AP Environmental Science Land and Water Use — Worked Answer Explanations

Unit 5 · 12 questions explained

Below is a complete answer key for our AP Environmental Science Land and Water Use practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Land and Water Use practice test and come back here to review, or head back to the Land and Water Use unit overview.

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Question 1 · Easy
The concept of sustainable agriculture includes which of the following practices?
- A
  Applying maximum synthetic fertilizer to maximize annual yield
  Why not A: Maximizing synthetic fertilizer input depletes soil organic matter, causes nitrogen runoff, and is not sustainable.
- B
  Rotating crops to maintain soil nutrients and reduce pest buildupCorrect
- C
  Irrigating with flood methods to ensure deep root hydration
  Why not C: Flood irrigation is water-inefficient and contributes to salinization; it is not a sustainable practice.
- D
  Concentrating livestock in large feedlots to minimize land use
  Why not D: CAFOs (feedlots) create concentrated nutrient pollution and are energy-intensive; they are industrial, not sustainable.
Explanation
Crop rotation — alternating different crops (e.g., corn, soybeans, wheat) across seasons — prevents pest and disease buildup (different crops host different pests), maintains soil health (legumes fix nitrogen), reduces need for synthetic fertilizer and pesticides, and prevents soil depletion. Other sustainable practices: cover crops, no-till farming, integrated pest management (IPM), and composting.

Key takeaway
Sustainable agriculture: crop rotation, cover crops, no-till, IPM, composting → maintains soil health and reduces chemical inputs.
Question 2 · Easy
The maximum amount of fish that can be harvested from a fishery without causing long-term population decline is called the:
- A
  Ecological footprint
  Why not A: Ecological footprint measures human demand on natural systems as a whole, not the fishery-specific harvest limit.
- B
  Maximum sustainable yield (MSY)Correct
- C
  Carrying capacity of the marine ecosystem
  Why not C: Carrying capacity is the maximum population size the environment can support; MSY is the derived harvest quota from that biological reality.
- D
  Biomagnification threshold
  Why not D: Biomagnification describes pollutant concentration up the food chain; it is unrelated to harvest limits.
Explanation
Maximum sustainable yield (MSY) is the largest catch that can be taken from a fish stock over an indefinite period. It corresponds to harvesting at the population's maximum growth rate (N = K/2 in the logistic model). Exceeding MSY leads to stock collapse. Modern fisheries management uses stock assessments to set catch limits at or below MSY.

Key takeaway
MSY = maximum harvest without long-term population decline. In logistic growth, MSY occurs at N = K/2.
Question 3 · Medium
Industrial monoculture agriculture maximizes yield per acre by growing a single crop variety across large areas. Which of the following is a significant environmental drawback of this approach?
- A
  It requires more labor per unit of food produced than traditional polyculture.
  Why not A: Industrial monoculture actually reduces labor per unit through mechanization; this is not a drawback.
- B
  Genetic uniformity increases vulnerability to pests, diseases, and climate variability.Correct
- C
  It produces lower food yields compared to diverse crop systems.
  Why not C: Monoculture often achieves high yields in the short term through specialization and mechanization; yield is not typically a drawback.
- D
  It relies on traditional organic fertilizers that are difficult to distribute at scale.
  Why not D: Industrial monoculture typically uses synthetic fertilizers, not traditional organic ones.
Explanation
Monocultures plant genetically identical or near-identical crops across vast areas. If a pathogen or pest evolves to overcome that variety's defenses, the entire crop is at risk (e.g., Irish Potato Famine, 1845). Genetic uniformity eliminates natural variation that could provide resistance. Additionally, monocultures degrade soil biodiversity and require heavy pesticide and fertilizer inputs.

Key takeaway
Monoculture risk: genetic uniformity → single pest/pathogen can devastate entire crop; also degrades soil and requires heavy chemical inputs.
Question 4 · Medium
A farmer in an arid region uses drip irrigation rather than flood irrigation. The primary environmental benefit of drip irrigation is:
- A
  It adds more nitrogen to the soil than flood irrigation.
  Why not A: Drip irrigation delivers water, not nitrogen; fertilizer application is a separate process.
- B
  It dramatically reduces water use and prevents soil salinization from evaporation.Correct
- C
  It requires no energy input since gravity drives the flow.
  Why not C: Drip systems require pumps and pressurization; they are not gravity-only.
- D
  It eliminates the need for pesticide application.
  Why not D: Irrigation method has no direct effect on pest management.
Explanation
Flood irrigation saturates fields, evaporating large volumes of water. As water evaporates from soils in arid climates, dissolved salts are left behind, causing soil salinization that can eventually render soil infertile. Drip irrigation delivers water directly to the root zone with 90%+ efficiency, reducing water waste and minimizing evaporation-driven salt buildup.

