38 free MCQs on Organisms and Populations with worked answers and explanations. 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 quiz →Below are 38 practice questions on Organisms and Populations, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Organisms and Populations notes.
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.
Q1.
Ecology is best defined as the study of:
Answer: B. Organisms in relation to their environment
Why: Ecology is the scientific study of organisms and their interactions with each other and with their physical (abiotic) environment.
Q2.
Which abiotic factor is considered ecologically the most important, affecting enzyme kinetics and metabolic rate?
Answer: C. Temperature
Why: Temperature is regarded as the single most important abiotic factor because it directly affects enzyme activity and the metabolic rate of organisms.
Q3.
Organisms that can tolerate and thrive across a wide range of temperatures are called:
Answer: B. Eurythermal
Why: Eurythermal organisms can tolerate and survive in a wide range of temperatures, unlike stenothermal organisms restricted to a narrow range.
Q4.
Mammals that maintain a constant body temperature regardless of the surrounding temperature are called:
Answer: C. Homeotherms (regulators)
Why: Mammals and birds are homeotherms (regulators) that maintain a relatively constant internal body temperature through physiological mechanisms.
Q5.
Dormancy during winter to escape cold conditions, as seen in bears, is called:
Answer: B. Hibernation
Why: Hibernation is a state of winter dormancy that animals like bears enter to escape cold conditions.
Q6.
A stage of suspended development commonly seen in zooplankton and insects is called:
Answer: C. Diapause
Why: Diapause is a stage of suspended development that helps organisms like zooplankton and insects survive unfavourable conditions.
Q7.
The number of individuals of a species per unit area at a given time is called:
Answer: A. Population density
Why: Population density is the number of individuals of a population present per unit area or volume at a given time.
Q8.
A graphical representation of the proportion of individuals of different ages in a population is called:
Answer: B. Age pyramid
Why: An age pyramid (age distribution) is a plot of the proportion of individuals of different age groups, showing the age structure of a population.
Q9.
Exponential population growth occurs when resources are:
Answer: B. Unlimited
Why: Exponential growth occurs under ideal conditions of unlimited food and space, producing a J-shaped growth curve.
Q10.
The shape of the logistic population growth curve is:
Answer: B. S-shaped (sigmoid)
Why: The logistic growth model produces an S-shaped (sigmoid) curve as the population growth rate slows near the carrying capacity.
Q11.
In the logistic growth equation, the symbol K represents:
Answer: B. The carrying capacity
Why: In the logistic growth model, K represents the carrying capacity, the maximum population size the environment can sustain.
Q12.
An interaction in which both participating species benefit is called:
Answer: C. Mutualism
Why: Mutualism is an interspecific interaction where both participating species derive benefit, such as lichens (fungus and algae).
Q13.
An interaction in which one species benefits and the other is unaffected is called:
Answer: B. Commensalism
Why: Commensalism is an interaction in which one species benefits while the other species is neither harmed nor helped.
Q14.
An organism that depends on a host for nutrition, often harming it, exhibits which interaction?
Answer: C. Parasitism
Why: Parasitism is an interaction where a parasite derives nutrition from its host, generally causing the host harm.
Q15.
Cuckoos (koels) laying eggs in a crow's nest is an example of:
Answer: C. Brood parasitism
Why: Brood parasitism is a special form of parasitism in birds where the parasitic species lays eggs in the host's nest, letting the host incubate and raise its young.
Q16.
Mycorrhizae represent a mutualistic association between fungi and:
Answer: C. Plant roots
Why: Mycorrhizae are mutualistic associations between fungi and plant roots, helping the plant absorb nutrients more efficiently.
Q17.
Migratory birds such as Siberian cranes travelling to Indian wetlands in winter best illustrate which response to environmental stress?
Answer: C. Migrate
Why: Migration allows organisms to temporarily move away from a stressful habitat to a more favourable one and return once conditions improve.
Q18.
Most invertebrates, fish, amphibians, and reptiles, whose body temperature varies with the environment, are best described as:
Answer: B. Conformers (poikilotherms/ectotherms)
Why: Conformers (poikilotherms or ectotherms) cannot regulate their internal temperature, which changes along with ambient environmental temperature.
Q19.
Why do most organisms behave as conformers rather than regulators with respect to temperature?
Answer: B. Thermoregulation is metabolically expensive, so only a few groups like mammals and birds can afford it
Why: Maintaining a constant internal environment (thermoregulation) is energetically costly; most organisms (about 99%) instead conform to ambient conditions rather than regulate.
Q20.
In the exponential growth equation dN/dt = rN, the term r represents:
Answer: C. The intrinsic rate of natural increase
Why: In the exponential growth model, r is the intrinsic rate of natural increase, reflecting the difference between birth and death rates under ideal conditions.
Q21.
As a population approaches its carrying capacity under the logistic growth model, the growth rate:
Answer: C. Gradually decreases and approaches zero
Why: As population size N approaches carrying capacity K, the term (K - N)/K approaches zero, so the growth rate slows and eventually approaches zero.
Q22.
Gause's competitive exclusion principle states that:
Answer: B. Two closely related species competing for identical limited resources cannot coexist indefinitely
Why: Gause's principle states that when two species compete for exactly the same limited resources, one will eventually outcompete and exclude the other from that habitat.
Q23.
