- A fishery is harvesting a fish population at its Maximum Sustainable Yield (MSY). A government body, facing economic pressure, authorizes a slight increase in the annual fishing quota. What is the most likely long-term consequence for the sustainability of this fishery?
Answer: The fish population will decline because the harvesting rate will exceed the maximum population recovery rate.
The Maximum Sustainable Yield (MSY) represents the highest rate at which a resource can be harvested without impairing its ability to be renewed. Harvesting at a rate even slightly above the MSY means that removals exceed the population's maximum rate of growth, leading to a decline in the population size and eventual collapse of the fishery. Choice A assumes an unrealistic evolutionary response. Choice C describes a short-term gain followed by a long-term collapse. Choice D introduces a secondary ecological effect that is less certain and does not negate the direct impact of overharvesting.
- As part of a rewilding project, a large herbivore like the European bison is reintroduced to a temperate grassland that has become dominated by dense, uniform grasses. What is a likely secondary ecological effect of this reintroduction?
Answer: Creation of a mosaic of grazed and ungrazed patches, increasing habitat heterogeneity and wildflower diversity.
Large herbivores act as ecosystem engineers by altering the physical environment. Their selective grazing creates patchiness in the vegetation structure. This increased heterogeneity (variety of habitats) provides niches for other species, such as wildflowers, that may have been outcompeted by the dominant grasses. This increases overall biodiversity. Desertification (A) would only occur under extreme overgrazing. While some soil compaction (B) may occur, the creation of habitat heterogeneity is a more significant ecological effect. Bison are herbivores and do not compete with predators (D).
- High levels of persistent organic pollutants (POPs) like PCBs are found in polar bears in the Arctic, thousands of kilometers from any major industrial source. What is the best explanation for this observation?
Answer: The pollutants are transported to the Arctic via atmospheric currents, enter the marine food web, and biomagnify to high concentrations.
This phenomenon is explained by two key ecological processes. First, POPs are subject to long-range atmospheric transport, a process known as 'global distillation,' where they evaporate in warmer climates and condense in colder polar regions. Second, once in the Arctic, they enter the marine food web and, due to their persistence and fat-solubility, biomagnify to very high concentrations in top predators like polar bears. Choice A is biologically impossible. Choice C is less likely to be the widespread cause than global transport. While glacier melt (D) can release some pollutants, it doesn't explain the initial transport to the Arctic or the extreme concentration in top predators via biomagnification.
- The removal of sea otters, a keystone species, from a kelp forest ecosystem leads to a surge in the sea urchin population, which then overgrazes the kelp. If the sea otters are reintroduced, what is the most likely sequence of events contributing to the restoration of ecosystem stability?
Answer: Sea otter predation reduces sea urchin numbers, allowing kelp to regrow, which in turn increases habitat for other species.
The correct answer describes a trophic cascade. The reintroduction of the top predator (sea otters) controls the population of the primary consumer (sea urchins). This reduction in herbivory allows the primary producers (kelp) to recover, restoring the habitat structure and supporting greater biodiversity. Choice A reverses the cause and effect. Choice C misidentifies the mechanism; sea otters increase biodiversity by controlling herbivores, not through competition. Choice D suggests migration, which is a less direct and certain outcome than the immediate effects of predation.
- The concept of a 'keystone species' is a powerful model in ecology. Which statement represents a critical limitation or a potential misuse of this concept in conservation planning?
Answer: Focusing conservation efforts exclusively on a single keystone species may lead to neglecting other important species and ecosystem processes.
This question addresses the Nature of Science (NOS) aspect of models. Models are simplifications. The keystone species concept is useful but can lead to a 'silver bullet' approach where conservation plans focus too narrowly on one species. A holistic approach that considers habitat, connectivity, and other species is more robust for maintaining overall ecosystem health. Choice B is incorrect; the definition of a keystone species is that its effect is disproportionately large relative to its abundance. Choice C is a common misconception; keystone species can be herbivores, plants, or ecosystem engineers. Choice D is false; the concept is supported by extensive evidence.
- A river ecosystem is monitored for effects of agricultural runoff by tracking populations of mayfly larvae (require high oxygen) and sludge worms (tolerate low oxygen). Which finding would most strongly indicate a decline in the river's health and a move away from sustainability?
Answer: A decreasing population of mayfly larvae and an increasing population of sludge worms.
Mayfly larvae are sensitive indicator species for high water quality (especially high dissolved oxygen). Sludge worms are tolerant indicator species for pollution (low dissolved oxygen). The pattern in choice C—sensitive species declining while tolerant species increase—is the classic biological indicator of eutrophication and oxygen depletion caused by nutrient runoff. Choice B would indicate improving water quality. Choice A suggests a stable, perhaps moderately impacted, state. Choice D would suggest a severe toxic pollution event that harms all life, rather than the specific effects of eutrophication.
