2025 AP AP Environmental Science Practice Paper with Answers

Based on the table, as the nitrogen load increases steadily from 120 metric tons/year in 2012 to 340 metric tons/year in 2020, the eelgrass shoot density declines consistently from 450 shoots/m² down to 85 shoots/m². This demonstrates an inverse (negative) relationship between nitrogen load and eelgrass shoot density.

1 point for correctly describing the trend/relationship.
- Acceptable responses: 'As nitrogen load increases, eelgrass density decreases/declines'; 'Eelgrass shoot density and nitrogen load have an inverse/negative relationship.'
- Do not accept vague statements such as 'eelgrass density changes as nitrogen changes' without specifying direction.

The independent variable is the factor that is systematically manipulated by the researchers across experimental treatments, which is the nitrate concentration (amount of nitrate/nitrogen added to the mesocosm tanks).

1 point for identifying the independent variable.
- Acceptable responses: 'Concentration of nitrate'; 'Nitrate levels'; 'Amount of dissolved nitrogen/nitrate added to the tanks.'
- Do not accept dependent variables such as 'eelgrass biomass', 'algal turbidity', or controlled variables such as 'water salinity'.

Vegetation and soil microbes within the riparian buffer intercept and absorb dissolved nitrogen and phosphorus from agricultural runoff before it enters the water. The reduction in nutrient loading limits cultural eutrophication and subsequent algal blooms. When smaller algal populations die, there is a lower amount of organic matter for aerobic decomposers (bacteria) to consume, thereby decreasing cellular respiration rates among decomposers and preventing the depletion of dissolved oxygen.

1 point for explaining the ecological mechanism linking nutrient uptake by the buffer to reduced decomposition and higher DO.
- Acceptable responses must include both:
1. Buffer plants take up/filter nutrients (nitrates/phosphates) from runoff.
2. Reduced nutrient input prevents large algal blooms/die-offs, resulting in less aerobic decomposition/bacterial respiration and higher dissolved oxygen.
- Do not accept statements solely mentioning 'plants produce oxygen via photosynthesis into the water' without connecting the buffer to runoff filtration/eutrophication mitigation.

A valid hypothesis states a testable relationship between the independent variable (stream salinity/road salt concentration) and the dependent variable (macroinvertebrate taxa richness/diversity).

Acceptable examples:
• As road salt concentration/salinity increases, macroinvertebrate taxa richness will decrease.
• Increased salinity from road runoff will reduce macroinvertebrate species diversity.
• Macroinvertebrate taxa richness is correlated with stream salinity levels.

1 point for a testable hypothesis linking the independent variable (salinity / road salt levels) to the dependent variable (macroinvertebrate richness / diversity / abundance).

Notes:
- Do NOT accept a question (e.g., 'Does salt runoff affect insects?').
- Do NOT accept vague references without identifying both variables (e.g., 'Salinity affects the ecosystem').

The dependent variable is the measurable response that changes in reaction to the manipulated independent variable (salt concentration). In this experiment, the researchers measured the number (or percentage) of surviving mayfly nymphs (or mayfly mortality/survival rate).

Acceptable answers include:
• Number/count of surviving mayfly nymphs
• Mortality rate/survival rate of mayflies
• Mayfly survival

1 point for correctly identifying the dependent variable as the number of surviving mayflies / mortality rate of mayflies / mayfly survival.

Notes:
- Do NOT accept 'salinity' or 'NaCl concentration' (independent variable).
- Do NOT accept controlled variables such as 'water temperature' or 'dissolved oxygen'.

In an experiment, the independent variable is the factor that is manipulated or categorized as the explanatory variable across test sites. In this investigation, the researchers are examining how varying concentrations of dissolved nitrates (or agricultural runoff) affect eelgrass canopy density. Therefore, the dissolved nitrate level (or nutrient concentration) is the independent variable.

Award 1 point for correctly identifying the independent variable:
- Dissolved nitrate concentration / levels
- Amount/presence of agricultural nutrient runoff

Reject: Eelgrass canopy density (this is the dependent variable), number of coastal bays (sample sites).

