AP Environmental Science Unit 1 Review: Ecosystems

6–8% of AP multiple choice
11 topics
AP exam review verified for 2027

Unit 1 covers ecosystem structure and function: species interactions (predation, symbiosis, competition), terrestrial and aquatic biome distribution, the four major biogeochemical cycles, primary productivity (GPP vs. NPP), trophic levels, the 10% rule, and food web dynamics including feedback loops.

Fiveable's AP Environmental Science Unit 1 review covers 11 topics, with guided practice questions for the unit.

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AP Environmental Science unit 1 topics

1.1

Introduction to Ecosystems

Explains how resource availability shapes predator-prey relationships, three types of symbiosis (mutualism, commensalism, parasitism), intraspecific and interspecific competition, and resource partitioning as a strategy for coexistence.

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1.2

Terrestrial Biomes

Covers the nine major terrestrial biomes from tundra to tropical rainforest, their climate signatures, characteristic organisms, and why biome boundaries shift with global climate change.

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1.3

Aquatic Biomes

Distinguishes freshwater biomes (streams, rivers, ponds, lakes, wetlands) from marine biomes (oceans, coral reefs, marshes, estuaries) and explains how salinity, depth, turbidity, nutrients, and temperature control resource distribution.

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1.4

The Carbon Cycle

Traces carbon through sources and sinks, contrasts short-term reservoirs (living organisms, atmosphere) with long-term reservoirs (fossil fuels), and explains how fossil fuel combustion rapidly transfers stored carbon to the atmosphere as CO2.

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1.5

The Nitrogen Cycle

Explains the five steps of the nitrogen cycle (fixation, nitrification, assimilation, ammonification, denitrification), the role of bacteria at each step, and why the atmosphere is the major nitrogen reservoir.

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1.6

The Phosphorus Cycle

Describes phosphorus movement through rock, soil, water, and organisms with no atmospheric phase; explains why phosphorus limits freshwater productivity and how agricultural runoff causes eutrophication.

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1.7

The Hydrologic (Water) Cycle

Traces water through evaporation, transpiration, condensation, precipitation, surface runoff, infiltration, and groundwater storage; identifies the ocean as the primary surface reservoir and the sun as the energy driver.

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1.8

Primary Productivity

Defines gross primary productivity (GPP) and net primary productivity (NPP), explains the formula NPP = GPP minus respiration, introduces units (kcal/m2/yr), and describes how light attenuation limits aquatic photosynthesis.

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1.9

Trophic Levels

Organizes producers, primary consumers, secondary consumers, tertiary consumers, and decomposers into trophic levels and explains how energy flows upward while matter cycles through biogeochemical cycles.

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1.10

Energy Flow and the 10% Rule

Applies the 10% rule to calculate energy available at each trophic level, explains energy loss through the second law of thermodynamics, and uses energy pyramids to visualize why food chains rarely exceed four or five links.

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1.11

Food Chains and Food Webs

Distinguishes linear food chains from complex food webs, explains how removing or adding a species triggers trophic cascades, and identifies positive and negative feedback loops in food web dynamics.

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Practice AP Environmental Science unit 1 questions

Try AP-style questions on unit 1, then use the review notes below for anything you miss.

Example stimulus-based MCQs

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Stimulus-based practice question

In a coastal kelp forest, 10 years of data show sea urchin population density and kelp biomass. In year 4, disease sharply reduced the sea otter population. The researchers want to test how the decline in this secondary consumer affected energy flow from kelp to sea urchins.

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Question

Which statement is the appropriate null hypothesis for the investigation?

The decline in sea otters will have a significant positive effect on kelp biomass.

The decline in sea otters will have no significant effect on kelp biomass.

The decline in sea otters will have no significant effect on sea urchin population density.

The decline in sea otters will have no significant effect on energy available to tertiary consumers.

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Stimulus-based practice question

The figure below shows the average net primary productivity for six different terrestrial biomes, along with their typical latitude ranges.

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Question

Which statement accurately describes the pattern of net primary productivity across the biomes?

