Population Ecology Study Notes

Population Ecology

Levels of Study

• Individual/Organism: The basic unit of ecology.

• Population: A group of individuals that interbreed or interact with each other within a defined space and time.

• Community: Comprises multiple populations that interact within a specific area.

• Ecosystem: An assemblage of living organisms and their environment functioning as a unit.

• Global: The highest level, encompassing all ecosystems and their interactions at a planetary scale.

Populations vs. Metapopulations

• Metapopulation:

  • A network of discrete subpopulations linked via gene flow.

  • Acknowledges the fragmented nature of populations and the discontinuous nature of habitats.

Example 1 of Metapopulations

• Students attending classes at various campuses:

  • Some students study at CSUF and Cerritos, while others attend CSULB and Cerritos, with very few at both CSUF and CSULB.

Example 2 of Metapopulations

• Local extinctions can occur within these subpopulations.

• Subpopulations can be repopulated from other populations, indicating a connection between them.

Describing Populations

Major Descriptors:

• Range: The geographical area where a population lives; specific examples include:

  • California Floristic Province,

  • Various mountain ranges (e.g., Klamath Ranges, San Gabriel Mountains).

• Distribution: How individuals within a population are spaced out. Types include:

  • Clustered Distribution:

  • Regular Distribution: Even distribution of individuals across a space.

Size, Density, and Demographics

• Size: Indicates the total number of individuals in a population.

• Density: Population density is expressed as individuals per unit area.

  • Showcases population figures for various U.S. cities, highlighting densities from 0-500 persons per square mile.

• Demography: Population characteristics such as age, sex ratio, and birth/death rates.

  • Differences noted between developed countries (e.g., Sweden) and developing countries (e.g., Honduras) are illustrated with specific population data projections for 2010 and 2050.

7 Ways to be “Rare”

• Characteristics of populations that make them rare include:

  • Population Size: Small

  • Geographic Range: Restricted

  • Habitat Specificity

• Based on the study by Rabinowitz, 1981, a population can be categorized across multiple criteria resulting in different modes of rarity.

Population Growth: Inflows and Outflows

• Population Growth is influenced by inflows and outflows, specifically:

  • Inflows: Births and immigration

  • Outflows: Deaths and emigration

• The formula relating these factors:
  $ext{Population Size} = ext{Births} + ext{Immigration} - ext{Deaths} - ext{Emigration}$

Growth Rate

• The rate at which the population size grows at time 't' is given by:
  $ext{Growth Rate} (r) imes ext{Current Population Size} (N)$

• Different growth rates can be categorized as high, moderate, low, and very low.

Logistic vs. Exponential Growth

• Logistic Growth: This model shows a reduction in the rate of population change as the population approaches carrying capacity.

  • Carrying Capacity: The maximum population size supported by resources.

• Exponential Growth: Illustrates an increase in the rate of population growth as the population size increases.

Factors Affecting Population Growth

Abiotic Factors

• Natural disasters (e.g., floods, fires) that affect growth regardless of density.

Biotic Factors

• Influences that vary based on population density include:

  • Habitat availability,

  • Soil nitrogen availability,

  • Predation risk,

  • Competition,

  • Disease.

Population Stability Example

• Population Stability Scenario:

  • Example involving 1000 sea turtles, which are 50% female.

  • Each female lays an average of 100 eggs per clutch, with 10 clutches over her lifetime.

  • Question: What is the average offset of offspring needed to sustain the population?

Observations from Sea Turtle Predation

• Predation levels post-hatching have minimal effect on the overall population size despite high mortality rates during this phase.

Survivorship Curves

• Type I: High survivorship until old age, then rapid decrease.

• Type II: Steady decline in survivorship throughout life.

• Type III: High early mortality rates, but survivors live long lives.

Fecundity

• Definition: The average number of female offspring produced by each female, crucial to life table studies.

• Age-Specific Fecundity: The average number of female offspring produced by females in a particular age class.

Importance of Fecundity Data

• Data on survivorship and fecundity are utilized to calculate the growth rate of a population.

Life-History Continuum

• Describes the variations in reproductive strategies:

  • High fecundity linked to fast growth and early sexual maturity.

  • Low fecundity tends to correlate with high survivorship and slower growth.

Evolution of Life History Strategies

• Strategies aim to maximize reproductive output through tradeoffs among growth, reproduction, and survival.

• Pivotal to understanding ecological and evolutionary pressures.

Big-Bang Reproduction Example

• Described by mayflies, featuring:

  • Sexual maturity at one year,

  • Lifecycle lasts only a few minutes to a day,

  • High reproductive effort in a short time leading to adult mortality immediately post-mating.

Comparison of Reproductive Strategies

• Examples:

  • Greater Bulldog Bat: slow investment with one offspring yearly.

  • House Mouse: fast reproduction at one month maturity, producing large litters.

Reproductive Investment and Survival

• Discussion on costs and benefits of varying reproductive strategies, exploring ecological trade-offs, such as:

  • Big-bang reproducers like Pacific salmon, high investment but short survival,

  • Beech trees' growth variability correlated with seed production.

Offspring Size Consideration

• Example with lizards producing different egg sizes:

  • Larger eggs provide more resources for embryos.

  • Trade-off is noted as larger eggs mean fewer total offspring due to physiological constraints.