Ecology Lesson Test: 40 Practice Questions

Answer: A

Correct answer: The study of how organisms interact with one another and their environments. The word ecology comes from the Greek oikos, meaning house, so it is the study of living things in their home. Ecologists look at interactions among organisms, such as predation and competition, and between organisms and nonliving factors like water, light and temperature. A good way to recognize the definition is to look for both living things and their environment. Chemistry, astronomy and geology may inform ecology, but none of them is centered on those relationships.

• A (correct): Ecology focuses on relationships among living things and environmental conditions.
• B: This describes astronomy, which studies objects in space rather than living things and their surroundings.
• C: This describes chemistry; ecology focuses on organisms and their environments.
• D: Classifying rocks and minerals is part of geology and mineralogy, not ecology.

Answer: B

Correct answer: All red foxes of one species in a particular forest. Ecologists organize life into levels: an organism, a population, a community, an ecosystem and the biosphere. A population is all members of one species living in the same area at the same time, so they can interbreed and compete with each other. The tempting mistake is choosing a mix of species living together; that is a community. Adding nonliving factors such as soil and water moves the description to the ecosystem level.

• A: Several interacting species form a community.
• B (correct): One species occupying the same area fits population.
• C: These are abiotic components rather than a population.
• D: This is closer to the biosphere.

Answer: B

Correct answer: Dissolved oxygen concentration. Abiotic factors are the nonliving physical and chemical conditions of an environment, such as temperature, light, pH and dissolved gases. Biotic factors are living or once-living things. A quick test is to ask whether the item is or was alive; oxygen dissolved in water never was, even though organisms depend on it. Do not be misled because oxygen is linked to life: plants and animals affect it, but it remains a chemical condition of the pond.

• A: Snails are living animals, so they are biotic factors.
• B (correct): Dissolved oxygen is a chemical condition of the water.
• C: Mosquito larvae are living organisms, so they are biotic factors.
• D: Water lilies are living plants, so they are biotic factors.

Answer: D

Correct answer: Primary consumer. Trophic levels are counted by feeding steps from the producer. Grass makes its own food through photosynthesis, so it is the producer. Any animal that eats a producer directly is a primary consumer, also called an herbivore in this case. A secondary consumer would eat the rabbit, and a tertiary consumer would eat that predator. A common slip is to confuse the rabbit with a decomposer, but decomposers break down dead matter rather than eating living plants.

• A: A tertiary consumer eats secondary consumers.
• B: Grass itself is a producer; the rabbit does not make food by photosynthesis.
• C: The rabbit is not chemically breaking down remains as its main role.
• D (correct): A rabbit eating a producer is a primary consumer.

Answer: D

Correct answer: A fungus growing on dead leaves. Decomposers, mainly fungi and bacteria, obtain energy by breaking down dead organisms and wastes. Fungi release digestive enzymes into the material and then absorb the dissolved nutrients, which returns elements such as nitrogen and phosphorus to the soil for plants. To recognize a decomposer, look for an organism feeding on dead matter rather than catching or grazing on living things. Animals eating living food are consumers, even if they eat small organisms like plankton.

• A: A fish feeding on plankton is a consumer.
• B: A hawk catches and eats live prey, so it is a predator and consumer.
• C: A rabbit eats living plants, so it is an herbivorous consumer.
• D (correct): Many fungi chemically break down dead matter.

Answer: C

Correct answer: Food energy passes from the mouse to the owl. In food chains and food webs, arrows point in the direction energy moves, from the organism eaten to the organism that eats it. So grass → mouse means the mouse eats grass, and mouse → owl means the owl eats the mouse. A helpful phrase is that the arrow means "is eaten by." The tempting error is reading the arrow as "eats," which reverses the relationship and would make the mouse the hunter.

• A: Food-web arrows do not indicate metamorphosis.
• B: The chain does not specify climate.
• C (correct): Arrows follow food energy toward the consumer.
• D: The prey normally does not hunt its predator in this example.

