Population Ecology Review Flashcards

Population Ecology

Human World Population

• 2023: Approximately 7.9 billion people, more than double the population since 1965.
• 2050: Projected 9.8 billion people. The question is raised whether the planet can support this many people and if everyone would have the same level of support.

Exponential Growth of Human Population

• Growth rate is determined by the formula: Growth : rate = Birth : rate – Death : rate
• Major factors determining population size.

Ecology

• Ecology is the study of the interactions of living things with each other and their environment.

Arithmetic and Exponential Growth

• Demonstrates how populations grow.

Limits to Population Growth

• Asks whether a species population can grow unlimitedly.
• Population size is affected by birth rate, death rate, resources, competition, interactions with other species and conditions.

Limits to Population Growth: Carrying Capacity

• Carrying capacity: the maximum population size that can be supported indefinitely in a given environment.
• Populations may grow exponentially but are limited by environmental resources such as food, water, shelter, and space.
• No population can grow unlimitedly indefinitely because resources are finite.

Exponential vs. Logistic Growth

• Exponential growth: Occurs in populations not limited by environmental resources before reaching their carrying capacity.
• Logistic growth: Occurs in populations limited by environmental resources as they approach their carrying capacity.
• K = carrying capacity

Complex Growth Patterns

• Other more complex growth patterns can be observed but they also have to do with carrying capacity

Density-Dependent vs. Density-Independent Factors

• Density-dependent factors: Influence population growth rates based on population size (density).
• Density-independent factors: Influence population growth rates regardless of population size (density).

Density-Dependent Factors

• Population-limiting factors that increase with population size: Limited food supply, increased risk of disease, increase in waste levels.
• Results in: Decrease in birth rates and/or increase in death rates.

Density-Independent Factors

• Influence population growth rates regardless of population size:
  • Droughts
  • Temperature extremes
  • Natural disasters
• The severity of effects may depend on population size; for example, competition for resources increases if a population is already close to carrying capacity and conditions worsen.

Limits to Population Growth for Humans

• 1960s: Highest population growth rate at 2.1\%. Current growth rate is 1.1\%.
• Death rates continue to decline, and birth rates are declining (by choice).

Carrying Capacity for Humans

• The question of when we will reach carrying capacity is raised, with a range of future projections; this is difficult to predict.

Resources and Competition

• Critical factors to determine how far away we are from reaching carrying capacity.
• Net primary productivity (NPP): the amount of vegetable matter produced in a given amount of time.
• Human food resources depend on the Net primary productivity (NPP)

Net Primary Productivity

• The amount of vegetable matter produced in a given amount of time. Human food resources depend on net primary productivity.

Signs Indicating Human Population Is Not Near Carrying Capacity Yet

• Enough food is produced to feed around 11 billion people.
• We currently use between 1/4 to 1/3 of the total land Net Primary Productivity.
• The maximum Net Primary Productivity of the Earth could theoretically support around 21 billion people if humans were solely supported by photosynthetic products.

Essential Resources

• Other resources besides food are essential to sustain populations:
  • Clean water
  • Clean air
  • Energy for heating, producing, and preserving food
• Resource use produces pollution, which affects air and water quality.

Earth's Water Distribution

• Freshwater accounts for 2.5\%, oceans 96.5\%, and other saline water 0.9\%.
• Of freshwater, glaciers and ice caps make up 68.7\%, groundwater 30.1\%, and surface/other freshwater 1.2\%.

Non-Renewable Energy Resources

• We rely on non-renewable energy resources: one-time stock resources that cannot be easily replaced, such as:
  • Fossil fuels: buried remains of ancient plants transformed by heat and pressure.
    • Coal
    • Oil
    • Natural gas
• Use of fossil fuels depends on population size and average lifestyle.

American Use of Resources

• Americans make up 5\% of Earth’s population but consume 24\% of global energy.
• Consume 815 billion food calories per day, which is 200 billion more calories than needed (enough to feed 80 million additional people).

Resource Consumption Comparison

• The average American uses as much as:
  • 2 Japanese or Spaniards
  • 3 Italians
  • 6 Mexicans
  • 13 Chinese
  • 31 Indians
  • 128 Bangladeshis
  • 307 Tanzanians
  • 370 Ethiopians

Energy Consumption Comparison

• The World Bank (2011) stated that 3.7 billion people (roughly 50\% of the total world population) consume 11\% less energy than the US alone.

Factors Impacting Human Population

• Wars
• Disease: Flu, HIV, Ebola, Covid (e.g., the 1918-1919 flu pandemic killed about 5\% of the total world population).
• Natural disasters (random?)

Human Population Growth Rate

• 1800: 1 billion people
• 1930: 2 billion people
• 1970: 4 billion people
• Current growth rate: ~1.2\% per year
• Current population: ~7.9 billion people
• 86 million people added each year: more than combined populations of California, Texas, and New York.
• 250,000 people added every day.
• Three people added every second.
• Growth : rate = Birth : rate – Death : rate

Demographic Transition

• The period when birth rates are dropping toward lowered death rates.
• Pre-industrial revolution: high birth and death rates.
• 18th century (industrialization): deaths decreased due to advanced disease treatment and prevention.
• Transition time affects population size.
• Developed countries: short transition, low growth.
• Less developed countries remain in demographic transition.

Signs of Not Reaching Carrying Capacity for Humans

• The rate of population growth decreases as a consequence of lack of resources, leading to more deaths and fewer births.
• For humans, death rates continue to decline (resources not limiting yet).
• In developed countries, birth rates are declining (by choice).

Estimated World Water Use

• Presents a graph of estimated world water use over time, broken down by reservoir losses, municipal use, industry use, and agriculture.

Population Dispersion

• How organisms are distributed in space:
  • Clumped distribution
  • Uniform distribution
  • Random distribution

Clumped Distribution

• High densities in resource-rich areas, low densities elsewhere.
• Globally, humans are clumped around resources (waterways).
• Plants and animals clump around food and energy sources.

Uniform Distribution

• Spacing between individuals tends to be equal.
• Examples: spacing between human houses on a local level, territorial species.

Random Distribution

• Shown by nonsocial species with the ability to tolerate a wide range of conditions.
• No compelling factor brings individuals together or pushes them apart.
• Trees with windblown seeds.

Population Definition

• All the individuals of a species in a given area.

Studying a Population

• To study a population, one must consider:

  • What is the size of the population you start with?
  • What factors affect the birth rate?
  • What factors affect the death rate?
  • Population : size = Birth : rate – Death : rate

Measuring Population Size

• Population ecologists normally have to estimate the size of a population of interest.
  • Direct census: Individuals are counted or surveyed.
  • Mark-recapture method: Estimates the size of more mobile or inconspicuous species.

Mark-Recapture Method Example

1. Capture and mark 100 salamanders.
2. Release them into the pond.
3. In a second visit to the pond, take another sample of 100 salamanders from the lake.
4. If 50 of the 100 salamanders were already marked
   → 50\% of the sampled population was marked
   → the whole population should be around 200.

Exponential vs. Logistic Growth

• Exponential growth: Pattern of growth seen in populations that are not limited by environmental resources (before reaching their carrying capacity).
• Logistic growth: Pattern of growth seen in populations limited by environmental resources (when they reach their carrying capacity).

Limits to Population Growth for Humans

• 1960s: 2.1\% (highest population growth rate).
• Current growth rate = 1.1\%. Human population still growing but closer to a stable number.
• Three scenarios projected by UN: High-growth, Medium-growth, Low-growth (Very difficult to predict).
• Death rates continue to decline.
• Birth rates are declining (by choice).