Population Ecology
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AP Biology › Population Ecology
A rabbit population in a grassland was monitored monthly. It increased from 50 to 80 to 130 over the first three months. In month 4, a viral disease outbreak occurred; counts then were 70 in month 4 and 60 in month 5. No major changes in predators or food were observed. Which factor most directly explains the population decrease after month 3?
Increased mortality due to disease reduced population size despite prior growth
The population entered exponential growth, which temporarily lowers population size
A constant immigration rate stopped, eliminating the only source of population increase
Carrying capacity increased, causing the population to overshoot and then stabilize lower
New advantageous traits spread, reducing reproduction and shrinking the population
Explanation
This question assesses the skill of analyzing population ecology trends by identifying factors causing population declines after growth periods. The rabbit population grew from 50 to 130 over three months, likely due to favorable conditions, but the Month 4 viral disease outbreak directly increased mortality, leading to decreases to 70 and then 60 despite no changes in predators or food. This density-dependent factor, disease, reduced net population growth by elevating death rates. The timing of the decline aligns precisely with the outbreak, distinguishing it from other potential causes. A tempting distractor is choice B, suggesting carrying capacity increased causing an overshoot and stabilization lower, but this misconstrues the data as there was no overshoot followed by stability, only a direct decline. In such analyses, correlate timing of events with population changes to pinpoint causal factors accurately.
A biologist monitors a yeast population in a closed flask with abundant sugar and stable temperature. Cell counts (millions/mL) are: 2 at 0 h, 4 at 2 h, 8 at 4 h, 16 at 6 h, and 32 at 8 h. Waste products remain low during this interval, and no cells are removed from the flask. Which explanation best accounts for the growth pattern observed from 0 to 8 hours?
Linear growth because the same number of cells is added each interval
Cyclic growth driven by seasonal changes in temperature and resources
Logistic growth caused by density-dependent limits reached early in the trial
Exponential growth with a constant doubling time over successive intervals
Growth decline because carrying capacity is exceeded by hour 4
Explanation
This question tests the skill of analyzing population ecology by identifying growth models from temporal data. The yeast population doubles consistently every two hours from 2 to 32 million cells/mL, reflecting exponential growth with a constant per capita rate under unlimited resources like abundant sugar and low waste. This pattern occurs because each cell divides at a steady rate without density-dependent constraints during the observed period. The closed flask and stable conditions support unchecked multiplication, typical of early exponential phases. A tempting distractor is choice C, linear growth, which assumes constant absolute additions rather than proportional increases, misconceiving the multiplicative nature of biological reproduction. When evaluating growth, plot the data on semi-log scales to check for linearity, which indicates exponential patterns transferable to other microbial studies.
A population of freshwater snails is counted monthly in a small pond with no immigration or emigration. After a nutrient runoff event, algae increases sharply. Snail abundance rises from 120 to 210 over two months, then remains near 205–215 for the next four months while algae levels fall to a steady, moderate level. Predatory fish abundance does not change during the study. Which explanation best accounts for the snails leveling off after the initial increase?
Stabilization near carrying capacity due to density-dependent resource limitation
A plateau caused by long‑term genetic changes reducing reproduction rate
A sustained linear increase because births exceed deaths by a fixed amount
A shift to constant exponential growth as food becomes more abundant over time
A decline caused primarily by increased emigration to neighboring ponds
Explanation
This question tests the skill of analyzing population ecology by explaining stabilization in population trends. The snail population rises sharply from 120 to 210 after nutrient influx boosts algae, then levels off around 205–215 as algae moderates, suggesting density-dependent resource limitation has reached carrying capacity. This plateau occurs because increased density heightens competition, reducing per capita reproduction or survival to balance births and deaths. No changes in predators or migration support that internal factors, not external, drive the stabilization. A tempting distractor is choice A, shift to exponential growth, which assumes ongoing unlimited resources, but this misconceives the density-dependent feedback from declining algae. For similar scenarios, monitor resource levels alongside population counts to identify when carrying capacity is approached, a strategy applicable to various ecosystems.
A mosquito population in a wetland was estimated weekly. For weeks 1–5, the population rose from 2,000 to 2,500 to 3,100 to 3,900 to 4,800. In week 6, a cold snap occurred, and the population dropped to 1,200 in week 7. By week 9, it returned to 4,600 even though food availability and standing water were similar to earlier weeks. Which explanation best accounts for the sharp decline between weeks 5 and 7?