Answer: A

Correct answer: Photosynthesis. During photosynthesis, plants, algae and some bacteria use light energy to combine carbon dioxide and water into glucose, releasing oxygen. The step that captures CO₂ into sugar is called carbon fixation, and it is the main way carbon enters living things from the air. Most other biological processes, such as respiration and fermentation, release CO₂ instead. Animals depend on this captured carbon only indirectly, by eating plants or other animals.

• A (correct): Photosynthetic carbon fixation incorporates CO₂ into organic matter.
• B: Movement uses energy but does not directly fix CO₂ into sugars.
• C: Eating is not the direct carbon-fixation reaction.
• D: Fermentation does not perform oxygenic carbon fixation.

Answer: B

Correct answer: Carrying capacity. Every habitat has limited food, water, space and shelter, so a population cannot grow forever. As numbers rise, competition, disease and predation increase until births and deaths roughly balance. The level where this leveling off happens is the carrying capacity, often shown as the flat top of an S-shaped growth curve. Predation is tempting because it limits populations, but it is one cause of the limit, not the name for the maximum itself.

• A: A niche is an ecological role/resource use.
• B (correct): Carrying capacity reflects environmental limits on sustained abundance.
• C: Mutation rate describes genetic changes, not capacity.
• D: Predation is one possible limiting interaction.

Answer: B

Correct answer: Parasitism. Symbiotic relationships are classified by who benefits and who is harmed. In mutualism both partners gain, in commensalism one gains and the other is unaffected, and in parasitism one gains while the host is harmed. A tick drinks the deer's blood and can spread disease, so the deer clearly suffers. Commensalism is the usual trap, but it requires that the host experience no real harm, which is not the case here.

• A: Commensalism would not clearly harm the host.
• B (correct): A parasite benefits at the host’s expense.
• C: Photosynthesis is an energy-conversion process.
• D: Mutualism requires benefits to both in the described interaction.

Answer: A

Correct answer: A nonnative organism that spreads and causes ecological harm. An invasive species must meet two conditions: it is not native to the area, and it spreads in a way that causes ecological or economic harm. Introduced species often lack natural predators or diseases in their new range, letting them outcompete natives. Zebra mussels in the Great Lakes are a well-known example. Being numerous is not enough; a common native species is not invasive, and a nonnative plant kept in a pot that never spreads is not either.

• A (correct): Nonnative spread plus harm fits the invasive concept.
• B: Native origin alone is not invasive.
• C: Introduced without spread/harm does not establish invasive status.
• D: A native stable species does not become invasive just because it is abundant.

Answer: A

Correct answer: An ecosystem. Each ecological level adds something. A population is one species, and a community is all interacting species in an area. An ecosystem adds the abiotic environment, such as water, soil and temperature. Because this list includes rainwater and soil temperature along with oak trees, ants and birds, the community level is too narrow. When you see both living and nonliving factors in one place, the broadest fitting level is the ecosystem.

• A (correct): Ecosystem includes interacting organisms and their abiotic environment.
• B: Multiple species and abiotic conditions are present.
• C: Community omits abiotic rainwater/temperature.
• D: Many entities are mentioned, not one organism.

Answer: D

Correct answer: Habitat: pond shore; niche: insect-eating predator. A habitat is the physical place where an organism lives, like an address. A niche is the organism's role, including what it eats, what eats it and how it uses resources, much like a job or profession. For a frog, the pond shore is the place, and catching insects describes its role. The tempting answer swaps these two ideas, so check whether each label names a location or a function.

• A: These labels are reversed.
• B: Photosynthesis is not a frog habitat.
• C: Population count and weather instruments do not define its role.
• D (correct): Habitat is the place; niche is ecological role and resource use.

Answer: B

Correct answer: Anchovy. Count feeding steps from the producer. Algae make food by photosynthesis, so they are the producer. Zooplankton eat algae and are primary consumers. Anchovies eat zooplankton, making them secondary consumers, and tuna eat anchovies, making them tertiary consumers. A common mistake is to number consumers starting with the producer, which shifts every label by one. Remember that the producer is not counted as a consumer.

• A: Tuna eats the secondary consumer and is tertiary in this chain.
• B (correct): Anchovy eats the primary consumer and is secondary.
• C: Zooplankton eats algae and is the primary consumer.
• D: Algae is the producer.

