Population Ecology

Learning Objectives – Population Ecology Part 1

  • Define geographic range and describe:

    • **Factors influencing a species

  • Climate

  • Physiological adaptations

  • Nutrient/food availability

  • Interspecific interactions

  • Dispersal ability/mobility

  • Historical factors (such as glaciations)

    • **Factors influencing distribution of populations within species

  • Microclimate conditions

  • Resource availability

  • Interspecific interactions (local and anthropogenic)

  • Mobility and accessibility of areas

    • Different distributions within a population:

  • Clumped distribution: most common, often results from social interaction and availability of resources.

  • Uniform distribution: less common, may arise due to resource scarcity.

  • Random distribution: rare, can be difficult to distinguish from clumped distribution.

  • Define life history traits and explain their importance to individual fitness:

    • Life history traits: phenotypes affecting life history strategy.

    • Life history strategy: age-specific schedules of survival and reproduction.

  • Describe the contents and assumptions of a life table and its questions:

    • Contents include age structure, survival/mortality rates, and fecundity.

    • Assumptions:

    • Individuals of the same age group behave similarly.

    • The population is all female.

  • Estimate age-specific survival and mortality rates from population cohort data.

  • Estimate reproductive output and population growth rates from life table data.

  • Explain how life tables indicate population growth.

Population Ecology: Distribution

  • Key questions:

    • What limits species

    • How do different factors affect population distribution?

Examples of Salamander Species

  • Northern: Plethodon glutinosus

  • White-spotted: Plethodon cylindraceus

  • Atlantic Coast: Plethodon chlorobryonis

  • Savannah: Plethodon savannah

  • Others: Plethodon variolatus, Plethodon chattahoochee, Plethodon ocmulgee, Plethodon grobmani, Plethodon mississippi, Plethodon kiamichi, Plethodon kisatchie, Plethodon sequoyah, Plethodon albagula.

Geographic Range Factors

  • Geographic ranges are limited by physiological adaptations to climate

  • Example: Sugar maple (Acer saccharum)

    • South: Atlantic Ocean

    • North, West, East: limited by aridity and seasonal temperatures.

Additional Range Limiting Factors

  • Nutrient/Food availability: e.g., Monarch Butterfly range limited by milkweeds (Asclepias).

  • Interspecific interactions: relationships with other species can dictate presence in certain areas.

  • Dispersal ability: populations may be confined to areas hard to access.

  • Historical factors: such as climatic events (e.g., glaciations).

Population Distribution Within Species Ranges

  • Microclimate variations influence temperature and moisture.

  • Nutrient availability often found in patches.

  • Interspecific interactions influence local populations, can change due to anthropogenic effects.

  • Dispersal ability can limit individual distribution due to area accessibility.

Distribution Patterns of Individuals Within Populations

  • Clumping: Most common; driven by social behavior, safety in numbers, mating, and resource allocation.

  • Uniform Distribution: Seen in cases of resource scarcity, leading to territorial behaviors.

  • Random Distribution: Rare; ideal for dispersal via wind – can occur in plant populations.

Clumped/Aggregated Distribution Causes

  • Social tendencies to form groups (e.g., silversides).

  • Limited dispersal abilities lead to aggregation.

  • Resources may be clumped together.

Uniform/Evens Spacing Causes

  • Territoriality among species (e.g., king penguins, creosote bush).

  • Resource competition leads to evenly spaced distributions.

Life History Traits

  • Definition: Traits that determine life history strategies for an organism.

  • Life History Strategy: Schedules of survival and reproduction determined by energy distribution across survival, growth, and reproduction at various ages.

  • Life History Traits Examples:

    • Life Span: Mayfly (1 day) vs. Bristlecone Pine (5000 years).

    • Offspring Size: Kiwi bird laying eggs half the mother

    • Offspring Number: 300 million eggs (e.g. fish) versus 1 offspring every 5 years for others.

    • Maturity: Dwarf surfperch mature at birth vs. Magicicadas (17 years).

The Darwinian Demon

  • Hypothetical organism that reproduces at birth, produces infinite offspring, and lives indefinitely.

Trade-offs Among Life History Traits

  • Limited energy leads to trade-offs between:

    • Storage, maintenance, repair, growth, parental care, reproduction.

Life History Continuum

  • High-fecundity vs. low-fecundity traits: Influenced by life spans, offspring size, maturity, and survivorship.

  • High juvenile mortality favor delayed reproduction, larger offspring over time.

  • High adult mortality promotes early maturity and high offspring production rate with smaller size.

Life Tables

  • Assumptions:

    • Lowest/highest mortality risk per age group.

    • Population growth rate and future projections.

    • Age-specific reproduction investments.

  • Types:

    • Cohort Life Table: Tracks individuals from a cohort.

    • Static Life Table: Captures age structure and reproductive rates at a single moment in time.

Life Table Calculations

  • Dynamic Variables:

    • n_x: Number alive at age x.

    • lx: Proportion surviving to age x ( lx = nx / n0 ).

    • d_x: Number dying in age interval.

  • To calculate age-specific mortality rate ( qx ): qx = dx / nx.

Estimating Reproductive Output

  • Use life tables to estimate fecundity:

    • lx mx indicates female offspring per year.

    • Age-specific fecundity expressed as m_x (number of offspring per individual).

Population Growth Projections

  • Net Reproductive Rate ( R0 ): R0 = \sum lx mx.

  • If R_0 > 1, the population is growing.

Population Growth under Conditions

  • Density-Independent Growth: Characterized by unlimited resources, where growth rate remains constant and population size increases exponentially.

  • Logistic Growth: Growth rate declines as the population approaches carrying capacity (K), represented mathematically as:

    • \frac{dN}{dt} = rN\frac{(K-N)}{K}.

    • \frac{dN}{dt} = rate of population change.

Key Characteristics of Logistic Growth

  • Early growth is rapid due to low density; slows as resources become limited.

  • Carrying Capacity (K): Maximum population size that the environment can sustain.