Population Ecology: Principles, Dynamics, and Life History Strategies

Population Ecology

What is a Population?

In biology, a population is a group of individuals of the same species living in the same area. These individuals interact, share resources, and face similar environmental challenges.

• Population studies help us understand how species survive, adapt, and persist over time.

• Populations change due to births, deaths, immigration, and emigration.

• Human impacts, such as habitat destruction, can endanger populations.

Population Dynamics: Density and Dispersion

Population dynamics examine how and why populations change in size and composition over time.

• Population density: Number of individuals per unit area or volume.

• Dispersion: How individuals are spaced within the area (clumped, uniform, random).

Dispersion Pattern	Description	Example
Clumped	Individuals aggregate in patches	Wolves in packs
Uniform	Evenly spaced	Penguins nesting
Random	Unpredictable spacing	Wind-dispersed plants

Population Size Change

Population size changes due to:

• Births (individuals born)

• Deaths (individuals die)

• Immigration (individuals enter)

• Emigration (individuals leave)

Formula for change in population size:

• Change in population size = Births + Immigration - Deaths - Emigration

Demographics: Life Tables and Survivorship Curves

Demographics study the vital statistics of populations, especially birth and death rates.

• Life table: Summarizes survival and reproductive rates at each age.

• Survivorship curve: Graphs the proportion of individuals surviving at each age.

Type	Description	Example
Type I	Low death rates early, high late	Humans
Type II	Constant death rate	Ground squirrels
Type III	High death rates early, low late	Oysters, fish

Population Growth Models

Population growth can be modeled mathematically to predict changes over time.

Exponential Growth

• Occurs when resources are unlimited.

• Population grows faster as it gets larger.

• Equation:

$\frac{dN}{dt} = rN$

• N = population size

• r = intrinsic rate of increase

Logistic Growth

• Occurs when resources become limited as population density increases.

• Population growth slows as it approaches carrying capacity (K).

• Equation:

$\frac{dN}{dt} = rN \left( \frac{K-N}{K} \right)$

• K = carrying capacity (maximum population size environment can support)

Density-Dependent and Density-Independent Factors

Population growth is regulated by factors that may depend on population density.

• Density-dependent factors: Effects increase as population density increases (e.g., competition, disease).

• Density-independent factors: Effects are unrelated to density (e.g., weather, natural disasters).

Life History Strategies: Semelparity vs. Iteroparity

Life history strategies describe how organisms allocate resources to growth, reproduction, and survival.

• Semelparity: Organisms reproduce once, then die (e.g., salmon).

• Iteroparity: Organisms reproduce multiple times over their lifetime (e.g., humans, most mammals).

Trade-Offs in Life History

Organisms face trade-offs between quantity and quality of offspring, survival, and reproduction.

• High investment per offspring often means fewer offspring but higher survival (K-selected).

• Low investment per offspring means many offspring but lower survival (r-selected).

Strategy	Offspring Number	Parental Care	Environment
r-selected	Many	Low	Unstable
K-selected	Few	High	Stable

Stability and Fluctuations

Population sizes can fluctuate due to interactions with other species (e.g., predator-prey cycles) and changes in birth or death rates.

• Understanding these dynamics helps predict and manage population changes, especially for conservation.

Key Terms and Concepts

• Population density: Number of individuals per unit area/volume

• Dispersion: Pattern of spacing among individuals

• Carrying capacity (K): Maximum population size environment can support

• Exponential growth: Unlimited resources, rapid increase

• Logistic growth: Limited resources, growth slows near K

• Semelparity: Single reproductive event

• Iteroparity: Multiple reproductive events

• r-selected species: Many offspring, little care

• K-selected species: Few offspring, much care

Additional info: These notes expand on the original points by providing definitions, equations, and examples for key concepts in population ecology, as well as structured tables for comparison.