Net primary productivity shows a perfect negative correlation with latitude across all biomes.

Biomes located near the equator exhibit higher productivity than biomes at higher latitudes.

Extreme environments like deserts and tundras show the highest rates of carbon assimilation.

Net primary productivity remains relatively constant across all latitudes shown in the chart.

Example FRQs

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FRQ

Freshwater lake eutrophication and nutrient cycling

3. Lake Mendota is a temperate freshwater lake ecosystem that supports diverse aquatic life. The lake has experienced increased algal blooms in recent years due to agricultural runoff from surrounding farmland. Scientists are studying the lake's energy flow and nutrient cycling to develop management strategies.

Lake Mendota Biomass and Nutrient Data

Trophic Level	Biomass (kg/m²)	Phosphorus Input Source	Annual P Load (kg/yr)
Producers (Phytoplankton)	0.45	Agricultural runoff	2,850
Primary Consumers (Zooplankton)	0.082	Urban stormwater	640
Secondary Consumers (Small fish)	0.015	Atmospheric deposition	210
Tertiary Consumers (Large fish)	0.0028	Natural sources	180

Identify one abiotic factor, other than nutrients, that would limit primary productivity in Lake Mendota.

Describe the process of denitrification and explain how it removes nitrogen from the lake ecosystem. The nitrogen cycle in Lake Mendota involves multiple processes including nitrogen fixation, nitrification, and denitrification.

Explain how excess nitrogen and phosphorus from agricultural runoff leads to decreased dissolved oxygen levels in Lake Mendota. Agricultural runoff containing excess nitrogen and phosphorus enters Lake Mendota from surrounding farmland, leading to increased algal blooms.

Figure 1. Lake Mendota Trophic Structure and Energy Flow

Calculate the amount of energy, in kcal/m²/yr, that is available to secondary consumers (small fish) in Lake Mendota. Show your work. Based on Figure 1, the net primary productivity of phytoplankton in Lake Mendota is 8,400 kcal/m²/yr. Zooplankton (primary consumers) receive energy from the phytoplankton, and small fish (secondary consumers) receive energy from the zooplankton. Energy transfer between trophic levels follows the 10% rule.

Calculate the percent change in biomass between the primary consumer level and the secondary consumer level. Show your work. Based on Table 1, the biomass of primary consumers (zooplankton) in Lake Mendota is 0.082 kg/m², and the biomass of secondary consumers (small fish) is 0.015 kg/m².

Propose a realistic agricultural practice that farmers could implement to reduce phosphorus runoff into Lake Mendota without significantly reducing crop yields. Local farmers want to reduce nutrient runoff into Lake Mendota while maintaining crop productivity.

Calculate the amount of phosphorus, in kg/yr, that would be prevented from entering Lake Mendota after the riparian buffer zones are installed. Show your work. Based on Table 1, agricultural runoff currently contributes 2,850 kg of phosphorus per year to Lake Mendota. The local watershed district plans to install a 50-meter-wide vegetated riparian buffer zone along all streams feeding into the lake. Research shows that properly maintained riparian buffers remove 65% of phosphorus from agricultural runoff before it reaches the lake.

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FRQ

Nitrogen cycling and phytoplankton distribution in lakes

1. A freshwater lake ecosystem contains phytoplankton (photosynthetic algae), zooplankton (small herbivorous animals), small fish, and large predatory fish. Energy flows through this ecosystem as organisms consume those at lower trophic levels. The productivity of the ecosystem depends on the availability of limiting nutrients, particularly nitrogen and phosphorus.

Describe one process in the nitrogen cycle by which atmospheric nitrogen ( $\text{N}_2$ ) is converted into a form usable by phytoplankton.

Explain why phytoplankton biomass is highest near the surface of the lake despite the low nutrient concentrations at shallow depths, as shown in Figures 1 and 2.

Figure 1. Dissolved Nutrient Concentrations at Different Depths in a Freshwater Lake (Nitrogen and Phosphorus)

Based on the data in Figure 1, identify the nitrogen concentration at a depth of 20 meters.

