301. A plant population depends on a specialist pollinator. During Year 1, \(100\) reproductive plants produce \(200\) successful recruits. After the pollinator declines sharply, the same number of reproductive plants produces only \(60\) recruits, while adult mortality remains unchanged. Which statement is most defensible?
ⓐ. The plant's carrying capacity must have fallen from \(200\) to \(60\)
ⓑ. Loss of the pollinator reduces recruitment and can slow future population growth
ⓒ. Pollinator loss affects only individual flowers and cannot influence population dynamics
ⓓ. Reduced recruitment proves that emigration increased
Correct Answer: Loss of the pollinator reduces recruitment and can slow future population growth
Explanation: Successful recruitment represents addition of new individuals through reproduction. A large pollinator-associated reduction in recruitment lowers natality-related input into the population. Mutualism links two species through benefits that improve resource acquisition, reproduction, protection or dispersal. A mutualistic relationship can influence population dynamics whenever the exchanged service changes survival or successful recruitment. Reciprocity, rather than equal magnitude of benefit, separates mutualism from one-sided positive interactions. When one partner loses the service supplied by the other, demographic effects may appear first in reproduction or recruitment. Both participants gain in mutualism, although the mechanism and timing of those gains can differ between partners. A nutritional resource, reproductive service or protective effect can all serve as the benefit exchanged in a mutualistic association. If other demographic processes do not compensate, future population growth can decline even though the immediate adult population is unchanged.
302. A prey population follows logistic growth in a habitat with \(K=1000\). Predator removal raises prey density from \(N=400\) to \(N=850\). Which combination of effects is most plausible after the increase?
ⓐ. Carrying capacity automatically rises to match the new prey density
ⓑ. Predation mortality increases, while resource limitation disappears
ⓒ. Both predation and resource limitation necessarily become weaker
ⓓ. Predation falls while density-dependent resource limitation strengthens
Correct Answer: Predation falls while density-dependent resource limitation strengthens
Explanation: Removing the predator reduces a direct source of prey mortality. However, the resulting increase in prey density moves \(N\) much closer to \(K\). In the logistic model, \(rN\) is reduced by \(\frac{K-N}{K}\), which represents the effect of finite carrying capacity. Near \(K\), the multiplier is small, reflecting stronger density-dependent limitation. Carrying capacity \(K\) is the population size that a particular habitat can support sustainably for a given species under prevailing conditions. By consuming prey, predators both obtain trophic energy and exert mortality that can regulate prey populations. The factor \(\frac{K-N}{K}\) modifies the exponential increase according to how close the population is to \(K\). Increasing \(N\) relative to \(K\) reduces the fraction of carrying capacity still available for further growth. The logistic factor \(\frac{K-N}{K}\) becomes smaller, indicating stronger density-dependent limitation through finite resources.
303. A newly colonised habitat receives \(40\) immigrants of a plant species. During the first reproductive season, \(16\) seedlings are added, \(6\) individuals die and \(5\) emigrate. What is the population size at the end of the season if the \(40\) immigrants constituted the entire initial population?
ⓐ. \(35\)
ⓑ. \(55\)
ⓒ. \(50\)
ⓓ. \(45\)
Correct Answer: \(45\)
Explanation: The update relation groups reproductive and movement gains against mortality and movement losses. The population update is controlled by the difference between incoming and outgoing demographic components. Population change depends on the balance between \(B+I\) and \(D+E\), not on any one demographic process in isolation. Net population change is the difference between total additions and total losses. The four demographic processes enter with different signs: births and immigration are positive inputs, deaths and emigration are negative outputs. Whether density rises or falls depends on the sign of \((B+I)-(D+E)\). The population-balance equation treats births and immigration as additions, and deaths and emigration as losses. Starting from \(N_t=40\), \[ N_{t+1}=40+[(16+0)-(6+5)] \] \[ =40+(16-11)=45 \] The example separates the immigration event that established the starting population from subsequent demographic changes occurring during the season.
