19.1 Population Ecology and Dynamics

Population ecology explores how groups of organisms interact with their environment and each other. This field examines factors influencing population size, growth, and distribution, from food availability to predation.

Understanding population dynamics is crucial for conservation and resource management. By studying how populations change over time, scientists can predict trends, assess ecosystem health, and develop strategies to protect biodiversity.

Population dynamics in ecosystems

Population characteristics and structure

• Population consists of individuals of the same species living in a particular geographic area at the same time
• Key population characteristics include size, density, distribution pattern, age structure, and sex ratio
• Population dynamics describe changes in size and structure over time due to births, deaths, immigration, and emigration
• Metapopulations comprise spatially separated populations of the same species interacting through dispersal
• Population dispersion patterns classify as uniform, random, or clumped, influenced by resource availability and species behavior (ant colonies, schooling fish)
• Demographic tools analyze population structure and predict future trends
  • Life tables track survival and reproduction rates across age groups
  • Survivorship curves illustrate mortality patterns (Type I: humans, Type II: birds, Type III: oysters)

Ecological interactions and population ecology

• Population ecology examines how populations interact with their environment and other species
• Competition occurs when organisms vie for limited resources (food, space, mates)
  • Intraspecific competition happens between members of the same species
  • Interspecific competition occurs between different species
• Predation involves one organism consuming another (lions hunting zebras)
• Mutualism describes relationships where both species benefit (clownfish and sea anemones)
• Commensalism occurs when one species benefits while the other is unaffected (remora fish attaching to sharks)
• Parasitism involves one organism living on or in a host, causing harm (tapeworms in mammals)

Factors influencing population growth

Biotic and abiotic influences

• Biotic factors affect population growth
  • Food availability limits population size (rabbits in grasslands)
  • Predation regulates prey populations (lynx and snowshoe hare cycles)
  • Disease outbreaks can cause rapid population declines (chytrid fungus in amphibians)
• Abiotic factors impact population dynamics
  • Temperature influences metabolic rates and reproduction (insects in varying climates)
  • Rainfall affects plant growth and habitat quality (savanna ecosystems)
  • Habitat quality determines resource availability (coral reefs for marine life)

Population growth concepts

• Intrinsic rate of increase (r) represents maximum growth rate under ideal conditions
• Carrying capacity (K) defines maximum sustainable population size in a given environment
• r- and K-selection describe different life history strategies
  • r-selected species produce many offspring with little parental care (bacteria, insects)
  • K-selected species have fewer offspring with extensive parental investment (elephants, whales)
• Population regulation occurs through negative feedback mechanisms
  • Stabilizes population size around carrying capacity
  • Examples include increased competition and reduced reproduction at high densities

Interpreting population growth curves

Types of population growth

• Exponential growth occurs with unlimited resources, resulting in J-shaped curve
  • Observed in bacterial cultures or invasive species in new environments
• Logistic growth incorporates carrying capacity, producing S-shaped curve
  • Population approaches equilibrium as resources become limiting
  • Logistic growth equation: $\frac{dN}{dt} = rN(\frac{K-N}{K})$
    • dN/dt: rate of population change
    • r: intrinsic growth rate
    • N: current population size
    • K: carrying capacity

Complex growth patterns

• Real populations exhibit complex growth due to environmental fluctuations and species interactions
• Population cycles show oscillating growth curves
  • Predator-prey systems (wolves and moose on Isle Royale)
  • Host-parasite interactions (measles outbreaks in human populations)
• Boom-and-bust cycles occur when populations overshoot carrying capacity
  • Example: algal blooms followed by population crashes in aquatic ecosystems
• Human population growth historically followed exponential curve
  • Now showing signs of slowing in many regions due to demographic transition

Density-dependent vs density-independent factors

Density-dependent factors

• Intensify as population density increases
• Competition for resources becomes more intense in crowded populations
  • Intraspecific competition for territories in birds
  • Plant competition for sunlight and nutrients
• Predation pressure often increases with prey density
  • Functional and numerical responses of predators
• Disease transmission rates typically rise in dense populations
  • Spread of respiratory infections in urban areas
• Parasitism can increase with host density
  • Tick infestations on deer in overpopulated forests

Density-independent factors

• Affect populations regardless of their density
• Extreme weather events impact populations indiscriminately
  • Hurricanes destroying coastal habitats
  • Droughts affecting plant and animal communities
• Natural disasters can cause widespread population declines
  • Volcanic eruptions disrupting ecosystems
  • Wildfires altering landscape and wildlife populations
• Human-induced habitat destruction affects species regardless of density
  • Deforestation impacting tropical biodiversity
  • Urban development fragmenting natural habitats
• Climate change acts as both density-dependent and density-independent factor
  • Alters resource availability and habitat suitability
  • Shifts species ranges and phenology

Implications for population management

• Allee effects describe reduced fitness at low population densities
  • Difficulties finding mates in sparse populations
  • Reduced group vigilance against predators
• Relative importance of factors varies across species and ecosystems
• Understanding these factors crucial for wildlife management and conservation
  • Designing effective protected areas
  • Implementing sustainable harvest strategies
• Predicting impacts of environmental changes on populations
  • Modeling species responses to climate change
  • Assessing vulnerability of endangered species to habitat loss