Glossary

45.1 Population Demography

4 min read•june 14, 2024

Population ecology examines how groups of organisms interact with their environment and each other. It explores methods for measuring populations, distribution patterns, and that track survival and reproduction rates. These tools help scientists understand population dynamics and predict future trends.

Population growth is influenced by birth rates, death rates, and migration. Growth models, like exponential and logistic, help predict population changes over time. Factors like and density-dependent regulation play crucial roles in shaping population dynamics and long-term sustainability.

Population Ecology

Methods for population measurement

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  • Population size: total number of individuals in a population
    • Mark and recapture method involves capturing, marking, and releasing a sample of individuals, then recapturing another sample and counting marked individuals to estimate population size using the : N=M×CRN = \frac{M \times C}{R}
      • NN: estimated population size
      • MM: number of individuals marked in the first capture
      • CC: total number of individuals in the second capture
      • RR: number of marked individuals in the second capture
    • divides the area into equal-sized quadrats, counts individuals within randomly selected quadrats, and estimates population size by extrapolating the average number of individuals per to the total area
  • : number of individuals per unit area or volume calculated by dividing population size by the total area or volume occupied (plants in a forest, fish in a lake)

Patterns of population distribution

  • occurs when individuals are evenly spaced due to strong competition for resources or territorial behavior (penguins nesting on a beach)
  • occurs when individuals are randomly distributed in the environment due to abundant resources and no strong competition or territorial behavior (dandelions in a field)
  • occurs when individuals are clustered together in groups due to patchily distributed resources or social behavior (schools of fish, herds of elephants)

Life tables and mortality rates

  • summarizes the survival and reproductive rates of a population at different age intervals
    • lxl_x: proportion of individuals surviving to age xx
    • dxd_x: proportion of individuals dying between age xx and x+1x+1
    • qxq_x: (probability of dying) between age xx and x+1x+1, calculated as qx=dxlxq_x = \frac{d_x}{l_x}
  • Mortality rate: proportion of individuals in a population that die within a specific time period calculated using age-specific mortality rates from life table data (infant mortality rate, adult mortality rate)
  • : the reproductive potential of an individual or population, often measured as the number of offspring produced per unit time

Survivorship curves across species

  • shows low mortality rates during early and middle life, followed by a rapid decline in survivorship in later life, typical of large mammals including humans
  • shows constant mortality rate throughout the lifespan, typical of small mammals, birds, and reptiles
  • shows high mortality rates during early life, followed by low mortality rates for survivors, typical of fish, marine invertebrates, and many plants (sea turtles, oak trees)
  • Survivorship curves reflect different life history strategies and adaptations to environmental pressures and influence population growth rates and

Population Dynamics

Factors affecting population growth

  • (birth rate) influenced by intrinsic physiological and genetic factors that affect reproductive capacity and extrinsic environmental factors such as resource availability, predation, and competition
  • Mortality (death rate) influenced by intrinsic genetic factors, senescence, and disease and extrinsic environmental factors such as predation, competition, and abiotic stressors (temperature, humidity)
  • : movement of individuals into a population from other areas
  • : movement of individuals out of a population to other areas
  • : the difference between immigration and emigration rates

Exponential and logistic growth models

  • assumes a constant per capita growth rate where population size increases exponentially over time according to the equation: Nt=N0ertN_t = N_0 e^{rt}
    • NtN_t: population size at time tt
    • N0N_0: initial population size
    • rr: intrinsic growth rate
    • tt: time
    • Limitations: does not account for resource limitations or
  • assumes a density-dependent per capita growth rate where population size increases exponentially at first, then slows down and reaches a carrying capacity according to the equation: Nt=KN0N0+(K−N0)e−rtN_t = \frac{KN_0}{N_0 + (K - N_0)e^{-rt}}
    • KK: carrying capacity (maximum sustainable population size)
    • Limitations: assumes a constant carrying capacity and does not account for time lags or oscillations in population size (predator-prey cycles)

Carrying capacity and population growth

  • Carrying capacity (KK): maximum population size that an environment can sustain indefinitely, given the available resources (food, water, shelter)
  • As population size approaches the carrying capacity, the per capita growth rate decreases due to density-dependent factors like increased competition for limited resources, increased predation, parasitism, and disease transmission, and accumulation of waste products and toxins
  • When population size exceeds the carrying capacity, the population may experience a crash or decline due to overexploitation of resources and increased mortality rates (reindeer on St. Matthew Island, Alaska)

Population regulation factors

  • Density-dependent factors: factors that affect population growth rate in relation to population density, such as competition for resources, predation, and disease
  • : factors that affect population growth rate regardless of population density, such as natural disasters, climate change, and human activities

Population characteristics and dynamics

  • Age structure: the proportion of individuals in different age groups within a population, which influences future population growth
  • : the average time between two consecutive generations in a population, affecting the rate of population growth and evolution
  • : the pattern of growth, reproduction, and survival that a species has evolved in response to its environment, influencing population dynamics
  • : the process of estimating future population size and structure based on current demographic data and trends