Density-dependent competition increased gradually, causing a slow approach to carrying capacity by week 7.
Emigration increased because mosquitoes left to find mates, lowering the population temporarily.
A density-independent abiotic event increased mortality, producing a rapid population decrease.
Individuals developed cold resistance, which immediately reduced the population size after week 5.
Birth rates rose above death rates, causing the population to fall abruptly after week 5.
Explanation
This question tests population ecology analysis of sudden population changes and their causes. The mosquito population shows steady growth until a cold snap causes an abrupt 75% decline (4800→1200), characteristic of density-independent mortality where environmental factors affect populations regardless of their size. The rapid recovery to previous levels confirms this was an acute environmental event rather than a lasting change in carrying capacity. Choice A incorrectly suggests gradual density-dependent competition, but the sharp decline and quick recovery indicate an external factor rather than resource limitation. When populations crash suddenly then recover quickly, look for density-independent factors like weather events rather than competition or predation.
A bird population in a city park was estimated annually. The park’s area and tree cover remained constant. The population was 90 birds in year 1, 95 in year 2, and 92 in year 3. In year 4, a nearby construction project removed nesting sites outside the park, and the park population rose to 140 in year 5 and 155 in year 6. Which explanation best accounts for the increase after year 4?
Density-independent drought reduced resources, leading to higher birth rates after year 4.
Decreased mortality from predators in the park caused the population to double immediately.
Individuals evolved higher fecundity in response to construction, increasing births within one year.
Increased immigration into the park raised population size after outside nesting habitat was reduced.
A sudden rise in carrying capacity occurred because park area increased substantially after year 4.
Explanation
This question requires population ecology analysis of immigration effects on local populations. The bird population remains stable (90-95 birds) until nearby habitat destruction drives immigration into the park, causing a 50% increase (92→140→155). This demonstrates how habitat loss in surrounding areas can concentrate populations in remaining suitable habitats through immigration rather than reproduction. Choice B incorrectly attributes the increase to decreased predation, but such a sudden doubling requires immigration rather than just reduced mortality. When analyzing urban wildlife populations, consider how changes in surrounding habitats affect movement patterns and local densities.
A plant population in a meadow was monitored for 12 years. After a wildfire in year 1, the population increased from 50 to 120 to 210 individuals by year 4. From years 5–12, the population fluctuated narrowly between 190 and 230 individuals, while soil nutrients and available space gradually decreased as vegetation cover increased. Which interpretation best describes the population dynamics from years 1–12?
The population stabilized because individuals evolved to match resource availability, preventing overshoot.
The population fluctuated because immigration and emigration dominated, even though the meadow was closed.
The population approached a carrying capacity after initial growth, then fluctuated around an equilibrium size.
The population showed exponential growth throughout because it never stopped increasing after the wildfire.
The population declined due to density-independent disturbance during years 5–12, causing repeated crashes.
Explanation
This question requires population ecology analysis of post-disturbance succession dynamics. The plant population shows rapid growth after wildfire (50→120→210), then stabilizes and fluctuates around 190-230 as vegetation cover increases and resources become limiting. This pattern represents initial colonization followed by equilibrium around carrying capacity as the meadow matures. Choice A incorrectly identifies this as continuous exponential growth, missing the clear stabilization phase where population fluctuates within a narrow range. Following disturbances, expect rapid initial growth in open habitats followed by stabilization as competition intensifies and resources become limiting.
A bird population on an island was counted annually. For four years, the population stayed near 1,200 birds. In year 5, a hurricane reduced the population to 700 birds. In years 6–8, the population increased to 900, then 1,050, then 1,170 birds. Habitat area and food availability returned to pre-hurricane levels by year 6, and no new predators arrived. Which factor most likely caused the sharp decline in year 5?
