Population Dynamics Study Notes

Definition: Group of interacting individuals of the same species living in a particular area.
- Interactions include sexual reproduction and competition.

Distribution: Geographic area where individuals of a species occur.
Abundance: Number of individuals in a given area.
Ecologists seek to understand factors determining the distribution and abundance of species.

Limitations of distributions and abundances are caused by:
- Habitat Suitability: Refers to how well an environment meets the needs of a species.
- Historical Factors: Previous events that shape current distributions.
- Dispersal: Movement of individuals away from their birth site to new locations.
Many species have a patchy distribution across their geographic range.
Dispersion of individuals within a population is determined by:
- Location of essential resources.
- Competition among individuals.
- Dispersal mechanisms.
- Behavioral interactions.

Populations are dynamic; their distribution and abundance can change over time and across space.
Understanding these dynamics aids in population management for harvest or conservation.

Estimating abundance is complex when the total area occupied by a population is unknown.
Difficulties include:
- Knowing how far organisms can travel.
- Making estimations based on the best available knowledge of the species.

Changes in abundance can occur over time and space, varying significantly among species.
Some species exhibit dramatic fluctuations in abundance year-to-year; others remain relatively stable.

Clonal Example: Aspen trees produce clones from root buds, potentially giving rise to a grove of genetically identical individuals, complicating the definition of individual trees.

Factors Influencing Suitability:
- Abiotic features include moisture, temperature, pH, sunlight, and nutrients.
- Species differ in tolerance to these abiotic conditions; some can tolerate wide ranges, others have narrow ranges.

Disturbance: Events that kill or damage organisms, creating growth opportunities for others.
- Example: Some species thrive in areas that experience periodic fires.

Dispersal Limitations: May prevent species from reaching suitable habitats.
Example: Hawaiian Islands' only native mammal, the hoary bat.
Types of Dispersal:
- By animals: Hitchhikers, fruits consumed and dispersed.
- By wind: Light seeds with wings or plumes (e.g., dandelions).
- By water: Seeds that can float, like those of the red mangrove.

Historical northward movement of species as glaciers retreated.
Recorded movement rates from tree pollen data indicate potential shifts in habitat due to current climate change.
- Example: Maple trees reached their northeastern range 6,000 years ago, while hemlock did not reach its limits until 2,000 years ago.

Factors Influencing Dynamics:
- Immigration
- Birth
- Emigration
- Death
- These factors can contribute to the increase, decline, or maintenance of populations.

Types of Survival Curves:
- Type I: High survival among young (e.g., humans).
- Type II: Constant survival rate (e.g., corals).
- Type III: High mortality among young (e.g., many plants).

Age distributions can be classified as:
- Stable
- Declining
- Increasing
Population Stages:
- Stage 1: High birth rate and short life expectancy.
- Stage 2: High birth rate, low death rates, longer life expectancy.
- Stage 3: Declining birth rates, low death rates, elderly population.
- Stage 4: Low birth rates, low death rates, higher dependency ratios.

Logistic Growth: Characterized by a sigmoidal curve, accounts for carrying capacity.
Exponential Growth: Rapid population increase in favorable environments with available resources.

Factors limiting population growth are classified into:
- Density-Dependent Factors: Disease and predation.
- Density-Independent Factors: Environmental occurrences, such as floods and fires.

Fertility rates have decreased worldwide, influenced by education, urbanization, and contraceptive accessibility.
- Current trends show that about two-thirds of the global population lives in regions with fertility below the replacement rate of 2.1.
- Notable highest fertility rates are seen in African countries like Niger and Somalia.
- Countries like South Korea, with rates as low as 0.8, illustrate the global disparity.

Events affecting population numbers include:
- Predation
- Disease
- Competition
- Facilitation
These forces contribute to maintaining population numbers within a specific range to avoid extinction or unlimited growth.

Dynamic equilibrium is the norm within populations; intrinsic and extrinsic factors affect their stability.
Concepts of compensatory and additive mortality highlight varying effects of predation and other mortality agents on population dynamics.

The risk of extinction increases in small populations, influenced by:
- Fluctuations in growth rates and population sizes.
Chance events can drastically affect small population survival rates and genetic variation, leading to higher extinction risk.