Unit 3 - Populations: Comprehensive Study Notes

Specialist vs. Generalist Species

  • Learning Objective: ERT-3.A - Identify differences between generalist and specialist species.
  • Essential Knowledge: ERT-3.A.1 - Specialist species are advantaged in constant habitats, while generalist species are advantaged in changing habitats.
  • Specialist Species:
    • Smaller range of tolerance.
    • Narrower ecological niche, making them more prone to extinction.
    • Specific food requirements (e.g., bamboo for pandas).
    • Less ability to adapt to new conditions.
    • Use a specific set of resources.
    • Easily affected by changing conditions.
    • Have an advantage when conditions are more constant.
  • Generalist Species:
    • Larger range of tolerance.
    • Broader niche makes them less prone to extinction and more likely to be invasive.
    • Broad food requirements.
    • High adaptability.
    • Adaptable to many environments.
    • Less likely to become extinct.
    • Use a variety of resources.
    • High range of tolerance.
    • Have an advantage when conditions change.
  • Explanation: The breadth of a species' niche affects its vulnerability to environmental changes. Specialists thrive in stable environments but are highly susceptible to extinction when conditions change, while generalists can adapt more readily.

K-selected & r-selected species

  • Learning Objective: ERT-3.B - Identify differences between K- and r-selected species.
  • Essential Knowledge:
    • ERT-3.B.1 - K-selected species: Large, few offspring, high energy investment per offspring, long maturation, long lifespan, reproduce multiple times, high competition.
    • ERT-3.B.2 - r-selected species: Small, many offspring, minimal energy investment per offspring, early maturation, short lifespan, may reproduce only once, low competition.
    • ERT-3.B.3 - Biotic potential: Maximum reproductive rate in ideal conditions.
    • ERT-3.B.4 - Many species exhibit traits of both r- and K-selection or change strategies based on conditions.
    • ERT-3.B.5 - K-selected species are more adversely affected by invasive species than r-selected species. Most invasive species are r-selected.
  • K-selected Species (“quality”):
    • Few offspring.
    • Heavy parental care to protect offspring.
    • Usually reproduce many times.
    • Example: Most mammals, birds.
    • Long lifespan, long time to sexual maturity = low biotic potential = slow population growth rate.
    • More likely to be disrupted by environmental change or invasives.
  • r-selected Species (“quantity”):
    • Many offspring.
    • Little to no care.
    • May reproduce only once.
    • Example: Insects, fish, plants.
    • Shorter lifespan, quick to sexual maturity = high biotic potential = high population growth rate.
    • More likely to be invasive.
    • Better suited for rapidly changing environmental conditions.
  • Traits of K-selected Species:
    • Long life span.
    • Long time to reproductive maturity.
    • Few reproductive events.
    • Few offspring.
    • Large offspring.
    • Present parental care.
    • Slow population growth rate.
    • Density-dependent population regulation.
    • Stable population dynamics, near carrying capacity.
  • Traits of r-selected Species:
    • Short life span.
    • Short time to reproductive maturity.
    • Many reproductive events.
    • Many offspring.
    • Small offspring.
    • Absent parental care.
    • Fast population growth rate.
    • Density-independent population.
    • Highly variable population dynamics.
  • Spectrum: Species exist on a spectrum between r- and K-selected. (oyster, fish, frog, hare, large cat, chimpanzee)
  • Invasiveness & Disturbances:
    • K-selected: Low biotic potential makes it hard for population to recover after a disturbance. High parental care means death of parent = death of offspring. Invasives outcompete for resources.
    • R-selected: High biotic potential means rapid population recovery after disturbance. Low parental care means death of parent doesn’t impact offspring. More likely to be the invasive.

