♻️AP Environmental Science Unit 3 – Populations

Key Population Concepts

• Population refers to a group of individuals of the same species living in a specific area at a given time
• Population size is the total number of individuals in a population and can change over time due to births, deaths, immigration, and emigration
• Population density measures the number of individuals per unit area (square kilometers or hectares)
• Carrying capacity is the maximum population size an environment can sustain given available resources (food, water, shelter)
• Exponential growth occurs when a population grows at a constant rate, doubling in size at regular intervals (bacteria)
  • Characterized by a J-shaped curve on a population growth graph
• Logistic growth happens when a population's growth slows down as it approaches the carrying capacity
  • Represented by an S-shaped curve on a population growth graph
• Age structure refers to the distribution of individuals in a population across different age groups (pre-reproductive, reproductive, and post-reproductive)

Population Growth and Dynamics

• Populations can experience growth, stability, or decline depending on the balance between births, deaths, immigration, and emigration
• Birthrate is the number of live births per 1,000 individuals in a population per year
• Death rate is the number of deaths per 1,000 individuals in a population per year
• Natural increase rate is the difference between the birthrate and the death rate, expressed as a percentage
• Doubling time is the time required for a population to double in size, calculated using the rule of 70 (70 divided by the annual growth rate)
• Survivorship curves depict the proportion of individuals in a population that survive to each age
  • Type I: high survival until old age, then rapid decline (humans, elephants)
  • Type II: constant mortality rate throughout life (birds, lizards)
  • Type III: high mortality in early life stages, followed by low mortality for survivors (fish, oysters)
• Life tables summarize age-specific survival and mortality rates for a population

Factors Affecting Population Size

• Biotic factors are living components of the environment that influence population size (predation, competition, parasitism)
• Abiotic factors are non-living components of the environment that affect population size (temperature, rainfall, sunlight)
• Resource availability, such as food, water, and shelter, can limit population growth
• Predation can reduce population size by increasing mortality rates
• Competition for resources among individuals of the same species (intraspecific) or different species (interspecific) can limit population growth
• Disease outbreaks can significantly reduce population size by increasing mortality rates
• Natural disasters (earthquakes, floods, wildfires) can cause sudden declines in population size
• Human activities, such as habitat destruction, pollution, and overharvesting, can negatively impact population size

Human Population Trends

• The global human population has grown exponentially, reaching 7.8 billion in 2020
• The human population growth rate has slowed in recent decades but remains positive
• Fertility rate is the average number of children born to a woman during her reproductive years
• Replacement level fertility is the fertility rate needed to maintain a stable population size (2.1 children per woman)
• Life expectancy is the average number of years an individual is expected to live, based on current mortality rates
• Infant mortality rate is the number of deaths of infants under one year old per 1,000 live births
• The human population is unevenly distributed, with high concentrations in urban areas and certain regions (Asia, Africa)
• Migration, both within and between countries, can significantly affect population distribution and growth

Demographic Transition Model

• The demographic transition model describes the changes in birth rates, death rates, and population growth as a society develops
• Stage 1: Pre-industrial society with high birth rates, high death rates, and slow population growth
• Stage 2: Developing society with high birth rates, declining death rates, and rapid population growth
  • Improved healthcare and sanitation reduce mortality rates
• Stage 3: Industrializing society with declining birth rates, low death rates, and slowing population growth
  • Increased education and economic opportunities lead to lower fertility rates
• Stage 4: Developed society with low birth rates, low death rates, and stable or declining population growth
• Stage 5: Post-industrial society with birth rates below replacement level, leading to population decline (Japan, Germany)
• The model helps explain the population dynamics observed in different countries and regions over time

Population Ecology and Interactions

• Population ecology studies the factors that influence population size, distribution, and dynamics
• Intraspecific interactions occur between individuals of the same species (competition, cooperation)
• Interspecific interactions occur between individuals of different species (predation, mutualism, parasitism)
• Predator-prey relationships involve one species (predator) feeding on another (prey), affecting the population sizes of both
  • Predators can help regulate prey populations and maintain ecosystem balance
• Symbiotic relationships involve close associations between two species
  • Mutualism: both species benefit (flowers and pollinators)
  • Commensalism: one species benefits while the other is unaffected (sharks and remora fish)
  • Parasitism: one species (parasite) benefits at the expense of the other (host)
• Keystone species have a disproportionately large impact on their ecosystem relative to their abundance (sea otters, wolves)
• Invasive species are non-native organisms that can disrupt ecosystems and negatively impact native populations (kudzu, zebra mussels)

Environmental Impacts of Population Growth

• Population growth can lead to increased demand for resources (food, water, energy) and strain on ecosystems
• Habitat loss and fragmentation occur as human populations expand and convert natural areas for agriculture, urbanization, and infrastructure
• Deforestation, particularly in tropical regions, contributes to biodiversity loss and climate change
• Overexploitation of natural resources (overfishing, poaching) can lead to population declines and ecosystem imbalances
• Pollution (air, water, soil) from human activities can harm wildlife populations and degrade habitats
• Climate change, exacerbated by human population growth and resource consumption, can alter species distributions and disrupt ecosystems
• Biodiversity loss accelerates as human populations grow and encroach on natural habitats
• Increased human-wildlife conflicts arise as human populations expand into animal habitats (crop raiding, livestock predation)

Sustainable Population Management

• Sustainable population management aims to balance human needs with environmental conservation
• Family planning programs provide education and access to contraceptives to help individuals make informed reproductive choices
• Empowering women through education and economic opportunities can lead to lower fertility rates and more sustainable population growth
• Sustainable resource management practices (renewable energy, sustainable agriculture) can reduce the environmental impact of growing populations
• Protected areas and conservation efforts help preserve biodiversity and maintain ecosystem services in the face of population pressures
• Sustainable urban planning and green infrastructure can minimize the environmental footprint of cities
• Education and awareness campaigns can promote sustainable lifestyles and consumption patterns
• International cooperation and agreements (Paris Agreement, Sustainable Development Goals) aim to address global population and environmental challenges
• Adaptive management strategies allow for flexibility and adjustment in response to changing population dynamics and environmental conditions