Figure 2. Phytoplankton Biomass at Different Depths in a Freshwater Lake

Based on the data in Figure 2, describe the relationship between depth and phytoplankton biomass below 5 meters.

Describe how much biomass of large predatory fish could be supported by the phytoplankton, assuming 10% energy transfer efficiency between each trophic level. Show all work. The phytoplankton in the lake have a total biomass of 50,000 kg. Energy is transferred through the food chain as zooplankton consume phytoplankton, small fish consume zooplankton, and large predatory fish consume small fish.

A group of students hypothesized that increased phosphorus availability would increase phytoplankton growth rates. To test this hypothesis, students collected water samples from the lake and designed an experiment. They obtained six identical 1-liter glass bottles and filled each with lake water containing natural phytoplankton populations. The students added different amounts of dissolved phosphorus to each bottle and placed all bottles under identical light conditions for one week. They measured phytoplankton density at the beginning and end of the experiment.

Identify the independent variable in the students' investigation.

ii.

Identify one variable that was held constant in the experimental design.

Phytoplankton Species Richness in Two Lake Zones After Nutrient Addition

Lake Zone	Species 1	Species 2	Species 3	Species 4	Species 5	Species 6
High nutrient zone	X	X
Low nutrient zone	X	X	X	X	X	X

Students studying phytoplankton biodiversity in a eutrophic lake (high nutrient levels) compared species richness between a zone receiving agricultural runoff (high nutrient zone) and a zone without direct runoff (low nutrient zone). Their data are shown in the following table. An 'X' indicates that the species was present in that zone.

Explain why the low nutrient zone would be more resilient to environmental disturbances, such as temperature changes or disease outbreaks, than the high nutrient zone would.

ii.

Explain one reason why excess phosphorus from agricultural runoff could lead to decreased dissolved oxygen concentrations in the lake, negatively affecting fish populations.

Describe one environmental consequence of deforestation in the watershed surrounding the lake on the phosphorus cycle.

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FRQ

Estuary nutrient cycling and phytoplankton community dynamics

2. A coastal estuary ecosystem supports diverse marine life and serves as a critical nursery habitat for commercial fish species. The estuary receives freshwater input from a river that drains an agricultural watershed. Recent monitoring has detected changes in water quality parameters and shifts in the phytoplankton community structure.

Figure 1. Nitrogen Cycle and Estuary Food Web in a Coastal Estuary Ecosystem (with Agricultural Runoff Input)

Identify the bacterial process shown in Figure 1 that converts ammonium ( $NH_4^+$ ) to nitrate ( $NO_3^-$ ) in the estuary sediments.

Identify the trophic level that phytoplankton occupy in the estuary food web shown in Figure 1.

Figure 2. Water Quality and Biological Data from Estuary Monitoring (2015 and 2024)

Identify whether the nitrate concentration in the estuary increased or decreased between 2015 and 2024, based on the data in Figure 2.

Explain why the increased nitrate levels from agricultural runoff led to the increase in phytoplankton biomass observed in the estuary between 2015 and 2024.

Describe one negative environmental effect of the decreased dissolved oxygen levels on the estuary ecosystem, based on the data shown in Figure 2.

Propose one realistic solution that farmers in the watershed could implement to reduce nitrogen runoff into the river that feeds the estuary.

Describe one characteristic of a wetland ecosystem that enables it to remove excess nitrogen from water flowing through it.

Justify the solution you proposed in part (F) by explaining one additional environmental benefit, other than reducing nitrogen runoff to the estuary.

Describe how the phosphorus cycle differs from the nitrogen cycle in terms of atmospheric reservoirs.

Describe how energy availability changes as energy flows from phytoplankton to zooplankton to fish in the estuary food web, referencing the second law of thermodynamics.

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AP Environmental Science unit 1 practice test

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Unit 1 review notes

1.1

Species Interactions and Resource Availability

Resource availability determines how species interact. When resources are limited, species compete; when one species relies on another, symbiotic or predator-prey relationships form. Understanding these interactions explains population dynamics and ecosystem stability.

