Ecosystem Dynamics: Population, Biomass, Productivity and Succession

Population Dynamics

The dynamics of a population depend on several parameters: birth rate (b), mortality, emigration and immigration. These are measured as the number of births, deaths and migrations per unit of time relative to the total population of individuals.

The population change rate is calculated as: r = b - m + (immigration - emigration).

Population growth can be represented by survival curves, age pyramids and growth curves. The theoretical form of growth is exponential, while real growth is limited by environmental resources.

Ecosystem Functioning and Parameters

Ecosystems are quantified using key parameters:

• Biomass: the mass of living organisms per unit of surface or volume.
• Production: the biomass increment over time, such as fresh fish weight or dry matter gained per unit of time.
• Net Primary Production (NPP): NPP = GPP - respiration - renewal of excreted material.
• Productivity rate: production divided by biomass, multiplied by 100.
• Renewal rate: production divided by biomass, multiplied by 100.
• Efficiency: NPP divided by GPP, multiplied by 100.

Net Ecosystem Production (NEP)

NEP indicates the state of an ecosystem:

• NEP = 0: stable ecosystem, dominated by K-strategy species that adapt slowly and reproduce with low fecundity.
• NEP < 0: degraded or overexploited ecosystems (logging, fire, pollution).
• NEP > 0: expanding, immature ecosystems in balance, not yet dominated by strategic species, with R-strategy species that reproduce fast.

Ecological Pyramids

Ecological pyramids, such as production pyramids, typically form a pyramid shape with about 10% energy transfer between levels. Biomass pyramids can take diverse forms, including normal or inverted shapes, and carry limited information value.

Primary Productivity of Biomes

• Terrestrial biomes: approximately 100 to 300 gC/m²/year.
• Marine aquatic systems: neritic zones lie over the continental shelf with high productivity, while pelagic zones are deep open-ocean areas with low productivity.

Ecological Succession

Primary Succession

Primary succession occurs in a new area where no ecosystem has existed before, such as a volcanic island emerging from the sea. The process is slow: bacteria, lichens and arachnids gradually erode the rock and build soil.

Secondary Succession

Secondary succession occurs in a pre-existing ecosystem destroyed by fire, logging or other disturbance.

Stages of Succession

Early stages are dominated by pioneer species with an R strategy, low biomass and little production. The climax stage is dominated by species with K strategies, high biomass, specialized habitats and niches. One species, one habitat and one niche create stable competition within the ecosystem.

Biotic Relationships

Intraspecific Relationships

Relationships within one species include family bonds, monogamous or polygamous mating systems (one male with one female, one male with many females, or many males with many females), and colonial, social or gregarious associations.

Interspecific Relationships

• Predation: +, -
• Competition: -, -
• Parasitism: +, -
• Symbiosis: +, +

Statistical Study of Biocenoses

1. Sampling: using plots or transects.
2. Inventory: recording all species present.
3. Calculating parameters:

• Frequency of a species = number of samples containing the species / total number of samples
• Density of a species = number of individuals / surface or volume
• Abundance of a species = number of individuals / total number of individuals
• Dominance of a species = number of individuals of the species / total number of individuals in the ecosystem

Ecosystem Diversity: Simpson Index

The Simpson index is calculated as D = 1 - Σ pi², where pi is the proportion of individuals belonging to species i.