101. Consider two scenarios:
Case 1: Species P and Q use the same feeding site and both show reduced reproductive success.
Case 2: Species R and S use different portions of the same habitat and maintain normal reproductive performance.
The best interpretation is that
ⓐ. both cases demonstrate competitive exclusion
ⓑ. Case 1 shows competition; Case 2 shows resource partitioning
ⓒ. Case 1 is mutualism, whereas Case 2 is parasitism
ⓓ. neither case provides information about ecological interactions
Correct Answer: Case 1 shows competition; Case 2 shows resource partitioning
Explanation: In Case 1, both species experience a fitness cost while using the same feeding site, which is consistent with competition. In Case 2, different use of the shared habitat reduces overlap and allows normal performance, illustrating how resource partitioning can weaken competition. Competition should be inferred from depressed fitness, not from mere proximity, similarity or resource overlap. Competition includes both exploitation of shared resources and direct interference with resource use. Competitive release is revealed when a restricted species expands after a competitor is removed; resource partitioning instead promotes coexistence by separating resource use. The contrast shows that coexistence can result from ecological differentiation rather than from the complete absence of potentially shared resources.
102. A parasite obtains nutrients from its host and, as a consequence, reduces the host's growth and reproductive success. The interaction is represented by
ⓐ. \((+,+)\)
ⓑ. \((+,0)\)
ⓒ. \((+,-)\)
ⓓ. \((-,-)\)
Correct Answer: \((+,-)\)
Explanation: In parasitism, the parasite benefits by obtaining resources while the host is harmed. The host may experience reduced survival, slower growth, lower reproduction or a decline in population performance. A sign pair is a compact description of benefit, harm or neutrality, not always a complete classification of the mechanism. The defining asymmetry of parasitism is benefit to the parasite coupled with a biological cost to the host. The host can be harmed through reduced growth, reproduction or survival even when it remains alive. Different ecological interactions can share the same outcome signs, so mechanism must be considered alongside the sign pattern. Parasites profit from the association, whereas the host pays a fitness cost. This produces the sign combination \((+,-)\), although the same sign pattern can also occur in predation and must be interpreted together with the biological mechanism.
103. A parasitised animal survives but shows slower growth, produces fewer offspring and becomes easier for predators to capture. Which conclusion is best supported?
ⓐ. Parasitism affects only host nutrition and never reproduction or vulnerability
ⓑ. Parasitism benefits both parasite and host as long as the host survives
ⓒ. A surviving host cannot be considered harmed
ⓓ. Parasites can reduce fitness without immediately killing the host
Correct Answer: Parasites can reduce fitness without immediately killing the host
Explanation: Harm in parasitism does not require immediate host death. Parasites can reduce host growth, survival probability and reproductive output, and they may also increase the host's vulnerability to predators. The interaction remains parasitic whenever parasite benefit is linked to reduced host fitness. A parasite may lower host fitness without killing the host quickly; impaired growth or reproduction is enough. Predation supplies energy to the consumer while imposing mortality that may keep prey populations below otherwise attainable densities. A reduction in predator abundance can increase prey survival and alter subsequent population growth. These effects decrease host fitness while allowing the parasite to continue benefiting from the association.
104. A host population evolves a defence that reduces infection by a particular parasite. Several generations later, parasite variants capable of overcoming this defence become more common. This sequence is most consistent with
ⓐ. reciprocal host-parasite co-evolution
ⓑ. complete absence of natural selection
ⓒ. a permanent host defence that removes further selection on the parasite
ⓓ. competitive release between unrelated habitats
Correct Answer: reciprocal host-parasite co-evolution
Explanation: A host defence can impose selection on the parasite population by favouring variants able to bypass or overcome that defence. In turn, parasite adaptations can impose further selection on the host. Host resistance can favour parasite variants that overcome the defence, creating reciprocal selection across generations. Parasitism is recognised by the parasite's gain and the host's loss in survival, growth or reproduction. The parasite-host relationship is unequal: parasite success increases at the expense of host fitness. As hosts evolve resistance, parasites that counter that resistance gain an advantage, producing continuing host-parasite co-evolution. Host harm need not be immediately lethal for an interaction to qualify as parasitism; a fitness reduction is sufficient. This reciprocal evolutionary response is characteristic of host-parasite co-evolution and helps explain why many parasite-host relationships show highly specific adaptations.
