Population Dynamics Overview

Population Dynamics

• Population: a group of individuals of the same species living in a defined area at a specific time.
• Key characteristics of a population:
  • Size: number of individuals.
  • Natality: birth rate.
  • Mortality: death rate.
  • Sex ratio: proportion of males to females.
  • Age structure: distribution of individuals across different age groups.
• Recruitment: the number of new individuals reaching breeding age plus immigration.
• Immigration: the influx of new individuals into a population.
• Emigration (Dispersal): individuals leaving a population.
• Exponential Growth Curve: a population's rate of growth accelerates over time.
• Exponential growth is rare in nature because limiting factors, such as starvation, predation, and disease, restrict growth.

Sigmoid Growth Curve

• Sigmoid Growth Curve: a population's growth rate initially increases rapidly, then slows down as it approaches carrying capacity.
• Inflection point (IP): the point on the curve where the population is still growing but at a slower rate.
• Carrying capacity (K): the maximum number of individuals that an environment or habitat can sustain.
• Variation in environmental factors can influence carrying capacity over time.
• If carrying capacity is exceeded, the population growth rate drops.

Density-Dependent and Independent Factors

• Density-dependent mortality factor: a factor that increases mortality as a population becomes denser (e.g., predation, disease, starvation).
• Density-independent mortality factor: a factor that increases mortality regardless of population density (e.g., hurricanes, floods, fires, catastrophic events).

Sex Ratio

• Sex ratio: the relative abundance of each sex within a population.
• Primary sex ratio: the sex ratio at fertilization.
• Secondary sex ratio: the sex ratio at birth.
• Tertiary sex ratio: the sex ratio of juveniles.
• Quaternary sex ratio: the sex ratio of adults.
• The sex ratio can be influenced by various factors, such as mortality rates and management practices (e.g., "Don't Take a Jake Rule" for turkeys).

K-selected and r-selected Species

• K-selected species: large, long-lived organisms that produce few offspring and invest heavily in parental care (e.g., elephants, whales).
• r-selected species: small, short-lived organisms that produce many offspring with high mortality and little parental care (e.g., rodents, fish, insects, amphibians).

Population Growth Equations

• Population growth can be calculated using equations that account for births, deaths, immigration, and emigration.
• r = (b - d) + (i - e)
  • r = actual increase or decrease of the population.
  • b = natality (birth rate).
  • d = mortality (death rate).
  • i = immigration.
  • e = emigration.
• Additive mortality: assumes that mortality factors act independently and their effects are additive (e.g., 40 rabbits die from predation and 40 from starvation for a total of 80 deaths). This is often unrealistic in nature, as factors can interact and influence each other.