Populations

Outline

  • Defining Ecology

  • Properties of a Population

    • Density

    • Size (and Change of Size over Time)

    • Age Structure and Survivorship Curves

    • Reproductive Strategies

  • Exponential versus Logistic Growth

  • Human Population Growth

Defining Ecology

  • Ecology is defined as the study of interactions among organisms and the environment. Ecologists analyze these interactions at several levels, including populations, communities, ecosystems, and the biosphere.

Review: Organization of Life

  1. Organism

    • Definition: A single living individual.

    • Example: One acacia tree.

  2. Population

    • Definition: A group of the same species of organisms living in the same place and time.

    • Example: Multiple acacia trees.

  3. Biosphere

    • Definition: The global ecosystem; parts of the planet and its atmosphere where life is possible.

  4. Ecosystem

    • Definition: The living and nonliving components of an area.

    • Example: The savanna.

  5. Community

    • Definition: All populations that occupy the same region.

    • Example: All populations in a savanna.

What is a Population?

  • Population: A subset of individuals of one species that occupies a particular geographic area and, in sexually reproducing species, interbreeds.

  • Populations typically fall into one of three distribution patterns:
    a. Clumped Distribution
    b. Uniform Distribution
    c. Random Distribution

Habitat and Population Density

  • Habitat: The physical location where members of a population normally live, e.g., palm trees in a desert.

  • Population Density: The number of individuals of a species per unit area or unit volume of a habitat.

Estimating Population Size

  • Ecologists use various methods to count population members, including:

    • Aerial photos

    • Sampling small subsets

    • Mark-recapture methods

Importance of Determining Population Size

  • Population ecologists study the factors affecting how a population changes, which is vital for managing population size. Conditions affecting population size include:

    1. Growth: A population grows when it adds more individuals than it loses.

    2. Stability: A population remains stable when it adds the same number of individuals as it loses.

    3. Decline: A population shrinks when it loses more individuals than it adds.

  • Practical applications include controlling animal pests.

Factors Affecting Population Size

  • Births:

    • Birth Rate: Number of new individuals produced per individual in a defined time period.

    • Affected by:

    • Number of reproductive episodes per lifetime

    • Number of offspring per reproductive episode

    • Age at first reproduction

    • Population age structure

    • Migration into the population

    • Availability of dispersal mechanisms and suitable habitats.

  • Deaths:

    • Death Rate: The number of deaths per unit time, scaled by population size.

    • Affected by:

    • Accidents

    • Genetic/infectious diseases

    • Predation

    • Nutrient availability

    • Migration out of the population

    • Availability of dispersal mechanisms.

Age Structure and Birth Rate

  • A population’s age structure significantly impacts its birth rate:

    • Populations with many young, reproducing individuals tend to have a high birth rate and grow.

    • Populations with mostly older individuals have lower birth rates and may stabilize or decline.

Life Tables and Survivorship Curves

Life Tables provide data on the probability of surviving to any given age, e.g.,

Age (years)

Number of Survivors (out of 1000)


0

1000


1

324


2

278


3

242


4

217


5

193


6

172


7

154


8

138


9

122


10

110


11

98


12

87


13

75


14

71


15

63


16

57

Types of Survivorship Curves

  1. Type I: Species invest energy caring for young; most individuals survive to reproduce.

  2. Type II: Species have an approximately equal probability of dying at any age.

  3. Type III: Species invest little energy in raising young; few individuals survive to reproduce.

Reproductive Strategies

  • Reproductive strategies reflect a trade-off between the quantity and quality of offspring, generally classified into two patterns:

    1. Opportunistic Life History:

    • Characterized by producing many offspring (often type III survival rates). Examples include weeds and insects.

    1. Equilibrium Life History:

    • Characterized by producing fewer, high-quality offspring (often type I survival rates).

Population Growth Patterns: Exponential vs Logistic

  • Exponential Growth:

    • Defined as a rapid increase in population size over time, exemplified by a J-shaped curve on a graph.

    • Example: A seal population tracked over forty years continues to grow due to no resource limitations.

    • Limitation: Exponential growth is unsustainable due to resource depletion.

  • Logistic Growth:

    • Characterized by slowing growth as a population approaches its carrying capacity (K), the maximum number of individuals the habitat can support indefinitely.

Cycles in Population Growth

  • Population growth can exhibit cycles, influenced by factors such as predation, for example, the lemming population fluctuating due to stoat predation.

Factors Limiting Population Size

  • Density-dependent Factors:

    • Effects increase as population density grows, primarily biotic factors:

    • Competition for resources

    • Increased disease spread

    • Higher predation risks.

  • Density-independent Factors:

    • These factors limit populations at all densities, typically abiotic:

    • Natural disasters

    • Environmental pollution

    • Habitat destruction.

Human Population Growth

  • Human populations have experienced exponential growth, recently unrestrained by environmental resources or other limiting factors.

  • Growth trends reflect increasing birth rates and decreasing death rates, particularly in less developed countries.

Variability in Birth and Death Rates

  • Birth and death rates vary significantly among human populations:

    • High birth rates and low death rates contribute to population growth in developing regions.

    • Low-income countries primarily face infectious disease challenges, whereas high-income countries deal with chronic diseases associated with aging.

Age Structures and Growth Rates

  • Age distributions impact projected population trends:

    • Countries like India have high proportions of young individuals, suggesting continued growth.

    • Contrarily, the U.S. and China show lesser proportions in young demographics, indicating stabilization or decline in their populations.

Human Overexploitation of Resources

  • The ecological footprint of countries quantifies how lifestyle choices strain available resources. Understanding this footprint helps mitigate resource over-exploitation.

Inequity in Resource Distribution

  • Resource consumption is unevenly distributed among countries, emphasizing the need for sustainable practices to prevent depletion of resources.