C4.1 Populations and communications

What is a population?

A: A group of individual organisms of the same species living in an area, interacting through breeding and competition.

Q: Why is random sampling used to estimate population size?

A: Because counting every individual is often impractical. Random sampling ensures every individual has an equal chance of being selected, reducing bias.

Q: How is population size estimated using quadrats?

1. Place quadrats randomly in a habitat.

2. Count organisms per quadrat.

3. Calculate:

   Population = Mean count per quadrat × Total area divided Quadrat area

Note: Only for sessile organisms (e.g., plants).

Q: What does standard deviation indicate in quadrat sampling?

• Low SD: Even distribution, reliable estimate.

• High SD: Clumped distribution, higher sampling error.

Q: How is the Lincoln index used to estimate population size?

Population size=M x N divided R

• M: Marked initially.

• N: Total recaptured.

• R: Recaptured with marks.
  Assumptions: No migration, births/deaths, marks remain intact.

Q: What is carrying capacity?

A: The maximum population size an environment can support, limited by resources (e.g., food, water, space).

Q: Compare density-dependent and independent factors.

• Density-dependent: Effect increases with population size (e.g., competition, disease).

• Density-independent: Effect is unrelated to population size (e.g., frost, fire).

Q: Describe the three phases of sigmoid population growth.

1. Exponential: Rapid growth, abundant resources.

2. Transitional: Growth slows as resources deplete.

3. Plateau: Population stabilizes at carrying capacity.

Q: Give examples of intraspecific competition and cooperation.

• Competition: Gannets fighting for nest sites.

• Cooperation: Penguins huddling for warmth.

Q: Name six types of interspecific relationships.

1. Herbivory (bison/grass).

2. Predation (anteater/ants).

3. Competition (ivy/oak).

4. Mutualism (coral/zooxanthellae).

5. Parasitism (tick/deer).

6. Pathogenicity (anthrax/kudu).

Q: Describe one mutualistic relationship.

A: Legumes & Rhizobium bacteria – Bacteria fix nitrogen for the plant; plant provides sugars and shelter.

Q: Why are invasive species harmful?

A: They outcompete endemic species due to lack of natural predators, leading to niche reduction or extinction (e.g., red lionfish in the Caribbean).

Q: How can interspecific competition be tested?

1. Field manipulation (remove one species).

2. Lab experiments (grow species together/apart).

3. Random sampling (test species associations).

Q: Why use logarithmic scales in population graphs?

A: Exponential growth appears as a straight line, making trends clearer (e.g., collared dove population).