Population Demography: Videos & Practice Problems

Demography is the statistical study of population characteristics, including size, structure, and distribution, influenced by birth rates, death rates, and migration. Life tables summarize demographic data, focusing on a cohort's survivorship, mortality, and reproductive rates, often emphasizing females in sexually reproducing species. Survivorship curves graphically represent life tables, illustrating three types: type 1 (low early mortality), type 2 (constant mortality), and type 3 (high early mortality). Understanding these concepts is crucial for analyzing population dynamics and ecological interactions.

Population Demography

Population demography is the statistical analysis of population characteristics and trends, focusing on aspects such as population size, structure, and distribution. It also examines the processes that influence population changes, including birth rates, death rates, and migration patterns. Understanding these elements is crucial for analyzing how populations evolve over time.

Key tools in demography include life tables and survivorship curves. Life tables provide a detailed breakdown of the mortality rates and survival probabilities within a population, allowing researchers to predict future population dynamics. Survivorship curves graphically represent the number of individuals surviving at each age, illustrating different patterns of mortality across species.

By studying these demographic tools, one can gain insights into the health, longevity, and reproductive strategies of populations, which are essential for effective resource management and conservation efforts.

Life Tables

Life tables are essential tools in demography that provide a comprehensive statistical overview of a population's survivorship, mortality, and reproductive rates. A cohort, defined as a group of individuals of the same species born around the same time, is typically analyzed in these tables. In studies of sexually reproducing organisms, life tables often focus on females, as they are the ones who produce offspring and significantly influence population dynamics.

For instance, consider a life table for female fruit flies, Drosophila melanogaster, which examines a cohort of 500 females aged between 0 and 10 days. The table tracks these fruit flies until all have died, collecting various statistical data along the way. Key components of a life table include:

• Age Interval: The specific age range being analyzed.
• Number of Organisms Alive: The count of individuals alive at the beginning of each age interval.
• Overall Survivorship: The proportion of the original cohort that remains alive at the start of each age interval.
• Overall Mortality Rate: The death rate between age intervals, calculated as the number of deaths divided by the initial number of organisms.
• Fecundity: The reproductive capacity, often represented as the average number of female offspring produced per female.

To illustrate, if 500 female fruit flies are observed and 134 die in the first age interval (0-10 days), the mortality rate can be calculated as:

Mortality Rate = \(\frac{134}{500} = 0.268\) or 26.8%.

In the next age interval (20-30 days), if 280 organisms are still alive, the survivorship is calculated as:

Survivorship = \(\frac{280}{500} = 0.560\) or 56%.

Continuing this process, if the survivorship at the start of the 30-40 day interval is 0.248, the number of organisms alive can be determined by:

Number Alive = \(0.248 \times 500 = 124\).

Understanding life tables is crucial for analyzing population dynamics and predicting future trends in species populations. As you practice these concepts, you'll gain a deeper insight into the factors that influence survival and reproduction within a population.

Which of the following statements about life tables is true?

• A. Life tables usually only consider males in the population because they typically survive the longest.
• B. Life tables provide detailed information on a population’s birth & death rates but not survivorship data.
• C. Life tables can be used to estimate future population size by examining age-specific mortality/fecundity rates.
• D. Life tables never include data on males in the population.
• E. Life tables are exclusively used for studying human populations and are not applicable to other species.

Answer: (C)

Based on the data in this life table, at what age do female European hedgehogs reach sexual maturity and become capable of sexual reproduction?

• A. 0-1 years.
• B. 1-2 years.
• C. 2-3 years.
• D. 3-4 years.

Answer: (B)

If a cohort of female European Hedgehogs has a starting population of 2,500 and a survivorship of 0.343 after 2 years, how many hedgehogs make it to 2 years old?

• A. 883.
• B. 503.
• C. 1723.
• D. 353.

Answer: (A)

Survivorship Curves

Survivorship curves are graphical representations of life tables, illustrating the number of living individuals in a cohort across different age groups. On a survivorship curve, the y-axis typically represents the number of individuals, often using a logarithmic scale to accommodate a wide range of values, while the x-axis represents age groups.

There are three primary types of survivorship curves: type 1, type 2, and type 3. Each type reflects different mortality patterns throughout the lifespan of a species.

A type 1 survivorship curve, exemplified by humans, shows low mortality rates during early life, allowing most individuals to survive into adulthood. However, mortality rates increase significantly in later life stages.

In contrast, a type 2 survivorship curve, represented by species such as the northern cardinal, displays a constant mortality rate throughout the lifespan. This results in a straight-line graph, indicating that the likelihood of survival remains relatively stable at all ages.

