Population Ecology: Key Concepts, Growth Models, and Factors

Population

A population is a group of living things (same species) living in the same place at the same time.

Habitat

The environment where a population lives (like a desert or forest).

Population Density

How many individuals live in a certain space (like 50 rabbits per acre).

Population Distribution

How individuals are spread out in their environment.

Clumped Distribution

Live close together (like fish in schools).

Uniform Distribution

Spread evenly (like penguins on nests).

Random Distribution

No pattern (like plants growing wherever seeds land).

Estimating Population Size

Scientists estimate population size using methods like aerial photos or counting in small areas.

Mark and Recapture

Tag some animals, release them, and see how many tagged ones you catch again.

Population Change Factors

Populations grow or shrink depending on births, deaths, immigration, and emigration.

Birth Rate (b)

Births per individual per time (like 0.3 = 3 births per 10 individuals).

Death Rate (d)

Deaths per individual per time.

Growth Rate (r)

r = b - d. If r is positive, the population grows. If r is negative, it shrinks.

Growth Increment (G)

G = r × N, where N = current population size.

Population Over Time

Next population = current population + G.

Exponential Growth

When resources are unlimited, the population grows super fast, resembling a 'J' curve.

Logistic Growth

When resources become limited, growth slows down and levels off.

Carrying Capacity (K)

The max number the environment can support.

Density-Dependent Factors

Effects get stronger as the population grows (like lack of food, disease, competition, predation).

Density-Independent Factors

Happen no matter the population size (like fires, floods, or storms).

Example Calculations

If a population of 100 snails has 60 births and 40 deaths: b = 60/100 = 0.6, d = 40/100 = 0.4 → r = 0.2 G = 0.2 × 100 = 20 → new population = 120.

Carrying Capacity Example

If N1 = 50, b = 0.6, d = 0.1, and K = 100: r = 0.5, G = rN1 × (K - N1) / K = 0.5 × 50 × (50 / 100) = 12.5. New population = 62.

Real-World Example — Wolves in Yellowstone

When wolves were reintroduced to Yellowstone, their population grew fast (exponential growth) but then slowed and balanced out near carrying capacity (logistic growth).