🔬 Biology · Class 12 · NEET
Organisms and Populations - Practice Questions with Answers 75 free MCQs on Organisms and Populations, each with its own worked answer and explanation. How organisms interact with abiotic factors and respond to stress, plus population attributes, growth models, and interspecific interactions such as predation, competition, and mutualism.
Take the timed Organisms and Populations chapterwise test → 75 practice questions on Organisms and Populations , sorted Easy → Hard. Try each one first, then open its answer page for the worked explanation. Want the full theory first? Read the Organisms and Populations notes .
Exponential (J-curve) vs Logistic (S-curve) Growth time Population (N) Exponential (unlimited resources) K (carrying capacity) Logistic (limited resources) Logistic growth slows as N approaches K; exponential growth never slows down Exponential growth produces an ever-steepening J-curve with no limit, an unrealistic long-term model; logistic growth produces an S-curve that levels off at the carrying capacity K, as growth rate (K−N)/K shrinks to zero once resources are fully used.
Easy - 25 questions Q1.
The population density equation N<sub>t+1</sub> = N<sub>t</sub> + [(B + I) - (D + E)] shows that density increases when which terms dominate?
A Natality and immigrationB Mortality and emigrationC Death and immigrationD Emigration and natalityShow answer & explanation →
Q2.
Which chemical defence is produced by plants such as tobacco and Cinchona to deter herbivores?
A Chlorophyll and starchB Nicotine and quinineC Cellulose and ligninD Glucose and sucroseShow answer & explanation →
Q3.
The intrinsic rate of natural increase of a population is represented by which symbol?
A Symbol KB Symbol NC Symbol rD Symbol eShow answer & explanation →
Q4.
The majority of animals and nearly all plants cannot maintain a constant internal environment and change with the surroundings; such organisms are called:
A RegulatorsB MigratorsC Partial regulatorsD ConformersShow answer & explanation →
Q5.
In the exponential growth curve dN/dt = rN, when plotted against time the curve is shaped like which letter?
A The letter JB The letter SC The letter UD The letter VShow answer & explanation →
Q6.
Ecology is best defined as the study of:
A The classification and naming of plant speciesB Organisms in relation to their environmentC The diagnosis and treatment of human diseasesD The internal structure and function of cellsShow answer & explanation →
Q7.
Which abiotic factor is considered ecologically the most important, affecting enzyme kinetics and metabolic rate?
A LightB Soil pHC TemperatureD Wind speedShow answer & explanation →
Q8.
Organisms that can tolerate and thrive across a wide range of temperatures are called:
A StenothermalB EurythermalC EurybioticD HomeothermalShow answer & explanation →
Q9.
Mammals that maintain a constant body temperature regardless of the surrounding temperature are called:
A Conformers, whose body temperature varies with surroundingsB Poikilotherms, animals with variable body temperatureC Homeotherms (regulators)D Ectotherms, relying on external heat sourcesShow answer & explanation →
Q10.
Dormancy during winter to escape cold conditions, as seen in bears, is called:
A AestivationB HibernationC DiapauseD MigrationShow answer & explanation →
Q11.
A stage of suspended development commonly seen in zooplankton and insects is called:
A HibernationB AestivationC DiapauseD MigrationShow answer & explanation →
Q12.
The number of individuals of a species per unit area at a given time is called:
A Population densityB Carrying capacityC NatalityD Biotic potentialShow answer & explanation →
Q13.
A graphical representation of the proportion of individuals of different ages in a population is called:
A Growth curveB Age pyramidC Survivorship curveD Carrying capacity curveShow answer & explanation →
Q14.
Exponential population growth occurs when resources are:
A Limited and scarceB UnlimitedC Decreasing rapidlyD Used only by competitorsShow answer & explanation →
Q15.
The shape of the logistic population growth curve is:
A J-shapedB S-shaped (sigmoid)C Straight lineD U-shapedShow answer & explanation →
Q16.