Predator-prey relationship: The predator eats the prey; predator and prey populations cycle together, with predator numbers lagging behind prey numbers.
Symbiosis types: Mutualism: both species benefit. Commensalism: one benefits, one is unaffected. Parasitism: one benefits, one is harmed.
Intraspecific vs. interspecific competition: Intraspecific competition occurs within a species; interspecific competition occurs between species competing for the same limited resource.
Resource partitioning: Species reduce competition by using resources differently in space, time, or method, allowing coexistence rather than competitive exclusion.
Keystone species: A species with a disproportionately large effect on ecosystem structure relative to its abundance; removing it can collapse the food web.

Can you give one example each of mutualism, commensalism, and parasitism, and explain how resource partitioning allows two competing species to coexist?

Interaction	Species A outcome	Species B outcome	Example
Mutualism	Benefits (+)	Benefits (+)	Legume roots and Rhizobium bacteria
Commensalism	Benefits (+)	Unaffected (0)	Barnacles on a whale
Parasitism	Benefits (+)	Harmed (-)	Tapeworm in a host
Competition	Harmed (-)	Harmed (-)	Two plant species competing for soil nitrogen
Predation	Benefits (+)	Harmed (-)	Wolf eating a rabbit

1.2

Terrestrial Biomes

A biome is defined by its climate, especially temperature and precipitation, which determines the characteristic plants and animals found there. Biome boundaries are not fixed; they shift as climate changes. The global distribution of resources like water and timber also varies by latitude, altitude, and soil type.

Major terrestrial biomes: Taiga, temperate rainforest, temperate seasonal forest, tropical rainforest, shrubland, temperate grassland, savanna, desert, and tundra. Each has a distinct climate profile.
Tundra and permafrost: Found at high latitudes; permanently frozen subsoil (permafrost) limits plant growth and stores large amounts of carbon.
Tropical rainforest productivity: High year-round temperature and rainfall produce the highest net primary productivity of any terrestrial biome and the greatest biodiversity.
Biome distribution factors: Climate, latitude, altitude, nutrient availability, and soil type together determine where nonmineral resources like water and timber are found.
Dynamic biome boundaries: Biome ranges have shifted in the past and are projected to shift again as global temperatures and precipitation patterns change.

Given a description of annual temperature range and precipitation, can you identify the correct terrestrial biome and name one characteristic plant or animal adaptation?

Biome	Climate signature	Key vegetation
Tundra	Very cold, low precipitation	Mosses, lichens, sedges
Taiga	Cold winters, moderate precipitation	Coniferous trees (spruce, fir)
Temperate seasonal forest	Distinct seasons, moderate precipitation	Deciduous broadleaf trees
Tropical rainforest	Warm year-round, very high precipitation	Broadleaf evergreen trees, lianas
Desert	Low precipitation, extreme temperature swings	Cacti, xerophytes

1.3

Aquatic Biomes

Aquatic biomes are divided into freshwater and marine systems. The distribution of organisms and resources in aquatic biomes depends on salinity, depth, turbidity, nutrient availability, and temperature. Algae are the primary photosynthetic organisms in most aquatic systems.

Freshwater biomes: Streams, rivers, ponds, lakes, and freshwater wetlands. Wetlands are especially productive and filter pollutants.
Marine biomes: Oceans, coral reefs, marshes, and estuaries. Estuaries are highly productive transition zones between fresh and salt water.
Photic zone: The upper layer of water where enough sunlight penetrates for photosynthesis. Most red light is absorbed in the top 1 m; blue light can reach beyond 100 m in clear water.
Salinity and turbidity: Salinity determines which organisms can survive; turbidity (suspended particles) limits light penetration and reduces photosynthesis.
Coral reefs: Among the most biodiverse marine ecosystems; depend on warm, clear, shallow water and the mutualistic relationship between coral polyps and zooxanthellae algae.