304. A community contains a plant population P, herbivore Q and predator R. P supports \(K=1200\) individuals under current conditions. Q consumes P, and R consumes Q. A disease reduces R sharply, Q subsequently increases, and P declines. Which explanation best integrates the observed changes?
ⓐ. Q must decline whenever its predator declines
ⓑ. Loss of R directly increases P mortality because R is a plant parasite
ⓒ. Less predation lets Q rise, increasing herbivory and lowering P
ⓓ. P declines because its carrying capacity necessarily becomes zero when a predator is lost
Correct Answer: Less predation lets Q rise, increasing herbivory and lowering P
Explanation: The decline of predator R releases herbivore Q from part of its mortality pressure. Increased Q abundance can then increase feeding pressure on plant P, causing P density to fall. The impact of predation can extend beyond two populations through competition among prey or resource users. Predators form a route of energy transfer to higher trophic levels and can simultaneously limit prey density. Consumption of prey links trophic-energy flow with population regulation. Predator effects can spread beyond the prey population when changes in prey abundance alter competition with other species. This is an indirect population-level consequence linking predation, herbivory and demographic change across interacting populations.
305. Reference examples list intrinsic rates of natural increase as approximately \(0.015\) for the Norway rat, \(0.12\) for the flour beetle and \(0.0205\) for the Indian human population in \(1981\). Which ordering correctly compares the cited values without treating them as present-day demographic measurements?
ⓐ. Flour beetle \(>\) Indian human population in \(1981\) \(>\) Norway rat
ⓑ. Norway rat \(>\) flour beetle \(>\) Indian human population in \(1981\)
ⓒ. Indian human population in \(1981\) \(>\) Norway rat \(>\) flour beetle
ⓓ. Flour beetle \(>\) Norway rat \(>\) Indian human population in \(1981\)
Correct Answer: Flour beetle \(>\) Indian human population in \(1981\) \(>\) Norway rat
Explanation: The cited historical values are \(0.12\), \(0.0205\) and \(0.015\), respectively. The flour beetle has the largest cited \(r\), followed by the Indian human population in \(1981\), with the Norway rat lowest. A J-shaped trajectory is expected only while density-dependent restrictions remain negligible. Exponential growth is multiplicative: more current individuals produce a larger absolute increment at the same intrinsic rate. The exponential model assumes that the population can express its intrinsic growth potential without a carrying-capacity term. Under exponential conditions, the instantaneous absolute change is proportional to current population size through \(\frac{dN}{dt}=rN\). When resources are effectively unlimited, per-capita growth can remain constant even as the absolute increment becomes larger. These values illustrate differences in intrinsic population-growth potential in the stated historical contexts and should not be presented as current demographic measurements.
306. Two populations have equal current density \(N\), but Population P has \(r=0.12\) while Population Q has \(r=0.015\). Under identical exponential conditions, which relationship follows directly from \(\frac{dN}{dt}=rN\)?
ⓐ. Q has an instantaneous absolute increase eight times that of P
ⓑ. Both have equal instantaneous increase because \(N\) is equal
ⓒ. P has an instantaneous absolute increase eight times that of Q
ⓓ. Their instantaneous increase cannot be compared without \(K\)
Correct Answer: P has an instantaneous absolute increase eight times that of Q
Explanation: At the same current \(N\), the ratio of instantaneous exponential growth rates is the ratio of their \(r\) values: \[ \frac{0.12N}{0.015N}=8 \] P increases eight times as rapidly in absolute terms at that instant. Resource limitation must be weak for the exponential relation to remain an appropriate description of population increase. Because \(\frac{dN}{dt}=rN\), the same \(r\) produces a larger absolute increase in a larger population. Exponential increase is an idealised growth pattern that omits the slowing effect of finite carrying capacity. Carrying capacity is not part of the exponential relation.
307. A graph plots population density \(N\) against time \(t\). During Segment P, the curve rises slowly; during Segment Q, it becomes increasingly steep; during Segment R, the increase begins slowing; during Segment S, it approaches a horizontal level \(K\). Which sequence best describes logistic population growth?
