Population dynamics is the branch of ecology that studies how and why populations change in size and composition over time. It examines the interplay of birth, death, immigration, and emigration.
The change in population size () over a time period is given by:
Where:
If , the population grows. If , it declines.
Under ideal conditions with unlimited resources, populations grow exponentially:
This produces a J-shaped curve. The growth rate accelerates as increases. Examples: bacterial cultures, introduced species with no predators.
In reality, resources are limited. As population density increases, environmental resistance slows growth. The logistic growth equation is:
This produces an S-shaped (sigmoidal) curve:
Carrying Capacity () is the maximum number of individuals of a species that a given environment can sustainably support, given available food, water, space, and other resources.
Factors that restrict population growth are called limiting factors. They are classified into two types:
These are biotic factors whose effect intensifies as population density increases. They act as negative feedback mechanisms:
| Factor | Mechanism |
|---|---|
| Intraspecific competition | Increased competition for food/space at high density |
| Predation | Predator populations increase when prey is abundant |
| Disease & Parasitism | Pathogens spread more easily in dense populations |
| Stress & Aggression | Overcrowding increases stress hormones, reducing reproduction |
These are abiotic factors that affect population size regardless of density:
A sudden frost kills the same proportion of a population whether it has 10 or 10,000 individuals.
Species have evolved different reproductive strategies based on their ecological niche:
| Feature | r-Selected Species | K-Selected Species |
|---|---|---|
| Growth rate | High | Low |
| Body size | Small | Large |
| Offspring number | Many | Few |
| Parental care | Little/none | Extensive |
| Lifespan | Short | Long |
| Population size | Variable, often below | Stable, near |
| Environment | Unstable, unpredictable | Stable, predictable |
| Examples | Bacteria, insects, annual plants | Elephants, whales, humans |
r-strategists maximize reproductive rate to exploit temporary resources. K-strategists invest in quality offspring and compete effectively in stable, resource-limited environments.