A valid scientific hypothesis must propose a directional or causal relationship between the independent variable (dissolved nitrate concentration) and the dependent variable (eelgrass canopy density). For example: 'If dissolved nitrate levels in coastal water increase, then eelgrass canopy density will decrease.'

Award 1 point for a testable hypothesis linking dissolved nitrate levels to eelgrass canopy density, such as:
- Increasing dissolved nitrate concentrations will decrease eelgrass canopy density.
- Increasing dissolved nitrate concentrations will increase eelgrass canopy density.
- Variations in dissolved nitrate levels will alter eelgrass canopy density.

Reject: Phrasing written as a scientific question rather than a statement/hypothesis (e.g., 'Does nitrate affect eelgrass?').

Regulating ecosystem services are benefits obtained from the natural regulation of ecosystem processes. Coastal eelgrass beds act as natural wave attenuators, slowing water currents and absorbing wave energy to prevent coastline erosion. Alternatively, they regulate water quality by filtering particulates or sequestering carbon in marine sediments (blue carbon).

Award 1 point for a valid description of a regulating ecosystem service provided by eelgrass beds, such as:
- Coastal erosion control / shoreline stabilization by trapping sediment with roots and reducing wave energy.
- Water purification / filtration by trapping suspended particulate matter and taking up excess dissolved nutrients.
- Climate regulation / carbon sequestration via long-term storage of blue carbon in estuarine sediment.
- Flood mitigation / storm surge buffering along coastal zones.

Reject: Provisioning services (e.g., fish nursery for harvest) or cultural services (e.g., ecotourism).

When epiphytic microalgae cover the surface of eelgrass blades, sunlight availability for eelgrass chloroplasts decreases. This reduced light infiltration impedes photosynthesis, causing eelgrass die-offs and reducing primary productivity at the base of the food web. Herbivorous fish that rely on eelgrass for forage lose their nutritional supply and protective habitat, which lowers reproduction and increases mortality.

Award 1 point for explaining the cause-and-effect relationship between shading and herbivorous fish decline:
- Shading decreases photosynthesis/growth/biomass of eelgrass (primary producer), leading to reduced food/caloric availability for herbivorous fish.
- Shading leads to eelgrass die-offs, destroying nursery habitat/shelter from predators, thereby increasing fish mortality.

Note: The response must connect reduced light/photosynthesis of the primary producer to the trophic impact on the herbivorous fish.

Agricultural nonpoint source pollution of nitrates can be mitigated through targeted conservation practices. Planting riparian buffer zones (strips of native vegetation along stream banks) allows plant roots and soil microbes to assimilate and denitrify excess nitrates in surface and subsurface runoff before it reaches receiving waters. Other viable methods include precision fertilizer application, planting winter cover crops, or applying conservation tillage.

Award 1 point for a correct description of an agricultural management practice to reduce nitrate runoff, such as:
- Planting vegetated riparian buffer strips along agricultural ditches/streams to absorb excess nutrients before runoff reaches waterways.
- Implementing precision agriculture / split fertilizer applications to match crop uptake needs and prevent fertilizer overapplication.
- Planting off-season cover crops (such as clover or rye) to take up residual soil nitrogen after main crops are harvested.
- Constructing artificial wetlands to denitrify agricultural drainage water before discharge.

Reject: Vague answers such as 'use less fertilizer' without describing a specific method or mechanism (e.g., soil testing, precision placement).

The rain shadow effect occurs on the leeward side (the eastern slope in this model), where descending dry air creates arid or semiarid climatic conditions.

Acceptable identification point (1 point):
• The leeward side
• The eastern side / eastern slope

Reject: Windward side / western slope.

According to the theory of island biogeography, smaller islands support smaller populations with higher extinction rates, and islands located farther from the mainland experience lower rates of immigration/colonization. Therefore, Island X will support the lowest species richness.

Acceptable identification point (1 point):
• Island X
• The small, distant island

Reject: Island Y.

At a convergent boundary where an oceanic plate meets a continental plate, the denser oceanic plate sinks beneath the continental plate at a subduction zone.

Acceptable identification point (1 point):
• Subduction zone
• Ocean trench / deep-sea trench

Reject: Divergent boundary, transform fault, mid-ocean ridge.

