Population Ecology

Hierarchy of Scales in Ecology

• Ecological studies operate at various scales, from individual organisms to global ecosystems.

• Focus for this discussion: Population Ecology

Definition of Population

• Population in Biology: A group of individuals of the same species that live in a particular place at a particular time.

Factors Determining Population Size

• Key Factors:
    - Number of individuals born into the population.
    - Number of individuals that immigrate into the population.
    - Factors decreasing population size:
      - Death of individuals.
      - Emigration (individuals leaving the population).

• Correct Answer to Clicker Question: E. All of the above.

Measurement of Population Size

• Population size is often expressed as Population Density:
    - Definition: Number of individuals per unit area.   - Population density and size are often used interchangeably.

Techniques for Estimating Population Size

• Population counting can be straightforward for large, obvious animals in open habitats, such as elephants.

• Advancements: The use of drones has improved aerial surveys for counting populations.

• Challenges: For non-obvious or smaller organisms, estimating population size may require alternative techniques due to difficulties in direct observation.

Capture and Marking Techniques

• Mark-Recapture Method:
    - Involves capturing individuals, marking them, then releasing them to estimate population size later.
    - Example of marking:
      - Catching birds using a mist net.
      - Banding individuals with unique color combinations for identification.

Mark-Recapture Formula

• Formula:
  $N = \frac{(M \times C)}{R}$
    - Where:
      - N = Estimated population size.
      - M = Number of individuals captured and marked in the first instance.
      - C = Total number of individuals captured in the second instance.
      - R = Number of marked individuals recaptured in the second instance.

Class Example of Mark-Recapture

• Each student assigned a unique number for a mark-recapture simulation.

• Scenario details:
    - Initial capture: 27 students were marked.
    - Second capture: 47 students were recaptured, with 5 being marked from the first capture.
    - Estimated population size calculated using the formula, yielding approximately 253.

Assumptions of Mark-Recapture Technique

1. Marked and unmarked individuals have the same probability of being captured.
        - Example: Past trapping experiences influencing future capture.

2. Marked individuals have mixed back into the population after being released.
        - Implication: If marked individuals stay near the trapping area, they are more likely to be recaught.

3. No individuals have entered or exited the population during the study period (through births, deaths, or migration).      - The timing between captures is critical to minimize assumption violations.

Population Dynamics over Time

• Understanding population change requires mathematical modeling.

• Simple Population Growth Model:
  $\frac{dN}{dt} = r_{max} N$
    - Where:
      - N = Population size.
      - $\frac{dN}{dt}$ = Rate of change of population size over time.
      - $r_{max}$ = Intrinsic rate of natural increase for the species.

• Exponential Growth:
    - Characterized by slow initial growth that accelerates as population numbers increase.

Real-World Example of Exponential Growth

• Kruger National Park, South Africa: Elephant populations exhibited exponential growth following protection measures against poaching.

Carrying Capacity and Population Dynamics

• Carrying Capacity (K): The maximum sustainable population size in a given environment.

• Enhanced growth model incorporating carrying capacity:
  $\frac{dN}{dt} = r_{max} N \left(\frac{K - N}{K}\right)$
    - Implications:
      - When N is small, population grows rapidly with little resistance from K.
      - As N approaches K, growth slows, and potential decline occurs if N exceeds K.

Biological Implications of Population Dynamics

• Factors affecting carrying capacity include competition, predation, disease, and resource availability.

• Overpopulation issues may lead to resource depletion and aggression among individuals (e.g., elephants in Kruger National Park).

Cyclical Population Fluctuations

• Classic example from snowshoe hare and its predator, the lynx.
    - Observed cyclical population trends indicate interdependence between predator and prey.

Human Population Dynamics

• Current Global Population: Estimated at 8,286,000,000 as of recent data.

• Human population growth historical overview demonstrates rapid increases reflective of exponential growth, with key milestones in reaching billion thresholds.

Birth and Death Rates in Human Populations

• Comparative analysis of demographic trends in countries suggests correlations between socioeconomic status, access to healthcare, and population age distribution, influenced by birth and death rates.

• Demographic Transition: Refers to the pattern whereby death rates decline followed by a decline in birth rates as a nation develops.

Conclusion

• Key factors governing population size include numerous biological, environmental, and anthropogenic factors. Understanding these dynamics can inform conservation efforts and potential ecological impacts of population changes.