or removed, but the underlying soil remains intact. Common examples include forests recovering after a wildfire, abandoned agricultural fields, or areas cleared by logging. Because the soil, with its organic matter and seed bank, is already present, this type of succession proceeds more rapidly. Pioneer species in secondary succession can immediately utilize the existing resources, leading to quicker ecosystem recovery.
Lichen’s Role in Primary Succession
Lichens serve as examples of pioneer species in primary succession, playing a foundational role in initiating ecological development on barren surfaces. These organisms are a symbiotic partnership between a fungus and either an alga or a cyanobacterium. The fungus provides structure and protection, while the alga or cyanobacterium performs photosynthesis, producing energy for the partnership. This symbiotic relationship allows lichens to survive in harsh conditions, including nutrient-poor environments and exposed rock surfaces.
Their ability to colonize bare rock is important for establishing ecosystems. Lichens adhere directly to rock surfaces and, through a combination of chemical and physical processes, begin to break down the rock. They secrete organic acids, which chemically weather the rock by dissolving minerals. Additionally, their growth and decay contribute to the physical disintegration of rock particles. This breakdown, combined with trapped dust and decaying lichen matter, initiates the formation of a rudimentary layer of soil.
The nascent soil layer created by lichens provides a substrate that can retain moisture and nutrients. As this thin soil develops, it allows for the establishment of more complex plant life, which can further accumulate organic matter. Over extended periods, these early colonizers pave the way for grasses, then shrubs, and eventually larger woody plants like trees, leading to a more diverse and complex climax community.
Lichen’s Presence in Secondary Succession
While lichens are important in primary succession, their role in secondary succession is not as a primary pioneer species. In areas undergoing secondary succession, existing soil provides immediate opportunities for other, faster-growing plants to colonize. Grasses and herbaceous plants emerge first, rapidly covering disturbed ground due to the presence of an established seed bank and available nutrients in the soil. These plants quickly dominate the initial stages of recovery, outcompeting lichens for space and resources.
Lichens can be present within areas experiencing secondary succession, but they do not initiate the ecological recovery in the same way. They might colonize exposed surfaces such as dead wood, fallen logs, or rocks that become uncovered within a disturbed forest. Their presence on these substrates contributes to local biodiversity and nutrient cycling, but it does not drive the overall successional trajectory as it does on newly formed, barren land.