What four factors most directly control the distribution of marine resources like fish populations, and how does light availability differ between the top 1 m and depths below 100 m?

System	Salinity	Key productivity factor	Example biome
Freshwater	Very low	Nutrient input, light	Lake, river, wetland
Estuarine	Variable (brackish)	Nutrient mixing, tidal flow	Salt marsh, estuary
Marine (coastal)	High	Upwelling, nutrient availability	Coral reef, continental shelf
Open ocean	High	Nutrient scarcity limits productivity	Pelagic zone

1.4

The Carbon Cycle

The carbon cycle moves carbon between the atmosphere, living organisms, soil, oceans, and geological reservoirs. The key distinction for the exam is between short-term reservoirs (living organisms, atmosphere) and long-term reservoirs (fossil fuels, deep ocean sediments). Human combustion of fossil fuels rapidly transfers long-stored carbon into the atmosphere as CO2.

Short-term carbon reservoirs: Living organisms and the atmosphere hold carbon for years to decades; carbon moves quickly through photosynthesis and cellular respiration.
Long-term carbon reservoirs: Fossil fuels, deep ocean sediments, and carbonate rocks store carbon for millions of years; burning fossil fuels bypasses the slow natural release.
Photosynthesis and respiration: Photosynthesis removes CO2 from the atmosphere and stores it in organic molecules; cellular respiration releases CO2 back to the atmosphere.
Decomposition: Decomposers break down dead organic matter, releasing CO2 (aerobic) or methane (anaerobic) back to the atmosphere.
Fossil fuel combustion: Burning coal, oil, and natural gas rapidly moves millions of years of stored carbon into the atmosphere, increasing atmospheric CO2 concentrations.

Trace a carbon atom from atmospheric CO2 through photosynthesis, a food chain, decomposition, and back to the atmosphere. Then explain why fossil fuel combustion disrupts this cycle.

1.5

The Nitrogen Cycle

The atmosphere is the major nitrogen reservoir, holding nitrogen as N2 gas that most organisms cannot use directly. Bacteria are essential at every step of the nitrogen cycle, converting nitrogen into usable forms and returning it to the atmosphere. Most nitrogen reservoirs hold compounds for relatively short periods.

Nitrogen fixation: Bacteria (including Rhizobium in legume root nodules and free-living cyanobacteria) convert atmospheric N2 into ammonia (NH3), making nitrogen available to plants.
Nitrification: Soil bacteria convert ammonia to nitrite and then to nitrate (NO3-), the form most easily absorbed by plant roots.
Assimilation: Plants absorb nitrate or ammonium and incorporate nitrogen into proteins and nucleic acids; consumers get nitrogen by eating plants.
Ammonification: Decomposers break down nitrogen-containing organic matter and release ammonia back into the soil.
Denitrification: Bacteria in low-oxygen environments convert nitrate back to N2 gas, returning nitrogen to the atmosphere and completing the cycle.

List the five main steps of the nitrogen cycle in order and name the type of organism responsible for each step.

1.6

The Phosphorus Cycle

Phosphorus cycles through rock, soil, water, and living organisms but has no atmospheric phase. Rock weathering is the primary natural source of phosphate. Because phosphorus is often scarce in soils and water, it frequently limits plant and algae growth. Excess phosphorus from agricultural runoff causes eutrophication in aquatic systems.

Major reservoir: Rock and sediments containing phosphate minerals are the primary long-term phosphorus reservoir; weathering slowly releases phosphate into soil and water.
No atmospheric phase: Unlike carbon and nitrogen, phosphorus does not cycle through the atmosphere, so it moves only between land, water, and organisms.
Phosphate uptake: Plant roots absorb phosphate (PO4^3-) from soil; phosphorus is incorporated into DNA, RNA, and ATP in all living organisms.
Limiting nutrient: Phosphorus availability often limits primary productivity in freshwater systems because it is scarce and does not cycle through the air.
Eutrophication: Excess phosphate from fertilizer runoff or sewage triggers algal blooms; when algae die and decompose, oxygen is depleted, creating hypoxic dead zones.