Survivorship Curves

  • Learning Objective: ERT-3.C - Explain survivorship curves.
  • Essential Knowledge:
    • ERT-3.C.1 - Survivorship curve: Line displaying relative survival rates of a cohort (group of same-aged individuals) from birth to maximum age.
    • ERT-3.C.2 - K-selected species typically follow Type I or Type II curves, and r-selected species follow a Type III curve.
  • Survivorship Curve: A line that shows survival rate of a cohort in a population from birth to death.
    • Faster drop in line = quicker die-off of individuals.
    • Slower drop in line = longer average lifespan.
  • Type I (mostly K-selected):
    • High survivorship early in life due to high parental care.
    • High survivorship in midlife due to large size & defensive behavior.
    • Rapid decrease in survivorship in late life as old age sets in.
    • Example: Most mammals (humans).
  • Type II (in between r & K):
    • Steadily decreasing survivorship.
    • Example: Birds
  • Type III (mostly r-selected):
    • High mortality (low survivorship) early in life due to little to no parental care.
    • Few make it to midlife; slow, steady decline in survivorship in midlife.
    • Even fewer make it to adulthood; slow decline in survivorship in old age.
    • Example: Insects, fish, plants.

Carrying Capacity

  • Learning Objective: ERT-3.D - Describe carrying capacity. ERT-3.E - Describe the impact of carrying capacity on ecosystems.
  • Essential Knowledge:
    • ERT-3.D.1 - When a population exceeds its carrying capacity (K), overshoot occurs, leading to resource depletion.
    • ERT-3.E.1 - A major ecological effect of population overshoot is dieback due to resource scarcity.
  • Carrying Capacity (K): The maximum number of individuals in a population that an ecosystem can support based on limiting resources.
    • Limiting resources: Food, water, habitat (nesting sites, space).
  • Overshoot: When a population briefly exceeds carrying capacity.
    • Example: Deer breed in fall, give birth all at once in spring; sudden spike in population = overshoot.
  • Consequence of Overshoot: Resource depletion.
    • Ex: Overgrazing in deer.
  • Die-off: Sharp decrease in population size when resource depletion leads to many individuals dying.
    • Example: Many deer starve with too many new fawns feeding in spring.
  • Die-off Example: Reindeer of St. Paul Island:
    • Introduced in 1910.
    • Growth was gradual (1910-1930’s), then exponential (1930’s-1937).
    • Carrying capacity was overshot.
    • Sharp die-off led to population crash as food resource (lichen) were severely depleted.
  • Predator-Prey Cycles:
    • Hare population increase due to low predator population (lynx).
    • Lynx population increase due to increase in food (hare).
    • Increasing lynx population limits hare population; leads to die-off.
    • Hare die-off decreases lynx food source, leading to die-off.

Population Growth & Resource Availability

  • Learning Objective: ERT-3.F - Explain how resource availability affects population growth.
  • Essential Knowledge:
    • ERT-3.F.1 - Population growth is limited by environmental factors, especially available resources and space.
    • ERT-3.F.2 - Resource availability is limited and finite.
    • ERT-3.F.3 - Abundant resources accelerate population growth.
    • ERT-3.F.4 - Shrinking resource base increases mortality and decreases fecundity, causing population decline.
  • Population Characteristics:
    • Size (N): Total number of individuals in a given area at a given time. Larger = safer from population decline.
    • Density: Number of individuals/area. Ex: (12 panthers/km^2). High density = higher competition, disease outbreak, food source depletion.
    • Distribution: How individuals in population are spaced out compared to each other. Random (trees).
    • Sex Ratio: Ratio of males to females. Closer to 50:50, the more ideal for breeding.
  • Density-Dependent Factors: Factors that influence population growth based on size:
    • Ex: Food, competition for habitat, water, light, disease.
  • Density-Independent Factors: Factors that influence population growth independent of their size:
    • Ex: Natural disasters (flood, hurricane, tornado, fire).
  • Biotic Potential: Maximum potential growth rate, with no limiting resources.
    • Logistic growth = initial rapid growth, then limiting factors limit population to K.
  • Calculating Population Change: $$"Population Size = (Immigrations + births) - (Emigrations + deaths)