105. Many parasites show strong host specificity. Which explanation best connects this pattern with evolution?
ⓐ. Host specificity prevents any evolutionary response by the host
ⓑ. Host specificity means parasites no longer undergo natural selection
ⓒ. A host-specific parasite can survive equally well on every unrelated species
ⓓ. Reciprocal selection can favour parasite adaptation to a particular host
Correct Answer: Reciprocal selection can favour parasite adaptation to a particular host
Explanation: Long-term interaction between a parasite and a particular host can favour increasingly specialised mechanisms of attachment, infection, resource use or avoidance of host defences. At the same time, the host may evolve resistance. A host defence changes the selective environment for the parasite, and successful counter-adaptations can in turn alter selection on the host. Host specificity and specialised attachment or transmission traits can evolve together with reduction of structures that are less useful inside a protected host environment. Parasites obtain a benefit from the association while the host experiences a reduction in biological performance. Such reciprocal selection can generate both host specificity and continued co-evolution rather than evolutionary independence.
106. A parasite has greatly reduced sensory structures, possesses strong attachment organs and produces a very large number of offspring. These features are best interpreted as
ⓐ. adaptations that increase dependence on free-living movement
ⓑ. specialisations associated with successful parasitic existence
ⓒ. evidence that parasites are always anatomically more complex than hosts
ⓓ. characteristics that prevent the parasite from exploiting host resources
Correct Answer: specialisations associated with successful parasitic existence
Explanation: Many parasites show structural simplification together with specialised features that improve survival within or on a host. Unnecessary sensory structures may be reduced, while adhesive organs or suckers help maintain attachment. High reproductive capacity increases the chance that at least some offspring complete the often complex transmission cycle. Host harm in parasitism may be expressed through survival, growth or reproduction rather than immediate death. Many parasites combine host-specific adaptations with attachment or transmission specialisations, while unnecessary sensory or digestive structures may be reduced. These changes represent functional adaptation to parasitic life rather than general anatomical advancement.
107. Match each parasitic adaptation with its most appropriate functional significance.
| Adaptation | Functional significance |
|---|
| P. Adhesive organ or sucker | 1. Greater probability that some descendants complete transmission |
| Q. Loss of unnecessary sensory organs | 2. Firm attachment to the host |
| R. Loss of digestive system in some parasites | 3. Reduced need for structures unnecessary in a protected host environment |
| S. High reproductive capacity | 4. Reliance on readily available host-derived nutrients |
Select the correct matching.
ⓐ. P-1, Q-3, R-2, S-4
ⓑ. P-3, Q-4, R-1, S-2
ⓒ. P-4, Q-2, R-3, S-1
ⓓ. P-2, Q-3, R-4, S-1
Correct Answer: P-2, Q-3, R-4, S-1
Explanation: Attachment organs help parasites remain firmly associated with the host. Structures that are unnecessary in a protected parasitic environment may become reduced, while some parasites can lose a digestive system because nutrients are obtained directly from the host. A protected life on or within a host can favour specialised attachment and transmission mechanisms along with simplification of structures no longer needed independently. A parasite exploits host resources or services in a way that lowers some component of host fitness. The defining direction of parasitism is asymmetrical: parasite fitness rises while host fitness is reduced. Parasite adaptation often includes precise host use, firm attachment and efficient transmission, accompanied in some forms by structural reduction. High reproductive capacity compensates for the low probability that every offspring will successfully reach the next required host or stage.