Lastly, a type 3 survivorship curve is characterized by high mortality rates early in life, with most individuals failing to reach adulthood. However, those that do survive past a certain age experience a significant decrease in mortality rates. The American bullfrog serves as a prime example of this type.

It is important to note that survivorship is not strictly limited to these three categories; many populations exhibit characteristics that fall between these types. This spectrum of survivorship reflects the complexity of life histories in various species, highlighting that real-world data can show a range of survivorship patterns beyond the classic types.

Population Demography Example 1

In ecology, survivorship curves are graphical representations that illustrate the number of individuals surviving at each age for a given species. A type 2 survivorship curve is characterized by a constant mortality rate throughout the lifespan of the species, meaning that individuals have an equal chance of dying at any age.

To identify species that exhibit a type 2 survivorship curve, we can analyze the provided examples:

1. **Oak Trees**: These trees produce thousands of acorns, but very few grow into mature trees. This indicates a high mortality rate early in life, aligning with a type 3 survivorship curve.

2. **Rats**: The statement indicates that rats do not have a significantly higher death rate at any specific age, suggesting a consistent mortality rate throughout their lives. This is characteristic of a type 2 survivorship curve.

3. **Humans in the USA**: Humans typically experience low mortality rates early in life, with higher rates later on. This pattern corresponds to a type 1 survivorship curve.

4. **Frogs**: Frogs produce many tadpoles, most of which face predation or starvation, indicating a high mortality rate early in life, consistent with a type 3 survivorship curve.

5. **Wrens**: These small songbirds have an equal chance of dying throughout their lifespan, which aligns with a type 2 survivorship curve.

From this analysis, both rats and wrens exhibit type 2 survivorship curves. Therefore, the correct answer to the question is option D, which includes both species 2 and 5. Understanding these patterns helps in studying population dynamics and the ecological strategies of different species.

A carp has many offspring at once, but the majority of them are eaten by predators in their first year of life. As carp develop & grow, they have fewer predators in later life. Which survivorship curve do carp exhibit?

• A. Type I.
• B. Type II.
• C. Type III.
• D. Impossible to tell.

Answer: (C)

Based on the following survivorship curve for moose, how many moose in an initial cohort of 3,500 individuals would you expect to survive to age 20?

• A. 100 moose.
• B. 350 moose.
• C. 35 moose.
• D. Impossible to tell.

Answer: (B)

Here’s what students ask on this topic:

What is population demography and why is it important?

Population demography is the statistical study of population characteristics and trends, including size, structure, and distribution. It examines processes that drive population changes, such as birth rates, death rates, and migration patterns. Understanding demography is crucial because it helps predict population growth or decline, informs public policy, and aids in resource allocation. It also provides insights into ecological interactions and the sustainability of populations, which is essential for conservation efforts and managing natural resources.

What are life tables and how are they used in population demography?

Life tables are comprehensive statistical tables that display demographic information about a population's survivorship, mortality, and reproductive rates. They often focus on a cohort, a group of same-species individuals born around the same time. Life tables typically include data on females in sexually reproducing organisms, as females have a greater impact on population dynamics. These tables help researchers understand population trends, predict future changes, and make informed decisions about conservation and management strategies.

What are survivorship curves and what do they represent?

Survivorship curves are graphical representations of life tables, showing the number of living individuals in a cohort at each age group. There are three general types of survivorship curves: Type 1 (low early mortality, high late mortality), Type 2 (constant mortality throughout life), and Type 3 (high early mortality, low late mortality). These curves help visualize population dynamics and understand the survival patterns of different species, which is essential for ecological studies and conservation efforts.

How do birth rates, death rates, and migration patterns influence population demography?

Birth rates, death rates, and migration patterns are key factors that influence population demography. Birth rates determine the number of new individuals added to a population, while death rates indicate the number of individuals lost. Migration patterns, including immigration and emigration, affect the distribution and size of populations. Together, these factors shape population dynamics, influencing growth, decline, and the overall structure of populations. Understanding these processes is essential for predicting future trends and making informed decisions about resource management and conservation.

What is the significance of focusing on females in life tables for sexually reproducing organisms?

In life tables for sexually reproducing organisms, focusing on females is significant because females produce offspring and thus have a greater impact on population dynamics. By analyzing female data, researchers can better understand reproductive rates, population growth, and the potential for future generations. This focus helps in making accurate predictions about population trends and informs conservation and management strategies to ensure the sustainability of species.