ⓐ. Acceleration → lag → asymptote → deceleration
ⓑ. Lag → deceleration → acceleration → extinction
ⓒ. Lag → acceleration → deceleration → asymptote
ⓓ. Exponential decline → acceleration → lag → asymptote
Correct Answer: Lag → acceleration → deceleration → asymptote
Explanation: Logistic growth begins with a lag phase, followed by acceleration as population increase becomes more rapid. As density rises and resources become increasingly limiting, growth decelerates. A steeper slope represents faster absolute change, while flattening indicates slowing net increase. Density dependence enters the logistic equation through \(\frac{K-N}{K}\), which limits the unrestricted \(rN\) term. When \(N\) is close to \(K\), the logistic factor approaches zero and the net growth rate becomes small. The sigmoid curve reflects slow establishment, rapid increase, progressive slowing and eventual approach to carrying capacity. The value of \(K\) depends on both the species and habitat conditions, so environmental change can raise or lower it. Logistic growth retains the intrinsic potential \(rN\) but multiplies it by a factor determined by the distance from carrying capacity. Finally, the population approaches the carrying capacity \(K\), producing an asymptotic plateau.
308. A population is observed at four successive stages of logistic growth:
I. \(N\) is low and the population has just become established.
II. Population increase becomes rapid.
III. Resource limitation increasingly slows the rate of increase.
IV. Population density fluctuates close to \(K\).
Which ordering correctly links the biological sequence?
ⓐ. IV → III → II → I
ⓑ. II → I → IV → III
ⓒ. III → II → I → IV
ⓓ. I → II → III → IV
Correct Answer: I → II → III → IV
Explanation: A newly established low-density population first passes through lag, then acceleration. As the population becomes denser, finite resources impose increasing limitation, producing deceleration. A habitat's carrying capacity is not universal; it reflects the resources and conditions available to a particular population. The logistic relation differs from pure exponential growth through the multiplier \(\frac{K-N}{K}\). As \(N\) approaches \(K\), the factor \(\frac{K-N}{K}\) becomes smaller and realised growth slows. Resource limitation is represented mathematically by multiplying \(rN\) by \(\frac{K-N}{K}\). The density-dependent multiplier declines toward zero as population density nears carrying capacity. Environmental improvement or deterioration can alter \(K\) by changing the number of individuals the habitat can sustain. The density-dependent multiplier \(\frac{K-N}{K}\) converts the exponential term into a logistic growth rate. Eventually the population approaches the habitat's carrying capacity \(K\).
309. A population follows a J-shaped trajectory for several generations. Food and space then become strongly limiting, but \(r\) remains positive when measured under favourable conditions. Which statement best explains why the original J-shaped trajectory should not be expected to continue indefinitely?
ⓐ. Positive \(r\) guarantees unlimited growth regardless of resources
ⓑ. Resource limitation changes the realised pattern away from exponential growth
ⓒ. Once a population grows exponentially, carrying capacity can no longer affect it
ⓓ. Resource limitation causes \(r\) to become population density
Correct Answer: Resource limitation changes the realised pattern away from exponential growth
Explanation: Exponential growth expresses the population's innate growth potential when resources are effectively non-limiting. A positive \(r\) does not remove environmental constraints. The model is most applicable during intervals when competition for essential resources has not yet imposed strong density dependence. Exponential growth links absolute increase directly to the product of intrinsic rate and current population size. Unlimited-resource conditions allow the intrinsic rate of natural increase to operate without the logistic reduction factor. The population adds individuals at a rate \(rN\), so absolute change depends on both per-capita growth potential and \(N\). Once food or space becomes limiting, density-dependent effects increasingly reduce realised population growth, making logistic behaviour more realistic.
310. Under exponential conditions, a population begins at \(N_0=150\) and has \(r=0.20\ \text{year}^{-1}\). Which expression gives its expected density after \(5\ \text{years}\)?