108. A human liver fluke completes its life cycle using a snail and a fish before reaching the definitive human host. The snail and fish are best described as
ⓐ. two predators of the parasite
ⓑ. two definitive hosts
ⓒ. two intermediate hosts
ⓓ. two unrelated vectors that merely carry the parasite externally
Correct Answer: two intermediate hosts
Explanation: In the stated example, the human liver fluke depends on \(2\) intermediate hosts, a snail and a fish, during its complex life cycle. Intermediate hosts participate in developmental stages of the parasite before it reaches the definitive host. Completion of a complex parasite life cycle depends on reaching each required host or vector in the correct sequence; blocking an essential transfer can stop the cycle. An association remains parasitic when the parasite gains and the host pays a measurable biological cost. Their role is biologically different from that of an organism acting mainly as a transmission vector.
109. Which statement correctly distinguishes an intermediate host from a vector in parasite life cycles?
ⓐ. An intermediate host is always unaffected, whereas a vector is always killed
ⓑ. Intermediate hosts support parasite development; vectors transmit parasites
ⓒ. A vector is necessarily the definitive host of every parasite
ⓓ. Intermediate hosts occur only in plants, while vectors occur only in animals
Correct Answer: Intermediate hosts support parasite development; vectors transmit parasites
Explanation: An intermediate host supports part of a parasite's developmental cycle before the parasite reaches its definitive host. A vector, by contrast, is important in transmitting the parasite from one host to another. When a parasite requires successive hosts or a vector, failure at one necessary transmission step prevents normal completion even if earlier development occurs. Parasitism does not require rapid host death; chronic loss of growth or reproductive success is sufficient host harm. The host can remain alive for a long period and still be harmed if parasite exploitation lowers its fitness. The human liver fluke example highlights intermediate hosts, whereas the malarial parasite depends on a mosquito for transmission.
110. A mosquito acquires the malarial parasite from an infected person and later transmits it to another human. In this transmission sequence, the mosquito is functioning as
ⓐ. a competing species
ⓑ. an intermediate plant host
ⓒ. a transmission vector
ⓓ. a commensal partner
Correct Answer: a transmission vector
Explanation: The mosquito is essential for transmission of the malarial parasite from one human host to another. In this context, its ecological role is that of a vector. Parasite success is achieved at the host's expense, distinguishing parasitism from commensalism or mutualism. Complex life cycles are chain-dependent: interruption of one obligatory host or vector stage can prevent the parasite from reaching its final stage. Reduced host condition, fecundity or survival can all express the negative side of a parasitic interaction. A parasite may develop normally through early stages yet fail to complete its life cycle if transfer to a required host or vector is blocked. This distinction is important because not every organism associated with a parasite should automatically be labelled in the same way; the role depends on how it participates in the parasite's life cycle and transmission.
111. Which set contains only ectoparasites?
ⓐ. Lice on humans, liver flukes and malarial parasites
ⓑ. Liver flukes, malarial parasites and Cuscuta
ⓒ. Lice, ticks and fish ectoparasitic copepods
ⓓ. Ticks on dogs, Cuscuta and liver parasites
Correct Answer: Lice, ticks and fish ectoparasitic copepods
Explanation: Ectoparasites live on or feed from the external surface of their hosts. Lice on humans, ticks on dogs and ectoparasitic copepods on marine fishes all satisfy this condition. The parasite-host relationship is defined by unequal fitness consequences rather than by a requirement that the host be killed. Host exploitation provides resources to the parasite while diminishing the host's ability to survive or reproduce. A living host may continue to support a parasite even while suffering cumulative losses in growth and reproductive output. Parasites living within organs, tissues or cells are classified as endoparasites rather than ectoparasites.
112. Parasite P remains attached to the external surface of its host, whereas parasite Q lives within the host's liver. The correct classification is
ⓐ. P is an endoparasite and Q is an ectoparasite
ⓑ. P is an ectoparasite and Q is an endoparasite
ⓒ. both P and Q are ectoparasites
ⓓ. both P and Q are commensals
Correct Answer: P is an ectoparasite and Q is an endoparasite
Explanation: The distinction is based on location relative to the host. Ectoparasites occupy the external surface, while endoparasites live inside host tissues, organs or cells. Parasitism is identified from the combination of parasite benefit and host harm, not from the duration of the association alone. Fitness loss to the host is the essential negative consequence that accompanies the parasite's gain. The host may show subtle costs such as slower growth or fewer offspring even when mortality is unchanged. Resource extraction by a parasite can reduce host performance without producing an immediate lethal effect. A parasite attached externally is ectoparasitic, whereas one living in the liver is endoparasitic.
