ENVR101 Ecological Communities and Ecosystems

Population Dynamics

Populations increase with births and immigration and decrease with deaths and emigration. They may reach a stable size where births, immigration, deaths, and emigration are balanced, or vary due to internal and external regulatory mechanisms or regular cycles.

Population regulation is influenced by density-dependent and density-independent factors:

  • Density-dependent factors: become more intense as population density increases (e.g., competition for resources, predation, disease).

  • Density-independent factors: affect population size regardless of density (e.g., weather events, climate change, pollution).

Life History Strategies

Species exhibit different life history strategies:

  • R-selected species: have high reproductive rates, short lifespans, early maturity, and minimal parental care (e.g., rabbits).

  • K-selected species: have low reproductive rates, long lifespans, high parental investment, and slow maturity (e.g., Kakapo).

Species-Area Relationship

The species-area relationship describes how the number of species increases with the area of the habitat being considered. Factors influencing this relationship include:

  • Habitat heterogeneity

  • Isolation and connectivity

  • Time and ecological succession

  • Environmental factors

Maintenance of Local Diversity

Local diversity can be maintained through:

  • Equilibrium dynamics: Communities reach a climax state and return to it after disturbance.

  • Non-equilibrium dynamics: Disturbance is a part of maintaining diversity.

Diversity peaks at a balance of disturbance levels, as too little disturbance allows dominant species to outcompete others, while too much prevents species establishment.

Natural Disturbances

Naturally occurring disturbances include fire, flooding, storms, volcanoes, and drought. Human-induced disturbances such as deforestation, pollution, climate change, and urbanization also affect diversity.

Climate change exacerbates the intensity, frequency, and scale of disturbances.

Effects of Disturbances on Ecosystems

Disturbances can enhance or reduce biodiversity, leading to species loss and disrupted ecosystem processes. Succession occurs after disturbances, involving a series of colonization and replacement until a climax community is reached.

Primary and Secondary Succession

  • Primary succession: occurs after major disturbances where there is no soil. Pioneer species colonize the area, altering the soil and microhabitat.

  • Secondary succession: occurs after minor disturbances where soil and some plants remain. A source population exists nearby.

Primary succession happens on longer scales while secondary succession happens on shorter scales.