Natural Disruptions to Ecosystems
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AP Environmental Science › Natural Disruptions to Ecosystems
A prescribed burn is used in a pine savanna; what is the primary ecological goal?
Mimic natural fire regimes to reduce fuel loads, recycle nutrients, and maintain fire-adapted species composition.
Convert the ecosystem to primary succession by removing soil organic matter and exposing bedrock for lichens.
Eliminate all insects permanently, preventing any future herbivory and increasing ecosystem stability indefinitely.
Increase soil salinity to limit plant growth, thereby reducing competition and promoting longleaf pine regeneration.
Explanation
Prescribed burns in pine savannas mimic natural fires, reducing fuels and recycling nutrients. This maintains fire-adapted species and prevents intense wildfires. Goals include preserving biodiversity and ecosystem structure. Fires promote understory diversity by limiting woody encroachment. Without them, succession shifts habitats. Such management sustains disturbance-dependent ecosystems.
A heat wave causes fish die-off in a pond; which mechanism best explains the event?
Warmer water holds more dissolved oxygen, but fish suffocate because oxygen becomes too concentrated in gills.
Warmer water holds less dissolved oxygen, and increased respiration can outpace oxygen supply, stressing fish and causing mortality.
Heat waves eliminate sunlight penetration, stopping photosynthesis permanently and causing oxygen to rise to toxic levels.
Fish die because heat waves increase salinity in freshwater ponds to ocean levels within hours, causing osmotic shock universally.
Explanation
Heat waves warm pond water, reducing dissolved oxygen capacity while increasing metabolic demands. This leads to hypoxia, stressing and killing fish. Warmer conditions favor respiration over oxygenation. Die-offs occur when oxygen supply fails. Mitigation includes shading or aeration. Such events highlight thermal disruptions in aquatics.
Following a severe drought, many trees die; which short-term change is most likely in stream ecosystems?
Higher sediment input from reduced root stabilization, increasing turbidity and potentially reducing benthic macroinvertebrate diversity.
Immediate increase in stream pH because drought adds carbonate ions, causing long‑term alkalinization of all watersheds.
Lower water temperature from increased shading, raising dissolved oxygen and boosting trout populations in newly shaded channels.
Permanent elimination of decomposition because drought kills all bacteria, preventing leaf litter breakdown for centuries.
Explanation
Droughts kill trees, reducing root stabilization along streams and increasing sediment erosion into water. This raises turbidity, which can harm aquatic life like macroinvertebrates by blocking light and clogging habitats. Short-term changes include decreased biodiversity in benthic communities. Riparian vegetation normally filters sediments, so its loss exacerbates impacts. Recovery depends on regrowth and rainfall patterns. Such disruptions illustrate linkages between terrestrial and aquatic ecosystems.
After a wildfire, invasive grass increases fire frequency; what feedback does this represent?
Neutral feedback: fire frequency is unaffected by vegetation changes because ignition is controlled only by lightning rates.
Negative feedback: more grass reduces fire probability by increasing soil moisture and decreasing fine fuels.
Positive feedback: invasive grass promotes more frequent fires, which further favors the grass over native shrubs.
Abiotic buffering: grass converts $CO_2$ to oxygen, preventing combustion and stabilizing climate at the site.
Explanation
Invasive grasses creating fine fuels promote more fires, which further advantage the grass in a positive feedback loop. This alters native shrub communities negatively. Feedbacks amplify changes in disturbance regimes. Native systems may shift irreversibly. Management aims to break such cycles. This illustrates invasion-disturbance interactions.
A forest experiences repeated low-intensity fires; which trait is most favored in dominant tree species?
Complete dependence on animal pollination only, because fire eliminates wind and prevents airborne pollen transfer.
Thin bark and shallow roots, increasing water uptake and ensuring rapid mortality that opens space for competitors.
Low seed production, because fires reduce competition and guarantee survival of every seed that is produced.
Thick bark and high canopy, protecting cambium from heat and allowing survival through frequent surface fires.
Explanation
In fire-prone forests, traits like thick bark protect trees from heat damage during low-intensity fires. High canopies elevate vulnerable parts above flames. These adaptations favor survival in repeated disturbances. Fire regimes select for such species over time. Thin-barked trees are outcompeted. Ecosystem dynamics depend on these traits.
A coastal city rebuilds after repeated hurricanes; which planning approach best reduces future ecological and human risk?
Increasing groundwater extraction to lower sea level locally, preventing storm surge and improving freshwater availability simultaneously.