Why does the phosphorus cycle have no atmospheric component, and what environmental problem results when excess phosphorus enters a freshwater lake?

Cycle	Atmospheric reservoir?	Major long-term reservoir	Key limiting factor role
Carbon	Yes (CO2)	Fossil fuels, ocean sediments	Greenhouse gas; climate driver
Nitrogen	Yes (N2)	Atmosphere	Limits productivity in many terrestrial systems
Phosphorus	No	Rock and sediments	Limits productivity in most freshwater systems
Water	Yes (water vapor)	Oceans	Limits productivity in arid terrestrial biomes

1.7

The Hydrologic Cycle

The hydrologic cycle is powered by solar energy and moves water between the atmosphere, land surface, and subsurface reservoirs. The oceans are the primary surface reservoir. Water moves through evaporation, transpiration, condensation, precipitation, surface runoff, infiltration, and groundwater flow.

Evapotranspiration: The combined loss of water from soil and open water surfaces (evaporation) and from plant leaves (transpiration); returns water vapor to the atmosphere.
Precipitation: Water returns to Earth's surface as rain, snow, sleet, or hail after condensation in the atmosphere.
Surface runoff and infiltration: Precipitation either flows over the land surface into streams (runoff) or soaks into the soil (infiltration) to recharge groundwater.
Groundwater: Water stored in underground aquifers; a smaller reservoir than the oceans but critical for drinking water and irrigation in many regions.
Ocean as primary reservoir: Oceans hold the vast majority of Earth's surface water; ice caps and glaciers are the next largest reservoirs, followed by groundwater.

Trace a water molecule from the ocean surface through evaporation, precipitation, surface runoff, and infiltration back to groundwater. What energy source drives this entire cycle?

1.8

Primary Productivity

Primary productivity measures how fast producers convert solar energy into organic matter. Gross primary productivity (GPP) is the total rate of photosynthesis; net primary productivity (NPP) is what remains after producers use energy for their own respiration. NPP is the energy available to the rest of the ecosystem.

GPP vs. NPP: GPP = total photosynthesis rate. NPP = GPP minus autotrophic respiration. NPP represents the biomass actually available to consumers.
Units of productivity: Measured in energy per unit area per unit time, such as kcal/m2/yr or gC/m2/yr.
Light and aquatic productivity: Most red light is absorbed in the top 1 m of water; blue light penetrates beyond 100 m only in the clearest water. This limits where photosynthesis can occur in aquatic systems.
Photic zone: The layer of water with enough light for photosynthesis; depth varies with turbidity and water clarity.
Terrestrial vs. aquatic NPP: Tropical rainforests have the highest terrestrial NPP; open ocean has low NPP per unit area due to nutrient scarcity, though its total area makes it globally significant.

If a forest ecosystem has a GPP of 8,000 kcal/m2/yr and producers use 3,000 kcal/m2/yr for respiration, what is the NPP, and what does that value represent ecologically?

1.9

Trophic Levels and the 10% Rule

Energy enters ecosystems at the producer level and flows upward through consumers, but only about 10% transfers from one trophic level to the next. The remaining 90% is lost as heat through metabolic processes, which is explained by the second law of thermodynamics. This limits food chain length and determines how much biomass each level can support.

Trophic levels: Producers (autotrophs) form the base; primary consumers eat producers; secondary consumers eat primary consumers; and so on up to apex predators.
10% rule: Approximately 10% of energy at one trophic level is transferred to the next; the rest is lost as heat through respiration and metabolic activity.
Energy pyramid: A diagram showing decreasing energy at each successive trophic level; always widest at the producer base and narrowest at the top consumer level.
Second law of thermodynamics: Energy conversions are never 100% efficient; heat is always lost, which explains why energy decreases as it moves up trophic levels.
Conservation of matter: Unlike energy, matter is not lost; nutrients cycle through biogeochemical cycles and are recycled by decomposers back into the system.

If producers in an ecosystem fix 10,000 kcal/m2/yr, how much energy is available to secondary consumers? Show your calculation using the 10% rule.

1.11

Food Chains and Food Webs

A food chain shows a single linear path of energy transfer; a food web shows the full network of overlapping food chains in an ecosystem. Because species are interconnected, adding or removing one species triggers feedback effects throughout the web. Positive feedback amplifies change; negative feedback dampens it.

Food web: An interlocking model of multiple food chains showing how energy and matter flow through an ecosystem among producers, consumers, and decomposers.
Decomposers: Bacteria and fungi break down dead organic matter, releasing nutrients back into the soil and water for producers to use again.
Trophic cascade: When a top predator is removed, prey populations increase, which can reduce vegetation and alter the entire ecosystem structure.
Negative feedback loop: A change in one part of the food web triggers a response that reduces the original change, stabilizing the system.
Positive feedback loop: A change in one part of the food web amplifies the original change, potentially destabilizing the system.

If a keystone predator is removed from a food web, describe the sequence of changes that would occur at two other trophic levels and identify whether this is a positive or negative feedback loop.

Key terms

Term	Definition
Predator-Prey Relationship	A dynamic interaction where the predator hunts and consumes the prey; predator and prey population sizes cycle together over time.
symbiosis	A close, long-term interaction between two species; includes mutualism (both benefit), commensalism (one benefits, one unaffected), and parasitism (one benefits, one harmed).
Resource Partitioning	The division of limited resources among species by using them differently in space, time, or method, reducing competition and allowing coexistence.
Biome	A large geographic region defined by its climate, characteristic plant communities, and animal populations; examples include tundra, tropical rainforest, and savanna.
Biogeochemical cycles	Natural processes that cycle elements such as carbon, nitrogen, phosphorus, and water through living organisms and their nonliving environment.
nitrogen fixation	The conversion of atmospheric N2 into ammonia by bacteria, making nitrogen available for plant uptake and incorporation into biological molecules.
Denitrification	The bacterial conversion of nitrate (NO3-) back to nitrogen gas (N2) in low-oxygen environments, returning nitrogen to the atmosphere.
Phosphorus	An element that cycles through rock, soil, water, and organisms with no atmospheric phase; often the limiting nutrient in freshwater systems and essential for DNA, RNA, and ATP.
net primary productivity	The rate of energy storage by producers after subtracting their own respiration; calculated as GPP minus respiration, measured in kcal/m2/yr.
Trophic efficiency	The percentage of energy transferred from one trophic level to the next; approximately 10%, meaning 90% is lost as heat through respiration.
Food Web	A model of interlocking food chains showing how energy and matter flow among producers, consumers, and decomposers in an ecosystem.
Feedback Loops	Processes in which a change in an ecosystem triggers a response that either amplifies the change (positive feedback) or dampens it (negative feedback), affecting food web stability.
Photic Zone	The upper layer of a water body where sunlight is sufficient for photosynthesis; depth is limited by turbidity and water clarity.
evapotranspiration	The combined water loss from soil and open water surfaces through evaporation and from plants through transpiration; a key process in the hydrologic cycle.
Keystone species	A species with a disproportionately large effect on ecosystem structure relative to its abundance; its removal can trigger a trophic cascade.

How to review unit 1

Topics students miss most

Miss rates come from Fiveable multiple-choice practice.

1.11

Food Chains and Food Webs

40%4,864 tries

1.2

Terrestrial Biomes

32%5,818 tries

1.4

The Carbon Cycle

31%4,268 tries

Common unit 1 mistakes

Confusing GPP and NPP

GPP is the total rate of photosynthesis; NPP is what remains after producers use energy for their own respiration. NPP is always less than GPP. On the exam, questions about energy available to consumers require NPP, not GPP.

Applying the 10% rule in the wrong direction

The 10% rule means energy decreases going up trophic levels. To find energy at a higher level, multiply by 0.1. To find the energy needed at a lower level to support a higher level, divide by 0.1. Students often multiply when they should divide.

