Ecology & Population

Ecology Basics

  • Ecology: study of interactions between organisms & their environments
  • Organizational levels:
    • Individuals
    • Populations: interbreeding individuals of one species
    • Communities: interacting populations of different species
    • Ecosystems: living + non-living components within an area

Population Ecology

  • Focus: population size, growth patterns, environmental interactions
  • Key question: how do resources & interactions regulate population size?

Population Growth Models

  • Exponential growth:
    • Rate proportional to current size; continuous acceleration
    • Occurs while resources abundant
  • Logistic growth:
    • Starts exponential, slows as resources limit
    • Levels at carrying capacity KK (maximum sustainable population)
    • Equation (conceptual): dNdt=rN(1−NK)\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right)
  • Stabilized populations hover around KK

Carrying Capacity & Management

  • Maximum Sustainable Yield (MSY): harvest at 12K\tfrac{1}{2}K to maintain growth
  • Difficult because KK fluctuates with environmental variables
  • Sub-K management reduces risk of overexploitation

Factors Limiting Population Size

  • Density-dependent:
    • Disease & parasites
    • Predation pressure
    • Competition for food, water, habitat
  • Density-independent:
    • Weather extremes: floods, droughts, fires, climate shifts
    • Habitat destruction

Population Oscillations

  • Many populations cycle through boom & bust due to interacting factors

Life History & Reproductive Strategies

  • Key traits:
    • Age at first reproduction
    • Survival & reproductive probabilities
    • Litter size & frequency
    • Longevity
    • Reproductive vs. growth investment
  • Strategy trade-offs: growth vs reproduction, offspring size vs number, parental care vs litter size

Survivorship Curves

  • Type I: low early mortality (e.g., humans)
  • Type II: constant mortality
  • Type III: high early mortality (e.g., many fishes, plants)
  • Life-history traits align with curve type (e.g., Type III → early reproduction, many offspring)

Human Population Dynamics

  • Historical milestones: 11 billion reached ~1800; rapid rise post-industrial & medical advances
  • Demographic transition:
    • Stage 1 ("3rd World"): high birth & death rates
    • Transition: death rate drops first → population spike; birth rate later declines
    • Stage 2 ("1st World"): low birth & death rates, stabilized growth
  • Global carrying capacity uncertain; depends on water, food, energy, health care resources

Resource Stress Indicators

  • Water stress: many regions approaching or exceeding sustainable groundwater use
  • Food insecurity: significant portions of populations in crisis (IPC ≥ Phase 3)
  • Economic signals: rising debt, lower savings, stagnating wages highlight resource allocation pressures

Future Considerations

  • World population projected 9−119−11 billion by 2100; growth rate slowing but still positive
  • Approaching KK will intensify density-dependent limitations: competition, disease, habitat scarcity
  • Effective management requires balancing resource use, technological innovation, and equitable distribution