Ecological succession describes the predictable process of change in the species structure of an area over time. Biodiversity is the variety of life, including the number of different species present in a community. The intuitive answer is often yes, but the reality is a non-linear process with critical limitations. Clarifying this relationship requires understanding the driving forces behind community shifts and the conditions that ultimately cap or reduce the number of species an environment can support.
Defining the Process of Ecological Succession
Ecological succession is the orderly and progressive sequence of species replacement following the initial colonization of a new habitat or the disturbance of an existing one. This process involves species modifying their environment in ways that make it more suitable for a new set of species to establish themselves.
The starting conditions of the process determine whether it is categorized as primary or secondary succession. Primary succession begins in areas entirely devoid of life and soil, often following a major geological event. Examples include the colonization of bare rock exposed by a retreating glacier or the cooling of a new lava flow from a volcano.
Secondary succession is a faster process that occurs in an area where a pre-existing community has been disturbed, but the soil remains intact. A wildfire, an abandoned agricultural field, or a clear-cut forest patch are common examples of where secondary succession begins. Because the soil, nutrients, and seed bank are already present, the ecosystem has a significant head start in recolonizing the area.
The General Pattern of Biodiversity Change
The relationship between successional time and species richness follows a distinct, non-linear pattern. Initially, biodiversity is low, as only a few hardy pioneer species, like mosses and lichens, can colonize the barren or recently disturbed ground. During the middle stages of succession, there is a rapid and significant increase in the total number of species.
This dramatic rise occurs as intermediate communities develop, with grasses and shrubs establishing themselves alongside the early-successional species. The peak in biodiversity is observed during these mid-successional stages, which feature a mix of fast-growing, opportunistic species and slower-growing, more competitive species.
As the community progresses toward a late-successional or stable state, traditionally known as the climax community, the species count plateaus or slightly declines. The final, stable community is dominated by a few highly competitive species, usually long-lived trees in a forest ecosystem.
Key Factors Driving Increased Diversity
The initial increase in species richness during early and intermediate succession is driven by several ecological mechanisms. One of the most significant factors is the increase in habitat heterogeneity, or the physical complexity of the environment. As small, low-lying plants are replaced by taller shrubs and trees, the vertical structure of the habitat grows, creating new layers like the canopy, understory, and forest floor.
This complex physical structure provides a greater number of specialized microhabitats and ecological niches. Different species of animals, insects, and microorganisms can now specialize in utilizing resources at various heights, light levels, and moisture gradients.
Early pioneer species play a crucial role by changing the environment to make it more hospitable for later species. They break down rock, add organic matter, and fix nitrogen, leading to an increase in soil depth and nutrient availability. This modification of resource availability allows for the establishment of species that require richer soil and more stable conditions.
Conditions That Limit or Decrease Biodiversity
While succession promotes diversity, there are specific conditions that can limit or even decrease the number of species. A primary limiting factor occurs in the late stages of succession due to a process called competitive exclusion. As the ecosystem matures, a few species become highly dominant and competitively superior, effectively monopolizing resources like light and nutrients.
These dominant species, such as tall, shade-tolerant trees in a mature forest, outcompete and eliminate many of the opportunistic species that thrived in the intermediate stages. This leads to the slight decline in overall species richness observed in the most stable, late-successional communities.
If the successional process is constantly reset by frequent, severe disturbances, like intense annual fires or continuous overgrazing, the community remains in a perpetually early-successional state with low diversity. Conversely, the introduction of invasive species can halt or reverse the natural increase in diversity. An aggressive invasive species can rapidly dominate an ecosystem, outcompeting native plants and animals for resources, which ultimately reduces the number of native species that can be supported.