AP Environmental Science Unit 1: The Living World: Ecosystems — Free Review Games.

A food web shows the complex, interconnected feeding relationships among organisms in an ecosystem, linking multiple food chains together.

Producers (autotrophs) convert inorganic materials into organic compounds through photosynthesis or chemosynthesis, forming the base of food webs.

The 10% rule states that roughly 10% of energy is transferred between trophic levels. The remaining 90% is lost primarily as heat through cellular respiration.

The phosphorus cycle is primarily sedimentary. Phosphorus moves through rocks, soil, water, and organisms but does not have a significant gaseous phase in the atmosphere.

Decomposers (bacteria, fungi) break down dead organisms and waste, returning essential nutrients to the soil where they can be used by producers again.

NPP = GPP - R. Net primary productivity is the energy fixed by producers minus the energy they use for their own respiration. It represents energy available to consumers.

Nitrogen fixation is performed by certain bacteria (e.g., Rhizobium in root nodules) that convert atmospheric N2 into ammonia (NH3), making nitrogen available to plants.

Tropical rainforests have the highest NPP of any terrestrial biome due to abundant sunlight, rainfall, warm temperatures year-round, and high biodiversity.

Denitrification is performed by anaerobic bacteria that convert nitrates (NO3-) back to nitrogen gas (N2), returning it to the atmosphere and completing the nitrogen cycle.

A trophic cascade is an ecological phenomenon where changes at one trophic level ripple through others. For example, removing top predators can cause herbivore populations to increase, which then overgraze vegetation.

The thermocline is the middle layer in a thermally stratified lake where temperature drops rapidly with depth, separating the warm epilimnion above from the cold hypolimnion below.

Aquatic GPP depends primarily on light penetration (which decreases with depth and turbidity) and nutrient availability (especially nitrogen and phosphorus for algal growth).

Fossil fuel combustion and deforestation transfer carbon from long-term geological and biological reservoirs to the atmosphere as CO2, which traps heat and drives global warming.

In open ocean ecosystems, phytoplankton (producers) have less standing biomass than zooplankton (consumers) at any given time because phytoplankton reproduce so rapidly that their biomass is constantly being consumed.

Mycorrhizal fungi extend their hyphae far beyond the root zone, greatly increasing the plant's ability to absorb phosphorus, water, and other nutrients. In return, the plant provides the fungi with sugars from photosynthesis.

A keystone species has effects on the ecosystem far greater than would be expected based on its abundance or biomass alone. Sea otters, for example, control sea urchin populations and thereby maintain kelp forests despite being relatively rare. Choice A describes a dominant species — characterized by high biomass or abundance — which is a fundamentally different concept from a keystone species, whose influence is defined by disproportionate impact regardless of abundance.

Primary consumers (herbivores) occupy the second trophic level and feed directly on producers — plants, algae, and other autotrophs. Choice A describes producers. Choice C describes tertiary consumers. Choice D describes decomposers or detritivores. Correctly placing organisms within trophic levels is foundational to analyzing energy flow through any ecosystem.

Phosphorus is the most common limiting nutrient in freshwater ecosystems because it is tightly bound to sediments and has no gaseous phase to replenish supplies from the atmosphere. Nitrogen is more commonly the limiting nutrient in marine ecosystems. Carbon is rarely limiting because it dissolves readily from the atmosphere into water. Sulfur is not a primary limiting nutrient. The distinction between phosphorus limitation (freshwater) and nitrogen limitation (marine) is a key concept in aquatic ecology.

Transpiration is the process by which plants release water vapor through their stomata (leaf pores). Together with evaporation, it forms evapotranspiration — a major pathway by which water returns from land surfaces to the atmosphere. Choice A describes condensation. Choice B describes percolation or infiltration. Choice D describes evaporation specifically. In forested watersheds, transpiration can account for a large fraction of total water loss from the landscape.

Heterotrophs (from Greek: 'other feeders') must consume organic molecules made by other organisms to obtain energy. All animals, fungi, and most bacteria are heterotrophs. Photoautotrophs (Choice A) use sunlight to produce food through photosynthesis. Chemoautotrophs (Choice B) synthesize organic compounds using energy from inorganic chemical reactions. Lithotrophs (Choice D) use inorganic compounds as electron donors. Choices A, B, and D are all forms of autotrophs that produce their own organic compounds.

