19.1 Population Demographics and Dynamics Study Notes

By the end of this section, you will be able to:
- Describe how ecologists measure population size and density.
- Describe three different patterns of population distribution.
- Use life tables to calculate mortality rates.
- Describe the three types of survivorship curves and relate them to specific populations.

Populations are dynamic entities characterized by:
- Fluctuating size and composition influenced by various factors:
- Seasonal and yearly changes in the environment.
- Natural disasters (e.g., forest fires, volcanic eruptions).
- Competition for resources within and between species.
Demography: The statistical study of populations, employing mathematical tools to describe and investigate changes over time.
Originally developed to study human populations, demography's tools apply to all living populations.
Life Tables: Initially designed by life insurance companies to estimate the life expectancy of individuals in a population.

Defined by:
- Population Size: Total number of individuals in a population.
- Population Density: Number of individuals per unit area.
Variability:
- Large population sizes can be distributed densely or sparsely.
- Smaller populations can also exhibit dense or sparse distributions.
Impact on adaptation and interaction:
- Population size affects potential for genetic variation and adaptation.
- Density influences interactions within the population, such as competition for food and mate availability.
General trend: Smaller organisms often have higher population densities than larger organisms.

Counting Individuals: Most accurate method is direct counting, not always feasible (logistical/economic constraints).
Sampling Techniques: Tailored to organism characteristics.
- For immobile or small organisms (e.g., plants, slow-moving organisms), a Quadrat is utilized:
- A quadrat is a square frame (wood, plastic, or metal) randomly placed to count individual organisms within its margins.
- Sufficient random placements are needed for a reliable estimate of population size and density.
- For mobile organisms (e.g., mammals), Mark and Recapture Method is often employed:
- Involves marking a sample of captured individuals and releasing them.
- A second sample is captured, and the number of marked individuals in this sample is counted.
- The assumption is that in larger populations, a smaller percentage of marked individuals will be recaptured.
- Example calculation:
  - If 80 field mice are captured, marked, and released, then 100 field mice are recaptured where 20 were marked, the population size (N) can be determined using:
    ( ext{number marked in first catch} imes ext{total number in second catch}) / ext{number marked in second catch} = N
- Inserting numbers:
  N = \frac{80 \times 100}{20} = 400
- Estimated total individuals: 400.

Distribution refers to the spacing of individuals within a habitat at a specific time.
Three broad categories of distribution patterns:
- Random Distribution: Individuals are spread unpredictably. Example: Dandelion plants with wind-dispersed seeds.
- Clumped Distribution: Individuals are grouped together. Example: Social structures in animals like fish schools or elephant herds; seed dropping in oak trees.
- Uniform Distribution: Individuals are evenly spaced. Example: Plants that inhibit nearby growth (e.g., sage) or territorial animals like penguins.
Importance of Distribution: Reflects biological behaviors and interactions, impacts population estimates:
- Effects on mating success: Lower density species might have difficulty finding mates compared to clumped social species.

Beyond size and density, Demography provides insights into:
- Statistical changes in populations over time:
- Birth rates.
- Death rates.
- Life expectancies.
These population characteristics are typically shown in Life Tables.

Life tables supply critical data concerning:
- Life history of organisms and age-specific life expectancy.
- Modeled after insurance industry actuarial tables for human life expectancy.
Key components:
- Probability of an age group dying before the next birthday.
- Percentage of surviving individuals dying at specific age intervals (mortality rate).
- Life expectancy at particular age intervals.
Example Life Table (Dall Mountain Sheep):
- Age intervals (Column A), number dying (Column B), the number surviving (Column C), mortality rate calculations (Column D), and life expectancies (Column E).
Mortality Rate Calculation:
- \text{mortality rate} = \frac{\text{number of individuals dying}}{\text{number of individuals surviving}} \times 1000
- Example calculation:
- Between ages 3 and 4, 12 individuals die out of 776 remaining from an original 1000 sheep:
- \text{mortality rate} = \frac{12}{776} \times 1000 \approx 15.5
- Results show age-specific mortality rates and pattern changes with age.

Survivorship Curves: Graphical representation showing the number of individuals surviving at each age interval over time.
Three Types of Survivorship Curves:
- Type I Curve: Low mortality in early and middle years, with higher mortality in older individuals. Characteristic of organisms that produce few offspring and provide extensive care (e.g., humans, most mammals).
- Type II Curve: Mortality is constant across all stages of life; mortality is equally likely at any age. Typical of many bird populations.
- Type III Curve: High early-age mortality, with lower mortality rates for older individuals. These organisms often produce many offspring but give little to no care (e.g., certain trees, marine invertebrates).
Significance: Allows comparison of life histories across different populations, indicating survival strategies based on reproductive patterns.

Data referenced from Edward S. Deevey, Jr., “Life Tables for Natural Populations of Animals,” The Quarterly Review of Biology 22, no. 4 (December 1947): 283-314.