In the logistic growth equation, the symbol K represents:
A The intrinsic rate of natural increaseB The carrying capacityC The death rateD The initial population sizeShow answer & explanation →
Q17.
An interaction in which both participating species benefit is called:
A ParasitismB CommensalismC MutualismD AmensalismShow answer & explanation →
Q18.
An interaction in which one species benefits and the other is unaffected is called:
A MutualismB CommensalismC PredationD ParasitismShow answer & explanation →
Q19.
An organism that depends on a host for nutrition, often harming it, exhibits which interaction?
A CommensalismB MutualismC ParasitismD AmensalismShow answer & explanation →
Q20.
Cuckoos (koels) laying eggs in a crow's nest is an example of:
A MutualismB CommensalismC Brood parasitismD AmensalismShow answer & explanation →
Q21.
Mycorrhizae represent a mutualistic association between fungi and:
A Insect larvaeB Bacteria in soilC Plant rootsD Bird nestsShow answer & explanation →
Q22.
Which is regarded as the most important abiotic factor?
A TemperatureB the soil texture onlyC only wind velocityD a bit of humidityShow answer & explanation →
Q23.
Animals that keep a constant internal temperature are called?
A RegulatorsB the conformer typeC only migratorsD a set of hoardersShow answer & explanation →
Q24.
A J-shaped curve represents which kind of growth?
A Exponential growthB a logistic slow riseC only linear patternD the flat steady lineShow answer & explanation →
Q25.
Number of individuals per unit area is called?
A Population densityB a natality rate measureC only the sex ratioD the age pyramidShow answer & explanation →
Medium - 25 questions Q26.
The fig tree and its specific pollinator wasp show such tight mutual dependence that it best illustrates which phenomenon?
A Brood parasitism in plantsB Co-evolution of partnersC Competitive exclusionD Chemical mimicryShow answer & explanation →
Q27.
The Mediterranean orchid Ophrys uses petals resembling a female bee to achieve pollination. This strategy is best described as:
A Amensalism with the beeB Mutual nutrition exchangeC Sexual deceit by the orchidD Parasitism of the beeShow answer & explanation →
Q28.
At high altitudes people initially feel altitude sickness because the air has low oxygen; the body acclimatises mainly by:
A Lowering the breathing rateB Decreasing the heart rateC Reducing haemoglobin bindingD Producing more red blood cellsShow answer & explanation →
Q29.
A population growing under the logistic model grows fastest (maximum rate of increase) when its size N is approximately:
A Half of KB Equal to KC Near zeroD Greater than KShow answer & explanation →
Q30.
The Monarch butterfly is distasteful and toxic to predatory birds because it:
A Has very bright warning wing coloursB Gains a chemical from its larval food plantC Flies faster than predatorsD Produces a loud warning soundShow answer & explanation →
Q31.
Migratory birds such as Siberian cranes travelling to Indian wetlands in winter best illustrate which response to environmental stress?
A RegulateB ConformC MigrateD SuspendShow answer & explanation →
Q32.
Most invertebrates, fish, amphibians, and reptiles, whose body temperature varies with the environment, are best described as:
A Regulators (homeotherms), maintaining constant temperatureB Conformers (poikilotherms/ectotherms)C Migrants that move to avoid unfavorable conditionsD Mainly aestivators during the driest seasonsShow answer & explanation →
Q33.
Why do most organisms behave as conformers rather than regulators with respect to temperature?
A Active regulation is energetically inexpensive and readily available to nearly all organisms according to standard textbooksB Thermoregulation is metabolically expensive, so only a few groups like mammals and birds can afford itC Conforming organisms are sometimes thought to lack much measurable metabolic activity in general practiceD All organisms are sometimes thought to be biologically forced to conform with few exceptions as frequently describedShow answer & explanation →
Q34.