Because phosphorus has no stable gaseous form at normal environmental temperatures and pressures, it cannot cycle through the atmosphere the way carbon (as \(\text{CO}_2\)) and nitrogen (as \(\text{N}_2\)) can. Instead, phosphorus enters ecosystems almost exclusively through the slow weathering of phosphate-containing rocks and minerals, making the phosphorus cycle much slower and more localized than gaseous cycles. Choice A is incorrect — unlike carbon or nitrogen, phosphorus is not meaningfully returned via precipitation. Choice C refers to nitrogen cycling. Choice D is incorrect; mining of phosphate rock for fertilizer is the primary human perturbation of the phosphorus cycle.

Detritivores (such as earthworms, millipedes, pill bugs, and certain beetles) physically consume and shred dead organic matter (detritus), breaking it into smaller pieces and greatly increasing surface area for microbial attack. Choice A describes decomposers (bacteria and fungi), which use extracellular enzymes and chemical processes to break down organic molecules. Both groups are critical to nutrient cycling, and both are found in terrestrial and aquatic ecosystems, making Choice D incorrect. In practice, detritivores and decomposers work together — detritivores fragment material that decomposers then chemically mineralize.

NPP is calculated as \(\text{NPP} = \text{GPP} - R_a = 8{,}500 - 3{,}200 = 5{,}300\ \text{kcal/m}^2\text{/yr}\). GPP represents total energy fixed by photosynthesis; plants use some of that for their own cellular respiration (\(R_a\)), and the remainder is NPP — energy stored in biomass and available to consumers and decomposers. Choice A incorrectly adds the two values instead of subtracting. Choices C and D each present only one of the given values without performing the subtraction.

With 10% ecological efficiency at each level: primary consumers receive \(50{,}000 \times 0.10 = 5{,}000\ \text{kcal}\); secondary consumers receive \(5{,}000 \times 0.10 = 500\ \text{kcal}\). Approximately 90% of energy is lost as heat through respiration, movement, and metabolic waste at each trophic transfer. Choice A gives the energy available at the primary consumer level — a common error of applying only one transfer instead of two. This rapid energy loss explains why food chains rarely exceed four to five trophic levels.

Ammonification (also called mineralization) is performed by decomposer bacteria and fungi that break down proteins and nucleic acids in dead organic matter and animal waste, releasing ammonia (\(\text{NH}_3\)) or ammonium (\(\text{NH}_4^+\)). Choice A describes nitrogen fixation. Choice C describes nitrification. Choice D describes denitrification. These four processes — nitrogen fixation, ammonification, nitrification, and denitrification — form the core of the nitrogen cycle and are frequently tested on the AP exam.

Cellular respiration by the combined activity of all living organisms — plants, animals, fungi, and bacteria — collectively releases far more carbon as \(\text{CO}_2\) than any other single process. While fossil fuel combustion is now ecologically significant and is increasing atmospheric \(\text{CO}_2\) concentrations, it remains smaller than total biological respiration in absolute volume. Volcanic outgassing and carbonate weathering are very slow geological processes. The near-balance between photosynthesis (removing \(\text{CO}_2\)) and respiration (releasing it) kept atmospheric \(\text{CO}_2\) relatively stable for thousands of years before industrialization.

Liebig's Law of the Minimum states that growth is controlled not by the total resources available, but by the single resource that is scarcest relative to the organism's requirement. For instance, if phosphorus is present at only 20% of required levels while all other nutrients are at 100%, productivity is limited to roughly 20% of potential regardless of all other abundances. This principle explains why adding a single limiting nutrient can dramatically boost productivity. Choice D is incorrect — the total quantity of all nutrients combined is not the relevant measure; only the most limiting one determines the ceiling on growth.