Answer: B

Correct answer: Nutrients can cycle, while usable energy flows through and disperses as heat. Matter and energy behave differently in ecosystems. Atoms of carbon, nitrogen and phosphorus are reused again and again as decomposers return them to soil, water and air. Energy enters mainly as sunlight and, at each transfer, much is lost as heat from metabolism, which organisms cannot reuse. That is why ecosystems need a constant energy input. The misconception that energy is destroyed fails because energy is conserved; it simply becomes less useful.

• A: Energy transforms and disperses; it is not literally destroyed.
• B (correct): Matter is recycled while usable energy becomes less available across transfers.
• C: Energy is not regenerated as sunlight by biogeochemical cycles.
• D: Decomposition releases nutrients, not all original useful energy.

Answer: C

Correct answer: 50. The 10% rule says only about one tenth of the energy at one trophic level is passed to the next; the rest is used for life processes or lost as heat. Starting with producers, multiply by 0.10 once to reach primary consumers and again to reach secondary consumers. A common error is stopping after one step, which gives the primary consumer level. Count the arrows between levels before multiplying.

• A: 500 is the primary-consumer level after one transfer.
• B: 5 would require three transfers to reach tertiary consumers.
• C (correct): Two transfers: 5,000 → 500 → 50.
• D: That is the producers’ initial energy, not secondary consumer supply.

Answer: A

Correct answer: 20 rabbits per km². Population density is the number of individuals per unit of area, so the count always goes on top of the fraction and the area goes on the bottom. Checking units helps: rabbits divided by square kilometers gives rabbits per square kilometer. Flipping the fraction gives area per rabbit, and multiplying the numbers produces units that make no sense. Density lets ecologists compare populations living in areas of different sizes.

• A (correct): Divide individuals by area: $\frac{240}{12}$ = 20 rabbits/km².
• B: This is not individuals per area.
• C: This repeats area rather than calculating density.
• D: Multiplication gives wrong units/quantity.

Answer: C

Correct answer: 130. Population size changes through four flows. Births and immigration add individuals, while deaths and emigration remove them. Write the change as births plus immigrants minus deaths minus emigrants, then add it to the starting size. Here the gains are 40 and the losses are 30, for a net gain of 10. The tempting mistakes come from adding only the gains or forgetting one of the losses, so list all four terms before calculating.

• A: This includes additions but ignores losses.
• B: This fails to subtract some departures.
• C (correct): 120 + 30 + 10 − 25 − 5 = 130.
• D: This misses some population flows.

Answer: C

Correct answer: Nitrogen fixation. Nitrogen gas makes up about 78% of the air, but its strong triple bond makes it unusable by most organisms. Nitrogen-fixing bacteria, including those in legume root nodules, convert N₂ into ammonia or ammonium that plants can use to build proteins and nucleic acids. Lightning and industrial fertilizer production also fix some nitrogen. Denitrification is the tempting distractor, but it does the reverse, returning nitrogen compounds to the atmosphere as gas.

• A: Denitrification generally returns nitrogen toward atmospheric gases.
• B: Biomagnification concerns contaminant concentrations.
• C (correct): Certain microbes fix nitrogen gas into reduced nitrogen compounds.
• D: Transpiration releases water vapor from plants.

Answer: B

Correct answer: Weathering rock can release phosphate that enters food webs. Unlike the carbon and nitrogen cycles, the phosphorus cycle has no significant gas phase. Phosphorus is stored mostly in rocks and sediments, and weathering slowly releases phosphate into soil and water, where plants absorb it. Animals obtain it by eating plants, and decomposers return it to the soil. Because the cycle is slow and lacks an air reservoir, phosphorus often limits growth. Living things need it for DNA, ATP and cell membranes.

• A: The common phosphorus cycle lacks a major gaseous reservoir.
• B (correct): Rock weathering can release phosphate into soil/water.
• C: Phosphorus is part of nucleic acids and ATP.
• D: Chemical processes rearrange atoms; they do not create new phosphorus atoms.