Saying phosphorus cycles through the atmosphere

Phosphorus has no atmospheric phase. It moves only between rock, soil, water, and organisms. Confusing phosphorus with nitrogen or carbon on a cycle question is a common error.

Mixing up nitrogen cycle steps and the bacteria involved

Nitrogen fixation converts N2 to ammonia; nitrification converts ammonia to nitrate; denitrification converts nitrate back to N2. Students often reverse the direction of denitrification or forget that each step requires specific bacteria.

Treating biome boundaries as fixed

Biome distributions are dynamic and have shifted in the past. Climate change is projected to shift biome boundaries again. Describing biomes as permanently fixed ignores a key essential knowledge point and a common exam application.

How this unit shows up on the AP exam

Cycle diagram interpretation and explanation

The AP exam frequently presents a diagram of a biogeochemical cycle and asks you to identify a specific process, label a reservoir, or explain how human activity alters the cycle. Practice tracing carbon, nitrogen, and phosphorus through their steps and naming the organisms or processes responsible at each stage.

10% rule and energy pyramid calculations

Quantitative questions in Unit 1 most often involve calculating energy at a given trophic level using the 10% rule or solving for GPP or NPP given the other values. Show your work clearly, include correct units (kcal/m2/yr), and be prepared to explain why energy decreases using the laws of thermodynamics.

Food web analysis and species removal scenarios

A common task asks you to read a food web, identify trophic levels, and predict the effect of removing or adding a species. Your response should name specific organisms affected, describe the direction of population change at each trophic level, and identify the type of feedback loop involved.

More ways to review

Topic study guides

Open the individual guides for Unit 1 when you want a closer review of one topic.

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Practice questions

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FRQ practice

Practice free-response reasoning and compare your answer with scoring guidance.

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Cram archive videos

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Official unit cheatsheet

Open the Fiveable one-page unit review, then explore visual cheatsheets for a quick refresher.

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Score calculator

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Unit 1 printables

Print the unit cheatsheets ($3) for Unit 1. Yours to keep, no subscription needed.

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Frequently Asked Questions

What's on the APES Unit 1 progress check (MCQ and FRQ)?

The APES Unit 1 progress check includes both MCQ and FRQ parts drawn from all 11 unit topics. MCQ questions test recognition of biome characteristics, carbon cycle and nitrogen cycle pathways, trophic levels, and the 10% rule for energy flow. The FRQ portion typically asks you to explain a biogeochemical cycle or analyze a food web diagram. Practicing with those same topic areas on APES Unit 1 is the most direct way to prepare for the progress check.

How do I practice APES Unit 1 FRQs?

APES Unit 1 FRQs most often focus on the carbon cycle, nitrogen cycle, and energy flow through food chains and food webs. A typical question gives you a scenario, then asks you to trace how matter or energy moves, explain a disruption, or calculate energy transfer using the 10% rule. To practice, write out full cycle diagrams from memory, then narrate each step in complete sentences the way a real FRQ demands. You can find matched practice prompts at APES Unit 1.

Where can I find APES Unit 1 practice questions?

The best place to find APES Unit 1 practice questions, including multiple-choice and practice test sets, is APES Unit 1. The page organizes MCQ and FRQ practice by topic, so you can target specific areas like the carbon cycle, food chains and food webs, or biome identification. Working through topic-by-topic MCQs before taking a full unit practice test helps you spot gaps before they cost you points on the real exam.

How should I study APES Unit 1?

Start APES Unit 1 by mapping out the carbon cycle from scratch, since it anchors the whole unit and connects to climate topics later in the course. Then work through the nitrogen cycle, phosphorus cycle, and hydrologic cycle the same way. For ecosystems and biomes, focus on the key abiotic factors that define each one. Once the cycles click, tackle trophic levels and energy flow together, since the 10% rule shows up in calculations on both the progress check and the exam. Use APES Unit 1 to check your understanding topic by topic as you go.

Ready to review Unit 1?Start with the notes, check the topic cards, and use the practice or resource links when they are available for this course.

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