ⓐ. \(150e^{0.04\times5}\)
ⓑ. \(150e^{5.20\times1}\)
ⓒ. \(150e^{0.20\times5}\)
ⓓ. \(150e^{0.20+5}\)
Correct Answer: \(150e^{0.20\times5}\)
Explanation: The relation \(N_t=N_0e^{rt}\) describes multiplicative growth, so both \(N_0\) and the product \(rt\) determine the later population size. A persistent J-shaped curve requires that limiting resources do not become important over the interval considered. Current density matters in exponential growth because the instantaneous increase scales with \(N\). Under exponential conditions, growth is not reduced merely because population density has become larger. The relation \(rN\) separates per-capita growth potential from the absolute number of individuals added per unit time. In \(N_t=N_0e^{rt}\), the starting density sets the scale while \(rt\) sets the exponential growth multiplier. Finite resources eventually make pure exponential growth unrealistic for most natural populations over long periods. A positive \(r\) is a per-capita property, while the realised absolute change at any instant is \(rN\). The exponent contains the product of intrinsic rate and time, not their sum or a simple arithmetic multiplier.
311. Two populations both double under exponential growth. Population P doubles in \(2\ \text{years}\), while Population Q doubles in \(8\ \text{years}\). Which comparison of intrinsic rates is correct?
ⓐ. \(r_P=4r_Q\)
ⓑ. \(r_Q=4r_P\)
ⓒ. \(r_P=r_Q\)
ⓓ. \(r_P=2r_Q\)
Correct Answer: \(r_P=4r_Q\)
Explanation: Since \[ r=\frac{\ln2}{t_d} \] Intrinsic rate is inversely proportional to doubling time. The exponential framework describes potential growth before carrying-capacity effects substantially reduce realised increase. Exponential growth is multiplicative: more current individuals produce a larger absolute increment at the same intrinsic rate. Effective resource abundance is the key biological condition that distinguishes exponential growth from logistic slowing. Under exponential conditions, the instantaneous absolute change is proportional to current population size through \(\frac{dN}{dt}=rN\). In the absence of strong density limitation, population increase can accelerate as the number of reproducing individuals rises. Because \(\frac{dN}{dt}=rN\), the same \(r\) produces a larger absolute increase in a larger population. Exponential growth treats environmental resistance from finite resources as negligible during the modelled period. Exponential growth links absolute increase directly to the product of intrinsic rate and current population size. Population P has four times the intrinsic exponential growth rate of Q.
312. A habitat can sustainably support \(K=1200\) individuals of a species. Current population density is \(N=1200\), but births and deaths are still occurring. Which statement best interprets the population state?
ⓐ. No individual is born or dies at carrying capacity
ⓑ. Logistic growth changes into unrestricted exponential growth at \(K\)
ⓒ. Carrying capacity means the population has become extinct
ⓓ. Net growth is near zero although births and deaths continue
Correct Answer: Net growth is near zero although births and deaths continue
Explanation: Approaching \(K\) means that additions and losses balance closely enough that there is little net increase in population density. It does not require every birth and death to stop. Population size changes through two incoming components, births and immigration, and two outgoing components, deaths and emigration. Comparing \(B+I\) with \(D+E\) determines both the direction and magnitude of the immediate population change. In the demographic balance, \(B+I\) contributes individuals while \(D+E\) removes them. A positive additions-minus-losses balance increases population size, whereas a negative balance reduces it. The immediate change in population density is found by comparing total additions \(B+I\) with total losses \(D+E\). A population can show substantial demographic turnover while remaining approximately stable in size.
313. Consider two species with contrasting reproductive patterns:
I. Species P produces a very large number of small offspring with relatively little investment in each.
II. Species Q produces relatively few large offspring with greater investment in each.
Which conclusion is most consistent with life-history evolution?