Answer: A

Correct answer: Secondary succession. Succession is the gradual change in species after a disturbance. Secondary succession happens where soil, seeds or roots survive, as after a fire, flood or abandoned farm, so recovery can start quickly with grasses and shrubs. Primary succession begins on surfaces with almost no soil, like new lava or rock exposed by a retreating glacier, and must wait for pioneer species to build soil. The key clue is whether the soil remains.

• A (correct): Soil and biological remnants generally permit secondary succession.
• B: Nitrogen fixation is a nutrient transformation.
• C: Biomagnification is contaminant increase through a food chain.
• D: Primary succession starts on substrates with virtually no developed soil.

Answer: A

Correct answer: A pollinator obtains nectar while helping a flower reproduce. Mutualism is an interaction in which both species are better off. The pollinator gets food in the form of nectar, and the flower gets its pollen carried to another flower, allowing fertilization and seed production. To identify mutualism, ask what each partner gains; if one is harmed, it is parasitism or predation, and if both lose resources, it is competition. Many crops depend on this bee-and-flower partnership.

• A (correct): Both species receive benefits in the described exchange.
• B: The tick benefits while the dog is harmed, which is parasitism rather than mutualism.
• C: Predation harms the prey.
• D: Competition reduces each tree's access to light, so neither clearly benefits.

Answer: C

Correct answer: Decomposition of excess organic material consumes dissolved oxygen. This sequence is called eutrophication. Fertilizer adds nitrogen and phosphorus, nutrients that normally limit algal growth, so algae multiply rapidly. When the algae die, bacteria decompose the large mass of organic matter and use up dissolved oxygen through cellular respiration, creating low-oxygen dead zones where fish can suffocate. A common misconception is blaming the living algae directly; during the day they actually release oxygen, and the loss comes mainly from decomposition.

• A: Food-web energy does not flow backward, and this would not explain the drop in dissolved oxygen.
• B: Algae do not transform all lake water into carbon dioxide.
• C (correct): Microbial breakdown can use oxygen and create hypoxia.
• D: Nitrogen remains present in some forms, not removed from existence.

Answer: D

Correct answer: Soil moisture, temperature and nutrient conditions. A community includes only the living organisms in an area, while an ecosystem includes those organisms plus the nonliving conditions they interact with. So the missing piece is abiotic factors such as soil, water, temperature, light and nutrients. Adding more organisms, whether more individuals or a new species, keeps the description at the community level. Ask whether the addition is alive; only nonliving factors move it to an ecosystem.

• A: Labels do not supply environmental interactions.
• B: A new predator is still a living member of the community, so it adds no physical factors.
• C: Adding more individuals increases a population but adds no abiotic components.
• D (correct): Abiotic setting added to living populations makes an ecosystem description.

Answer: B

Correct answer: Rapid spread of a contagious disease in an overcrowded colony. Density-dependent factors grow stronger as a population becomes more crowded. Disease spreads faster when individuals are in close contact, and competition for food, space and mates also intensifies. Density-independent factors, such as storms, earthquakes, fires and cold snaps, affect a population no matter how large or small it is. To decide, ask whether the impact would be worse if more individuals were packed into the same area.

• A: Cold weather acts regardless of how crowded the population is, so it is density-independent.
• B (correct): Crowding can increase contact and some disease transmission.
• C: An earthquake’s occurrence is not determined by population density.
• D: A hurricane strikes regardless of population size, so it is a density-independent disturbance.

Answer: D

Correct answer: Energy pyramid tracks energy by trophic level; a numbers pyramid counts organisms and may be inverted. An energy pyramid shows how much energy is available at each trophic level, and because about 90% is lost at each transfer, it is always upright. A pyramid of numbers simply counts organisms, and organism size changes the shape. A single oak tree can feed thousands of caterpillars, making the base narrower than the level above. The misconception is assuming all ecological pyramids must be upright; only the energy pyramid always is.

• A: One large producer, such as a tree, can support many insects, so a numbers pyramid can be inverted.
• B: This reverses the two: energy pyramids measure energy, and numbers pyramids count organisms.
• C: Species richness is a different diversity measure.
• D (correct): This identifies the variables and why their shapes may differ.