Resource partitioning occurs when species evolve to use different resources or occupy distinct spatial or temporal niches. By consuming different types of vegetation (coarse grasses/tall shrubs vs. tender forbs/understory leaves) and foraging in different locations (open meadows vs. dense forest underbrush), the elk and mule deer minimize niche overlap. This reduction in direct interspecific competition allows both populations to maintain viable numbers simultaneously without one driving the other to local extinction via competitive exclusion.

1 point for an explanation that links resource partitioning to reduced competition/niche differentiation.

Acceptable explanations include:
- Resource partitioning reduces niche overlap/interspecific competition for limited resources.
- Species utilize different parts of the habitat/different food types, allowing both species to obtain sufficient energy without competing directly.
- Differentiation in feeding niches prevents competitive exclusion between the two species.

The scenario describes secondary succession (which begins with intact soil), whereas succession on bare lava rock is primary succession. Soil formation during primary succession requires hundreds to thousands of years for pioneer species like lichens and mosses to chemically and physically weather rock and accumulate organic matter. Because fertile soil, moisture-retentive substrate, microbial communities, and viable seed/root banks already exist following the wildfire, new plant species can germinate and establish immediately, drastically accelerating community recovery.

1 point for an explanation comparing secondary to primary succession based on the presence of soil/seed bank.

Acceptable explanations include:
- Soil and nutrients are already present after the fire, whereas primary succession requires extensive time for weathering of bare rock and soil formation by pioneer species.
- Pre-existing seeds, root systems, and fertile soil enable rapid colonization compared to bare lava that lacks organic matter/soil.

To reduce the loss of mangrove ecosystems, governments can establish legally enforced protected coastal reserves or zoning restrictions that ban clearing mangroves for aquaculture. Alternatively, governments can mandate or subsidize inland recirculating aquaculture systems (RAS) to shift aquaculture production away from sensitive coastal wetlands.

Award 1 point for proposing a realistic government solution to reduce mangrove clearing for aquaculture.

Acceptable responses include:
- Establish marine protected areas (MPAs) / zoning laws that prohibit commercial aquaculture development in coastal mangrove habitats.
- Provide financial subsidies / tax credits for aquaculture farms that transition to inland closed-loop recirculating systems.
- Require environmental impact assessments and mandatory permit caps on mangrove removal for commercial enterprises.
- Enact policies requiring replanting/restoration of an equivalent area of mangroves for any coastal footprint altered (compensatory mitigation).

Beyond storm surge protection, mangrove root networks provide essential nursery and breeding habitats that support diverse marine species, including commercial fish and invertebrates. They also enhance carbon sequestration (blue carbon) and filter terrestrial runoff before it reaches open waters.

Award 1 point for justifying the restoration by providing an additional ecological or environmental advantage.

Acceptable responses include:
- Provides nursery and breeding habitats for aquatic organisms / increases local marine biodiversity.
- Acts as a carbon sink / sequesters significant amounts of atmospheric carbon (blue carbon).
- Filters water pollutants, excess nutrients, and sediment from terrestrial runoff before reaching offshore coral reefs and seagrass beds.
- Traps sediment and prevents coastal shoreline erosion.

Agricultural runoff can be mitigated by planting riparian buffer strips of native trees and vegetation along field borders and stream banks, which physically trap sediment and biologically take up dissolved nitrates and phosphates before they enter surface waters. Other effective practices include applying cover crops during off-seasons, practicing precision fertilizer application, and utilizing contour plowing.

Award 1 point for describing a specific agricultural practice that reduces nutrient runoff.

Acceptable responses include:
- Planting riparian vegetative buffer strips along edges of water bodies to absorb dissolved nutrients and trap eroding sediment.
- Utilizing cover crops (such as clover or rye) during fallow periods to uptake residual soil nutrients and reduce surface runoff.
- Implementing precision agriculture / split fertilizer application to match fertilizer amounts precisely to crop needs, minimizing excess residue.
- Practicing no-till agriculture to maintain soil structure and minimize erosion that carries attached phosphorus into waterways.

A municipality can implement an urban forestry initiative or mandate green (vegetated) roofs on commercial buildings. This reduces ambient temperatures because vegetation provides direct shade to low-albedo surfaces and releases moisture into the atmosphere via evapotranspiration, which cools the surrounding air.

