Population Ecology: Principles, Patterns, and Human Impacts

Population Ecology

Levels of Ecological Organization

Population ecology examines the factors that affect population size and how and why it changes over time. It is a subfield within ecology, focusing on groups of individuals of the same species (populations) and their interactions with the environment.

• Population: A group of individuals of the same species living in a specific area.

• Community: All the populations of different species that interact in a given area.

• Ecosystem: The community of organisms in an area and the physical factors with which they interact.

Density and Dispersion Patterns

Population Density

Population density refers to the number of individuals per unit area or volume. Dispersion describes the pattern of spacing among individuals within the boundaries of the population.

• Clumped Distribution: Individuals aggregate in patches; often due to resource availability or social behavior.

• Uniform Distribution: Individuals are evenly spaced; often influenced by territoriality or competition.

• Random Distribution: The position of each individual is independent of others; occurs in the absence of strong attractions or repulsions.

Specific Habitat Requirements

Habitat Specialization

Some species have very specific habitat requirements, which can limit their distribution and abundance. For example, the Kirtland's Warbler requires young jack pine forests for breeding.

Survivorship Curves

Types of Survivorship

Survivorship curves show the proportion of individuals surviving at each age.

• Type I: High survivorship in early and middle life, followed by a rapid decline in survivorship in later life (e.g., humans, large mammals).

• Type II: Constant death rate over the organism's life span (e.g., birds, hydra).

• Type III: High death rates for the young, with a few individuals surviving to old age (e.g., oysters, elm trees).

Factors That Affect Population Size

Additions and Losses

Population size changes through the balance of additions and losses:

• Additions: Births and immigration (movement into a population).

• Losses: Deaths and emigration (movement out of a population).

Population Growth Models

Exponential Growth

Under ideal conditions, populations can grow rapidly. The exponential growth model assumes:

• Stable age distribution

• No predation, parasitism, or disease

• No competition (interspecific or intraspecific)

• Unlimited resources

Closed Population: No immigration or emigration occurs.

• If $(b - d) > 0$, population increases

• If $(b - d) < 0$, population decreases

Population growth rate:

• $r = b - d$

• $b$ = per capita birth rate

• $d$ = per capita death rate

• If $r > 0$, population grows; if $r < 0$, population declines; if $r = 0$, population is stable.

Exponential growth equation:

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

Where $N$ is population size, $r$ is the intrinsic rate of increase, and $t$ is time.

Example: Whooping Crane Population

The recovery of the whooping crane population demonstrates exponential growth under conservation efforts.

Carrying Capacity and Logistic Growth

Exponential growth cannot continue indefinitely due to resource limitations. The carrying capacity (K) is the maximum population size that a habitat can support.

Logistic Growth Model: Population growth slows as it approaches carrying capacity, producing an S-shaped (sigmoid) curve.

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

• When $N$ is small, growth is nearly exponential.

• As $N$ approaches $K$, growth slows and eventually stops.

Assumptions of the Logistic Model

• Each individual added to the population has the same negative effect on growth rate, regardless of population density.

• Populations adjust instantaneously to growth and approach carrying capacity smoothly.

Example: Paramecium Populations

Laboratory populations of Paramecium species demonstrate logistic growth, with population size leveling off at carrying capacity.

Life History Strategies

r-Selected vs. K-Selected Species

Species exhibit different reproductive strategies along a continuum from r-selection to K-selection.

Characteristic	r-Selected Species	K-Selected Species
Life Span	Short	Long
Mortality	High	Low
Survivorship Curve	Type III	Type I or II
Competition	Low	High
Body Size	Small	Large
Reproduction	Semelparous (one-time)	Iteroparous (repeated)
Offspring	Many, small	Few, large

Note: Most species fall somewhere along the continuum between r- and K-selection.

Regulation of Population Growth

Density-Dependent Factors

These factors intensify as population density increases, regulating population size.

• Competition for resources

• Territoriality

• Disease

• Predation

• Intrinsic factors (e.g., stress)

• Toxic wastes

Density-Independent Factors

These factors affect populations regardless of density.

• Fire

• Storms

• Floods

• Cold

• Drought

Population Dynamics

Metapopulations

A metapopulation consists of a group of spatially separated populations of the same species which interact through migration. Some subpopulations may go extinct, but migration can reestablish them.

Population Cycling

Some populations undergo regular boom-and-bust cycles, often due to predator-prey interactions (e.g., snowshoe hare and lynx).

Human Population Growth and Impact

Human Population Growth

The human population has grown exponentially, especially in the last few centuries, with doubling times decreasing rapidly.

Year	World Population	Doubling Time
10,000 B.C.	5–10 million	Stable
1650	500 million	~1550 years
1850	1 billion	200 years
1930	2 billion	80 years
1975	4 billion	45 years
2012	7 billion	~50 years (projected)

Human Disturbance

Human activities such as urbanization, agriculture, and pollution have significant impacts on ecosystems and population dynamics.

Age-Structure Pyramids

Age-structure diagrams show the distribution of various age groups in a population, which influences future population growth.

Pattern	Description
Rapid Growth	Broad base; high proportion of young individuals
Slow Growth	More even distribution across age groups
No Growth	Narrow base; low birth rates, aging population

Stabilization of the Human Population

Delays in stabilizing human population size make it more difficult to achieve sustainability, as overshooting carrying capacity can lead to resource depletion and environmental degradation.

Key Question: Why does delaying stabilization make it more difficult? Because the population may exceed the carrying capacity, causing long-term ecological damage and making recovery harder.

Additional info: Population ecology is essential for understanding conservation, resource management, and the impacts of human activity on the environment.