113. Cuscuta is classified as a parasitic plant because it
ⓐ. retains abundant chlorophyll and supplies carbohydrates to its host
ⓑ. draws nutrition from another plant and lacks chlorophyll and leaves
ⓒ. grows harmlessly on another plant only for physical support
ⓓ. benefits its host by increasing mineral absorption
Correct Answer: draws nutrition from another plant and lacks chlorophyll and leaves
Explanation: Cuscuta is a parasitic plant that has lost chlorophyll and leaves during evolution and depends on its host for nutrition. A parasitic association benefits the parasite at a measurable cost to host fitness. In parasitism, the parasite gains resources or reproductive advantage while host performance is reduced. The defining asymmetry of parasitism is benefit to the parasite coupled with a biological cost to the host. Parasites profit from the association, whereas the host pays a fitness cost. This nutritional dependence distinguishes it from an epiphyte, which may use another plant merely for support without extracting nourishment from it.
114. A female mosquito takes a blood meal required for reproduction. Using the ecological classification boundary for parasitism, it should
ⓐ. automatically be classified as a parasite because it consumes blood
ⓑ. be regarded as a parasite only when it feeds more than once
ⓒ. be classified as an endoparasite because blood is inside the host
ⓓ. not be regarded as a parasite merely because it takes a blood meal
Correct Answer: not be regarded as a parasite merely because it takes a blood meal
Explanation: The female mosquito provides a useful boundary case showing that blood feeding alone does not automatically make an organism a parasite. The interaction remains parasitic whenever parasite benefit is linked to reduced host fitness. Host damage can appear as depressed growth, survival or fecundity and does not require rapid mortality. Parasitism is recognised by the parasite's gain and the host's loss in survival, growth or reproduction. The biological cost to the host may be sublethal, including reduced growth or reproductive success. Classification depends on the biological nature of the relationship rather than on a single superficial feature such as obtaining blood from another organism.
115. Endoparasites often have complex life cycles but simplified morphology. Which explanation best reconciles these two features?
ⓐ. Internal life reduces some structures, while transmission requires specialised stages
ⓑ. Complex life cycles necessarily require increasingly complex adult anatomy
ⓒ. Structural simplification means endoparasites have no reproductive adaptations
ⓓ. Living inside the host removes the need for adaptations that permit transmission to another host
Correct Answer: Internal life reduces some structures, while transmission requires specialised stages
Explanation: The protected internal environment of a host can make some sensory or digestive structures unnecessary, favouring anatomical simplification. At the same time, successful movement between hosts may require multiple developmental stages, intermediate hosts or vectors. Each obligatory host or vector forms part of the transmission pathway, so breaking one indispensable link can halt completion of the life cycle. Specialisation for a particular host can be coupled with suckers, adhesive organs or transmission traits and with loss of functions supplied by the host environment. The parasite-host relationship is unequal: parasite success increases at the expense of host fitness. Structural simplification and life-cycle complexity are not contradictory features of endoparasitism.
116. A parasite produces enormous numbers of reproductive stages, although only a small fraction eventually reach the next suitable host. The high reproductive output is most directly advantageous because it
ⓐ. guarantees that every offspring survives
ⓑ. prevents the parasite from becoming host-specific
ⓒ. offsets heavy losses during transmission
ⓓ. eliminates the need for any intermediate host
Correct Answer: offsets heavy losses during transmission
Explanation: Parasite transmission can be inefficient because many offspring fail to encounter the correct host or survive all stages of the life cycle. High reproductive capacity increases the probability that at least some descendants successfully complete transmission. Host harm need not be immediately lethal for an interaction to qualify as parasitism; a fitness reduction is sufficient. Parasites obtain a benefit from the association while the host experiences a reduction in biological performance. Host harm in parasitism may be expressed through survival, growth or reproduction rather than immediate death. It functions as an adaptation to the risks associated with a complex parasitic life cycle.