- An abandoned farm field is colonized by grasses, followed by shrubs, and then trees over several decades. A nearby area of cooled lava from a recent volcanic eruption is slowly colonized by lichens and mosses. Which statement correctly compares these two processes? (HL only)
Answer: The farm field undergoes secondary succession, which is faster than the primary succession on the lava field because soil is already present.
Secondary succession occurs where an existing ecosystem has been disturbed but the soil remains intact, as in an abandoned field. Primary succession occurs on a new substrate without soil, such as a lava flow. Secondary succession is significantly faster because soil, containing nutrients and a seed bank, is already present. Choice A incorrectly identifies the farm field and the relative speed. Choice C reverses the definitions. Choice D incorrectly identifies both processes.
- In a marine food chain, phytoplankton absorb a persistent pollutant from the water at a concentration of 0.01 parts per billion (ppb). Zooplankton eat phytoplankton, small fish eat zooplankton, and tuna eat small fish. Assuming a 10-fold increase in pollutant concentration at each trophic level, what is the predicted concentration of the pollutant in the tuna?
Answer: 10.0 ppb
The food chain has four trophic levels: T1 (phytoplankton), T2 (zooplankton), T3 (small fish), and T4 (tuna). There are three trophic transfers where magnification occurs. The calculation is: 0.01 ppb (in T1) × 10 (to T2) × 10 (to T3) × 10 (to T4) = 10.0 ppb. Choice A represents only one transfer. Choice B represents two transfers. Choice D represents four transfers, a common error made by incorrectly counting the starting level as a transfer.
- Following excessive nutrient runoff into a freshwater lake, a massive algal bloom occurs. This is followed by a large-scale death of fish. What is the most direct cause of this fish mortality?
Answer: Decomposing bacteria consume large amounts of dissolved oxygen after the algae die.
The primary cause of fish kills in cultural eutrophication is hypoxia (low oxygen) or anoxia (no oxygen). When the large algal population dies, aerobic decomposer bacteria break them down, consuming vast quantities of dissolved oxygen in the process. This depletes the oxygen available for fish to respire. While some algae produce toxins (A) and blooms block light (C), the oxygen depletion from decomposition is the most common and direct cause of mass mortality. The nutrient concentrations themselves are not directly toxic to fish (D).
- In primary succession on bare rock, lichens are often the pioneer species. How do lichens most directly facilitate the establishment of later species like mosses and grasses? (HL only)
Answer: They secrete acids that chemically weather the rock and add organic matter upon decomposition, forming soil.
The critical role of pioneer species in primary succession is facilitation through environmental modification. Lichens facilitate the establishment of later species by initiating soil formation. They secrete weak acids that begin to break down the rock surface, and when they die and decompose, they contribute the first layer of organic matter. This process, repeated over long periods, creates the thin soil necessary for mosses and then grasses to take root. Releasing inhibitory chemicals (A) would hinder, not help, succession. Providing food (B) is a secondary role. Preventing erosion (C) is a function of later plants with root systems.
- A mature oak-hickory forest, considered a climax community, is subjected to a severe forest fire that clears most of the trees but leaves the soil intact. What is the most likely ecological change to occur in the first few years after the fire? (HL only)
Answer: The process of secondary succession will begin, with rapid growth of herbaceous plants and sun-loving pioneer trees.
A disturbance like a fire that leaves the soil intact initiates secondary succession. The first species to colonize are typically fast-growing, sun-loving pioneer species (e.g., grasses, weeds, certain fast-growing trees) whose seeds may have been dormant in the soil or are easily dispersed. The slow-growing, shade-tolerant climax species (oak, hickory) will only become re-established much later in the successional process (A is incorrect). Because soil is present, it is not primary succession (B is incorrect). While some forest soils can be damaged, a permanent conversion to grassland is not the most likely outcome; forests are generally resilient to fire (D is incorrect).
- Based on the passage, what is the most likely consequence for the filter-feeding fish?
Answer: The fish will be exposed to physical stress from the plastics and will also biomagnify the POPs from their food.
This scenario presents two threats. First, the plastic particles themselves can cause physical harm (e.g., digestive blockage, inflammation). Second, and more insidiously, plastics act as vectors for other toxins like POPs. The fish will ingest these POPs along with the zooplankton, and because POPs are persistent and fat-soluble, they will biomagnify in the fish's tissues. Choice A is incorrect as the POPs can be transferred even if the plastic is excreted. Choice C is incorrect as plastics are not digestible. Choice D is unlikely as detoxification systems can be overwhelmed, leading to accumulation.
- A single, large predatory fish is found to have a mercury concentration of 2.0 ppm. The small fish it consumes have an average mercury concentration of 0.4 ppm, and the zooplankton they consume have a concentration of 0.05 ppm. Which statement best explains these observations?
Answer: The predatory fish experiences biomagnification through its diet and also bioaccumulates mercury over its lifetime.
This question tests the distinction between two related concepts. Biomagnification refers to the increasing concentration of a toxin at successive trophic levels, which is seen here (0.05 -> 0.4 -> 2.0). Bioaccumulation is the buildup of a toxin within a single organism over its lifespan. The high concentration in the large (and presumably older) predatory fish is a result of both processes: it accumulates mercury from its environment and food, and the concentration is magnified up the food chain. Choices B and C are incomplete because both processes are occurring. Choice D is speculative and does not explain the pattern of concentration increase.