In the exponential growth equation dN/dt = rN, the term r represents:
A Total population size at the start of the observation periodB Carrying capacity of the surrounding habitatC The intrinsic rate of natural increaseD Time elapsed since the population began growingShow answer & explanation →
Q35.
As a population approaches its carrying capacity under the logistic growth model, the growth rate:
A Increases continuously without boundB Remains constant throughoutC Gradually decreases and approaches zeroD Becomes negative immediatelyShow answer & explanation →
Q36.
Gause's competitive exclusion principle states that:
A Two species can generally coexist indefinitely as long as they compete with roughly equal intensityB Two closely related species competing for identical limited resources cannot coexist indefinitelyC Interspecific competition generally benefits both competing species roughly equally over timeD Predation sometimes inevitably drives the prey species toward eventual total extinctionShow answer & explanation →
Q37.
Connell's field studies on barnacles (Chthamalus and Balanus) on rocky shores demonstrated that:
A Predation alone, rather than competition, determines barnacle distribution patternsB Competition for space can determine the distribution of competing species in natureC Both barnacle species coexist with equal abundance across every tidal zoneD A mutually beneficial relationship exists between the two barnacle speciesShow answer & explanation →
Q38.
How do predators contribute to maintaining species diversity in a community?
A By eliminating nearly every prey species gradually from the community over timeB By reducing the intensity of competition among prey species, sometimes acting as keystone speciesC By directly increasing the overall carrying capacity of all prey populations presentD Predators are sometimes thought to exert little measurable influence on overall community diversityShow answer & explanation →
Q39.
The use of the cactus moth to control the prickly pear cactus population in Australia is an example of:
A Mutualism benefiting both the cactus moth and the prickly pearB Biological control through predationC Commensalism, where the moth benefits without affecting the cactusD Amensalism, where the cactus is harmed with no effect on the mothShow answer & explanation →
Q40.
A large tree casting shade and suppressing the growth of a small plant beneath it, without itself being affected, illustrates:
A MutualismB CommensalismC AmensalismD ParasitismShow answer & explanation →
Q41.
Adhesive organs or suckers found in tapeworms are adaptations primarily related to:
A Improved locomotion specifically through open water environmentsB Clinging to the host in a parasitic lifestyleC Capturing prey actively using specialized grasping structuresD Photosynthesis occurring within specialized chloroplast-containing cellsShow answer & explanation →
Q42.
Why is soil porosity ecologically important for organisms in a habitat?
A It is sometimes thought to exert little measurable influence on either plant or animal life present in most textbook accountsB It determines water-holding capacity and aeration, which affect the type of vegetation and organisms presentC It mainly influences the superficial, visible colour of the topmost surface soil layer during normal conditionsD It is sometimes thought to directly determine the ambient temperature of the surrounding atmosphere as generally observedShow answer & explanation →
Q43.
Which interaction benefits both interacting species?
A MutualismB the predation typeC only competitionD a case of amensalismShow answer & explanation →
Q44.
In which interaction is one species harmed and other unaffected?
A AmensalismB the mutualism typeC only commensalismD a case of predationShow answer & explanation →
Q45.
A lichen is an example of which relationship?
A MutualismB a parasitic bondC only predationD the competitionShow answer & explanation →
Q46.
What does "r" denote in population growth?
A Intrinsic growth rateB a carrying capacity levelC only the death rateD the immigrationShow answer & explanation →
Q48.
Which growth model gives a sigmoid curve?
A Logistic growthB the exponential typeC only J-shaped modelD a linear increaseShow answer & explanation →
Q49.
A cuckoo laying eggs in a crow nest illustrates?
A Brood parasitismB a mutual symbiosisC only commensalismD the predation typeShow answer & explanation →
Q50.
Which adaptation helps the kangaroo rat survive deserts?
A Concentrated urineB a very dilute urineC only sweating a lotD the panting heavilyShow answer & explanation →
Hard - 25 questions Q51.
Why does a desert lizard bask on rocks in the morning but move into shade or burrows as the day heats up?
