- A rewilding project proposes reintroducing wolves to a large national park. How does this approach fundamentally differ from traditional conservation in a fenced wildlife sanctuary for an endangered deer species?
Answer: Rewilding prioritizes restoring ecosystem processes and trophic levels, while sanctuaries focus on species protection through active management.
The core philosophy of rewilding is to restore natural processes and self-regulating ecosystems, often by reintroducing keystone species or top predators to reinstate trophic cascades. Traditional sanctuaries often involve more intensive human management (like providing food or controlling predators) to protect a specific population, rather than restoring the entire system's autonomy.
- [HL] Which statement best describes the nature of a climax community?
Answer: It is a community in dynamic equilibrium, where species composition is relatively stable despite turnover of individuals.
A climax community is not static. While the overall structure, species composition, and biomass remain relatively constant over time, there is a constant turnover of individuals (death and replacement). It is best described as being in a state of dynamic equilibrium. Net primary productivity is low (close to zero) because gross production is balanced by ecosystem respiration, not at its highest possible value.
- The reintroduction of a top predator into an ecosystem as part of a rewilding initiative can increase long-term stability. However, what is a likely short-term consequence of such a reintroduction?
Answer: A period of population fluctuations as the food web reorganizes around the new predator.
Introducing a top predator is a major perturbation. Prey populations may initially crash, and their behaviour will change (e.g., avoiding certain areas). This will cause ripple effects throughout the food web. The system will likely experience a period of instability and oscillation before it settles into a new, more complex dynamic equilibrium. Stabilization is a long-term goal, not an immediate effect.
- Microplastics are a pervasive pollutant in marine environments. Which mechanism is a primary cause of physiological harm in a filter-feeding bivalve that ingests large quantities of microplastic particles?
Answer: The particles cause internal abrasion and can trigger a persistent, energy-consuming inflammatory response.
Ingested microplastics can cause physical damage (abrasion) to the delicate tissues of the digestive tract. The organism's immune system may also recognize these as foreign bodies, triggering a chronic inflammatory response. This response consumes significant energy, diverting it from growth and reproduction, and causing physiological stress.
- Following an algal bloom triggered by eutrophication, a massive die-off of algae occurs. What is the immediate biochemical process responsible for the subsequent development of an anoxic "dead zone"?
Answer: The decomposition of dead algae by aerobic bacteria consumes large amounts of dissolved oxygen.
The primary cause of anoxia (lack of oxygen) after an algal bloom is the massive amount of decomposition. Aerobic bacteria break down the dead organic matter (the algae), and this process of aerobic respiration consumes dissolved oxygen from the water faster than it can be replenished, leading to hypoxic or anoxic conditions.
- A logging company claims its operations are sustainable. Which of the following observations would provide the strongest evidence to refute this claim?
Answer: Satellite imagery shows a gradual but consistent decrease in total forest canopy cover over a decade.
Sustainability means meeting present needs without compromising the ability of future generations to meet theirs. Ecologically, this means the resource is not being depleted over time. A consistent decrease in canopy cover is direct evidence that the rate of harvesting exceeds the rate of regrowth, meaning the forest resource is being depleted. This directly refutes the claim of sustainability.
- [HL] During primary succession, pioneer species like lichens play a crucial role in facilitating the establishment of later species. What is the primary mechanism by which they achieve this?
Answer: They chemically and physically break down rock and contribute organic matter, initiating soil formation.
The most fundamental role of pioneer species in primary succession is soil creation. Lichens secrete weak acids that begin to chemically weather the rock surface. As they die and decompose, they contribute the first layer of organic matter (humus). This gradual process of weathering and organic accumulation creates the first primitive soil, which is essential for later, more complex plants to establish themselves.
- Based on the passage, what would be the most probable indirect effect of a disease that drastically reduces the lion population?
Answer: A decrease in the abundance and diversity of insect pollinators.
This describes a trophic cascade. A reduction in lions (top predator) would lead to an increase in wildebeest (primary consumer). More wildebeest would lead to more intense grazing, favouring the dominant tall grasses and reducing the abundance of forbs. A decrease in forbs (producers) would lead to a decrease in the insect pollinators that depend on them.
- A fish stock is managed based on the principle of Maximum Sustainable Yield (MSY), aiming to maintain the population at a size that produces the highest catch rate. Why might a management strategy strictly adhering to MSY fail to be ecologically sustainable in the long term?
Answer: It fails to account for natural fluctuations in population size and changing environmental conditions.
MSY models often assume a stable carrying capacity and predictable population growth, which is rarely the case in real ecosystems. Natural environmental fluctuations (e.g., changes in water temperature, food availability) can cause the population's carrying capacity and reproductive rate to vary. Harvesting at a fixed MSY level during a period of natural population decline can lead to over-exploitation and stock collapse, thus it is not truly sustainable.
- [HL] A grassland ecosystem in a region with enough rainfall to support a forest is prevented from doing so by frequent, naturally occurring fires. In this context, what is the ecological role of fire?
Answer: It is an abiotic factor that arrests succession, maintaining the ecosystem in a subclimax state.
Succession is the process of change in community structure over time. A climax community is the theoretical end point. When a recurring disturbance, like fire, prevents the community from reaching its potential climax state (a forest), it is said to be arrested. The resulting stable community (the grassland) is known as a subclimax or fire-climax community. Fire is an abiotic, density-independent factor.
