3.5 Population Growth and Resource Availability

Exponential vs. Logistic Growth

With unlimited resources, populations grow exponentially, producing a J-shaped curve as growth rate increases with population size. In reality, resources are limited, so growth slows as population nears carrying capacity (K) — the maximum population size an environment can sustain. This produces an S-shaped logistic growth curve, where growth rate declines as resources become scarce.

Carrying Capacity and Limiting Factors

Carrying capacity (K) is set by available resources like food, water, space, and shelter. When a population exceeds K, resources become depleted, causing die-offs until the population returns near K. Limiting factors (density-dependent or density-independent) regulate population size by affecting birth and death rates.

Density-Dependent vs. Density-Independent Factors

Density-dependent factors (competition, predation, disease) intensify as population density increases, strongly regulating growth near carrying capacity. Density-independent factors (natural disasters, weather, habitat destruction) affect populations regardless of size, causing sudden crashes unrelated to density.

Key terms

Exponential growth

Population growth with unlimited resources, producing a J-shaped curve as growth rate increases with population size.

Logistic growth

Population growth that slows as resources become limited, producing an S-shaped curve that levels off at carrying capacity.

Carrying capacity (K)

The maximum population size an environment can sustainably support based on available resources.

Density-dependent factors

Limiting factors (e.g., competition, predation, disease) whose effects intensify as population density increases.

Density-independent factors

Limiting factors (e.g., natural disasters, extreme weather) that affect populations regardless of their density.

Try an AP-style question

A population grows at 2% per year. Using the rule of 70, about how long does it take to double?

1. 35 years
2. 20 years
3. 70 years
4. 140 years