Demography Lecture Notes

Principles of Biology: Organisms to Ecosystems - Lecture 26: Demography

Review of Previous Material

  • Topics covered include:

    • Species Distributions

    • Population Models

    • Births and Deaths

    • Migration

    • Exponential Growth

    • Logistic Growth

    • Carrying Capacity

  • Credits cited: C. Darwin

Overview of New Material

  • Topics to be explored in this lecture include:

    • Life Tables

    • Survivorship Curves

    • Reproductive Rates

  • Credits cited: E. Gould

What is Demography?

  • Definition: Demography is the study of key population characteristics, specifically:

    • Birth rates

    • Death rates

    • Migration rates

  • These characteristics change over time and significantly influence population growth.

  • These population metrics are often referred to as vital statistics.

  • The exponential growth model and logistic growth model are key demographic models:

    • Exponential growth assumes that the population has:

    • No age structure

    • No size structure

Life Tables

  • Life Tables Definition: A demographic tool summarizing the survival and reproductive rates of individuals in specific age groups.

  • Cohorts:

    • Defined as a group of individuals of the same age.

    • Demographers often follow cohorts from birth until death to gather data.

    • Recorded metrics include the number of offspring produced by each female.

  • Key Constructive Notes:

    • Life tables are easier to construct for organisms that:

    • Can be marked

    • Do not move much (e.g., plants)

    • They are complicated for long-lived organisms.

  • Assumes that the cohort analyzed is typical, allowing demographic researchers to determine:

    • Survivorship: Likelihood of survival at different ages.

    • Fecundity: Number of offspring produced by a female.

    • Reproductive Rate: The average reproductive output relative to age group.

    • Generation time: The average time between the birth of individuals and their offspring.

  • Credited Source: Campbell Table 40.1

Survivorship Curves

  • Survivorship curves graphically represent the number of individuals in a cohort that survive at each age.

  • Types of Survivorship Curves:

    1. Type I Curve:

    • Characterized by low death rates in early and mid-life stages.

    • Features a sharp decline in survival during old age.

    • Common in species such as large mammals, which produce few offspring and offer parental care.

    • Credited Source: Campbell 40.17

    1. Type II Curve:

    • Mortality rates are relatively constant throughout the lifespan, indicating a steady risk of death regardless of age.

    1. Type III Curve:

    • Exhibits very high death rates at young ages, with a decline in mortality rates for the few individuals that survive to maturity.

    • Observed in species that produce numerous offspring with little to no parental care, e.g., marine invertebrates, many fishes, and long-lived plants.

    • Credited Source: Campbell 40.17

Practical Application Exercise (Lecture Attendance 26)

  • Question: A Type III survivorship curve, which drops steeply and then flattens, would most likely be observed in which species?

    • A: A crab that has short periods of increased mortality associated with molting, followed by periods of lower mortality.

    • B: An annual plant that has a constant death rate throughout its lifespan.

    • C: An elephant that produces few offspring and provides parental care, resulting in high survival rates until increased mortality in old age.

    • D: A marine coral that releases millions of eggs, most of which die young, with only a few surviving.

    • Credited Source: Campbell 40.17

Reproductive Rates

  • Definition: The reproductive output is often measured as the average number of female offspring produced by each female in a given age group.

  • Variability: Age-specific reproductive rates differ among species. Examples include:

    • Squirrels, which may produce 2-3 offspring each year for several years.

    • Certain trees that can produce thousands of offspring over hundreds of years.

    • Credited Source: Campbell Table 40.1

Trout Management

  • Challenges: Balancing conservation of native species, recreation, food production, and overall population structure and density in wildlife and fisheries management.

  • Stocking Practices: Some waterways are regularly stocked with hatchery fish of varying sizes.

  • Regulations:

    • Fishing regulations include size and bag limits, as well as tackle restrictions to manage native trout populations:

    • Red zone: catch-and-release with tackle restrictions.

    • Green zone: bag limit of 2 trout with tackle restrictions.

    • Xmas zone: bag limit of 2 trout allowing any legal tackle.

    • Credited Source: NMDGF

Future Topics of Study

  • Students are encouraged to further explore topics relevant to the lecture, including:

    • BIOL 2101: Molecules to Cells

    • BIOL 300C: Evolution

    • BIOL 310C: Principles of Ecology

    • BIOL 379: Conservation Biology

Study Review Questions

  • Questions include:

    • What is demography?

    • What does a life table summarize?

    • Why do demographers typically consider females only when constructing a life table?

    • How is reproductive output measured?

    • Why is considering the age of individuals important when assessing reproductive rates?

Review of Survivorship Curves

  • Exercise: Draw the basic shapes of Type I, Type II, and Type III survivorship curves, with axis labels.

  • Discuss the implications of each type of curve regarding survival rates throughout an organism's lifespan, along with examples of organisms corresponding to each survivorship type.