4.1 Natural Resources, Population Growth, and Human Impact
Key Takeaways
- Population change depends on four terms — births, deaths, immigration, and emigration; growth occurs whenever births plus immigration exceed deaths plus emigration.
- Exponential growth produces a J-shaped curve and occurs only when resources are effectively unlimited; logistic growth produces an S-shaped curve that levels off at the environment's carrying capacity.
- Carrying capacity is not a fixed number — irrigation, fertilizer, and sanitation raise it, while aquifer depletion, soil salinization, and pollution lower it.
- A renewable resource used faster than it regenerates behaves exactly like a non-renewable one, which is why the Ogallala Aquifer is treated as depletable even though rainfall recharges it.
- The demographic transition describes the shift from high birth and death rates to low birth and death rates as economies industrialize, explaining why some national populations stabilize while others continue to grow rapidly.
Why Resources and Population Sit Inside Domain I
Competency 004 is about science in society: how scientific principles inform personal and collective decisions. Population dynamics and resource use are the framework's chosen vehicle because they force students to combine data, probability, ethics, and economics. Exam items in this space usually present a decision — build a well, approve a wind farm, set a fishing limit — and ask which reasoning is scientifically sound.
The Four Terms That Change a Population
Any population changes by only four processes:
Population change = (births + immigration) − (deaths + emigration)
For a closed population such as bacteria in a sealed flask, immigration and emigration are zero, so the birth and death rates alone set the outcome. For a Texas county, migration frequently dominates: a county can have more deaths than births and still grow rapidly because people move in.
Two Growth Patterns
| Feature | Exponential growth | Logistic growth |
|---|---|---|
| Curve shape | J-shaped | S-shaped (sigmoid) |
| Condition | Resources effectively unlimited | Resources become limiting |
| Rate over time | Increases continuously | Rises, then slows, then approaches zero |
| Limit | None modeled | Levels off at carrying capacity (K) |
| Classroom example | Bacteria in fresh broth for the first several hours | Yeast in a closed flask over several days; deer on an island |
Exponential growth multiplies by a constant factor each interval. A bacterial culture that doubles every 30 minutes goes 1 → 2 → 4 → 8 → 16 cells; after 5 hours (10 doublings) a single cell has become 2¹⁰ = 1,024 cells. The pattern is memorable precisely because it is unsustainable — no real population sustains it, because something eventually runs short.
Logistic growth adds the limit. As numbers rise, food, space, nesting sites, or waste accumulation begin to matter. These density-dependent limiting factors intensify as crowding increases. Density-independent factors — a hard freeze, a wildfire, a hurricane — cut a population regardless of how crowded it was.
Carrying capacity (K) is the maximum population an environment can support indefinitely with its available resources. A population that overshoots K typically crashes below it before recovering, which is why deer populations without predators oscillate. Crucially, K is not a constant: irrigation, fertilizer, vaccination, and sanitation have repeatedly raised human carrying capacity, while aquifer overdraft, soil salinization, and topsoil loss lower it.
The Demographic Transition
Industrializing economies pass through a characteristic four-stage pattern:
- Pre-industrial — high birth rate, high death rate, slow net growth.
- Early industrial — death rate falls first (sanitation, food supply, medicine) while births stay high; growth is fastest here.
- Late industrial — birth rate falls (urbanization, education, access to family planning, lower child mortality); growth slows.
- Post-industrial — both rates are low; population stabilizes or declines slightly.
This model explains why global population growth rate has been falling since the late 1960s even while total population continues to rise — a distinction between a rate and a total that middle-school students routinely conflate and that exam items exploit.
Classifying Natural Resources
| Resource | Category | Renewal timescale | Consumption concern |
|---|---|---|---|
| Solar energy | Renewable (perpetual) | Continuous | Intermittency; storage; land area |
| Wind | Renewable (perpetual) | Continuous | Intermittency; siting; wildlife |
| Timber | Renewable (biological) | Decades | Harvest outpacing regrowth |
| Fish stocks | Renewable (biological) | Years | Harvest above the reproduction rate |
| Groundwater | Renewable but slow | Decades to millennia | Pumping faster than recharge |
| Topsoil | Renewable but very slow | Centuries per inch | Erosion outpacing formation |
| Coal | Non-renewable | 300+ million years | Finite supply; CO₂ and particulates |
| Petroleum and natural gas | Non-renewable | 100+ million years | Finite supply; emissions; spills and leakage |
| Uranium | Non-renewable | Geologic time | Finite ore; long-lived waste |
| Metals (copper, aluminum) | Non-renewable but recyclable | Geologic time | Ore grade decline; recycling recovers most value |
The Rate-of-Use Principle
The single most tested idea here is that the category label matters less than the ratio of use rate to renewal rate. The Ogallala Aquifer beneath the Texas High Plains recharges at a small fraction of the rate at which it is pumped for irrigation, so although groundwater is nominally renewable, this aquifer is managed as a depletable resource with measurable water-level declines. Conversely, a metal such as aluminum is geologically non-renewable but effectively conserved when recycled, since recycling aluminum uses roughly 5% of the energy of smelting it from bauxite ore.
Sustainable yield is the harvest rate a renewable resource can support indefinitely. Set the fishing quota below the stock's reproduction rate and the fishery persists; set it above and the stock collapses even though fish are renewable.
Using Science to Weigh Options
The framework is explicit that science informs decisions without dictating them. A defensible comparison of energy options examines cost per kilowatt-hour, land footprint, water use, lifecycle emissions, waste stream, and reliability — and then acknowledges that weighting those factors involves values, not just data. Teaching students to separate the empirical question ("How much water does this cooling system consume per megawatt-hour?") from the value question ("Is that trade-off worth it for our community?") is the instructional target of Competency 004, and it is what distinguishes a scientifically literate citizen from someone who simply has an opinion.
A yeast culture in a sealed flask grows rapidly for two days, then slows and levels off at about 8 × 10⁷ cells for the remaining week. Which statement best describes what happened?
A student concludes that because the global population growth rate has been declining since the late 1960s, the total world population must also be declining. What is the error?
An aquifer is pumped for irrigation much faster than rainfall can recharge it. Over decades, this resource is best described as: