Ecosystem Dynamics: Succession, Populations, and Growth

Ecological Succession: Ecosystem Transformation

Ecological succession is the sequence of changes that occur in an ecosystem, resulting from its own internal dynamics. It is a slow and gradual process, leading to the evolution of an ecosystem from an initial state of low biological diversity towards a final state of greater diversity.

General Changes in Succession Sequences

In all ecosystems, as the succession sequence progresses, the following phenomena generally occur:

• Increased species diversity
• Increased structural complexity
• Increased biomass
• Increased efficiency in energy use
• Lower net productivity (as more energy is used for maintenance)
• Increased ecosystem stability

The Climax Community Concept

The climax community is considered the ideal state of an ecosystem at the end of a succession process. It is a theoretical state where there is no further change in the composition of the community; it is stable and not replaced by another. However, the equilibrium of a climax community may be lost if environmental conditions change or if the number of individuals in populations fluctuates. In reality, a true climax is rarely reached, as ecosystems are not isolated and none are completely stable.

Primary vs. Secondary Succession

Ecological succession can be categorized into two types based on the starting point:

Primary Succession

This occurs in an area that has not been previously colonized by living organisms and where no soil exists. Examples include newly formed volcanic islands or bare rock exposed by retreating glaciers.

Secondary Succession

This occurs in an area where a community previously existed but has been partially or totally eliminated by a disturbance, such as a forest fire, logging, or severe flood. Soil is typically present.

Primary succession sequences are significantly slower than secondary succession because soil must first form before plant life can establish.

Population Dynamics and Self-Regulation

A population is a group of individuals of the same species living in a specific space. Demographic parameters used to characterize a population include its size, age distribution, birth rate, and mortality rate. Environmental resistance refers to all biotic and abiotic factors that limit population growth.

Population Growth Curves

In an ideal ecosystem where resources are unlimited, a population would grow exponentially. If this growth is represented as a function of time, it yields a J-shaped curve. However, in natural conditions with limited resources, a population initially grows exponentially until it reaches a point where environmental resistance slows its growth. The population then grows more slowly and eventually stabilizes, remaining more or less constant around a carrying capacity value. When this growth is represented, one obtains an S-shaped curve.

Reproductive Strategies in Populations

Based on their population growth patterns, species can exhibit two primary reproductive strategies:

r-Strategists

These are opportunistic or pioneer species that inhabit unstable ecosystems or the early stages of ecological succession. They typically have a short lifespan and a high rate of reproduction, giving rise to a large number of offspring. Their survival strategy relies on producing many offspring quickly to colonize new or disturbed environments.

K-Strategists

These are specialist species adapted to live in stable environments. Individuals are generally long-lived, and most reach adulthood. Although they may reproduce multiple times in their life, their rate of reproduction is low, and they produce few offspring. Their strategy focuses on investing more resources into fewer, higher-quality offspring with a greater chance of survival in competitive, stable environments.