Answer: C

Correct answer: Two deer of the same species seeking the same limited forage. The prefix intra- means within, so intraspecific competition occurs among members of the same species, while inter- means between, as in interspecific competition between different species. Competition requires a shared, limited resource such as food, water, mates or space. Members of the same species usually compete most intensely because they need exactly the same things. Predation and pollination involve two species and are not competition at all.

• A: Decomposition is nutrient recycling, not competition between members of a species.
• B: Pollination can be mutualism.
• C (correct): Competition within one species is intraspecific.
• D: This is predation, a feeding relationship between two different species.

Answer: D

Correct answer: Some microbes can be primary producers using chemical energy. Producers, or autotrophs, make organic molecules from inorganic carbon. Most use light in photosynthesis, but some bacteria and archaea use chemosynthesis, drawing energy from chemicals such as hydrogen sulfide. These microbes support entire food webs around deep-sea hydrothermal vents, where no sunlight reaches. The misconception is that all producers need sunlight. Remember too that producers also carry out cellular respiration to use the sugars they make.

• A: Chemosynthesis can support production without sunlight.
• B: Algae and microbes can also be producers.
• C: Photosynthetic producers also respire.
• D (correct): Certain microbes use chemical energy to make organic matter.

Answer: D

Correct answer: Primary succession involving pioneers and gradual soil formation. Primary succession starts on surfaces that have never had soil, such as cooled lava or bare rock. Pioneer species like lichens and mosses colonize first; they help break down rock and add organic matter when they die, slowly building soil. Over decades or centuries, grasses, shrubs and eventually trees can follow. Secondary succession is the tempting distractor, but it occurs only where soil already exists, as after a fire.

• A: Secondary succession requires existing soil and biological remnants, which new lava lacks.
• B: This is not a description of succession.
• C: Mature forest cannot generally form instantly on bare lava.
• D (correct): Bare substrate supports the concept of primary succession.

Answer: C

Correct answer: A contaminant building up within one organism over time. Bioaccumulation happens when an organism takes in a substance, such as mercury or a fat-soluble pesticide like DDT, faster than it can break it down or excrete it. The amount stored in its tissues rises over its lifetime. It is easily confused with biomagnification, which is the increase in concentration from one trophic level to the next in a food chain. Think of bioaccumulation as one body, and biomagnification as up the chain.

• A: This describes energy flow through a food chain, not the buildup of a contaminant.
• B: Light increase is an abiotic change.
• C (correct): Accumulation concerns one organism’s contaminant burden.
• D: A rise in fish numbers is population growth, not bioaccumulation.

Answer: D

Correct answer: Population stability can reflect approximately balanced gains and losses. Population size depends on the balance of additions and subtractions: births and immigration versus deaths and emigration. If these roughly cancel, the population stays steady even though individuals are constantly being born and dying. This is called dynamic equilibrium, and it is typical of populations near carrying capacity. A common misconception is that a stable number means nothing is happening, when in fact there is steady turnover.

• A: Zero capacity would not support the population.
• B: Predators and diseases can be present in a stable population; they are part of the losses balancing births.
• C: The stem states that births and deaths continue, so the population is not inactive.
• D (correct): Stable abundance does not imply biological inactivity.

Answer: D

Correct answer: 900 units. Energy transfer problems are solved by multiplying the starting amount by each transfer efficiency in turn. First find what primary consumers receive, 20% of the producer energy, and then take 5% of that result for secondary consumers. Converting percentages to decimals helps avoid errors. The usual trap is applying the second percentage to the original producer energy instead of to the primary consumer amount, which skips a trophic step.

• A: 4,500 takes 5% of producers, skipping the first transfer.
• B: 450 results from an incorrect percentage application.
• C: 18,000 is the energy at primary consumers, not secondary consumers.
• D (correct): 90,000 × 0.20 × 0.05 = 900 units.

Answer: C

Correct answer: Snakes may lose food; hawks may also be affected but have other prey. In a food web, an organism's sensitivity to losing a prey species depends on how many other food sources it has. Here snakes eat only mice, so they are directly vulnerable. Hawks eat mice, rabbits and snakes, so they can switch prey, although they may still feel some effect. Words like must, all and exactly signal claims that go beyond what a simple web can support, so prefer cautious predictions.