Density-dependent competition intensified gradually as the population approached carrying capacity
Long-term emigration steadily reduced the population, producing a single-year crash
Improved resource availability reduced births, lowering the population size suddenly
Genetic changes reduced fitness, causing an immediate population decline in one year
A density-independent disturbance abruptly increased mortality regardless of population density
Explanation
This question tests understanding of population ecology by distinguishing density-dependent from density-independent factors. The hurricane represents a density-independent disturbance that reduced the population from 1,200 to 700 birds regardless of population density - hurricanes affect mortality through physical destruction, not through population-mediated mechanisms. After the disturbance, the population recovered following a logistic pattern back toward the original carrying capacity as habitat recovered. Choice A incorrectly suggests gradual density-dependent competition, but the data shows an abrupt one-year decline rather than gradual change, which is characteristic of catastrophic events. When analyzing population crashes, examine whether the decline is sudden (suggesting density-independent factors) or gradual (suggesting density-dependent factors).
A fish population in a small lake was estimated each spring for 8 years. The population grew from 500 to 900 fish in the first two years, then increased more slowly to 1,050 fish by year 5. From years 5–8, estimates remained near 1,060–1,090 fish despite similar water temperature and no fishing. Surveys showed decreased dissolved oxygen near the bottom and reduced invertebrate prey density as fish density increased. Which growth pattern best describes this population over time?
Declining growth because the population experienced continuous net emigration
Exponential growth because resources increased as the population became larger
Linear growth because the population added the same number of fish each year
Boom-and-bust cycles because the population repeatedly crashed below initial size
Logistic growth because growth rate slowed as density-dependent limits intensified
Explanation
This question requires analyzing population ecology data to identify growth patterns over time. The fish population exhibits classic logistic growth: rapid initial increase (500→900), then slower growth (900→1,050), and finally stabilization (1,060-1,090). The density-dependent factors (reduced oxygen and prey availability) intensified as fish density increased, causing the growth rate to slow and eventually reach equilibrium at carrying capacity. Choice A incorrectly suggests exponential growth, which would show constant percentage increases rather than the observed slowing; exponential growth ignores resource limitations. To identify growth patterns, examine whether the rate of increase changes over time and look for environmental factors that correlate with population density.
A population of annual plants in a field was monitored after a wildfire. In year 1, 200 plants were counted. In year 2, 220 were counted. In year 3, 240 were counted. In year 4, 260 were counted. Rainfall and temperature were similar each year, and no major herbivore outbreaks occurred. Seed production per plant and seedling survival were also similar across years. Which pattern best illustrates the change in population size over time?
Linear growth because the population increased by a similar number each year
Irregular growth because density-independent factors changed unpredictably each year
Logistic growth because the population rapidly approached a stable carrying capacity
Exponential growth because the population size increased by a constant percentage
Negative growth because deaths exceeded births after the wildfire
Explanation
This question tests your ability to analyze population ecology patterns and distinguish between different growth models. The plant population shows a consistent increase of 20 plants per year (200→220→240→260), which represents linear growth where a constant number is added each time period. With similar environmental conditions and reproductive parameters each year, the population maintains steady additive growth rather than multiplicative growth. Choice A incorrectly identifies this as exponential growth, which would require constant percentage increases (like doubling) rather than constant numerical additions; this reflects confusion between additive and multiplicative patterns. When analyzing population data, calculate both absolute change and percentage change between time points to distinguish linear from exponential growth.
A deer population in a fenced reserve was estimated each year: Year 1 = 120, Year 2 = 180, Year 3 = 250, Year 4 = 290, Year 5 = 300, Year 6 = 298. Vegetation surveys showed increasing browse damage after Year 3. Which pattern best illustrates the deer population growth shown by the data?
A boom-and-bust cycle driven by alternating years of drought and heavy rain
A decline caused by increased genetic variation lowering population growth
A linear increase caused by a constant number of births added each year
Exponential growth with a constant per capita growth rate through Year 6
Logistic growth approaching a carrying capacity near 300 individuals
Explanation
This question assesses the skill of analyzing population ecology trends by identifying growth models from sequential population estimates. The deer population increased from 120 to nearly 300 over six years, with the rate of increase slowing as it approached 300, indicating density-dependent limitations from vegetation browse damage after Year 3. This slowdown reflects logistic growth, where factors like food scarcity reduce net growth as the population nears the carrying capacity of about 300 individuals. The stable counts in Years 5 and 6 further support that the population equilibrated at this level. A tempting distractor is choice C, which claims exponential growth with a constant per capita rate, but this overlooks the decelerating growth rate, a common misconception ignoring density dependence. For future problems, plot the data on a graph to visualize if the curve is J-shaped (exponential) or S-shaped (logistic).