- In a coastal marine ecosystem, sea otters are a keystone species that prey on sea urchins. Sea urchins graze on kelp, which forms large underwater forests. If a disease significantly reduces the sea otter population, what is the most likely long-term consequence for fish populations that use the kelp forest as a nursery?
Answer: Fish populations would decrease due to the loss of habitat and shelter provided by the kelp forests.
The removal of the keystone predator (sea otters) would lead to an increase in their prey (sea urchins). Increased sea urchin populations would lead to overgrazing and the destruction of the kelp forests. Since the fish populations use the kelp as a nursery, the loss of this critical habitat would cause a decline in fish populations. This is an example of a trophic cascade.
- A persistent organic pollutant (POP) is found in a lake. Analysis reveals its concentration in zooplankton is 0.05 ppm, in small fish that eat zooplankton it is 1.0 ppm, and in predatory birds that eat the fish it is 25 ppm. Which statement best explains these observations?
Answer: Bioaccumulation has occurred within each organism, and biomagnification has occurred across trophic levels.
Bioaccumulation is the build-up of a substance in an individual organism's tissues over its lifetime. Biomagnification (or bioamplification) is the increasing concentration of that substance in organisms at successively higher levels in a food chain. The data shows increasing concentration at each trophic level (biomagnification), which is a result of each organism bioaccumulating the pollutant from its food.
- A lake is suffering from advanced cultural eutrophication. Which of the following interventions would most directly address the root cause of this problem, rather than just treating its symptoms?
Answer: Restoring wetlands and creating buffer zones around agricultural fields that drain into the lake.
The root cause of cultural eutrophication is excessive nutrient (nitrate and phosphate) input from sources like agricultural runoff. Options A, B, and D treat the symptoms (algal blooms, low oxygen). Option C addresses the root cause by intercepting and filtering the nutrient-rich runoff before it can enter the lake and trigger the blooms.
- Scientists are assessing a new industrial chemical. Which combination of properties would make this chemical most likely to undergo significant biomagnification in terrestrial food webs?
Answer: Lipid-soluble, chemically stable, and not readily metabolized by organisms.
For a substance to biomagnify, it must be persistent in the environment (chemically stable), taken up by organisms, and retained in their tissues. Lipid-solubility causes it to be stored in fatty tissues rather than being excreted in urine (which is water-based). Being not readily metabolized means it stays in the body for a long time. These three properties lead to its accumulation and magnification up the food chain.
- [HL] During primary succession on bare rock, which of the following describes the general trend for gross production (GP), ecosystem respiration (R), and net production (NP) as the community progresses from pioneer species to a mature forest?
Answer: GP and R both increase, while NP is highest in early-to-mid succession and then declines.
In early succession, biomass is low, so GP and R are low. Because there are few consumers and decomposers, production exceeds respiration (P > R), so Net Production (NP = GP - R) is positive and biomass accumulates. As the community matures, biomass increases, leading to increases in both total GP and total ecosystem R. In a climax community, the system is in equilibrium, and R becomes nearly equal to GP, so NP approaches zero. Therefore, NP peaks during the intermediate stages of succession when biomass is accumulating rapidly.
- How does the process of eutrophication typically affect species richness and species evenness in an aquatic ecosystem?
Answer: Both species richness and species evenness decrease as conditions favour a few tolerant species.
Eutrophication leads to a decrease in biodiversity. Species richness (the number of different species) declines because many species cannot tolerate the low-oxygen conditions. Species evenness (the relative abundance of species) also declines sharply because a few nutrient-tolerant species, like certain types of algae, become extremely dominant, while all other species become rare or are eliminated.
- In an alpine meadow, a single species of grass accounts for 80% of the total biomass. In a nearby coral reef, a species of sea star with a very low total biomass prevents mussels from monopolizing all available rock space. Which statement correctly identifies the roles of the grass and the sea star?
Answer: The grass is a dominant species due to its high biomass, while the sea star is a keystone species.
A dominant species is one that is most abundant or has the highest biomass in a community (the grass). A keystone species has a disproportionately large effect on its ecosystem relative to its abundance or biomass (the sea star). The sea star's removal would lead to a drastic decrease in diversity, a classic indicator of a keystone species.
- [HL] An area of temperate forest is cleared for agriculture but is then abandoned. Over the next century, the area undergoes ecological succession. Which feature most clearly indicates that this is secondary, not primary, succession?
Answer: The process is initiated in an environment that already contains soil and organic matter.
The defining difference between primary and secondary succession is the starting condition. Primary succession begins on a substrate devoid of life and soil (e.g., bare rock after a volcanic eruption). Secondary succession occurs where an existing community has been disturbed, but the soil remains intact (e.g., an abandoned field). The presence of soil allows the process to happen much faster.
- A pesticide is applied to a field and has a concentration of 0.003 ppm in the soil. Earthworms bioaccumulate this to a concentration 10 times that of the soil. Shrews eat the earthworms, leading to a magnification factor of 20. Owls eat the shrews, resulting in a further magnification factor of 10. What is the approximate concentration of the pesticide in the owls?
Answer: 6.0 ppm
This requires a multi-step calculation. Concentration in earthworms = 0.003 ppm 10 = 0.03 ppm. Concentration in shrews = 0.03 ppm 20 = 0.6 ppm. Concentration in owls = 0.6 ppm * 10 = 6.0 ppm.