• A: Fewer mice means less grazing, so grasses would not be forced to vanish.
• B: The web gives no numbers, so no precise change in rabbits can be inferred.
• C (correct): Snakes rely on mice in this web, and hawks can use rabbits or snakes as alternatives.
• D: The web includes alternative hawk prey; absolute extinction is unsupported.

Answer: D

Correct answer: 80. Use the population change equation: final size equals starting size plus births plus immigrants minus deaths minus emigrants. Substitute the known values and solve for the unknown births. A quick check is the net change of 60; since losses total 40 and immigration adds 20, births must make up the remaining 80. The tempting answer of 60 is just the overall increase and ignores that some individuals left or died.

• A: This underestimates the required net gains.
• B: Sixty is the net population increase before accounting for losses and immigration.
• C: Twenty is the immigration count, not births.
• D (correct): 360 = 300 + B + 20 − 25 − 15, so B = 80.

Answer: B

Correct answer: The first has higher richness, but evenness is lower; overall diversity depends on the metric. Species diversity has two parts. Richness is simply the number of species, and evenness describes how equally individuals are spread among those species. A site dominated by one species has low evenness even if many rare species are present. Indices such as Shannon or Simpson combine both parts in different ways, so the two wetlands might rank differently depending on the index. Do not assume more species always means more diversity.

• A: Evenness requires counts of individuals in each species; water chemistry cannot provide it.
• B (correct): Richness and evenness are distinct components of diversity.
• C: 30 species is fewer than 60.
• D: Diversity indices weight richness and evenness differently, so more species does not guarantee higher diversity on every measure.

Answer: C

Correct answer: 1,000. Quadrat sampling estimates a population by finding the average density in small plots and scaling up to the whole area. First add the counts and divide by the total sampled area to get seedlings per square meter, then multiply by the field's area. This works only if the quadrats represent the field fairly. A common error is multiplying the total seedling count by the full area without first finding the density per square meter.

• A: 250 assumes density 1/$m^2$ rather than 4/$m^2$.
• B: 2,500 assumes 10 seedlings/$m^2$.
• C (correct): 20 seedlings ÷ 5 $m^2$ = 4/$m^2$; 4 × 250 = 1,000.
• D: 5,000 improperly multiplies all sampled seedlings by total area.

Answer: A

Correct answer: Herbivores may increase, leading to greater pressure on plants. Removing a top predator can trigger a trophic cascade. Without predation, herbivore numbers tend to rise, and more herbivores eat more plants, so plant abundance may fall. A classic case is the loss of wolves from Yellowstone, where growing elk herds overbrowsed willows and aspens until wolves were reintroduced. Effects pass down the chain through feeding links, so look for predictions that follow each link in order and avoid instant or extreme outcomes.

• A (correct): Predator release can raise herbivores, which can then reduce plants.
• B: Plants may experience more grazing, not necessarily immediate growth.
• C: Removing the predator does not directly imply herbivore disappearance.
• D: Food energy does not regenerate as sunlight.

Answer: B

Correct answer: An aquatic biomass pyramid can be inverted even if its energy pyramid is upright. A biomass pyramid shows how much living material exists at one moment, while an energy pyramid shows energy produced over time. Phytoplankton reproduce very quickly and are eaten almost as fast, so their standing mass can be smaller than that of the zooplankton feeding on them. Over a year, however, they still produce far more energy than the zooplankton receive. The misconception is assuming biomass and energy pyramids must have the same shape.

• A: Energy declines at each transfer, so an energy pyramid stays upright even when biomass is inverted.
• B (correct): High producer turnover can permit low standing producer biomass with greater consumer biomass.
• C: Algae remain producers because they photosynthesize, regardless of how much biomass is present at one moment.
• D: Aquatic organisms use trophic transfers.

Answer: A

Correct answer: A negative association exists, but other variables could contribute. Observational data can show that two variables change together, but not that one causes the other. Here, higher temperature goes with lower oxygen, a negative correlation. A plausible mechanism exists, since warm water holds less dissolved gas, but other factors such as nutrient runoff or algal blooms could also lower oxygen. Testing those factors, often with controlled experiments, is needed before claiming cause. Words like proven, only and every overstate the evidence.