Connell's field studies on barnacles (Chthamalus and Balanus) on rocky shores demonstrated that:
Answer: B. Competition for space can determine the distribution of competing species in nature
Why: Connell's experiments showed that Balanus outcompeted Chthamalus for space in the lower intertidal zones, providing field evidence that competition shapes species distribution.
Q24.
How do predators contribute to maintaining species diversity in a community?
Answer: B. By reducing the intensity of competition among prey species, sometimes acting as keystone species
Why: Predators can keep certain prey populations in check, reducing competitive pressure among prey species and thereby maintaining higher overall species diversity; some predators act as keystone species.
Q25.
The use of the cactus moth to control the prickly pear cactus population in Australia is an example of:
Answer: B. Biological control through predation
Why: The cactus moth was deliberately introduced as a predator/herbivore to control the invasive prickly pear cactus, an example of biological control.
Q26.
A large tree casting shade and suppressing the growth of a small plant beneath it, without itself being affected, illustrates:
Answer: C. Amensalism
Why: Amensalism is an interaction where one species (the small plant) is harmed or inhibited while the other species (the large tree) remains unaffected.
Q27.
Adhesive organs or suckers found in tapeworms are adaptations primarily related to:
Answer: B. Clinging to the host in a parasitic lifestyle
Why: Many endoparasites like tapeworms have adhesive organs or suckers as an adaptation to cling to their host's tissues, supporting their parasitic mode of life.
Q28.
Why is soil porosity ecologically important for organisms in a habitat?
Answer: B. It determines water-holding capacity and aeration, which affect the type of vegetation and organisms present
Why: Soil porosity, determined by grain size and aggregation, affects water-holding capacity and aeration of the soil, which in turn influences the vegetation and organisms that can thrive there.
Q29.
Why is the logistic growth model generally considered more realistic than the exponential growth model for natural populations?
Answer: B. It accounts for finite, limiting resources that slow growth as the population nears carrying capacity
Why: The logistic model incorporates the reality that resources such as food and space are finite, causing population growth to slow as it approaches carrying capacity (K), unlike the exponential model which assumes unlimited resources.
Q30.
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?
Answer: B. It behaves nearly like exponential growth, since (K-N)/K is close to 1
Why: When N is much smaller than K, the term (K-N)/K is close to 1, so dN/dt approximates rN, making early logistic growth resemble exponential growth before resource limitation slows it.
Q31.
Which statement best distinguishes hibernation from aestivation?
Answer: B. Hibernation is a winter dormancy to escape cold; aestivation is a summer dormancy to escape heat and desiccation
Why: Hibernation is dormancy adopted during winter to escape cold conditions (e.g., bears), while aestivation is dormancy adopted during summer to escape heat and desiccation (e.g., snails, certain fish).
Q32.
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?
Answer: B. Chthamalus would likely expand into the lower zone, since competitive exclusion by Balanus, not the physical environment alone, was restricting it
Why: Connell's experimental removals showed that Chthamalus was physiologically capable of surviving lower on the shore but was excluded there by superior competition from Balanus; removing Balanus allowed Chthamalus to occupy that zone, confirming competition as the cause of exclusion.
Q33.
A keystone predator is best described as one that:
Answer: B. Has a disproportionately large effect on community structure relative to its abundance, often maintaining prey diversity
Why: A keystone predator exerts an influence on community structure disproportionate to its numbers, often by controlling the abundance of a dominant prey or competitor, thereby helping to maintain overall species diversity.
Q34.
Why are eurythermal and euryhaline organisms generally able to occupy a wider geographic range than stenothermal or stenohaline organisms?
Answer: B. Their broader physiological tolerance to temperature or salinity variation lets them survive in more variable habitats
Why: Eurythermal and euryhaline organisms can physiologically tolerate a wide range of temperature or salinity respectively, allowing them to survive and establish populations across more variable and geographically extensive habitats than stenothermal or stenohaline species restricted to narrow tolerance ranges.
Q35.
Distinguishing an age pyramid that is broad at the base and narrow at the top, what does this shape typically indicate about a population?
Answer: C. A growing population with a large proportion of young individuals
Why: An age pyramid that is broad at the base (many young individuals) and narrows toward the top (fewer old individuals) typically indicates a rapidly growing population with high birth rates.
Q36.
Why might percent cover or pug marks be used instead of direct counts to estimate population density?
Answer: B. For very large, dispersed, or hard-to-count populations or elusive animals, indirect measures provide more practical density estimates
Why: When direct counting of every individual is impractical, such as for grasses (percent cover) or elusive animals like tigers (pug marks), ecologists use indirect measures as practical proxies for population density.
Q37.
How does diapause in zooplankton functionally differ from simple mortality during unfavourable conditions?
Answer: B. Diapause is a reversible, suspended developmental state allowing the organism to resume normal development once favourable conditions return, unlike mortality which is irreversible
Why: Diapause is a temporary, reversible suspension of development that helps the organism survive unfavourable periods; once conditions improve, the organism resumes its normal life cycle, unlike mortality which is a permanent loss of the individual from the population.
Q38.
Which combination correctly matches the interaction to its effect on the two species involved (species A, species B)?
Answer: A. Mutualism: A benefits, B benefits; Commensalism: A benefits, B unaffected; Amensalism: A harmed, B unaffected
Why: Mutualism benefits both species; commensalism benefits one while the other is unaffected; amensalism harms one species while the other remains unaffected, as in a large tree shading out a smaller plant.