Fundamentals of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 15. Megaevolution

15.2. Ecological Succession

One of the Fundamental properties of ecosystems is their dynamism. Ecosystem changes can depend on the source of the impact and are classified as either autogenic or allogenic. The latter are driven by external forces (primarily physical climatic and geological factors, such as erosion, sedimentary rock formation, and mountain building). Autogenic changes, on the other hand, are driven by processes operating from within the ecosystems themselves. In practice, it is nearly impossible to completely disentangle the Influence of External and internal factors.

Autogenic changes are referred to as ecological succession, or ecosystem development, which exhibits several distinct features:

1) it is driven by the biotic component of the ecosystem (the community), which alters the physical environment and determines The Nature of succession, its rate, and its developmental limits;

2) it represents an ordered Development of the ecosystem associated with changes in its species Structure and internal processes;

3) it is directional and predictable to a certain extent;

4) its culmination is The formation of a stable ecosystem characterized by maximum biomass and the highest ratio of interspecific interactions per unit of energy (the climax state).

The rate of succession processes and the attainment of the climax state vary across different ecosystems. In the Cytology/cytology/16.html">Early stages of ecological succession, gross primary production exceeds community Respiration costs (autotrophic succession). Conversely, when respiration costs exceed gross primary production, the process is termed heterotrophic succession (which is typical of ecosystems with low producer Abundance).

As succession progresses, respiration and productivity balance out, resulting in an increase in biomass within mature ecosystems and a net yield approaching zero over an annual cycle.

Ecosystem development is accompanied by profound structural changes in food webs, fostering tighter relationships and mutual adaptations between plants and animals. A key feature of succession is the tendency toward the closure of biogeochemical cycles for major elements (nitrogen, phosphorus, calcium): mature systems retain these components more efficiently within the exchange pool, suffering only minimal nutrient losses compared to immature or disturbed ecosystems.

Succession also significantly impacts the biological traits of the organisms comprising the ecosystem. Early in succession, organisms are typically small, with simple life cycles and high reproductive rates. Species richness is low, and the vast majority occupy broad ecological niches (pioneer species). These traits facilitate success in mineral-rich environments. As the ecosystem develops, biogenic substances accumulate, favoring larger organisms with more complex life cycles; the ecological niches of most species become narrower and more specialized. Population growth shifts from exponential to logistic, making negative feedback regulation critically important under these conditions. Thus, ecological succession tends toward the most complex and diverse structure achievable under existing environmental conditions (a state of overall Homeostasis).

Successions are divided into Primary and secondary types. Primary succession refers to The Development of an ecosystem on a substrate previously unoccupied by any community (such as volcanic islands or lava flows). Species replacement occurs because populations modify their environment, thereby creating favorable conditions for the populations of other species. This process continues until a balance between biotic and abiotic components is reached. Each successive stage lasts longer than the previous one and is characterized by a higher biomass-to-energy-flux ratio. Dominant plant species play a particularly influential role. Those present at the early and late stages of succession exhibit contrasting growth and reproductive strategies. Thanks to their high dispersal capacity, pioneer species rapidly colonize newly formed habitats. Late-successional species grow more slowly, but their shade tolerance and larger size give them a competitive edge over pioneers. In other words, pioneers thrive exclusively in vacant environments, whereas terminal communities require conditions engineered by the pioneers themselves.

Secondary succession is defined as ecosystem development within an area from which a prior community has been removed. It proceeds much faster than primary succession due to the pre-existence of conditions favorable for community development compared to a sterile substrate (a classic example is the regeneration of a forest community following a wildfire).

Any succession eventually approaches a relatively stable state. The terminal or stable community is known as the climax community. The climax is the final, relatively stable phase of natural ecosystem development that best corresponds to the environmental characteristics of a given locality during a specific geological period. A climax ecosystem is a mature system that has achieved a stable state with robust homeostasis, exhibiting a tendency toward equilibrium between production and respiration. According to V.N. Sukachev, climax states are not entirely static either—nature is inherently dynamic. Therefore, the climax community should be viewed as a relatively stable system.

Depending on the climatic zone, a distinction is made between climatic climax—a theoretical community toward which the entire development of an ecosystem in a given region is directed (characterized by equilibrium with climatic conditions). Such a theoretical community is realized only where physical environmental conditions are not extreme enough to override typical ambient factors. Where this is not possible, succession culminates in an edaphic climax—a relatively stable phase of ecosystem development shaped not only by climate but also by soil conditions. The more extreme the physical conditions of existence, the lower the probability that the ecosystem will attain a state of equilibrium with the climatic conditions.

Science also recognizes METABOLISM/2.html">THE CONCEPT OF catastrophic (cyclic, pyrogenic) climax—a terminal state of ecosystems caused by periodic disturbances such as fires. These disasters destroy climax communities, resetting succession to its initial state. The repetition of such events fosters a self-maintaining system whose continued existence depends on regular disturbances (such as African savannas or the California chaparral).

Successions sometimes assume a cyclic pattern, though certain Phases of the cycle may persist indefinitely. Secondary development may occur following specific disruptions. Succession is fundamentally distinct from ecosystem evolution (historical development) due to its cyclic nature. Evolution is irreversible and involves shifts in both macro- and micro-characteristics of ecosystems.

It is generally believed that successions are likely not a mechanism of ecosystem evolution, but rather a product of it, designed to ensure recovery following local ecological crises (such as fires or volcanic eruptions). Comparing the biosphere to an Organism, succession in a given ecosystem resembles the regeneration of damaged integumentary tissue. In most animals, this capacity is limited and cannot fully restore the organism to its prior state following severe trauma. Similarly, in ecosystems,

certain disruptions cannot be compensated for by succession alone, leading to ecological crises that escalate from local to planetary scales. A historical example is the Cretaceous ecological crisis, which resulted in a radical turnover of Earth's flora and fauna (the replacement of dinosaurs by mammals and birds; the proliferation of angiosperms and their coevolution with insects; The Emergence of social insects, etc.). This process spanned over 30 million years and was marked, on the one hand, by the punctuated collapse of prior ecosystems and, on the other, by the simultaneous evolution of numerous new forms.