Unit 2: Population Ecology

1. Population and Meta-population

Concept of Population

A population is a group of individuals of the same species that live in the same geographical area at the same time and are capable of interbreeding.

Concept of Meta-population

A meta-population is a "population of populations." It's a group of spatially separated populations of the same species that are connected by occasional dispersal (migration of individuals) between them.

The individual populations are called sub-populations.
These sub-populations may go extinct, but the empty habitat patch can be re-colonized by individuals from other sub-populations.
This dynamic of extinction and re-colonization allows the meta-population to persist.

Diagram: A map showing several "habitat patches" (circles). Some are filled (occupied), some are empty. Arrows between patches indicate "dispersal." One filled patch is shown fading to show "local extinction."

2. Life History Strategies (r- and K-selection)

This theory describes two main strategies organisms use to maximize reproductive success. These strategies exist on a continuum.

r-Selected Species

These species are adapted for rapid population growth in unstable environments. Their strategy is to produce many offspring quickly. They are associated with the 'r' (intrinsic rate of increase).

K-Selected Species

These species are adapted for survival in stable environments near the carrying capacity (K). Their strategy is to produce fewer, "higher quality" offspring and invest heavily in them.

Classic Exam Question: Compare and contrast r-selected and K-selected species.

Characteristic	r-Selected Species	K-Selected Species
Environment	Unstable, unpredictable	Stable, predictable
Survivorship Curve	Type III (high early mortality)	Type I (high late mortality)
Offspring	Many, small-sized	Few, large-sized
Parental Care	Little or none	Extensive
Lifespan	Short	Long
Examples	Insects, bacteria, weeds, oysters	Elephants, whales, humans, large trees

3. Characteristics of a Population

Populations have unique group attributes that individuals do not.

Population Density

The number of individuals per unit area or volume (D = N / S).

Crude Density: Number per total unit of space.
Ecological Density: Number per unit of habitat space.

Natality (Birth Rate)

The rate at which new individuals are produced.

Crude Birth Rate: Births per 1000 individuals per year.
Specific Birth Rate: Births per female of a specific age.

Mortality (Death Rate)

The rate at which individuals die.

Age Structure

The distribution of individuals among different age classes (pre-reproductive, reproductive, post-reproductive). This is visualized using an age pyramid.

Diagram: Three age pyramids.
1. Expanding (Rapid Growth): Wide base, narrow top.
2. Stable (Zero Growth): Rectangular shape.
3. Declining (Negative Growth): Narrow base, wider top.

4. Life Tables and Survivorship Curves

Life Tables

A life table is an age-specific summary of the mortality and survival patterns of a population. It follows a cohort (a group of individuals born at the same time) from birth until all are dead.

Key columns in a life table:

x = Age interval
nx = Number of individuals surviving to age x
lx = Proportion of individuals surviving to age x (nx / n0)
dx = Number of individuals dying during age interval x (nx - nx+1)
qx = Mortality rate per interval (dx / nx)

Survivorship Curves

A survivorship curve is a graph plotting the data from a life table (logarithm of the number of survivors, lx, vs. age). It shows the pattern of mortality.

Diagram: A graph with "Age" (x-axis) and "Log Number of Survivors" (y-axis).
1. Type I (Convex): Line starts high, stays high, then drops sharply at old age.
2. Type II (Diagonal): Line is a straight, constant downward slope.
3. Type III (Concave): Line drops sharply at a young age, then flattens out.

Type I (Late Loss): High survival in early/middle life, high mortality in old age. (e.g., Humans, elephants - K-selected).
Type II (Constant Loss): Constant mortality rate at all ages. (e.g., Many birds, rodents).
Type III (Early Loss): High mortality for the young, but high survival for those that make it past a certain age. (e.g., Oysters, insects, plants - r-selected).

5. Population Growth Models

Geometric Growth

Describes growth in populations with discrete, non-overlapping breeding seasons (e.g., annual plants). The population grows in steps.

Formula: N(t) = N(0) * λᵗ
Where N(t) = size at time t, N(0) = initial size, λ = geometric rate of increase, t = number of generations.

Exponential Growth (J-shaped curve)

Describes growth in populations with continuous reproduction in an unlimited environment. There are no limiting factors.

Formula: dN/dt = rN
Where dN/dt = rate of change in size, r = intrinsic rate of natural increase, N = population size.

This model results in a J-shaped curve. It cannot be sustained indefinitely.

Logistic Growth (S-shaped curve)

A more realistic model that incorporates limiting factors and the concept of carrying capacity (K).

Carrying Capacity (K): The maximum population size an environment can sustainably support.

Formula: dN/dt = rN * ( (K - N) / K )
Where (K - N) / K is "environmental resistance."

When N is small, (K-N)/K is ≈ 1, and growth is exponential.
When N approaches K, (K-N)/K approaches 0, and growth stops.

This model results in a sigmoid or S-shaped curve.

Diagram: A graph with "Time" (x-axis) and "Population Size (N)" (y-axis). Show the J-shaped exponential curve and the S-shaped logistic curve that levels off at "K (Carrying Capacity)".

6. Population Regulation (Limits to Population Growth)

These are the factors that cause the logistic (S-shaped) curve.

Density-Dependent Factors

Factors whose effect varies with population density. They become more intense as density increases. These are biotic factors that "regulate" a population around K.

Competition (Intraspecific): For limited resources (food, water, mates).
Predation: Predators may focus on abundant prey.
Disease and Parasitism: Spread more easily in dense populations.

Density-Independent Factors

Factors that affect a population regardless of its density. They are typically abiotic and do not regulate, but can cause sudden crashes.

Climate and Weather: Droughts, floods, extreme temperatures.
Natural Disasters: Fires, volcanic eruptions.