Ecology and Population Dynamics

Logistic Population Growth

  • Definition: Logistic population growth describes how populations grow in a limited environment.
    • Initial growth is exponential, followed by a slowdown as the population reaches its environment's carrying capacity.
    • This results in an S-shaped curve, also referred to as logistic growth.

Carrying Capacity

  • Carrying Capacity: The maximum number of individuals that an environment can sustain indefinitely based on available resources (e.g., food).
    • Varies by species and environment.
    • Acts as an upper limit for population growth.

Example: Green Borer Beetles

  • Starts with exponential growth until it reaches a carrying capacity (~300 adults in a barrel of grain).
  • After reaching capacity, the population stabilizes.

S-Shaped Curve

  • S-Curve Characteristics:
    • Exponential growth phase where the population number doubles rapidly.
    • Transition to stabilization as resources become limiting.
    • Inflection Point: The moment the growth rate shifts from increasing to decreasing.

Cases of Overshoot

  • Daphnia Example:
    • Initial exponential growth overshoot carrying capacity (~200 individuals) leading to a population decline and eventual stabilization at ~100-125 individuals.

Implications of Crowding

  • As populations grow and density increases, reproduction rates can decline due to limited resources and increased competition.
  • Density-dependent factors: Include reproduction rates, growth rates, and survival that depend on population density.

Example: Song Sparrows

  • Increased density leads to reduced offspring per female, possibly due to hormonal changes affecting reproduction.

Effects of Density on Organisms

  • High density can:
    • Decrease growth rates and body size.
    • Reduce survival rates, as seen in tadpoles and plants.
    • Results in unusual adaptations like wings in locusts under high density conditions.

Density Independent Factors

  • Definition: Factors affecting populations irrespective of their density.
    • Examples include natural disasters (fires, droughts, storms).
    • These factors can drastically reduce populations regardless of how many individuals exist.

Human Population Dynamics

  • Currently undergoing exponential growth:
    • Human population surpassed 8 billion with concerns about approaching carrying capacity.
  • Demographic Transition: Transition through various stages (pre-industrial, industrial, post-industrial) affecting birth and death rates.

Age Structure Graphs

  • Used to predict growth rates by analyzing the proportion of pre-reproductive to post-reproductive individuals.
    • Populations can be categorized as growing, stable, or declining based on the shape.

R-Selected vs. K-Selected Species

  • R-Selected Species:

    • High reproductive rates, small body size, little to no parental care (e.g., bacteria).
    • Type III survivorship curve where most offspring do not survive.
    • Influenced by density-independent factors.

  • K-Selected Species:

    • Lower reproductive rates, larger body size, significant parental care (e.g., mammals).
    • Type I or II survivorship curve, better buffer against environmental changes.
    • Population dynamics are more stable as they avoid extreme fluctuations.

Metapopulations

  • Describe interconnected populations that can interact through migration.
  • Source-Sink Dynamics:
    • Source populations: High reproduction and survival rates, often contribute migrants to other populations.
    • Sink populations: Lower rates of survival and reproduction, depend on immigration for maintenance.

Population Cycles

  • Populations like arctic hares and lynxes illustrate cycles driven by food availability and environmental factors.
  • Predator-prey dynamics show lagged responses where predator population changes follow those of prey, influenced by external factors such as weather and food supply.