• A (correct): The variables covary; testing other mechanisms and factors is needed.
• B: The data do not justify this universal reverse-causal statement.
• C: No species-specific survival evidence is supplied.
• D: Observational association alone cannot establish exclusivity.

Answer: C

Correct answer: Fewer nutrients become available from dead organic material. Plants need both energy and raw materials. Sunlight supplies energy, but elements such as nitrogen, phosphorus and potassium must come from the soil, and much of that supply is recycled from dead organisms and wastes by decomposers. If decomposition slows, nutrients stay locked in dead material, and producers run short even with plenty of light. The misconception is that light alone limits plant growth; any missing essential resource can become the limiting factor.

• A: Atoms are transformed and redistributed by cycles, not destroyed when decomposition slows.
• B: Sunlight comes from the sun; decomposers recycle nutrients, not light.
• C (correct): Reduced recycling can restrict mineral nutrient availability.
• D: Consumer nutrition does not reverse automatically.

Answer: A

Correct answer: It is nonnative and spreading; invasive harm is not yet established in the prompt. Scientists use precise labels. An introduced, or nonnative, species is one brought outside its natural range by people, and spreading shows it has become established. Calling it invasive also requires evidence of harm to native species, ecosystems or the economy. Because impacts have not been measured, the careful conclusion stops short of invasive while noting the risk. Absence of evidence is not evidence of harmlessness, so claims of never or cannot also overreach.

• A (correct): The information establishes introduction and spread, but not harm.
• B: Lack of measured impact does not prove permanent harmlessness.
• C: Spreading species may affect community dynamics; an unmeasured impact is not proof of no impact.
• D: Keystone status requires evidence of an outsized effect on community structure, not just spread.

Answer: C

Correct answer: 900 kcal. Secondary consumers are two feeding steps above the producers, so apply the stated 15 percent twice. Primary consumers get

kcal, and secondary consumers get

kcal.

• A: This uses the usual 10 percent rule instead of the 15 percent the model states.
• B: This is the energy at the primary consumers, only one step above the producers.
• C (correct): Two steps at 15 percent: 40,000 times 0.15 is 6,000, and 6,000 times 0.15 is 900.
• D: This applies 15 percent three times, which gives the tertiary consumers, not the secondary ones.

Answer: A

Correct answer: 45. Use change = (births + immigrants) − (deaths + emigrants). The herd grew by

Solving gives I = 45 immigrants.

• A (correct): Net change is +60, so 140 + I − 95 − 30 = 60, which gives I = 45.
• B: This leaves out the 30 emigrants, who are a loss that must be made up by more immigrants.
• C: This swaps the roles of births and deaths, treating the 140 births as a loss and the 95 deaths as a gain.
• D: This is the net change in herd size, not the number of immigrants; births, deaths and emigration also changed the herd.

Answer: C

Correct answer: Biomagnification. The pesticide concentration rises at every step from water to plankton to fish to birds. An increase in concentration from one trophic level to the next is biomagnification; buildup inside one organism over time would be bioaccumulation.

• A: Eutrophication is nutrient over-enrichment that feeds algal blooms, not a toxin building up in predators.
• B: A trophic cascade is a chain of population changes after a predator is gained or lost, not a rise in contaminant levels.
• C (correct): The concentration climbs with each feeding level because every predator eats many contaminated prey and retains the pesticide.
• D: Bioaccumulation is buildup within a single organism over its life; this data compares levels across the food chain.

Answer: B

Legumes such as alfalfa host nitrogen-fixing bacteria in root nodules that turn nitrogen gas into ammonium. When the alfalfa residue is decomposed, ammonium is released, and nitrifying bacteria convert it to nitrate, which corn roots absorb, so less fertilizer is needed.

