Population Ecology Lecture Notes

Population Ecology

Introduction

  • Definition of Population: A population is defined as a group of individuals from the same species that live in the same area at the same time.
  • Definition of Population Ecology: Population ecology is the study of how and why the number of individuals in a population changes over time.
    • It incorporates factors such as morphology, behavior, and physiology.
  • Darwin’s Important Population Thinking:
    • Recognizing variation among individuals in a population is a key aspect of understanding how populations change over time in response to their habitats.

Distribution and Abundance

  • Geographic Distribution:
    • Distribution and abundance are fundamental concepts in ecology. Both abiotic (non-living) and biotic (living) factors determine the range of populations.
    • Ranges are dynamic and are constantly fluctuating as abiotic and biotic factors change over time.

Spatial Distribution of Individuals in Populations

  • Spatial Distribution:
    • Types of Spatial Distribution:
    • Random:
      • Illustration: 15 red dots dispersed randomly.
      • Proximate Explanation: Random dispersal occurs due to factors like wind and currents affecting seeds, gametes, or larvae.
      • Ultimate Explanation: Average fitness of populations increases in varied environments as dispersal helps in distributing individuals.
      • Example: Dandelions are randomly distributed in a meadow due to wind dispersal.
    • Clumped:
      • Illustration: 15 red dots in a tight circular cluster.
      • Proximate Explanation: Individuals might form social groups for feeding, mating, and/or avoiding predators or due to patchy resources.
      • Ultimate Explanation: Individuals have higher fitness in groups, and selection favors traits that help in finding patchy resources.
      • Example: Bigeyes fish associate in schools to increase their chances of avoiding predation.
    • Uniform:
      • Illustration: 15 red dots in a structured grid (3 rows, 5 columns).
      • Proximate Explanation: Individuals tend to distance themselves to compete for resources like nutrients and nesting space.
      • Ultimate Explanation: Competition can reduce individual fitness; selection favors traits that minimize competition.
      • Example: Gannets compete for suitable nesting areas on rocky coasts, leading to uniform distribution.

Distribution and Abundance (Continued)

  • The overall distribution pattern in a habitat is influenced by several factors:
    • Proximate Causes: Physiological and behavioral mechanisms that determine how distributions occur.
    • Ultimate Causes: Evolutionary adaptations that have developed over time in response to environmental pressures.

Sampling Methods

  • Methods for Determining Population Size, Density, and Distribution:
    • Different methods are used depending on the species.
    • For sedentary species, populations can be counted along lines of known length (transects) or in known-size plots (quadrats).
    • For mobile species, the mark-recapture method can be applied: individuals are captured, tagged, and then released. The population size can then be estimated based on the number of recaptures.

Demography and Life History

  • The number of individuals in a population is affected by four processes:
    • Birth: New individuals entering the population.
    • Death: Individuals leaving the population.
    • Immigration: Movement of individuals into a population.
    • Emigration: Movement of individuals out of a population.
  • Population Growth:
    • Populations increase due to births and immigration and decrease due to deaths and emigration.

Trade-offs and Life History

  • Fitness Trade-offs: These occur because individuals have a limited amount of time and energy, making resource allocation critical.
  • Fecundity: Defined as the number of female offspring produced by each female in the population.
    • A female can prioritize maximizing fecundity, maximizing survival, or finding a balance between the two.
  • Life History: Refers to how individuals allocate resources for growth, reproduction, and survival-related activities.

Population Dynamics

  • Young vs. Old Populations:
    • A population heavily composed of young individuals with high survival rates and reproduction potential is likely to grow.
    • Conversely, a population predominantly made up of old individuals with low survival and reproduction rates is likely to decline.
  • Life Table: Biologists turn to life tables to analyze population dynamics and gather data on age structure and survivorship.

Age Class and Survivorship

  • Age Class: Refers to a group of individuals that are the same age.
  • Survivorship: The proportion of offspring produced that survive, on average, to a certain age.
  • Survivorship Curve: A graphical representation plotting the logarithm of the number of survivors against age.
    • Type I: High survivorship throughout life, with most reaching maximum lifespan.
    • Type II: Steady decline in survivorship across the lifespan.
    • Type III: High death rate in early life but high survivorship after maturity.

Population Growth Equations

  • Population Growth Rate:
    • Change in population size over time ( rac{ΔN}{Δt} = ext{Births} - ext{Deaths} + ext{Immigrants} - ext{Emigrants})
    • Per Capita Rate of Increase: Denoted by r.
    • Instantaneous Growth Rate: Expressed as rac{dN}{dt} = rN.
    • Intrinsic Rate of Increase: Denoted by r_{max}.
  • Exponential Growth conditions result from:
    1. Colonization of a new habitat.
    2. Recovery after devastation.

Logistic Growth

  • Logistic Growth Model:
    • As population density increases, births (BR) decrease, and deaths (DR) increase, leading to density-dependent (logistic) growth.
    • Carrying Capacity: The maximum number of individuals a habitat can sustain.
    • Density-Independence: Variations caused by abiotic factors like weather patterns.
    • Density-Dependence: Variations based on population size and biotic factors impacting fitness.

Density-Dependent Factors

  • Various factors that regulate population sizes can be categorized as density-dependent:
    • Competition for Resources:
    • e.g., Trees competing for light, water, and nutrients.
    • Disease and Parasitism:
    • e.g., Livestock susceptible to illness.
    • Predation:
    • e.g., An increase in hares raises the number of lynx that prey on them.
    • Toxic Wastes:
    • e.g., Ammonia or uric acid produced from excretion.
    • Social Behavior:
    • e.g., Cannibalistic behavior observed in blue crabs.

Density-Dependent Factors (Continued)

  • Types of Interactions:
    • Intraspecific Interactions: Competition among individuals of the same species, e.g., for food.
    • Interspecific Interactions: Competition, predation, or parasitism involving different species.

Research on Population Cycles

  • Research Question: What factors control the hare-lynx population cycle?
    • Hypotheses:
    • Bottom-Up Hypothesis: Food availability for hares controls the cycle.
    • Top-Down Hypothesis: Predation controls the cycle.
    • Interaction Hypothesis: A combined effect of food availability and predation controls the cycle.
    • Null Hypothesis: The cycle isn't controlled by predation or food availability.
    • Experimental Setup:
    • Survival rates of hares measured from 1987-1994 in seven similar boreal forest plots (1 km² each).
    • Controls Involved:
      • No lynx, extra food, 3 unmanipulated control plots.
      • One electrified fence excluding lynx while allowing hares.
      • Food supply introduced in other manipulated plots.
    • Predictions:
    • Hare populations in manipulated plots expected to be higher than control populations.
    • For the null hypothesis, hare populations would be the same across all plots.
    • Results:
    • Hare survival rates averaged over three years (1990-1992) demonstrated that rates were highest when both no lynx and extra food were present.
    • Conclusion: Hare populations are influenced by both predation and food availability, with combined limitations having a more pronounced effect than each factor independently.

Metapopulation Dynamics

  • Metapopulation: Defined as populations that are interconnected by migration.
    • These populations maintain a balance between extinction and recolonization.
  • Dynamics of Metapopulations: Focus on the birth and death rates of populations as a collective.
  • Dynamics of Single Populations: Concerned with the birth and death rates of individual organisms.

Human Populations

  • The age structure in human populations varies significantly across different countries:
    • Developed countries exhibit uniform age structures.
    • Developing countries typically display bottom-heavy age structures (more young individuals).
  • Age Pyramids: Visual representations that help depict the age structure within populations.
  • Projections: The analysis of age structures raises important public policy concerns and considerations.