- A farmer has a field adjacent to a stream that flows into a lake. To promote the sustainability of the lake ecosystem, which of the following agricultural practices is most effective?
Answer: Maintaining a buffer zone of native vegetation between the cultivated field and the stream bank.
A buffer zone (or riparian strip) of native plants is a key strategy for sustainable agriculture near waterways. The vegetation slows runoff from the field, and its roots absorb excess nutrients (like nitrogen and phosphorus) before they can enter the stream and cause eutrophication in the lake. Choice A would maximize nutrient runoff. Choice B would increase both nutrient and soil runoff. Choice D is an inefficient water-use strategy but is less directly related to eutrophication.
- Microplastics are considered a particularly insidious form of pollution in marine ecosystems. Which property most significantly contributes to their ability to be transferred up the food chain?
Answer: Their small size allows them to be ingested by primary consumers, such as zooplankton, at the base of the food web.
The key danger of microplastics is that their size range overlaps with the food particles of organisms at the base of the food web, like zooplankton. Once they enter the food chain at this low trophic level, they can be passed up to all subsequent levels. Choice A is incorrect as many plastics are buoyant. Choice C is incorrect as most plastics are not biodegradable. Choice D is incorrect as plastics are persistent solids and do not dissolve in seawater.
- The pesticide DDT biomagnifies significantly in food webs, while many modern herbicides break down quickly and do not. Which properties of DDT are most responsible for this difference?
Answer: Chemical persistence in the environment and high solubility in fatty tissues.
For a substance to biomagnify, two key properties are required. First, it must be persistent, meaning it does not break down easily in the environment. Second, it must be bioaccumulated, meaning it is taken up and stored in an organism's tissues, typically because it is fat-soluble (lipophilic) and not easily excreted. DDT has both of these properties. The properties in choices A, B, and D would all lead to a substance breaking down or being excreted, thus preventing biomagnification.
- The reintroduction of wolves into an ecosystem led to a decrease in the elk population. This allowed willow trees, which are browsed by elk, to recover along riverbanks. This vegetation recovery then led to an increase in songbird populations. This entire sequence of events is best described as:
Answer: A trophic cascade initiated by a keystone species.
This is a classic example of a trophic cascade, where the effects of a top predator (the wolf, a keystone species) cascade down through the food web to affect primary producers (willow trees) and other non-prey species (songbirds). It is not competitive exclusion (B), as that involves two species competing for the same limited resource. It is a stabilizing effect, not a destabilizing positive feedback loop (C). It is a change within an existing ecosystem, not primary succession (D).
- In many freshwater ecosystems, phosphorus is the limiting nutrient for algal growth. Based on this information, what is the most direct reason why using phosphate-free detergents helps prevent cultural eutrophication?
Answer: It removes the specific nutrient that controls the rate of algal reproduction, thereby preventing the initial algal bloom.
A limiting nutrient is a factor that restricts the growth of a population. Since phosphorus is the limiting nutrient for algae in these ecosystems, adding it stimulates a population explosion (algal bloom). The most effective way to prevent this is to restrict the input of the limiting nutrient. Therefore, removing phosphates from detergents directly addresses the specific cause of the bloom. Choice A is a consequence of preventing the bloom, not the mechanism. Choice C is incorrect; detergents do not directly affect decomposers. Choice D is plausible but not the primary reason; nitrogen can also cause blooms, and the goal is to prevent the bloom altogether by controlling the limiting factor.
- Which set of characteristics is most representative of a climax community, contributing to its high degree of stability? (HL only)
Answer: High species diversity, complex food webs, large organism size, and a large amount of standing biomass.
A climax community is a mature, stable ecosystem that is the culmination of ecological succession. Its stability comes from high species diversity (richness), which creates complex and redundant food webs. It is also characterized by large amounts of biomass (e.g., large trees) and well-developed soil with efficient nutrient cycling. Choices A, B, and D all describe characteristics of early successional stages, which are typically less stable and dominated by r-selected, pioneer species.
- A conservation group proposes to rewild a large area of degraded agricultural land by reintroducing beavers. Which observation would provide the strongest evidence that this rewilding effort is successfully increasing ecosystem stability and function?
Answer: The establishment of beaver dams, leading to diverse wetland habitats and an increase in local bird and amphibian species.
The goal of rewilding is to restore natural processes and increase biodiversity and stability. Beavers are 'ecosystem engineers'. Their dams create wetlands, which are highly diverse habitats. An increase in other species (birds, amphibians) is direct evidence of restored ecosystem function and complexity, which contributes to stability. A growing beaver population (A) only shows the species is viable, not that it is improving the ecosystem. A changed flow rate (C) is a physical effect but not direct evidence of ecological restoration. Increased competition (D) would suggest a negative impact on existing biodiversity.