Key takeaway
Drip irrigation: 90%+ water efficiency, reduces salinization and aquifer depletion compared to flood/furrow irrigation.
Question 5 · Medium
The Ogallala Aquifer underlies much of the U.S. Great Plains and is used for agricultural irrigation. It is being depleted at a rate 10–40 times faster than natural recharge. This situation is an example of:
- A
  Sustainable yield management
  Why not A: Sustainable yield means withdrawal ≤ recharge; extraction that exceeds recharge by 10–40× is the opposite of sustainable.
- B
  Overdraft (unsustainable groundwater use)Correct
- C
  The hydrologic cycle restoring the aquifer through precipitation
  Why not C: The Ogallala recharges extremely slowly (glacial-era water); the hydrologic cycle cannot keep pace with current extraction.
- D
  Salinization from excessive flood irrigation
  Why not D: Salinization is a soil degradation problem; this question is specifically about depletion of the water source itself.
Explanation
Overdraft occurs when groundwater is extracted faster than it naturally recharges. The Ogallala is largely fossil groundwater deposited during the last ice age; its modern recharge rate is nearly zero. Depletion threatens the agricultural productivity of 8 states (Nebraska to Texas). Once an aquifer is depleted, it can collapse and become unable to store water even if recharge occurs.

Key takeaway
Overdraft: extraction > recharge rate → aquifer depletion. The Ogallala is a classic case of unsustainable groundwater use.
Question 6 · Medium
Clear-cutting a tropical forest for cattle ranching causes which of the following environmental consequences MOST directly?
- A
  Immediate increase in soil fertility due to ash from burned vegetation
  Why not A: Burning does add some nutrients briefly (slash and burn), but tropical soils are inherently poor and fertility is short-lived.
- B
  Habitat loss and fragmentation, releasing stored carbon and reducing biodiversityCorrect
- C
  Reduction in atmospheric CO₂ because new grasses fix more carbon than trees
  Why not C: Grasses store far less carbon than mature forests; clearing forest is a major net CO₂ source, not a sink.
- D
  Increase in local precipitation due to reduced evapotranspiration
  Why not D: Reduced evapotranspiration from deforestation actually decreases moisture recycling and tends to reduce local precipitation.
Explanation
Deforestation directly causes habitat destruction (eliminating species that depend on the forest), carbon release (forest biomass and soil carbon oxidize), soil erosion (roots no longer anchor soil), reduced evapotranspiration (drying local climate), and biodiversity loss. Tropical forests store ~250 billion tons of carbon; releasing it accelerates climate change.

Key takeaway
Deforestation: habitat loss, carbon release, soil erosion, reduced local rainfall, biodiversity decline.
Question 7 · Medium
Strip mining for coal removes entire hillsides of overburden to access coal seams near the surface. The most significant long-term water quality problem associated with strip mining is:
- A
  Mercury contamination from detonation residues
  Why not A: Mercury contamination is more associated with gold mining (using mercury amalgamation); it is not the primary water quality concern from coal strip mining.
- B
  Acid mine drainage (AMD) from oxidation of iron sulfide mineralsCorrect
- C
  Radioactive leachate from uranium ore exposed during mining
  Why not C: Coal mining does not primarily expose uranium; uranium mining is a different operation.
- D
  Nitrogen runoff from explosives fertilizing nearby waterways
  Why not D: While nitrogen compounds are in some explosives, the dominant water quality issue from coal strip mining is acid drainage.
Explanation
When overburden containing iron sulfide minerals (pyrite, FeS₂) is exposed to oxygen and water, sulfuric acid forms — acid mine drainage. AMD can lower stream pH to <3, killing aquatic life and precipitating metals. It continues for decades or centuries after mining ends and is extremely difficult to remediate, making it the most persistent environmental legacy of strip mining.

Key takeaway
Acid mine drainage: pyrite + O₂ + H₂O → sulfuric acid → stream acidification → long-term ecosystem damage.
Question 8 · Medium
Urbanization increases impervious surface cover (roads, parking lots, buildings). Which hydrological consequence does this MOST directly cause?
- A
  Increased groundwater recharge from concentrated runoff
  Why not A: Impervious surfaces prevent infiltration, reducing groundwater recharge, not increasing it.
- B
  Reduced evapotranspiration and increased stormwater runoff volume and peak flowCorrect
- C
  Increased soil water retention due to compaction under pavement
  Why not C: Compaction reduces pore space; infiltration into compacted soil is slower, not increased.
- D
  Decreased flooding because drainage infrastructure channels water efficiently
  Why not D: Urban drainage systems can locally reduce flooding, but overall impervious cover increases total runoff and flood peaks downstream.
Explanation
Impervious surfaces prevent water infiltration into soil. Rain that would have been absorbed and slowly released (or evapotranspired) instead becomes immediate surface runoff. This increases peak stream flows, raising flood risk, and reduces groundwater recharge. Urban streams often show the 'flashy' behavior of high peaks after rain followed by very low base flows — a signature of impervious cover.