Award 1 point for proposing a realistic municipal solution and explaining how it lowers temperature.

Acceptable responses include:
- Mandating the installation of green roofs, which cool urban areas through evapotranspiration and shaded roof surfaces.
- Expanding urban street tree canopies / urban parks, which lower surface and air temperatures via shading and evapotranspirative cooling.
- Replacing conventional dark asphalt with high-albedo / reflective 'cool pavement', which reflects more solar radiation back into the atmosphere rather than absorbing and reradiating it as heat.

Note: Response must include both the realistic solution and the mechanism of cooling (e.g., increased albedo, shade, or evapotranspiration).

When anthropogenic \(\text{CO}_2\) dissolves into ocean water, it reacts with \(\text{H}_2\text{O}\) to form carbonic acid (\(\text{H}_2\text{CO}_3\)), which dissociates into bicarbonate (\(\text{HCO}_3^-\)) and free hydrogen ions (\(\text{H}^+\)). These free \(\text{H}^+\) ions bind readily with carbonate ions (\(\text{CO}_3^{2-}\)) to form additional bicarbonate, significantly reducing the concentration of free carbonate ions necessary for calcifying organisms to precipitate calcium carbonate (\(\text{CaCO}_3\)).

Award 1 point for explaining the chemical mechanism that impairs calcification.

Acceptable responses include:
- Added \(\text{CO}_2\) forms carbonic acid, releasing free hydrogen ions (\(\text{H}^+\)) that bond with carbonate ions (\(\text{CO}_3^{2-}\)), reducing the amount of free carbonate available for organisms to combine with calcium to form calcium carbonate (\(\text{CaCO}_3\)).
- Increased acidity / higher \(\text{H}^+\) ion concentration can dissolve existing calcium carbonate structures / shells faster than organisms can produce them.

Note: A complete response must link the elevated hydrogen ion/acidic condition to reduced availability of carbonate ions or dissolution of \(\text{CaCO}_3\).

Ground-level (tropospheric) ozone is a powerful respiratory irritant formed via photochemical reactions involving \(\text{NO}_x\) and volatile organic compounds in the presence of sunlight. Inhaling ground-level ozone damages epithelial cells lining the respiratory tract, exacerbates preexisting respiratory ailments such as asthma and chronic bronchitis, and reduces overall lung function.

1 point for a valid description of a negative human health effect caused by ground-level ozone exposure.

Acceptable responses include:
- Irritation/inflammation of lung tissue, throat, or airways
- Exacerbation/worsening of asthma or chronic bronchitis/emphysema
- Reduced lung capacity/difficulty breathing/coughing
- Eye/mucous membrane irritation

Do NOT accept: Non-human ecological impacts (e.g., damage to crop yields) or skin cancer/UV effects (which relate to stratospheric ozone depletion).

A municipality can incentivize energy conservation by adopting dynamic time-of-use pricing, where electricity rates are higher during peak afternoon hours, encouraging consumers to shift intensive electrical tasks to off-peak periods. Alternatively, the local government could offer financial subsidies or property tax rebates for homeowners who install smart thermostats or Energy Star-rated heat pumps and appliances.

1 point for proposing a realistic municipal government policy or economic incentive designed to reduce household electricity consumption.

Acceptable responses include:
- Providing financial subsidies, rebates, or tax credits for purchasing energy-efficient appliances (e.g., heat pumps, smart thermostats, high-efficiency insulation)
- Enacting time-of-use or tiered pricing structures that penalize excessive peak electricity usage
- Implementing municipal building codes that require stricter insulation and energy-conservation standards in residential construction
- Subsidizing home energy audits to help residents identify and seal thermal leaks

Do NOT accept: Vague personal actions (e.g., 'tell people to turn off lights') unless framed as an actionable government initiative (e.g., 'fund a public energy-conservation awareness campaign').

To calculate the avoided \(\text{CO}_2\) mass:

\[\text{Avoided } \text{CO}_2 = (4.5 \times 10^7\text{ kWh/year}) \times (0.90\text{ lb }\text{CO}_2/\text{kWh})\]
\[\text{Avoided } \text{CO}_2 = 40,500,000\text{ lb/year} = 4.05 \times 10^7\text{ lb/year}\]

1 point for the correct mathematical calculation with setup shown.