Key Terms to Review (34)

Age structure: Age structure refers to the distribution of individuals of different ages within a population. This distribution can provide insights into the reproductive potential, growth trends, and social dynamics of the population, impacting everything from resource allocation to social services.
Carrying Capacity: Carrying capacity refers to the maximum number of individuals of a species that an environment can sustainably support without degrading its resources. This concept is essential in understanding how populations interact with their environment and the limits that resources impose on population growth, reflecting the balance between biological and environmental factors.
Carrying capacity, or K: Carrying capacity, or K, is the maximum population size of a species that an environment can sustain indefinitely given the available resources such as food, habitat, water, and other necessities. It is determined by environmental resistance factors and biotic potential.
Clumped Distribution: Clumped distribution refers to a pattern where individuals in a population are grouped together in patches or clusters rather than being evenly distributed across a landscape. This type of distribution can result from various factors, including resource availability, social behaviors, and environmental conditions that favor certain areas over others.
Demography: Demography is the statistical study of populations, including their size, structure, and distribution. It involves analyzing data related to birth rates, death rates, migration patterns, and age distributions to understand population dynamics.
Density-dependent factors: Density-dependent factors are variables that affect a population's growth and health based on its density or size. These factors become more intense as the population increases, leading to increased competition for resources, higher mortality rates, and ultimately influencing the population's ability to grow. Understanding these factors is crucial for analyzing how populations respond to their environments and maintain balance within ecosystems.
Density-independent factors: Density-independent factors are environmental influences that affect population size regardless of the population's density. These factors can include natural disasters, climate conditions, and human activities that can cause sudden and significant changes in population numbers. They are important because they highlight how populations can be impacted by events that are outside of their control, and they help explain variations in population dynamics and growth limitations.
Emigration: Emigration is the act of leaving one country or region to settle in another, often motivated by various factors such as economic opportunities, political instability, or social conditions. This process plays a significant role in population dynamics, influencing both the demographic structure of the originating country and the destination country. Understanding emigration helps to analyze migration patterns, population growth, and the socio-economic impacts on both sending and receiving nations.
Exponential growth model: The exponential growth model is a mathematical representation of population growth that occurs when resources are unlimited, allowing a population to grow at a constant rate over time. This model reflects how populations can rapidly increase under ideal conditions, leading to a J-shaped curve when graphed. It emphasizes the potential for populations to expand exponentially, which is crucial for understanding population dynamics and ecological limits.
Fecundity: Fecundity is the reproductive capacity of an individual or population, often measured by the number of eggs or offspring produced. It plays a critical role in understanding population dynamics and evolutionary fitness.
Fecundity: Fecundity refers to the reproductive capacity of an individual or population, often measured by the number of offspring produced over a specific period. It plays a crucial role in understanding population dynamics and how different life history strategies can affect the survival and growth of populations. High fecundity can lead to rapid population growth, but it also involves trade-offs with other life history traits such as longevity and parental investment.
Generation time: Generation time refers to the average time it takes for a population to grow from one generation to the next, typically measured as the time between the birth of individuals and the birth of their offspring. This concept is crucial in understanding population dynamics, as it influences growth rates, reproductive strategies, and overall population structure. By analyzing generation time, researchers can better predict how populations will respond to environmental changes and resource availability.
Immigration: Immigration is the movement of individuals into a country or region from another country or region for the purpose of settling there, often to seek better opportunities. This phenomenon affects population demographics by influencing factors such as population size, density, and diversity. Immigration can have significant social, economic, and cultural impacts on both the arriving individuals and the existing population in the new area.
Life history strategy: Life history strategy refers to the evolutionary patterns and adaptations organisms develop regarding their growth, reproduction, and survival. These strategies involve trade-offs that determine how organisms allocate resources over their lifetimes, influencing population dynamics and species interactions within ecosystems. Understanding these strategies helps in analyzing population demography and the role of natural selection in shaping the life cycles of various species.
Life table: A life table is a demographic tool that provides a systematic way of representing the mortality and survival rates of a population at various ages. It summarizes the likelihood of individuals surviving from one age to the next, giving insights into population dynamics, reproductive strategies, and the overall health of the population being studied.
Life tables: Life tables are tools used to summarize the survival and reproductive rates of individuals within a population, typically categorized by age or stage. They help ecologists understand patterns of mortality and predict future population dynamics.
Lincoln-Petersen formula: The Lincoln-Petersen formula is a method used in ecology to estimate the size of wildlife populations. It involves capturing a sample of individuals, marking them, releasing them back into the population, and then recapturing another sample to see how many marked individuals are present. This technique helps researchers understand population dynamics, which is crucial for effective conservation and management strategies.