117. In brood parasitism, a parasitic bird gains reproductive advantage primarily by
ⓐ. laying eggs in another bird's nest for the host to incubate
ⓑ. destroying the host nest and building a larger nest nearby
ⓒ. feeding directly on the adult host as an ectoparasite
ⓓ. providing food to the host in exchange for nest space
Correct Answer: laying eggs in another bird's nest for the host to incubate
Explanation: In brood parasitism, the parasitic bird places its eggs in the nest of another species. The host then supplies incubation and often subsequent parental care, allowing the parasite to shift part of its reproductive investment onto the host. A parasite exploits host resources or services in a way that lowers some component of host fitness. The defining direction of parasitism is asymmetrical: parasite fitness rises while host fitness is reduced. An association remains parasitic when the parasite gains and the host pays a measurable biological cost. This relationship harms the host's reproductive success while benefiting the brood parasite.
118. Eggs of a brood-parasitic bird closely resemble host eggs in size and colour. If this resemblance becomes less accurate, the most likely immediate consequence is
ⓐ. reduced probability that the host detects the foreign egg
ⓑ. no change in brood-parasitic success because egg resemblance does not affect host rejection
ⓒ. reduced host discrimination against the less-matching foreign egg
ⓓ. increased probability that the host recognises and rejects the foreign egg
Correct Answer: increased probability that the host recognises and rejects the foreign egg
Explanation: Egg mimicry reduces the chance that a host recognises a parasitic egg as foreign. If resemblance in size or colour becomes poorer, detection and ejection should become more likely. Parasitism does not require rapid host death; chronic loss of growth or reproductive success is sufficient host harm. The host can remain alive for a long period and still be harmed if parasite exploitation lowers its fitness. Parasite success is achieved at the host's expense, distinguishing parasitism from commensalism or mutualism. Reduced host condition, fecundity or survival can all express the negative side of a parasitic interaction. The parasite-host relationship is defined by unequal fitness consequences rather than by a requirement that the host be killed. Host exploitation provides resources to the parasite while diminishing the host's ability to survive or reproduce. The trait improves the brood parasite's reproductive success by decreasing host rejection.
119. An interaction in which species P benefits while species Q experiences no detectable benefit or harm is represented by
ⓐ. \((+,0)\)
ⓑ. \((+,-)\)
ⓒ. \((+,+)\)
ⓓ. \((-,-)\)
Correct Answer: \((+,0)\)
Explanation: Commensalism occurs when one species benefits and the other is neither detectably helped nor harmed. The interaction is represented as \((+,0)\). The signs show who gains or loses, whereas the nature of feeding, exploitation or service distinguishes interactions with identical signs. Benefit, harm and neutrality can be coded by signs, but classification sometimes requires evidence beyond those outcomes. Sign notation identifies the direction of effects on each population; mechanism separates categories such as predation and parasitism. This distinguishes it from mutualism, where both partners benefit, and parasitism, where the benefiting species imposes a cost on the other.
120. An orchid grows as an epiphyte on a mango branch and gains access to better light without extracting nutrition from the mango. The relationship is classified as
ⓐ. commensal relationship
ⓑ. parasitic relationship
ⓒ. competitive relationship
ⓓ. mutualistic relationship
Correct Answer: commensal relationship
Explanation: The orchid benefits by obtaining physical support and improved access to light, while the mango is treated as neither benefited nor harmed. The interaction is commensalism. The \((+,0)\) pattern describes one-sided benefit while the second species shows no measurable fitness effect. An association is commensal only while one participant benefits and the other remains detectably unaffected. Physical association alone does not define commensalism; the key is benefit to one partner and neutrality for the other. Growing on another plant does not by itself imply parasitism; nutrient extraction or measurable harm would be required for that interpretation.
