PopulationBasics-1

Population Ecology Basics

• Populations exhibit basic principles governing growth, maintenance, and decline.

• Organisms continually interact with each other and their environment.

• Interactions occur at various organizational levels, influenced by selection pressures impacting population structure, genetic makeup, and ecosystem structure.

• Human populations also follow these ecological principles.

Changes in Organism Populations Over Time

• Species must adapt to survive environmental changes, or they face extinction.

• All populations possess genetic variability across traits such as size and speed.

• Humans exploit genetic variability through selective breeding (e.g., creating different breeds of animals).

• Example of selective breeding: Breeding large horses to produce offspring that are progressively larger.

• This process is known as Artificial Selection, which is faster as only the best traits are selected.

• Natural Selection operates similarly but more slowly, allowing for intermediates to survive.

• Fast rabbits, for instance, better evade predators and thus reproduce more, gradually shifting the population's traits over generations.

• Changes in gene frequency through selection are termed Evolution.

Mechanisms of Evolution

• Natural Selection alters gene frequencies but is not the only mechanism. Others include:

• Migration: Movement of individuals in and out of populations.

• Mutation: Changes in genetic code.

• Genetic Drift: Random changes in gene frequencies, often affecting small populations.

• Evolution requires time for the effects of selective pressures to manifest.

• Selective Pressures: Factors that influence survival and reproduction, categorized into four basic types:

• Finding shelter

• Finding food

• Avoiding predation

• Reproducing

• Evolutionary changes generally involve modifying existing structures or behaviors rather than creating new ones.

• For example, sharks have remained relatively unchanged due to the ocean's stability for hundreds of millions of years.

• Higher habitat diversity often leads to increased species diversity.

Population Dynamics

• Individual populations play dynamic roles in ecosystems.

• Growth rates vary widely among species, and limitations on growth maintain ecological balance.

• Example of exponential growth: A single female housefly can lead to over six trillion flies in a year without limits.

Mortality Factors Influencing Populations

• Factors limiting population growth are classified as Density-dependent or Density-independent:

• Density-independent: These affect population regardless of density; examples include weather and natural disasters.

• Density-dependent: These factors kill based on population density; for example, diseases spread more easily in denser populations.

• Key density-dependent factors:

  • Competition

  • Predation

  • Disease

  • Parasitism

Competition

• Competition for resources can be classified as direct or indirect:

• Direct Competition: Direct contact, such as fighting for mates or nest sites.

• Indirect Competition: Occurs without direct contact, e.g., depleted resources prevent others from having access.

• Interspecific competition: Between different species.

• Intraspecific competition: Within the same species.

• Competitive Exclusion Principle: No two species with identical needs can coexist indefinitely.

• Species coexistence requires resource partitioning: Share food, space, or active time to reduce competition.

Predation

• Predation facilitates energy flow through ecosystems and keeps prey healthy by eliminating the sick and less fit individuals.

• It regulates populations, allowing multiple species to coexist by controlling dominant grazers.

• If top predators are removed, herbivore populations can explode, degrading plant populations and overall productivity.

• Example: High deer populations leading to habitat degradation in Butler County.

• Disruption in productivity leads to declines in top predator populations due to reduced energy availability up the trophic pyramid.