Cultural eutrophication is driven by anthropogenic enrichment of water bodies with nitrogen and phosphorus — primarily from agricultural fertilizer runoff, livestock waste, and sewage effluent. These nutrients stimulate algal blooms; when the algae die and decompose, bacterial decomposition consumes dissolved oxygen, creating hypoxic or anoxic dead zones that kill fish and invertebrates. The causal chain is: excess nutrients → algal bloom → decomposition → oxygen depletion. While choices A, C, and D can all degrade lake ecosystems, they are not the primary cause of eutrophication.

Biomagnification occurs when a substance is lipophilic (fat-soluble) and resistant to metabolic breakdown or excretion. Because these compounds are stored in fatty tissues rather than eliminated, a top predator accumulates the toxin from every prey item consumed over its lifetime, concentrating it to levels many thousands of times higher than found in the surrounding water or in producers. This is why top predators like eagles, orca whales, and large tuna show the highest tissue concentrations of persistent organic pollutants. Choice B is precisely the opposite of what happens — concentrations increase, not decrease, with increasing trophic level.

Residence time is the average duration a substance spends in a particular reservoir. The deep ocean's long residence time — driven by the slow thermohaline circulation — means that carbon sinking to depth is isolated from the fast carbon cycle (the rapid exchanges among atmosphere, vegetation, and surface ocean) for very long periods. This makes deep ocean carbon uptake an important moderator of atmospheric \(\text{CO}_2\) concentrations on century timescales. Choice A is incorrect; thermohaline overturning takes roughly 1,000 years, so deep carbon is not quickly returned. Choice C is incorrect — it is the slow physical circulation, not chemical stability, that creates the long residence time.

Bottom-up control means the ecosystem is regulated from its energetic base — nutrient availability and primary productivity determine how much energy is available at each successive trophic level, thereby imposing upper limits on consumer populations throughout the food web. Choice A describes top-down control, as occurs in trophic cascades. Real ecosystems typically show elements of both control types simultaneously. Choice C describes nutrient cycling, which is related to bottom-up dynamics but is not itself the definition of bottom-up trophic control.

In tropical rainforests, warm and moist conditions accelerate decomposition dramatically. Nutrients released from decomposing litter are almost immediately captured by shallow root mats and dense mycorrhizal fungal networks before they can leach downward with rainfall. Nutrients are thus retained in living biomass rather than accumulating in the soil. Choice D is incorrect: high temperatures actually increase respiration rates, which reduces the fraction of GPP retained as NPP. Choice B is incorrect: rainfall itself delivers relatively few dissolved nutrients — the more important concern is nutrients being leached out by rainfall, not supplied by it.

Because phosphorus lacks a significant gaseous phase, once it washes from land to ocean and is buried in deep sediments, it is removed from biological cycling for geological time scales — only returned when tectonic uplift and subsequent weathering re-expose the rock. This is why phosphorus is so often the limiting nutrient in freshwater systems and why mining phosphate rock for fertilizer is essentially a non-renewable extraction of a finite geological resource. Choice A describes how the nitrogen and carbon cycles operate, not phosphorus. Choice C overstates the volcanic contribution to the phosphorus cycle on human timescales.

NPP represents energy available to primary consumers (trophic level 2). Applying 10% efficiency at each transfer: primary consumers (TL2) \(= 20{,}000 \times 0.10 = 2{,}000\ \text{kcal}\); secondary consumers (TL3) \(= 2{,}000 \times 0.10 = 200\ \text{kcal}\); tertiary consumers (TL4) \(= 200 \times 0.10 = 20\ \text{kcal}\). Note that NPP is the correct starting point, not GPP, since NPP is what is actually available to consumers. Choice A gives the energy at primary consumers — a common error from performing only one trophic transfer instead of three. Each additional trophic level requires one additional multiplication by 0.10.

When nitrogen inputs exceed ecosystem demand, nitrification accelerates and the highly mobile \(\text{NO}_3^-\) ion leaches through the soil profile. To maintain electrical charge balance, positively charged base cations — calcium (\(\text{Ca}^{2+}\)), magnesium (\(\text{Mg}^{2+}\)), and potassium (\(\text{K}^+\)) — are co-leached with the nitrate, depleting soil fertility and acidifying receiving streams. Research at Hubbard Brook Experimental Forest documented this process in detail. Choice A is incorrect because high nitrogen availability actually suppresses nitrogen fixation — organisms have no competitive advantage fixing \(\text{N}_2\) when inorganic nitrogen is already abundant. Choice D is incorrect; high nitrate does not shut down the nitrogen cycle.

