Biology Chapter on Populations and Communities

Study Notes

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.

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.

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.

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 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.

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.

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.

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.