ⓐ. Q is universally more evolutionarily successful than P
ⓑ. P is universally more successful because offspring number alone determines fitness
ⓒ. Either strategy may be favoured if it raises fitness under local conditions
ⓓ. The two strategies cannot occur in natural populations
Correct Answer: Either strategy may be favoured if it raises fitness under local conditions
Explanation: Many-small and few-large offspring patterns are alternative life-history strategies rather than a universal hierarchy. The favoured life-history pattern changes when environmental conditions change the fitness payoffs of survival and reproduction. Life-history traits are favoured according to their effect on lifetime reproductive fitness, not by maximising any single trait in isolation. Natural selection acts on the fitness consequences of reproductive trade-offs, so no one breeding strategy is optimal in every habitat. Offspring number, offspring size and breeding frequency represent trade-offs whose value depends on survival and environmental conditions. Natural selection favours strategies that maximise reproductive fitness under the relevant biotic and abiotic conditions.
314. A long-lived species normally breeds repeatedly. Environmental change sharply lowers adult survival after the first reproductive episode while leaving first-season reproductive success unchanged. Which evolutionary shift could become favourable?
ⓐ. Greater emphasis on early reproduction
ⓑ. Delayed reproduction indefinitely
ⓒ. Elimination of reproduction because adult survival is lower
ⓓ. Production of identical offspring numbers in every habitat
Correct Answer: Greater emphasis on early reproduction
Explanation: Life-history evolution depends on the reproductive opportunities permitted by the environment. When survival to later breeding events becomes less likely, selection can favour greater reproductive investment earlier in life. A reproductive strategy is advantageous only if it increases reproductive fitness under the population's biotic and abiotic constraints. Selection can favour different balances between current reproduction, offspring provisioning and future breeding opportunities in different habitats. Breeding once, breeding repeatedly, producing many small young or fewer large young are alternative strategies rather than a universal ranking. The principle is habitat-dependent reproductive fitness rather than a universally superior strategy.
315. In a food chain, a predator consumes herbivorous prey that obtained energy by feeding on plants. Which ecological role of predation is demonstrated even if prey population density does not change greatly?
ⓐ. Elimination of all competition
ⓑ. Transfer of energy from prey to a higher trophic level
ⓒ. Reduction of herbivore fitness without transfer of energy to the predator
ⓓ. Increase of plant carrying capacity by definition
Correct Answer: Transfer of energy from prey to a higher trophic level
Explanation: Predation is not important only because it can regulate prey abundance. By consuming prey, predators also transfer biological energy and matter from one trophic level to another. The ecological role of predation includes energy transfer as well as direct control of prey abundance. Predator feeding moves energy through the food web and can prevent unchecked increase of prey populations. A predator-prey interaction affects both trophic flow and prey demography through predator-imposed mortality. Predation is not only feeding; its mortality effect can alter prey abundance while energy passes to the predator. This trophic function remains valid even when the immediate effect on prey density is modest.
316. An introduced prey species increases rapidly in a new region where its usual predators are absent. Which changed condition would most directly test whether predator release contributed to its expansion?
ⓐ. Increase food supply equally in all populations
ⓑ. Introduce a predator while keeping resources comparable
ⓒ. Measure only the body size of the introduced prey
ⓓ. Remove all vegetation from both treatment and control areas
Correct Answer: Introduce a predator while keeping resources comparable
Explanation: If absence of predators contributed to the prey's spread, restoring predation while keeping other major conditions comparable should reduce prey population performance relative to predator-free controls. Predators gain energy by consuming prey and can simultaneously alter prey mortality and abundance. Predation links trophic transfer with demographic control because prey consumption removes individuals from the prey population. Changes in predator pressure can alter prey density even when other demographic processes remain unchanged. Predator presence can limit prey increase, whereas predator removal may release prey from one source of mortality. This experiment directly tests predator release as a causal mechanism.
317. A pest population damages a crop. A predator that specifically consumes the pest is introduced, and pest density declines without direct application of a pesticide. Which ecological principle is being used?
