Unit 3: Population and Community Ecology

A population is a group of individuals of a single species occupying a particular area at a specific time.

Key Characteristics:

• Density: The number of individuals per unit area or volume.
• Natality (Birth Rate): The rate at which new individuals are added to the population through reproduction.
• Mortality (Death Rate): The rate at which individuals are lost from the population through death.
• Dispersion: The spatial pattern of individuals within the habitat (clumped, uniform, or random).
• Age Distribution: The proportion of individuals in different age groups (pre-reproductive, reproductive, and post-reproductive).

Populations are regulated by various factors to maintain equilibrium with the environment.

• Population Dynamics: The study of how and why populations change in size and structure over time.
• Regulation: Includes density-dependent factors (competition, predation, disease) and density-independent factors (climate, natural disasters).
• Ecological Speciation: The process by which barriers to gene flow evolve between populations as a result of ecologically-based divergent selection.

Understanding an organism's role requires defining its physical space and functional role.

• Ecological Amplitude: The range of environmental conditions (temperature, moisture, etc.) within which a species can survive and function.
• Habitat: The physical place or specific environmental location where a species lives (its "address").
• Niche: The functional role or "profession" of a species in its ecosystem, including its use of resources and interactions with other species.

A plant community consists of different species populations interacting in a common habitat.

Community Attributes:

• Species Diversity: The variety and abundance of different species within the community.
• Dominance: One or a few species that exert a major influence over the community due to their size, numbers, or activity.
• Keystone Species: A species that has a disproportionately large effect on its community structure relative to its abundance. If removed, the community structure often collapses.

Boundaries between communities are often dynamic zones.

Ecotone: A transition zone between two distinct communities (e.g., the boundary between a forest and a grassland).

• Edge Effect: The tendency for an ecotone to have greater species diversity and higher population density than either of the adjacent communities.

Plant succession is the orderly and predictable process of change in the species structure of an ecological community over time.

Types of Succession:

• Primary Succession: Occurs in essentially lifeless areas where there is no soil (e.g., bare rock, volcanic ash).
• Secondary Succession: Occurs in areas where a community that previously existed has been removed or destroyed but the soil remains (e.g., after a forest fire or abandoned farmland).

Stages of Succession:

1. Pioneer Stage: The first species to colonize the area (e.g., lichens on bare rock).
2. Seral Stages: Intermediate stages of development.
3. Climax Community: The final, stable stage of succession that remains relatively unchanged as long as the environment remains stable.

7. Exam Focus: Tips and FAQs

Common Mistakes

• Mistake: Confusing Primary and Secondary Succession. Correction: Primary starts from scratch (no soil); Secondary starts where soil already exists.
• Mistake: Thinking an Ecotone has lower diversity. Correction: Due to the Edge Effect, Ecotones usually have higher diversity because they contain species from both adjacent communities.

Frequently Asked Questions

Q: What are the main attributes of a plant community?
A: Species composition, dominance, diversity, and physiognomy (visual appearance).

Q: What defines a climax community?
A: It is the final, stable stage of succession where the species composition is in equilibrium with the regional climate.