Biology Chapter on Populations and Communities

Populations and Communities Overview

• A population consists of individuals of the same species that interact and live in a defined area.
• A species is characterized by its ability to interbreed and produce fertile offspring.

Estimation of Population Size

• Population size is estimated rather than directly counted, particularly useful for large populations.
• Sampling involves selecting a random sample, where each member has an equal chance of being chosen.
• Quadrat sampling involves a fixed-size frame for estimating the size of sessile organisms.
• Sampling error represents the discrepancy between actual and estimated values.

Quadrant Sampling for Sessile Organisms

• Quadrat method is effective for counting non-moving organisms.
• Random sampling is achieved through marking boundaries and using random numbers for selection.
• Standard deviation can be used to measure variability in the data collected.

Capture-Mark-Release-Recapture Method

• Used for estimating population size of motile organisms.
• Process includes capturing, marking, releasing, and later recapturing individuals.
• The Lincoln index formula calculates population size: ( \text{Population size} = \frac{M \times N}{R} ), where:
  • M = initially marked individuals
  • N = total individuals captured in the second round
  • R = number of recaptured marked individuals
• Assumptions include no migration, no births or deaths during the study, and marks being visible without affecting survival.

Carrying Capacity and Competition

• Carrying capacity refers to the maximum population size that an ecosystem can sustain.
• Each habitat has unique carrying capacities based on available resources.
• Competition arises from limited resources such as water, food, light, and territory.

Density-Dependent and Density-Independent Factors

• Negative feedback controls population size, leading to periodic population fluctuations that stabilize over time.
• Density-independent factors affect populations regardless of their density, e.g., natural disasters like fires.
• Density-dependent factors only impact larger populations significantly:
  • Increased competition, predation, and spread of diseases or parasites can lead to greater mortality and reduced reproduction.

Population Growth Curves

• Sigmoid curves illustrate population growth dynamics with distinct phases:
  • Exponential growth occurs in the absence of effective density-dependent factors or in resource-rich environments.
  • Carrying capacity showcases the plateau phase when resources become limited.

Modeling Sigmoid Population Growth Curves

• Models can be created using small environments (mesocosms) with plentiful resources to observe growth.
• Experiments may utilize organisms like duckweed or yeast, focusing on how initial population numbers and resources influence carrying capacity.
• Examination of model strengths and limitations is essential to understand real-world applications.

Explore the dynamics of populations and communities in this quiz. Learn about population size estimation methods like sampling and the quadrat technique. Discover the capture-mark-release-recapture method and its applications in ecological studies.