Population Ecology

Population Ecology

  • Focus on understanding the dynamics of populations in relation to environmental factors and species interactions.

Learning Goals

  • Identify factors influencing species distribution.

  • Understand key emergent properties of populations.

  • Compute exponential population growth and concepts of carrying capacity.

  • Grasp key elements influencing life history evolution.

Geographic Range of Species

  • Every species has a geographic range that reflects where it can potentially exist.

  • Maximum range for most species in the U.S. is 4-24 states.

  • Cosmopolitan species: Found worldwide.

  • Endemic species: Restricted to small, specific areas.

Population Distribution

  • No species is truly ubiquitous; all are limited to specific habitats.

  • Suitable habitats are often clustered, leading to separated populations due to environmental discontinuities.

Population Definition

  • Populations consist of groups of individuals living in a specific habitat at a given time, typically separated by natural barriers.

  • Members of a population interact more with each other than with individuals from other populations.

Species Interaction Example: Channel Catfish

  • Prefer fast-moving, gravel-bottomed rivers;

  • Juvenile catfish thrive in turbid waters to avoid predation;

  • Adult catfish are adept swimmers; populations are concentrated in suitable river systems like those in Illinois.

Migration Behavior

  • Example: Salmon spawning upstream and migrating to ocean for growth.

  • Monarch butterflies demonstrate mass migration for overwintering and reproduction.

Emergent Properties of Populations

  • These arise from interactions with the environment and influence evolutionary processes.

  • Key aspects: Size, Density, Patterns of Dispersion, Age Structure, Sex Ratio, Variability.

Population Size

  • Defined as the number of individuals within a population at any given time (also called abundance).

Population Density

  • Number of individuals per unit area/volume.

  • Density often has more ecological impact than absolute numbers.

Dispersion Patterns

  • Three general patterns:

    • Clumped: Individuals grouped in favorable habitats.

    • Regular: Even spacing due to interactions like competition or territoriality.

    • Random: Absence of strong attraction or repulsion among individuals.

Age Distributions and Growth Impacts

  • Age structure affects future population growth; stable populations show greater proportion of individuals at certain ages.

  • Rapidly growing populations tend to have more young individuals.

Age Structure Examples

  • Comparative age structures in Kenya (rapid growth), U.S. (slow growth), and Italy (zero growth).

Implications of Ageing Population

  • Increased elderly leads to more retired individuals and higher healthcare costs; fewer young workers and consumers impact economies.

Sex Ratio

  • Proportion of individuals of each sex; critical for population growth.

  • Examples of species exhibiting varied sex ratios affecting reproduction, such as wasps and elk.

Variability Among Populations

  • Differences arise primarily from genetic and environmental factors.

Population Growth Modeling

  • First Law of Ecology: All populations can grow exponentially under ideal conditions.

Exponential Growth Equation

  • ( \frac{dN}{dt} = bN - dN ); where ( b ) is birth rate and ( d ) is death rate.

  • ( N(t) = N_0 e^{rt} ) where ( r ) is growth rate, and ( N_0 ) is initial population size.

Growth Rate Characteristics

  • Generally decreases with larger and longer-living organisms; influenced by reproduction types (egg layers vs. live bearers).

Carrying Capacity (K)

  • Maximum population size sustainable by environment under resource limitations.

Logistic Growth Model

  • Incorporates carrying capacity: ( dN/dT = r_{max} N (K-N)/K )

  • Commonly reflects actual population behaviors in lab settings, but may not accurately predict behaviors in varied environments due to factors like lag time.

Human Population Trends

  • Growth influenced by cultural practices, technology, and historical health improvements.

  • Historic vs. contemporary carrying capacities associated with agriculture and industrial developments.

Demography

  • Study of population structures, dynamics, mortality, and reproduction;

  • Malthus highlighted growth limitations based on resource availability.

Population Estimation Techniques

  • Complete enumeration or sampling methods such as mark-recapture.

Life Tables

  • Used to calculate mortality rates across cohorts and lifetime expectations, providing insights into overall population health and survival chances.

Survivorship Curves

  • Graphical representation of age-specific survival probabilities, reflecting different survival strategies in populations.

    • Type I: High survival in early and middle life.

    • Type II: Constant mortality risk regardless of age.

    • Type III: High early mortality, lower death rate as individuals age.

Considerations on Civilization Collapse

  • Contextualize environmental and economic sustainability in light of population growth and resource exploitation.