Logistic growth model: The logistic growth model is a mathematical representation of population growth that describes how populations grow rapidly at first, then slow down as they approach the carrying capacity of their environment. This model illustrates the concept of density-dependent factors, which limit population growth as resources become scarce and competition increases. It highlights the dynamic interplay between biotic potential and environmental resistance, ultimately leading to a stable population size around the carrying capacity.
Mark-recapture: Mark-recapture is a method used in ecology to estimate the size of animal populations by capturing a number of individuals, marking them, and then releasing them back into their environment. After a set period, researchers capture another sample of individuals to see how many are marked, allowing for population estimates based on the proportion of marked to unmarked individuals. This technique is crucial for understanding population dynamics and making informed conservation decisions.
Mortality rate: Mortality rate refers to the measure of the number of deaths in a given population during a specific time period, typically expressed per 1,000 individuals. This statistic is crucial for understanding population dynamics as it helps to quantify the impact of health conditions, environmental factors, and social influences on the survival of individuals within a population.
Natality: Natality refers to the birth rate within a population, specifically the number of live births per 1,000 individuals in a given time period. This metric is essential for understanding population growth and demographic changes, influencing factors such as resource availability, population density, and social structures within ecosystems.
Net migration: Net migration is the difference between the number of people entering a population and the number of people leaving that population over a specific period. This term reflects the balance of immigration and emigration, showing whether a population is growing or shrinking as a result of movement. It plays a crucial role in understanding population dynamics, economic impacts, and social changes within regions.
Population density: Population density is a measurement that represents the number of individuals living in a given area, usually expressed as individuals per square kilometer or square mile. This concept is crucial for understanding how populations interact with their environments, affecting resource availability, habitat quality, and social dynamics within species. High population density can lead to increased competition for resources, while low density can indicate a more dispersed distribution of organisms across a landscape.
Population projection: Population projection refers to the estimate of future population sizes and characteristics based on current demographic trends and statistical models. This process incorporates factors like birth rates, death rates, migration patterns, and age distribution to predict how a population will change over time. Understanding population projections is crucial for planning resources, services, and policies to meet future needs.
Population size (N): Population size (N) is the total number of individuals within a defined area at a given time. It is a fundamental parameter in population ecology used to understand population dynamics and health.
Quadrat: A quadrat is a square or rectangular plot used in ecological research to isolate a standard unit of area for study of the distribution of an item over a large area. It helps scientists assess the abundance and distribution patterns of species in a given habitat.
Quadrat sampling: Quadrat sampling is a method used in ecology to estimate the abundance and distribution of organisms within a specific area by surveying small, defined plots called quadrats. This technique helps researchers gather data on population density and species composition, which are critical for understanding ecological dynamics and population demography.
Random distribution: Random distribution refers to a pattern of spacing among individuals in a population where the locations of individuals are independent of one another. This concept is important because it helps to understand how resources, competition, and environmental factors influence the arrangement of organisms within an area. Random distribution can occur when resources are abundant and evenly available, leading to individuals being spaced out without any specific order or pattern.
Species dispersion patterns: Species dispersion patterns describe the spatial arrangement of individuals within a habitat. They are crucial for understanding population dynamics and interactions within ecological communities.
Survivorship curve: A survivorship curve is a graphical representation that shows the number of individuals surviving at each age for a given species or population. It illustrates the mortality rates and life expectancy across different life stages, helping to classify species into different categories based on their reproductive strategies and survival patterns.
Type I survivorship curve: A Type I survivorship curve is a graphical representation that illustrates the high survival rates of individuals throughout most of their life span, with a significant drop in survivorship occurring at older ages. This pattern is typically seen in species that invest heavily in their offspring, providing extensive parental care, which leads to low mortality rates during early and middle life. As individuals age, the likelihood of death increases, leading to fewer survivors by the end of their lifespan.
Type II survivorship curve: A Type II survivorship curve describes a pattern of survival where individuals have a relatively constant probability of dying at each age, resulting in a linear decline in the number of survivors over time. This curve is often observed in species that are subject to consistent environmental pressures throughout their lives, leading to an equal chance of death regardless of age. Understanding this curve is important for analyzing population dynamics and the life history strategies of different organisms.
Type III survivorship curve: A Type III survivorship curve is a graphical representation showing the high mortality rates of individuals at the early stages of their life cycle, followed by lower mortality rates for those that survive to adulthood. This type of curve is often seen in species that produce a large number of offspring but invest little in individual care, resulting in a significant number of young not reaching maturity. Understanding this curve helps to illustrate the reproductive strategies and population dynamics of various organisms within an ecosystem.
Uniform distribution: Uniform distribution refers to a probability distribution where all outcomes are equally likely. In the context of population demography, this concept can help understand how individuals within a population are spaced out across a given area, indicating that each individual has an equal chance of being found anywhere in that region. This can be important when examining factors like resource availability, competition, and mating patterns among species.
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