• A: Plants cannot break the triple bond in nitrogen gas; the alfalfa relies on bacteria in its root nodules to fix it.
• B (correct): This links fixation by nodule bacteria, release by decomposers and nitrification into a usable form for the next crop.
• C: Denitrification runs the other way, turning nitrate back into nitrogen gas that escapes to the air.
• D: Phosphorus is a separate nutrient with its own cycle; it cannot substitute for the nitrogen that corn needs.

Answer: C

Correct answer: climate. Climate is the long-term average of weather in a region, usually based on 30 or more years of data. A 40-year average rainfall is a climate statistic, while a single storm or a rainy week would be weather.

• A: Weather is the short-term state of the atmosphere, such as this week's rain, not a decades-long average.
• B: A habitat is the place where a particular organism lives, not a measure of atmospheric conditions.
• C (correct): Climate is the long-term average pattern of weather, usually measured over 30 years or more.
• D: A niche is the role an organism plays in its community, which has nothing to do with rainfall records.

Answer: A

Correct answer: 45,000 kcal. Tertiary consumers are three feeding steps above the producers. Each step down the pyramid holds 10 times as much energy, so secondary consumers had 450 kcal, primary consumers 4,500 kcal and producers 45,000 kcal.

• A (correct): Going down three steps reverses three 10 percent transfers, so multiply by 10 three times: 45 × 1,000 = 45,000.
• B: This multiplies by 10 only twice, which gives the energy at the primary consumers.
• C: This multiplies by 10 four times, counting one feeding step too many.
• D: This takes 10 percent three more times, moving up the chain instead of back down to the producers.

Answer: D

Correct answer: commensalism. Judge the effect on each partner separately. The bird benefits and the cactus is unaffected, which is the plus/zero pattern of commensalism.

• A: Mutualism needs both partners to benefit, but the cactus gains nothing here.
• B: In parasitism the host is harmed, and the cactus is not harmed by the nest.
• C: Predation involves one organism killing and eating another, which does not happen here.
• D (correct): The bird benefits from protection while the cactus is unaffected, the plus/zero pattern of commensalism.

Answer: B

Correct answer: A wildfire that sweeps across the entire field. A density-independent factor affects a population the same way no matter how crowded it is. A wildfire burns through the field whether there are few mice or many, while disease, predation and competition all intensify with crowding.

• A: Contagious disease passes more easily when animals are crowded, so it is density-dependent.
• B (correct): Fire kills mice regardless of how crowded they are, so it is a density-independent factor.
• C: Predators focusing on abundant prey hit dense populations harder, a density-dependent effect.
• D: Competition for food grows stronger as crowding increases, so it is density-dependent.

Answer: D

Correct answer: Cellular respiration by fungi and bacteria. Decomposers such as fungi and bacteria digest the dead wood and use its sugars for energy. Their cellular respiration releases the stored carbon back to the air as carbon dioxide.

• A: Photosynthesis removes carbon dioxide from the air; it does not release the carbon stored in the log.
• B: Nitrogen fixation converts nitrogen gas into ammonium and does not involve releasing carbon.
• C: Transpiration releases water vapor from leaves, not carbon dioxide from dead wood.
• D (correct): Decomposers break down the wood and release its carbon as carbon dioxide when they respire.

Answer: B

Correct answer: Transpiration. Plants take up water through their roots and release it as vapor from their leaves; this is transpiration. Together with evaporation from open water, it moves water from the land into the air.

• A: Condensation is water vapor cooling into liquid droplets, such as clouds, the opposite direction.
• B (correct): Transpiration is the process in which plants give off water vapor through their leaves.
• C: Precipitation is water falling from clouds as rain, snow, sleet or hail.
• D: Infiltration is water soaking into the ground, not vapor leaving leaves.

Answer: A

Correct answer: Several years of drought that reduce plant growth. Carrying capacity is set by limiting resources such as food and water. A drought reduces plant growth, so the island can support fewer goats; the other changes either leave the limit unchanged or raise it.

• A (correct): Less plant growth means less food, so the habitat can support fewer goats over the long term.
• B: A burst of births changes the population size, not the number of goats the habitat can support.
• C: Removing a competitor leaves more food for goats, which would raise carrying capacity, not lower it.
• D: More shrubs mean more food, which would raise carrying capacity rather than lower it.