BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.6. Community Ecology (Synecology)

10.6.2. The Course of Succession

Succession is most easily illustrated starting from a bare rock surface. This environment is initially uninhabitable for most organisms. Naturally, the first colonizers must be hardy autotrophs equipped with efficient dispersal mechanisms to deliver their propagules (diaspores) from other habitats. Such organisms, known as pioneers, include certain Lichens, Algae, and cyanobacteria. As they colonize the rock, they secrete leaching substances that provide essential mineral nutrients and promote weathering, thereby accelerating The breakdown of the substrate. Upon dying, these pioneer organisms decompose into SOM (soil organic matter), which accumulates in rock crevices and small depressions, serving as a food source for detritivores. The arrival of these detritivores marks the beginning of soil formation, yielding nutrients accessible to plants. Plants colonize the habitat in a specific sequence. First to appear are species tolerant of a wide range of conditions with long-range-dispersed seeds, known as generalists or opportunists. Each new Organism contributes to the accumulation of biomass and detritus in the ecosystem, consequently facilitating the establishment of subsequent colonists. Gradually, species that are relatively demanding environmentally, yet longer-lived and more competitive, make their appearance.

As succession proceeds, an increasing proportion of nutrients becomes tied up in the community's biomass, while the abiotic component of the ecosystem is correspondingly depleted of these nutrients. The amount of detritus produced also rises, and detritivores gradually assume The Role of primary consumers, relegating herbivores to a secondary role. Food webs restructure accordingly, with detritus becoming the principal source of nutrients. These and other successional trends are summarized in Table 10.5.

Class="center">Table 10.5. Changes in the ecosystem during a typical secondary succession

Succession is a clear example of organisms modifying their own environment and creating conditions that enable the settlement of other species. As vegetation develops, novel niches continually open up for herbivores, secondary consumers, and detritivores, until a complex community and a new ecosystem are finally established. This process is represented schematically in Fig. 10.17.

Fig. 10.17. Generalized sere diagram showing characteristic vegetation changes during succession.

10.13. What factors influence the Abundance and species diversity found in a given locality?

During later stages of succession, biotic interactions assume a primary role in shaping community structure, becoming increasingly complex (sec. 10.7.5). This is hardly surprising, given the concomitant rise in species diversity. For instance, tropical rainforests, which rank among the oldest climax ecosystems, are renowned for their staggering number of species and the extreme specialization of their interrelationships.

The idealized model of succession described above represents an optimal scenario for terrestrial environments. In reality, this process is frequently far less orderly and straightforward. Much depends on the availability of propagules from pioneer species. The situation is particularly complex during secondary succession, where early colonists are typically forced to interact immediately with species of later seral communities developing from the reserve of seeds and vegetative Organs surviving in the soil.

A climax community is frequently characterized by a single dominant species or a few co-dominant species. The term "dominance" in ecology is somewhat subjective, typically denoting the species with the maximum total biomass or productivity, although the absolute size of individual organisms is also taken into account.

The timeframes of succession—and consequently the degree of community maturity—are exceptionally difficult to determine. Initial stages can proceed very rapidly, after which the process often slows down, notably due to the absence of late-successional species in the surrounding area. For example, on a barren moraine left behind by a retreating glacier, well-developed soil and vegetation can be observed within 30–70 years, yet determining whether this constitutes a climax state remains problematic. Some ecologists even argue that, because the planet's climate is in a state of continuous flux, a corresponding final stage of ecosystem development cannot truly exist; in other words, an optimal, stable climax community must constantly evolve. Put differently, as observers of this process, we are far better informed about the Cytology/cytology/16.html">Early stages of succession than about the transformations that might unfold over 200–300 years—a timespan corresponding to the lifespan of certain trees that define The structure of climax communities in many terrestrial ecosystems.