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.