Population Ecology

Population Ecology

Learning Outcomes

• Understand how birth and death influence population size.

• Discriminate between density-dependent and density-independent limits on population growth.

Populations

• Definition: An interacting group of members of a species.

• Components of Populations:

  • Defined Area: The geographical range where individuals are found.

  • Density: The measure of how many individuals exist per area or volume.

  • Questions to consider:

    • How are individuals distributed?

    • How is population density measured at a defined time?

    • How might the size of the population change over time?

Range

• Most species have a limited geographical distribution.

• For example, even humans do not inhabit every area on the planet.

• Examples of Range:

  • Dolphins: Found in all oceans.

  • Devil’s Hole Pupfish: Exists in only one specific area.

• Definitions:

  • Endemic Species: A species limited to a defined location.

  • Cosmopolitan Species: A species that has a wide distribution.

• Ranges can change due to:

  • Changes in the environment.

  • Expansion into new areas, e.g., the cattle egret evolved in Africa and spread to South America in the 1930s and across most of North America by the 1960s.

Dispersion

• Definition: Spatial distribution of individuals within a population.

• Types of Dispersion:

  • Random Dispersion:

  • Description: Individuals are widely dispersed without noticeable pattern.

  • Example: Dandelions distributed in a meadow due to wind dispersal.

  • Clumped Dispersion:

  • Description: Individuals group together in social forms for feeding, mating, and predator avoidance.

  • Example: Bigeyes fish schooling together to avoid predation.

  • Uniform Dispersion:

  • Description: Individuals are evenly spaced due to competition for resources.

  • Example: Gannets competing for nesting areas along rocky coasts.

Changes in Populations

• Population changes are influenced by:

  • Births

  • Deaths

  • Immigration (movement into a population)

  • Emigration (movement out of a population)

• Measuring Population Size:

  1. Count individuals (full census).

  2. Estimate using representative samples.

Estimating Population Size

Research Tools

1. Capture a random sample of individuals from the population of interest and mark each captured individual.

2. Release the marked individuals to allow mixing with unmarked individuals.

3. Capture a second random sample. Count both marked individuals and total individuals in this sample.

4. Estimate total population size, N, using the equation: $N = \frac{n<em>1 \times n</em>2}{M}$ where:

   • $n_1$ = the size of the first sample (captured and marked)

   • $n_2$ = the size of the second sample (total number of individuals captured)

   • $M$ = the number of marked individuals recaptured in the second sample.

Demographics

• Definition: The statistical study of populations.

• Example: The common lizard (Zootoca vivipara) demonstrates a global range across Europe and regional ranges into Asia, with localized populations.

Life Tables

• Purpose: Predict future population size based on:

  • Life expectancy of individuals.

  • Likelihood of survival at a given age.

  • Timing and effectiveness of reproduction.

Environmental Effects on Life Histories

• An individual’s lifespan and reproductive outcomes depend heavily on environmental factors:

  • Food Availability

  • Weather Conditions

  • Predation

• These environmental variables influence:

  • When individuals start reproducing.

  • Number of reproductive events per year.

  • Number of offspring produced per brood.

Survivorship Curves

• Type 1 Curve:

  • Most individuals survive to reproductive age; few offspring produced; long lifespan.

  • Example: Humans.

• Type 2 Curve:

  • Constant risk of mortality across all ages.

  • Example: Most birds.

• Type 3 Curve:

  • High juvenile mortality with many offspring and low parental care.

  • Example: Mollusks, insects.

Life History Strategies

• Organisms allocate time and energy into different categories:

  • Growth

  • Maintenance

  • Reproduction

• This allocation is variable and depends on resources and mortality rates. The rate of population growth can be expressed as: $r = b - d$ where:

  • $r$ = rate of population growth

  • $b$ = rate of births

  • $d$ = rate of deaths.

Limits to Population Densities

• Questions: If resources (food, habitat) are unlimited, how large can populations grow?

• Exponential Growth:

  • Populations theoretically could grow indefinitely if resources are unlimited.

  • Examples of Population Growth:

  • (A) Elephant seals at Año Nuevo Island: 1,600 pups.

  • (B) Reindeer at St. Matthew Island: 6,000.

• Overpopulation can lead to crashes if resources become scarce or environmental pressures occur.

Carrying Capacity

• Definition: The maximum size of a population that the environment can sustain.

• Exponential growth can occur briefly until resources become limited.

• When the population approaches the carrying capacity (K), growth slows and follows a logistic growth curve.

r- vs. K-strategists

• r-strategists:

  • Characteristics include high rates of reproduction, adaptability to different habitats, production of many offspring, low parental investment, and fluctuating population sizes.

• K-strategists:

  • Characteristics include maintaining population sizes near the carrying capacity, narrow habitat ranges, high parental investment, and longer lifespans.

Population Regulation Factors

• Density-Dependent Factors (biotic):

  • Food scarcity leads to poor nutrition, slowing birth rates, and increasing death rates.

  • High population densities attract predators and promote disease spread.

• Density-Independent Factors (abiotic):

  • Natural disturbances like severe cold weather or hurricanes can decrease populations regardless of density.

Variation in Population Density

• Exploration of why some species are abundant while others are rare focuses on:

  • Generalist vs. Specialist Species:

  • Adaptation to different food sources.

  • Body Sizes: Smaller vs. larger body types can impact niche survival.

  • Sociality vs. Solitariness:

  • Interaction patterns can influence survival.

  • Native vs. Introduced Species:

  • The absence of evolved predators in introduced species may lead to an abundance, e.g., the garlic mustard in the eastern US.

Mosaics of Populations

• Populations are not uniformly distributed within their ranges and tend to be patchy.

• Gene Flow: Occurs in patches and is less likely between them, leading to local population dynamics.

• Metapopulation Dynamics:

  • Composed of small, isolated populations that can offer genetic exchange and recolonize.

  • Important to maintain corridors (physical connections) for species survival between patches.

  • Development can isolate populations, converting natural areas into equivalent "islands."

Metapopulation Example: Fritillaries

• Glanville fritillaries reside in isolated habitat patches in Finland.

• Process of Metapopulation Dynamics:

  1. Small isolated populations lead to potential local extinctions.

  2. Dispersal among individuals can restore populations or establish new ones.

Population Management

• Life history strategies are crucial for effective population management:

  • The cod population was historically abundant but declined due to overharvesting.

  • Regulations restricting fishing helped striped bass populations recover from severe overfishing.