A It is a regulator maintaining constant temperature metabolicallyB It is migrating to a new habitat permanentlyC It is a partial regulator using behaviour to manage body temperatureD It is entering diapause to survive heatShow answer & explanation →
Q52.
Tapeworms living in the human gut have lost their digestive system. This is best explained as:
A A defect caused by the hostB A temporary dormancy responseC Evidence of a beneficial mutualism with the hostD A parasitic adaptation as the host gives digested foodShow answer & explanation →
Q53.
Two closely related species coexisting on the same tree avoid competitive exclusion by feeding at different heights and times. This demonstrates:
A Resource partitioning allowing coexistenceB Complete competitive exclusion of speciesC Brood parasitism between themD Amensalism harming one speciesShow answer & explanation →
Q54.
If the per capita birth rate equals the per capita death rate in a population, the value of r is zero, which means the population will:
A Grow exponentiallyB Remain stable in sizeC Decline rapidly to extinctionD Exceed its carrying capacityShow answer & explanation →
Q55.
A mutualism where the partner is rewarded for a service, such as a plant offering nectar to a pollinator, differs from brood parasitism because in mutualism:
A One partner is always harmedB Neither partner is affectedC Both partners derive a benefitD The host unknowingly raises another's youngShow answer & explanation →
Q56.
Why is the logistic growth model generally considered more realistic than the exponential growth model for natural populations?
A It assumes resources remain largely unlimited at every stage of growth as commonly observed in practiceB It accounts for finite, limiting resources that slow growth as the population nears carrying capacityC It predicts that populations will usually continue to grow indefinitely without bound in many documented casesD It applies mainly to controlled laboratory populations and rarely to wild populations according to conventional understandingShow answer & explanation →
Q57.
If a population of size N is well below the carrying capacity K, the logistic growth equation dN/dt = rN(K-N)/K behaves most similarly to which model?
A It approaches a growth rate of zero almost immediately under these conditionsB It behaves nearly like exponential growth, since (K-N)/K is close to 1C It behaves like negative growth, with the population actively decliningD It is identical to the flattened logistic curve seen exactly at carrying capacity KShow answer & explanation →
Q58.
Which statement best distinguishes hibernation from aestivation?
A Hibernation occurs during the summer months; aestivation occurs during the winter months in routine practiceB Hibernation is a winter dormancy to escape cold; aestivation is a summer dormancy to escape heat and desiccationC Both terms describe the exact same dormancy phenomenon, just observed in different organisms overall in most casesD Hibernation applies mainly to insects, while aestivation applies mainly to mammals under typical conditionsShow answer & explanation →
Q59.
In Connell's barnacle studies, what would be expected if Balanus were experimentally removed from the lower intertidal zone where Chthamalus normally cannot survive due to competition?
A Chthamalus would show little change in its distribution, since competition was rarely truly the limiting factor in most cases under typical conditionsB Chthamalus would likely expand into the lower zone, since competitive exclusion by Balanus, not the physical environment alone, was restricting itC Both barnacle species would disappear largely and permanently from the entire rocky shore habitat according to standard textbooks in general practiceD Balanus would continue to dominate that lower zone even after being removed through deliberate experimental manipulation, as commonly describedShow answer & explanation →
Q60.
A keystone predator is best described as one that:
A Generally has the single largest body size out of any predator present within the ecosystem during normal conditionsB Has a disproportionately large effect on community structure relative to its abundance, often maintaining prey diversityC Usually and invariably reduces overall species diversity within every community where it is found, as generally observedD Affects mainly the size of its own population, with little measurable influence on any other species in typical laboratory settingsShow answer & explanation →
Q61.
Why are eurythermal and euryhaline organisms generally able to occupy a wider geographic range than stenothermal or stenohaline organisms?
