Ecological change over time involves a series of communities replacing one another until a final assemblage of species is established. This long-term development culminates in a self-sustaining stage known as the climax community. This community represents the point where the ecosystem reaches a dynamic equilibrium with its environment, maintaining its structure and composition until a significant external force intervenes.
Defining the Process of Ecological Succession
Ecological succession is the orderly, natural process where the species structure of an ecological community changes over time. This change occurs in a predictable sequence, as the organisms present in one stage modify the environment, making it more suitable for the next group of species to colonize. The entire process begins with pioneer species, which are the first organisms to colonize a barren or disturbed area.
Succession is categorized into two main types based on starting conditions. Primary succession occurs in environments that are newly formed and completely devoid of life and soil, such as a freshly cooled lava flow or rock exposed by a retreating glacier. Pioneer species like lichens and mosses must first break down the rock and create the initial organic matter necessary for soil formation.
Secondary succession, in contrast, begins where a previously existing community has been disturbed or destroyed, but the underlying soil remains intact. This occurs following events like a wildfire, a flood, or the abandonment of agricultural land. Because residual soil, nutrients, and seed banks are already present, secondary succession proceeds much faster than primary succession. Both successional pathways progress through intermediate stages, called seral communities, until the final, stable community is established.
Characteristics of the Climax Community
The climax community is a stable, mature ecosystem that maintains a dynamic equilibrium with its local conditions. This community is self-perpetuating, meaning the dominant species are capable of reproducing and replacing themselves without external colonizers. Species composition remains relatively constant over long periods unless a major disturbance occurs.
A defining trait of the climax community is its high level of species diversity and complex structure. It features multiple vertical layers and a greater variety of ecological niches, which support complex food webs rather than simple food chains. Organisms found in this stage are typically large, long-lived, and often shade-tolerant, a contrast to the small, fast-reproducing species of early succession.
From an energy standpoint, the climax community balances energy fixed through photosynthesis and energy lost through respiration. This balance means the ratio of production to respiration (P/R) approaches a value of one. While gross productivity is high due to the density of producers, net productivity approaches zero because of the energy required to maintain the large biomass and complex structure. This high biomass results in a community that is resilient and capable of recovering from minor environmental fluctuations. Examples include old-growth forests and established temperate grasslands.
Factors Influencing Stability and Maintenance
The composition of a climax community is primarily determined by external environmental forces. The most significant influence is the regional climate, which dictates the type of climax that develops. For instance, a region with high rainfall and warm temperatures leads to a forest climax, whereas a drier, colder climate results in a grassland or desert climax.
Other physical factors, known as edaphic factors, including soil composition, drainage, and topography, influence which species can thrive within a given climate. A sandy soil, for example, may support a different climax community than a clay soil, even if they are located in the same climatic zone. This acknowledges that many different stable communities can exist within the same broad region, known as the polyclimax theory.
Disturbance is a force that dictates the maintenance and stability of the final community. While minor disturbances, such as a single tree falling and creating a canopy gap, are often necessary to maintain diversity, larger, recurring disturbances can maintain an ecosystem in a state known as a fire climax. For example, in certain pine forests or grasslands, periodic fires are required to prevent the invasion of fire-sensitive species. Human activities, such as logging, grazing, or agriculture, can also perpetually prevent a community from reaching its natural climax, resulting in a type of arrested development known as a disclimax.