\(\text{NEP} = \text{NPP} - R_h\), where \(R_h\) is heterotrophic respiration (respiration by decomposers and consumers). A positive NEP means the ecosystem is accumulating organic carbon — a net carbon sink. A negative NEP means it is releasing more carbon than it fixes — a net carbon source. NPP already accounts for autotrophic respiration (\(\text{GPP} - R_a = \text{NPP}\)), so Choice A does not describe the difference between NPP and NEP. Young, regrowing forests often have high positive NEP; old-growth forests often have NEP near zero because the large organic matter pool fuels decomposer respiration that nearly offsets NPP.

Allochthonous inputs are organic materials produced outside the ecosystem and transported into it — primarily leaf litter, woody debris, and terrestrial invertebrates falling into the stream. In shaded headwater streams, the forest canopy intercepts most sunlight, severely limiting in-stream (autochthonous) photosynthesis. The food web therefore relies on terrestrial organic matter processed by shredder macroinvertebrates and microbial decomposers. This concept is central to the River Continuum Concept. Choice A describes autochthonous production and incorrectly claims headwater streams have high light availability — the opposite is true. Choice D is incorrect; chemosynthesis is energetically negligible in typical stream food budgets.

Fossil fuel combustion releases: (1) \(\text{CO}_2\), perturbing the carbon cycle; (2) nitrogen oxides (\(\text{NO}_x\)), formed when atmospheric \(\text{N}_2\) and \(\text{O}_2\) react at high combustion temperatures, perturbing the nitrogen cycle and contributing to acid rain and photochemical smog; (3) \(\text{SO}_2\), from sulfur impurities in coal and heavy oil, perturbing the sulfur cycle and causing acid deposition. Phosphorus (Choices C and D) is not significantly mobilized by combustion. While water vapor is a combustion product, the water cycle is not fundamentally altered by fossil fuel use in the same direct, globally significant way. Understanding that one human activity can simultaneously disrupt multiple biogeochemical cycles is essential for evaluating cumulative environmental impacts.

\(\text{NEP} = \text{NPP} - R_h\). In a vigorously regrowing forest, trees grow rapidly (high NPP) while the pool of dead wood and soil organic matter is still relatively small, keeping \(R_h\) low — resulting in a high positive NEP and net carbon accumulation. In old-growth forests, very large pools of dead wood and soil carbon support high decomposer respiration, so \(R_h\) nearly balances NPP and NEP approaches zero. Choice A is a common misconception: high biomass reflects past carbon accumulation, not current NEP. Choice D reverses the logic — a larger soil organic matter pool elevates \(R_h\), which reduces NEP rather than increasing it.

If the dissolved \(\text{N}:\text{P}\) ratio (8:1) is lower than the phytoplankton demand ratio (16:1), nitrogen is proportionally scarcer relative to biological need. Nitrogen will therefore be consumed first, leaving residual dissolved phosphorus — making nitrogen the proximate limiting nutrient. This condition gives nitrogen-fixing cyanobacteria such as Trichodesmium a competitive advantage, as they can draw on atmospheric \(\text{N}_2\) to supplement scarce dissolved nitrogen. Choice A describes the opposite scenario, which would occur if dissolved \(\text{N}:\text{P}\) were higher than 16:1. Choice D misunderstands evolutionary timescales; phytoplankton stoichiometry does not shift rapidly enough to track short-term nutrient dynamics.

The frog is a secondary consumer because it eats the grasshopper, which is the primary consumer that eats the producer (grass). The choice 'Grasshopper' is wrong because it feeds directly on a producer, making it a primary consumer rather than a secondary one. In general, trophic level assignment depends on the number of energy transfers away from the original producer, not on the size or type of organism.

About 90 percent of the energy at a given trophic level is lost as metabolic heat, used for respiration, or remains undigested, so only roughly 10 percent is transferred to the next level. The distractor '10 percent' describes the energy that IS transferred, not the energy that is lost, so it answers the opposite question. This large energy loss at each step explains why food chains rarely exceed four or five trophic levels and why top predators are relatively rare.