Acceptable setups and answers include:
- \((4.5 \times 10^7\text{ kWh}) \times (0.90\text{ lb/kWh}) = 40,500,000\text{ lb}\) (or \(4.05 \times 10^7\text{ lb}\))
- \(4.5 \times 10^7 \times 0.90 = 4.05 \times 10^7\)

Note: Correct mathematical representation without units is acceptable, but full work/setup must be shown.

The amount of grid electricity replaced by the solar array is \(360\text{ MWh}\).

The percentage reduction is:
\[\text{Percent reduction} = \left( \frac{360\text{ MWh}}{600\text{ MWh}} \right) \times 100 = 60\%\]

Alternatively, calculating new purchased grid electricity:
\[\text{New electricity purchased} = 600\text{ MWh} - 360\text{ MWh} = 240\text{ MWh}\]
\[\text{Percent change} = \frac{240 - 600}{600} \times 100 = \frac{-360}{600} \times 100 = -60\% \text{ (a } 60\% \text{ reduction)}\]

1 point for the correct calculation with setup shown.

Acceptable setups and answers include:
- \((360\text{ MWh} / 600\text{ MWh}) \times 100 = 60\%\)
- \(((240 - 600) / 600) \times 100 = -60\%\) or \(60\%\text{ decrease}\)
- \((360 / 600) \times 100 = 60\)

Acceptable values: 60%, 60, -60%, or 60% decrease.

Step 1: Calculate the annual electricity production per solar panel:
$$\text{Annual output per panel} = 2.0\text{ kWh/day} \times 365\text{ days/year} = 730\text{ kWh/panel/year}$$

Step 2: Calculate the required number of solar panels:
$$\text{Number of panels} = \frac{7.30 \times 10^6\text{ kWh/year}}{730\text{ kWh/panel/year}} = 10{,}000\text{ panels}$$

1 point for the correct setup to calculate the number of solar panels needed:
- \(\frac{7.30 \times 10^6\text{ kWh}}{2.0\text{ kWh/panel/day} \times 365\text{ days}}\)
- \(\frac{7{,}300{,}000}{730}\)
- \((7.30 \times 10^6) / (2.0 \times 365)\)

1 point for the correct calculation of the number of solar panels:
- \(10{,}000\)
- \(1.0 \times 10^4\)
(Note: Units are not required to earn the calculation point, but if provided, must be correct, e.g., panels)

Step 1: Use the percent change formula:
$$\text{Percent Change} = \frac{\text{Final Value} - \text{Initial Value}}{\text{Initial Value}} \times 100$$

Step 2: Substitute given values and solve:
$$\text{Percent Change} = \frac{126\text{ metric tons} - 840\text{ metric tons}}{840\text{ metric tons}} \times 100$$
$$\text{Percent Change} = \frac{-714}{840} \times 100 = -85\%\text{ (or an } 85\%\text{ decrease)}$$

1 point for the correct setup showing order of operations to calculate percent change:
- \(\frac{126\text{ metric tons} - 840\text{ metric tons}}{840\text{ metric tons}} \times 100\)
- \(((126 - 840) / 840) \times 100\)
- \((-714 / 840) \times 100\)

1 point for the correct calculation indicating a negative change/decrease:
- \(-85\%\)
- \(85\%\text{ decrease}\)
- \(-85\)
(Note: Stating only 85% without indicating a decrease or negative sign does not earn the calculation point)

Step 1: Calculate doubling time using the Rule of 70:
$$\text{Doubling Time } (T_d) = \frac{70}{\text{Growth Rate (\%)}} = \frac{70}{2.8} = 25\text{ years}$$

Step 2: Determine the target year:
Since \(70{,}000\) is exactly double the initial population of \(35{,}000\):
$$\text{Year} = 2024 + 25 = 2049$$

1 point for the correct setup showing the doubling time calculation added to the base year:
- \((70 / 2.8) + 2024\)
- \(25 + 2024\) where \(25 = 70 / 2.8\) is clearly shown

1 point for the correct calculation of the year:
- \(2049\)