Key takeaway
Urbanization → more impervious surface → less infiltration, less ET, more runoff volume and peak flow → increased flooding and reduced groundwater recharge.
Question 9 · Medium
Which of the following fishing practices has the MOST indiscriminate impact, capturing and killing non-target species (bycatch)?
- A
  Fly fishing with hook and line
  Why not A: Fly fishing is highly selective; anglers target specific species and can release non-target fish.
- B
  Bottom trawling with large nets dragged across the sea floorCorrect
- C
  Trap fishing with species-specific cage designs
  Why not C: Well-designed traps (e.g., lobster traps with escape gaps) have relatively low bycatch rates.
- D
  Aquaculture (fish farming) in enclosed pens
  Why not D: Aquaculture raises fish in controlled environments; bycatch doesn't apply since wild fish are not being caught.
Explanation
Bottom trawling drags huge weighted nets across the ocean floor, scooping up everything in their path — target species, non-target fish, sea turtles, corals, sponges, and invertebrates. It also destroys seafloor habitat that took centuries to form. Bycatch from trawling represents roughly 40% of global marine catch by some estimates. Purse-seine nets for tuna similarly catch dolphins, leading to 'dolphin-safe' labeling campaigns.

Key takeaway
Bottom trawling = highest bycatch and seafloor habitat destruction. Purse-seine nets also cause significant bycatch.
Question 10 · Medium
The 'tragedy of the commons' concept, as applied to fisheries, predicts that:
- A
  Fishers will voluntarily reduce their catch to protect long-term yields.
  Why not A: The tragedy of the commons predicts the opposite — individual rational self-interest leads to overexploitation.
- B
  Individual fishers, each acting in self-interest, will overfish the shared resource to collapse.Correct
- C
  Government subsidies will prevent overfishing by making fishing less profitable.
  Why not C: Government subsidies often do the opposite — they make overfishing more economically viable by reducing costs.
- D
  Fish populations self-regulate and prevent their own collapse.
  Why not D: Fish populations do not self-regulate against human fishing pressure; they can be driven to extinction.
Explanation
Garret Hardin's 'tragedy of the commons' (1968) describes how shared, unregulated resources are inevitably overexploited. Each fisher benefits fully from each additional fish caught but shares the cost of depletion with all others — creating an incentive to maximize individual catch regardless of the collective impact. Solutions include privatization, government regulation (catch limits), or community-managed quotas (Ostrom's commons governance).

Key takeaway
Tragedy of the commons: unregulated shared resources → individual incentives → overexploitation → collapse. Solution: regulation, quotas, or community governance.
Question 11 · Hard
Soil erosion is accelerated by all of the following EXCEPT:
- A
  Overgrazing by livestock that removes protective vegetation cover
  Why not A: Overgrazing removes vegetation that anchors soil, increasing erosion significantly.
- B
  Contour plowing across slopes rather than up and down hillsCorrect
- C
  Leaving fields bare (fallow) between planting seasons without cover crops
  Why not C: Bare soil has no root system to anchor particles and no canopy to break raindrop impact, greatly accelerating erosion.
- D
  Removing windbreaks (shelterbelts) from farmland edges
  Why not D: Windbreaks reduce wind speed across fields, protecting against wind erosion; removing them accelerates erosion.
Explanation
Contour plowing (plowing horizontal furrows across the slope) is an erosion-PREVENTION technique. It creates ridges that slow water runoff, allowing it to infiltrate rather than flowing straight downhill carrying topsoil. All other choices accelerate erosion. Contour plowing, no-till farming, terracing, and cover crops are standard soil conservation techniques.

Key takeaway
Contour plowing reduces erosion. Overgrazing, bare fields, and removing windbreaks all accelerate soil erosion.
Question 12 · Hard
Which of the following best describes the environmental impact of large-scale irrigation from surface water sources like rivers?
- A
  Increased river flow volume downstream due to field drainage returning water to rivers
  Why not A: Large-scale irrigation withdrawals typically reduce river flows, not increase them. Return flows exist but are less than withdrawals.
- B
  Reduced streamflow, altered sediment transport, and habitat loss for aquatic speciesCorrect
- C
  Removal of excess nutrients from rivers, improving downstream water quality
  Why not C: Irrigation return flows typically ADD nutrients (fertilizer) back to rivers; they do not remove nutrients.
- D
  Stabilization of riverbanks through water level control
  Why not D: Reduced flows and altered hydrology often destabilize riverbanks by reducing sediment deposition.
Explanation
Diverting river water for irrigation reduces downstream flow, sometimes dramatically (e.g., the Colorado River no longer reaches the sea; the Aral Sea shrank ~80% from river diversion). Reduced flows change temperature regimes, concentrate pollutants, reduce sediment delivery to deltas, and eliminate habitat for fish that depend on flow cues for migration and spawning. Return flows bring salts and fertilizer back, worsening water quality.

Key takeaway
River irrigation diversion: reduced flows → habitat loss, warmer water, concentrated pollutants, delta starvation (e.g., Colorado River, Aral Sea).