A They generally have higher reproductive rates regardless of the surrounding environmental conditions according to standard textbooksB Their broader physiological tolerance to temperature or salinity variation lets them survive in more variable habitatsC They usually outcompete stenothermal organisms even within stable, unvarying environments in general practice as commonly describedD Tolerance range has essentially no real relationship to an organism's geographic distribution in most textbook accountsShow answer & explanation →
Q62.
Distinguishing an age pyramid that is broad at the base and narrow at the top, what does this shape typically indicate about a population?
A A declining population with more old individuals than youngB A stable population with equal numbers across all agesC A growing population with a large proportion of young individualsD A population that has gone extinctShow answer & explanation →
Q63.
Why might percent cover or pug marks be used instead of direct counts to estimate population density?
A Direct counting is usually considered the more accurate method, so these indirect methods are rarely used in practice, as widely reportedB For very large, dispersed, or hard-to-count populations or elusive animals, indirect measures provide more practical density estimatesC Pug marks are used mainly to estimate population age structure, and rarely overall population density in standard practice under most conditions encounteredD Percent cover is a technique that is used mainly for measuring ambient water salinity levels as commonly observed in practice in many documented casesShow answer & explanation →
Q64.
How does diapause in zooplankton functionally differ from simple mortality during unfavourable conditions?
A Diapause is said to result in the permanent death of the organism, making it functionally identical to ordinary mortality in every way according to conventional understandingB Diapause is a reversible, suspended developmental state allowing the organism to resume normal development once favourable conditions return, unlike mortality which is irreversibleC Diapause is sometimes described as a phenomenon that occurs mainly within plant seeds, and rarely within zooplankton, in routine practice under typical conditionsD There is reported to be essentially no meaningful functional difference between diapause and outright mortality, according to standard textbooks in general practiceShow answer & explanation →
Q65.
Which combination correctly matches the interaction to its effect on the two species involved (species A, species B)?
A Mutualism: A benefits, B benefits; Commensalism: A benefits, B unaffected; Amensalism: A harmed, B unaffectedB Mutualism: A benefits, B harmed; Commensalism: A harmed, B benefits; Amensalism: both harmed during normal conditionsC Mutualism: both harmed; Commensalism: both benefit; Amensalism: both unaffected as generally observed in typical laboratory settingsD Mutualism: A unaffected, B harmed; Commensalism: both harmed; Amensalism: both benefit under usual circumstancesShow answer & explanation →
Q66.
Allen’s rule states mammals in cold climates have?
A Shorter extremitiesB the longer body limbsC only bigger ear sizeD a set of long tailsShow answer & explanation →
Q67.
In the logistic equation, growth halts when?
A N equals KB the N exceeds rC only N is zeroD a dN/dt risesShow answer & explanation →
Q68.
A population pyramid with a broad base indicates?
A Expanding populationB a declining group sizeC only stable numbersD the zero growth setShow answer & explanation →
Q69.
Paine’s Pisaster removal experiment demonstrated the role of?
A PredationB the mutualism roleC only competitionD a parasitism linkShow answer & explanation →
Q70.
Gause’s principle describes which concept?
A Competitive exclusionB a resource partition ideaC only niche overlapD the predator preyShow answer & explanation →
Q71.
Which desert plant adaptation limits daytime water loss?
A CAM photosynthesisB the C<sub>3</sub> fixation typeC only night floweringD a deep taproot useShow answer & explanation →
Q72.
Resource partitioning allows competing species to?
A Coexist togetherB the total exclusionC only rapid declineD a full competitionShow answer & explanation →
Q73.
Mycorrhiza between fungi and plant roots exemplifies?
A MutualismB a brood parasitismC only competitionD the amensalismShow answer & explanation →
Q74.
Individuals permanently leaving a population is termed?
A EmigrationB the immigration fluxC only natality riseD a mortality dropShow answer & explanation →
Q75.
An orchid growing on a mango tree branch shows?
A CommensalismB a mutual symbiosisC only parasitismD the predationShow answer & explanation →