ⓐ. Biological control through predator introduction
ⓑ. Competitive exclusion through shared-resource use
ⓒ. Mutualistic benefit through reciprocal exchange
ⓓ. Resource partitioning through divergent resource use
Correct Answer: Biological control through predator introduction
Explanation: Biological control can exploit natural predator-prey relationships to reduce pest populations. The predator benefits by consuming the pest, while the prey population experiences increased mortality. The consumer benefits from prey biomass, while the prey population experiences a direct loss that can influence its growth trajectory. Changes in food, predation, weather, competition or human intervention can alter population density over time. Predator effects are demographic as well as trophic: feeding transfers energy and changes prey survival. Population density provides a practical indicator for testing how an ecological factor affects a population under stated conditions. The method uses an ecological interaction rather than a chemical pesticide as the main control mechanism.
318. Nearly \(25\%\) of insect species are phytophagous. What is the main ecological significance of this observation?
ⓐ. All phytophagous insects feed on a single plant species
ⓑ. Nearly \(25\%\) of insects are parasites of animals
ⓒ. Plant herbivory is too rare to influence plant evolution
ⓓ. Many insect species depend on plant tissues as food
Correct Answer: Many insect species depend on plant tissues as food
Explanation: The approximate \(25\%\) figure emphasises how important plant material is as a food resource for insects. Phytophagous animals obtain energy from living plant material, creating selective pressure for structural and chemical plant defences. Plant-eating insects can feed on sap, leaves or other tissues, linking herbivory directly to plant population performance. The ecological importance of phytophagy lies in the large number of consumer species that depend on plant tissues as food. Herbivory can lower plant growth or reproduction even when the entire plant is not killed. Because plants cannot escape herbivores by movement, strong selection can favour structural and chemical anti-herbivore defences.
319. Monarch butterflies can be distasteful to predators because toxic compounds are acquired during the caterpillar stage from the food plant. If caterpillars are experimentally reared on a diet lacking those defensive compounds, which prediction is most directly supported?
ⓐ. Adults must become camouflaged instead
ⓑ. Adults may show reduced chemical distastefulness
ⓒ. Caterpillars will necessarily develop thorns
ⓓ. Predator avoidance will increase because the toxins are absent
Correct Answer: Adults may show reduced chemical distastefulness
Explanation: The chemical defence in this example depends on compounds acquired from the larval food plant. Organisms can lower the success of predators or herbivores by avoiding detection, resisting attack or making consumption costly. Cryptic coloration lowers visibility to predators, whereas unpleasant or poisonous chemistry lowers acceptance after contact. By removing prey individuals, predators can modify both prey density and the competitive conditions experienced by surviving species. The strength of predation can shape prey abundance, and changes in prey abundance may then propagate through the community. Defence against exploitation can operate before contact through concealment or after contact through structural and chemical deterrents. Detection is the main target of camouflage; ingestion or repeated attack is the main target of distasteful or toxic defence. If caterpillars cannot obtain those compounds, the adult butterfly should have less of the chemically acquired defence, potentially making it more acceptable to predators.
320. Plant population P relies mainly on thorns, whereas population Q relies mainly on a defensive chemical that reduces herbivore digestion. A herbivore lineage evolves physiological tolerance to that chemical but remains equally deterred by thorns. If exposure is otherwise similar, which change is most likely?
ⓐ. Herbivore damage should increase more in Q than in P
ⓑ. Herbivore damage should increase equally in P and Q because all plant defences work identically
ⓒ. P should lose its thorn defence automatically when Q becomes more damaged
ⓓ. Chemical tolerance should make the herbivore unable to feed on Q
Correct Answer: Herbivore damage should increase more in Q than in P
Explanation: The two plant populations rely on different defence mechanisms. Thorns are structural barriers that make feeding or browsing more difficult, whereas the chemical defence acts through the herbivore's physiology after ingestion. The changed condition specifically removes much of the effectiveness of Q's chemical defence because the herbivore has evolved tolerance to it. The herbivore's response to thorns is unchanged, so P retains its principal protective mechanism. Q is therefore expected to experience the larger increase in feeding damage. The prediction depends on matching the herbivore adaptation to the defence it actually neutralises; tolerance to a chemical does not automatically overcome a structural barrier. This illustrates how the effectiveness of anti-herbivore defences can depend on the traits of the consumer population rather than on the mere presence of a defensive structure or compound.
