Ecological succession describes the process of change in the species structure of an ecological community over time. It represents a fundamental concept in ecology, illustrating how ecosystems develop or recover from various influences. This gradual process involves a sequence of changes in the types of plants and animals inhabiting an area, ultimately shaping the landscape.
Primary Succession Explained
Primary succession begins in environments that are newly formed or newly exposed, where no life, and crucially, no soil, previously existed. Examples include areas of bare rock created by cooling lava flows, land uncovered by retreating glaciers, or newly formed sand dunes.
The first species to arrive in these harsh conditions are called pioneer species. Lichens are common pioneer species in primary succession. Mosses and certain microorganisms like bacteria and fungi also play this role. These pioneer species can adhere to bare rock surfaces and, through physical and chemical weathering, begin to break down the rock. As these organisms grow, reproduce, and eventually die, their decaying organic matter mixes with the weathered rock particles, gradually forming a thin layer of soil.
This nascent soil allows for the establishment of small, hardy plants like grasses and ferns, which further contribute to soil development. Over extended periods, often hundreds to thousands of years, the soil deepens and accumulates more nutrients. This progression enables larger and more complex plant forms, such as shrubs and eventually trees, to colonize the area, leading to a more diverse and stable ecosystem. A well-known example is the island of Surtsey off Iceland, which formed from a volcanic eruption in 1963, where scientists observe primary succession in action.
Secondary Succession Explained
Secondary succession occurs in areas that previously supported life but have undergone a disturbance, leaving the soil largely intact. Such disturbances can include natural events like wildfires, floods, hurricanes, or human activities such as logging and agricultural abandonment. The presence of existing soil and often a seed bank makes this process faster than primary succession.
In these disturbed environments, the initial colonizers are fast-growing plants like annual weeds and grasses. These species quickly establish themselves, taking advantage of the already present soil and residual nutrients. For instance, after a forest fire, ash from burned vegetation can return nutrients to the ground, facilitating rapid regrowth. An abandoned farm field is another common example, where weeds and grasses quickly reclaim the land.
As these early successional species grow, they further modify the environment, creating conditions suitable for other plants. Shrubs and small trees often follow, eventually being replaced by larger, more shade-tolerant trees. This sequence of plant communities continues until a relatively stable ecosystem, resembling the pre-disturbance community, is re-established. This entire process unfolds over decades to a few centuries.
Core Differences
A fundamental distinction between primary and secondary succession lies in their starting conditions. Primary succession begins on barren substrates without soil or life, such as bare rock or new volcanic land. Secondary succession, however, starts in disturbed areas where soil and some organic matter persist.
The presence or absence of soil is a defining factor. Primary succession requires the slow formation of soil from weathered rock and organic material. Secondary succession benefits from pre-existing soil, often containing seeds and plant parts, allowing for quicker regrowth.
Pioneer species also differ between the two types of succession. In primary succession, pioneer species are hardy organisms like lichens and mosses, surviving on nutrient-poor rock. For secondary succession, they are fast-growing plants such as grasses and weeds that rapidly colonize disturbed soil.
The time scale for these processes varies considerably. Primary succession is much slower, taking hundreds to thousands of years to develop a stable ecosystem. Secondary succession progresses more rapidly, completing stages within decades to a few centuries, aided by existing soil and biological foundation. Nutrient availability also influences this speed, being scarce in primary succession but higher in secondary succession.