Population Ecology Overview

Population Ecology

• Population Defined: A group of organisms of the same species. Defining populations can be challenging.
• Organism Types:
  • Unitary: Have a fixed form and predictable development (e.g., birds, humans).
  • Modular: Built from repeating units (e.g., plants, corals) whose structure and function can change.
  • Clonal: Modular organisms where individual units can survive independently.
    • Genet: The genetic individual (zygote) resulting in many independent modules forming an individual.
    • Ramet: The independent unit of a clonal organism.
• Counting Populations:
  • Census: Counting every individual. Inaccuracies due to undetected individuals. Useful for understanding overall population trends.
  • Sampling: Estimating population size from a portion of the population. Statistical methods are applied.
  • Mark-Recapture: A statistical method to estimate population size. Assumes no emigration or immigration.
    • N = total population size
    • M = initial number tagged
    • n = total number captured in second sample
    • m = number recaptured that were tagged

Population Dynamics

• Reproduction Strategies:
  • Semelparous: Reproduce only once in a lifetime (e.g., annual plants).
  • Iteroparous: Reproduce multiple times in a lifetime (e.g., deer). Age classes play a role in iteroparous population growth.
    • Cohort: All organisms born in the same period.
    • Static Life Table: Counts individuals in various age classes to understand population structure.
    • Survival Curves:
      • Type 1: High survival early, high mortality later (e.g., large mammals–K selected species).
      • Type 2: Steady decline in survival (e.g., birds–K selected species).
      • Type 3: Low survival early, high mortality early (e.g., insects–r selected species).
        • r-selected species: High reproductive output, often in unstable environments.
        • K-selected species: Stable populations, fewer offspring, greater parental investment in fewer offspring

Dispersal and Migration

• Dispersal: Movement of individuals or portions of a population, often driven by resources or overcrowding. Not directional.
  • Density-dependent dispersal: Dispersal due to high local population density.
  • Age-dependent dispersal: Young individuals often colonize new areas.
• Migration: Focused movement with a clear destination. Often repeated in a lifetime (e.g., migratory birds).
  • Repeated Migration: Multiple back and forth migrations.
  • Migration with return: Migration with a return to the original place.
  • One-time Migration: Migration from one location to another, and no return.

Population Growth and Competition

• Exponential Growth: Growth with unlimited resources. Growth rate (r) is dependent on natality and mortality.

  • Exponential: dN/dt = rN

• Logistic Growth: Growth limited by resources. Accounts for intraspecific competition for limited resources.

  • Logistic: dN/dt = rN(K-N/K)
    • K = Carrying capacity: maximum population size sustained by available resources.

• Competition:

  • Intraspecific Competition: Competition among members of the same species.
    • Interference competition: Direct physical exclusion from resources by individuals.
    • Exploitation competition: Competition by consuming resources, no direct physical exclusion.

• Grimes Plant Strategies: Categorizes plant species based on stress tolerance (S), ruderalism (R), and competitive ability (C).

  • Stress-tolerant: Dominate in stressful, sparsely populated environments.
  • Ruderal: Dominate in disturbed areas.
  • Competitive: Dominate in resource-rich, low-disturbance areas.

Predation and Herbivory

• Predation: Interactions between predators and prey.
  • True predators: Kill prey immediately. Consume multiple prey.
  • Grazers: Consume portions of prey, not necessarily killing them. Consume multiple prey.
  • Parasites: Consume portions of the host, do not kill host. Typically, only consume a single host during their lifetime, depending on the host life cycle.
• Predator-Prey Relationships: Population fluctuations interact in cyclical patterns.
• Optimal Foraging: Maximizing energy intake per unit of effort. Relates food intake to search and processing time. Predators often eat multiple prey types.

Functional Responses

• Functional response types:
  • Type 1: Predation rate increases linearly at low prey densities, saturates at high prey densities.
  • Type 2: Predation rate increases, then slows as handling time influences the rate.
  • Type 3: Low predation rate at low prey density, increases with increasing prey density, and plateaus.

Metapopulations

• Metapopulation: Spatial separation of populations of the same species with some level of interaction. Fluctuations in local populations are larger than the overall metapopulation. Patch size and habitat quality influence metapopulation dynamics.