Decomposers break down dead organisms and waste products, releasing nutrients such as nitrogen, phosphorus, and carbon back into the soil, water, or atmosphere for reuse by producers. The option 'Converting solar energy directly into chemical energy' describes photosynthesis performed by autotrophs, not the heterotrophic metabolism used by decomposers. Without decomposers, nutrients would remain locked in dead biomass, halting biogeochemical cycling and eventually limiting primary productivity.

GPP is the total amount of chemical energy producers capture through photosynthesis, while NPP equals GPP minus the energy producers use for their own respiration, so GPP is the larger, pre-respiration quantity. The choice describing 'the energy available to primary consumers only' is actually closer to NPP, since that is the energy stored as new biomass that herbivores can consume. Remembering the relationship \(\text{NPP} = \text{GPP} - R\) is essential for interpreting productivity data on the AP exam.

Nitrogen fixation converts inert atmospheric \(\text{N}_2\) gas into ammonia (\(\text{NH}_3\)), which can then be used directly by plants or further converted by nitrifying bacteria. Nitrate is incorrect as the direct fixation product because nitrate is instead formed later during nitrification, a separate microbial process. This distinction matters because fixation is the essential first step that makes atmospheric nitrogen biologically available to ecosystems.

Photosynthesis uses \(\text{CO}_2\), water, and sunlight to produce glucose and other organic carbon compounds that become plant biomass, temporarily removing carbon from the atmospheric pool. The option about 'fossilized carbon stored in sedimentary rock' describes a process that occurs over millions of years through geologic burial, not the immediate biochemical outcome of photosynthesis. This conversion of inorganic to organic carbon is the foundation of the biological portion of the carbon cycle.

Applying the 10 percent rule twice, from producers to primary consumers (\(50{,}000 \times 0.10 = 5{,}000\)) and then to secondary consumers (\(5{,}000 \times 0.10 = 500\)), yields \(500\ \text{kcal/m}^2/\text{yr}\). The value \(5{,}000\ \text{kcal/m}^2/\text{yr}\) is incorrect because it only accounts for a single trophic transfer, stopping at the primary consumer level. Students should always track how many trophic steps separate the given level from the target level before applying the 10 percent rule repeatedly.

Pyramids of energy are never inverted because the second law of thermodynamics guarantees that energy is lost as heat at every transfer, so each successive trophic level must always contain less usable energy than the one below it. Pyramids of numbers, in contrast, can invert in cases like a single tree hosting many insects, since that pyramid counts individual organisms rather than energy content. This makes the energy pyramid the most reliable model for understanding fundamental limits on food chain length and biomass support.

Sea otters act as a keystone predator whose removal triggers a trophic cascade, allowing urchin populations to boom and overgraze kelp, which restructures the entire community. Competitive exclusion is incorrect because it describes two species competing for the same limited resource until one is displaced, not a predator-prey chain effect propagating through multiple trophic levels. Recognizing keystone species is important because their disproportionate community-wide influence is not predictable from their abundance or biomass alone.

Denitrification is carried out by anaerobic bacteria that use nitrate as an alternative electron acceptor in the absence of oxygen, so it occurs mainly in waterlogged soils, wetlands, and low-oxygen sediments. Well-aerated, oxygen-rich soils instead favor nitrification, the oxidation of ammonium to nitrate, which requires oxygen rather than excluding it. This oxygen dependence explains why wetlands are often used in constructed treatment systems to remove excess nitrate from agricultural runoff.

Because phosphorus lacks a significant gaseous form, it moves between reservoirs mainly through the slow weathering of phosphate rock and transport in water, resulting in a much longer residence time and frequent local scarcity that limits primary productivity. The claim that phosphorus 'cycles much faster than carbon or nitrogen' is incorrect since the absence of an atmospheric pathway actually slows global redistribution rather than speeding it up. This is why phosphorus is often cited as the limiting nutrient in freshwater ecosystems, where even small anthropogenic inputs can dramatically stimulate algal growth.