Maintaining natural buffers like wetlands and dunes, limiting building in flood-prone zones, and using setback regulations to reduce exposure.
Hardening all shorelines with seawalls, eliminating wetlands and dunes to maximize developable land and shorten evacuation routes.
Replacing native coastal vegetation with lawns, which absorb wave energy better and require fewer nutrients than natural plants.
Explanation
Resilient coastal planning emphasizes preserving natural buffers like wetlands and dunes, which absorb storm energy and reduce flooding. Setback regulations prevent building in high-risk zones, minimizing human and ecological damage. This approach integrates ecosystem services for long-term sustainability. Hardening shorelines can exacerbate erosion elsewhere. Choice A is counterproductive as it eliminates protective features. Options C, D, and E are misguided: extraction does not lower sea levels, lawns are less effective than natives, and mangroves reduce surge, not increase it.
After an ice storm breaks branches, which factor most speeds forest recovery?
Increasing soil compaction with heavy machinery to stabilize roots, which increases infiltration and seedling survival.
Removing all understory plants to reduce competition, ensuring no new seedlings establish until mature trees return.
High genetic and species diversity, providing varied tolerances and regeneration strategies that increase resilience after disturbance.
Eliminating decomposers so fallen wood remains, preventing nutrient release that could favor early successional species.
Explanation
Ice storms break branches, creating openings that diverse species can exploit for recovery. High genetic and species diversity provides varied strategies, speeding regeneration. Resilient ecosystems bounce back faster with multiple tolerances. Decomposers aid nutrient recycling from debris. Factors like soil compaction hinder recovery. Biodiversity acts as insurance against disturbances.
After clear-cutting, a forest regrows; which indicator best shows progression toward late-successional conditions?
Increasing dominance of fast-growing sun-loving annuals and decreasing woody biomass, indicating early succession persists.
Complete absence of decomposers and fungi, indicating nutrient cycling has stopped and a climax community has formed.
Rising structural complexity (multiple canopy layers) and increasing shade-tolerant tree seedlings, indicating later successional stages.
Declining soil organic matter and increasing erosion rates, indicating stable late-successional equilibrium has been reached.
Explanation
Post-clear-cutting, progression to late succession shows increased structural complexity and shade-tolerant species. Canopy layers develop as pioneers give way. Indicators include rising woody biomass and understory changes. Succession rebuilds ecosystem functions over decades. Monitoring helps assess recovery. Disturbances like logging reset but allow regrowth.
A pest outbreak is controlled by introducing a predator; which risk is most associated with this biocontrol strategy?
Biocontrol increases atmospheric methane directly through predator respiration, causing immediate climate change at global scale.
Introduced predators may become invasive and prey on non-target species, disrupting food webs beyond the intended pest control.
Predators cannot survive in new habitats, so biocontrol guarantees no ecological side effects under any conditions.
Biocontrol always reduces biodiversity to zero because predators eliminate all organisms in the ecosystem indiscriminately.
Explanation
Biocontrol involves introducing natural predators to manage pest populations, but it carries risks if the predator becomes invasive. Introduced species may prey on non-target native species, disrupting food webs and potentially causing unintended declines in biodiversity. This can lead to cascading effects throughout the ecosystem, altering community structures. Careful assessment is needed to ensure the predator targets only the pest and does not thrive excessively in the new environment. Choice B exaggerates by claiming biodiversity drops to zero, which is unrealistic. Options C, D, and E are incorrect as biocontrol can have side effects, does not directly cause climate change via methane, and predators do not evolve into decomposers.
A lake experiences turnover after a cold front; which outcome is most likely for dissolved oxygen distribution?
Mixing redistributes oxygen and nutrients through the water column, often increasing oxygen in deeper layers temporarily.
Turnover eliminates oxygen entirely by forcing it into the atmosphere, leaving the lake anoxic for the rest of the year.
Turnover occurs only in tropical lakes, so a cold front cannot influence dissolved oxygen distribution in temperate systems.
Mixing permanently stratifies the lake, locking oxygen at the surface and preventing nutrient movement for decades.
Explanation
Lake turnover, triggered by cold fronts, mixes stratified layers, redistributing oxygen from surface to deeper waters. This can temporarily alleviate hypoxia in bottom layers, benefiting aquatic organisms. Nutrients are also circulated, potentially boosting productivity. Temperate lakes commonly experience this seasonal mixing. Choice B is false as turnover increases oxygen in depths, not eliminates it. Options C, D, and E are incorrect: turnover happens in temperate lakes, it destratifies rather than stratifies, and it does not directly affect salinity.