Unit 5: Ecology and Population Biology

Answer: C — Producers, such as plants and algae, convert sunlight into energy through photosynthesis, providing the base of the food web. This is why option C is correct. Option A is incorrect because primary consumers are herbivores that consume producers. Option B is incorrect because decomposers break down dead organic matter. Option D is incorrect because top predators are at the top of the food chain and consume other animals.

Answer: C — The correct answer is A because the question describes the classic transition from exponential to logistic growth, which occurs when a population encounters environmental resistance in the form of limiting resources. During the exponential phase, resources are abundant and the population grows at a constant rate. When the population size approaches the carrying capacity (K), resources like food and space become scarce, causing the growth rate to slow and stabilize at an equilibrium level around K. This is the defining characteristic of logistic growth modeled by the S-curve. Option B is incorrect because while predation can certainly affect population size, a single predator introduction would not necessarily result in the gradual, stabilizing pattern described. Predation typically causes more erratic fluctuations rather than a smooth transition to equilibrium. Additionally, the question asks what explains the transition mechanism itself, not just a change in population numbers. Option C is incorrect because genetic resistance to parasites would more likely cause continued population growth or increase the growth rate further, not cause it to decline and stabilize. This would move the population away from logistic growth, not toward it. Option D is incorrect because migration would represent a loss of individuals from the population but would not explain the stabilization at a new equilibrium level within the same ecosystem. Migration describes a change in distribution, not the mechanism of carrying capacity limitation.

Answer: C — The correct answer is A. When decomposer activity decreases due to drought, the rate of nutrient cycling slows. Dead organic matter (litter) accumulates because it is not being broken down as quickly, and therefore nitrogen remains locked in this litter layer rather than being released into the soil as bioavailable forms (like nitrate and ammonium). In the short term, this reduces nitrogen availability in the soil, which would limit plant growth since nitrogen is often a limiting nutrient. Over time, nitrogen would accumulate in the litter layer. Option B is incorrect because while some nitrogen can be lost through denitrification, this is not the primary effect of reduced decomposition—most nitrogen would remain in organic form in the litter. Option C is incorrect; reduced decomposition would actually decrease organic acid production, potentially raising pH, but this is not the most significant ecological consequence. Option D is incorrect and contradicts the premise—reduced nitrogen availability would limit plant growth, not increase herbivore populations. This question requires students to understand the interconnections between decomposition, nutrient cycling, and ecosystem limitation rather than simply recalling facts.

Answer: A — The correct answer is A. This question tests understanding of limiting factors and density-dependent vs. density-independent regulation. While food abundance increased, the songbird population did not respond proportionally, indicating that food is not the primary limiting factor for this population. Other limiting factors such as suitable nesting habitat, predation pressure, or disease can constrain population growth independently of food availability. This demonstrates that populations are regulated by multiple factors, not just one resource. Option B is incorrect because if grasshoppers were not nutritious, the spike in grasshoppers would have had no effect whatsoever—yet the question implies there could be some relationship. Option C misunderstands carrying capacity; carrying capacity is determined by limiting factors and can shift based on environmental conditions (the warm, wet season improved conditions). A stable bird population despite improved grasshopper availability indicates the limiting factor is elsewhere, not that the ecosystem has a fixed ceiling. Option D is partially true (songbirds do have longer generation times), but this doesn't explain why the population remained stable rather than beginning to increase; we would expect at least the beginning of a population increase over time. Option A best explains why an increase in one resource (grasshoppers) failed to produce a proportional increase in the dependent population (songbirds).

Answer: A — The correct answer is A) 10%. This reflects the general ecological principle of the 10% rule, where only about 10% of the energy from one trophic level is available to the next trophic level. The remaining 90% is lost through metabolic processes, heat production, movement, and other life functions. This is why energy pyramids narrow as you move up trophic levels and why there are typically fewer organisms at higher trophic levels. Option B (25%) is too high and does not reflect the actual efficiency of energy transfer in ecosystems. Option C (50%) is far too high and would suggest ecosystems retain and transfer more energy than they actually do. Option D (90%) is the inverse of the correct answer and represents the energy that is lost, not transferred. Understanding this energy loss principle is fundamental to comprehending why ecosystems can only support a limited number of top predators.