Transpiration is the process by which plants absorb water through their roots and release it as vapor through stomatal pores on their leaves, adding significant moisture to the atmosphere. The option describing condensation of vapor into precipitation instead refers to a separate atmospheric process that occurs after water has already entered the air, not the plant-driven release itself. In forested regions, transpiration combined with evaporation, called evapotranspiration, can account for a majority of atmospheric moisture input over land.

A complex food web offers multiple alternative pathways for energy flow, so if one species declines or disappears, consumers can often shift to other prey, buffering the ecosystem against collapse. Trophic efficiency between levels is set by thermodynamic losses during respiration and is not increased simply by having more feeding connections. This redundancy principle is a key reason biodiversity is often linked to ecosystem stability and resistance to disturbance.

Specialist species have a narrow niche, relying on a limited range of resources or conditions, which makes them highly efficient in stable environments but vulnerable to habitat or resource changes. A generalist species, in contrast, can use a wide variety of resources and tolerate diverse conditions, allowing it to persist even when its environment changes significantly. Understanding the specialist-generalist spectrum helps predict which species are most at risk during rapid environmental disturbance such as habitat loss or climate change.

Since \(\text{NPP} = \text{GPP} - R\), rearranging gives \(R = \text{GPP} - \text{NPP} = 12{,}000 - 7{,}500 = 4{,}500\ \text{kcal/m}^2/\text{yr}\), which is the energy producers use for their own metabolic maintenance. The value \(19{,}500\ \text{kcal/m}^2/\text{yr}\) is incorrect because it results from adding GPP and NPP together rather than subtracting them. Mastering this simple algebraic relationship allows students to quickly solve any productivity problem that provides two of the three variables.

Ecological efficiency is calculated as \(\frac{18{,}000}{200{,}000} \times 100 = 9\)... actually \(\frac{18{,}000}{200{,}000}=0.09\), giving 9 percent, which is close to but slightly below the standard 10 percent approximation. Wait, recompute: this matches option A, so option B claiming 18 percent misreads the ratio by failing to divide correctly and is therefore wrong. This exercise reinforces that actual measured ecological efficiencies in real ecosystems vary but commonly cluster near the textbook 10 percent value used for estimation.

Aerobic decomposer bacteria breaking down the large mass of dead algae consume dissolved oxygen faster than it can be replenished, producing hypoxic or anoxic conditions that suffocate fish and other aerobic organisms. The claim about 'toxic methane gas' is incorrect because methane production is associated with anaerobic decomposition in oxygen-depleted sediments and is not the primary direct cause of the fish kill in the water column. This oxygen-depletion mechanism, distinct from the initial nutrient trigger itself, is the critical link between eutrophication and dead zones in lakes and coastal waters.

Atmospheric \(\text{SO}_2\) reacts with water vapor and oxygen to form sulfuric acid (\(\text{H}_2\text{SO}_4\)), which falls as acid rain or acid deposition and can damage forests, acidify lakes, and corrode infrastructure. Stratospheric ozone depletion is incorrect because that process is driven mainly by halogenated compounds like CFCs reacting with ozone, not by sulfur emissions. This example shows how human perturbation of the sulfur cycle through fossil fuel combustion creates cascading impacts on soil chemistry and aquatic ecosystems far from the emission source.

When \(\text{GPP}/R\) is less than 1, total respiration by all organisms in the community exceeds the carbon fixed by photosynthesis, meaning the ecosystem releases more \(\text{CO}_2\) than it absorbs and thus functions as a net carbon source rather than a sink. The option describing carbon accumulation and a 'net carbon sink' actually corresponds to a ratio greater than 1, the opposite condition described in the question. This GPP/R ratio is a useful tool for assessing whether disturbed, decomposing, or heavily heterotrophic ecosystems, such as a polluted river reach, are net contributors to or absorbers of atmospheric carbon.

Mesopredator release occurs when the removal of a top predator eliminates the suppression it exerted on smaller, mid-level predators, allowing their populations and ecological impact to expand, often to the detriment of shared prey species. The option about 'migration of prey species to new territories' describes a behavioral response of prey rather than the population-level release of competing mid-level predators that defines this term. This concept builds on trophic cascade theory and highlights how apex predator loss can restructure entire communities through indirect, multi-level effects rather than simple direct predation changes alone.