RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
Ecology begins with the simplest question you can ask about a living thing: why does it live where it does, and not elsewhere? This chapter examines the physical factors that set those limits, the strategies organisms use to cope — regulation, conformity, migration, suspension — and then scales up to populations, their attributes, growth equations, and the ways two species interact. The exponential and logistic growth equations appear in almost every board paper, so learn what each symbol stands for.
The relationship between a cattle egret that feeds on insects stirred up by a grazing buffalo, where the buffalo is neither helped nor harmed, is an example of:
Answer
Commensalism is correct — one species benefits while the other is unaffected, written as (+ 0). The egret gains an easier food supply and the buffalo neither gains nor loses. Mutualism would require both to benefit, parasitism would require the buffalo to be harmed, and amensalism describes one species being harmed while the other is unaffected.
In the logistic growth equation dN/dt = rN (K − N)/K, what does K represent?
Answer
The carrying capacity of the habitat is correct — K is the maximum population size that the available resources of that habitat can support indefinitely. As N approaches K the term (K − N)/K approaches zero, so growth slows and the curve flattens into the sigmoid shape. The intrinsic rate of natural increase is r, not K.
An age pyramid with a very broad base and a narrow apex indicates a population that is:
Answer
Growing is correct — a broad base means a large proportion of pre-reproductive individuals who will enter the reproductive age group in the coming years, so the population will continue to increase. A bell-shaped pyramid indicates a stable population, and an urn-shaped one, narrow at the base, indicates a declining population.
The kangaroo rat of the North American deserts is able to survive without ever drinking water because it:
Answer
Meets its water requirement through internal fat oxidation and excretes concentrated urine is correct — metabolic water released when stored fat is oxidised supplies its needs, and its kidneys concentrate the urine so strongly that very little water is lost in excretion. It does not absorb water through the skin or store it in a bladder, and although aestivation is a genuine desert strategy in some animals, it is not the explanation here.
Assertion (A): Very small animals are rarely found in extremely cold polar regions. Reason (R): A small animal has a large surface area relative to its volume, so it loses body heat rapidly and must spend a great deal of energy generating more.
Answer
Both A and R are true and R is the correct explanation of A — heat is lost across the body surface, and the smaller the animal the greater its surface area in proportion to its body volume. In polar cold this means heat drains away very fast, and the animal would have to eat enormously to keep generating replacement heat. That energy budget is not sustainable, which is why polar mammals tend to be large-bodied.
Distinguish between a regulator and a conformer, giving one example of each.
Answer
A regulator maintains a constant internal body temperature and osmotic concentration by physiological, and sometimes behavioural, means, regardless of changes in the external environment — this is homeostasis. Birds and mammals are regulators; they sweat or pant to lose heat and shiver to generate it, keeping the body at a steady temperature. A conformer does not maintain constancy; its internal body temperature and osmotic concentration change along with the external environment. Most animals, including the majority of aquatic invertebrates, and nearly all plants are conformers, because for a small-bodied organism the energetic cost of regulation would be too high.
A laboratory population of beetles is started with 40 individuals and grows exponentially. Explain the equation used, and state what happens when this population is grown instead in a jar with a fixed amount of flour.
Answer
With unlimited resources the population grows exponentially, described by dN/dt = rN, where N is the population size at that moment, dN/dt is the rate of change in population size, and r is the intrinsic rate of natural increase, the difference between the per capita birth rate and the per capita death rate. Because the growth rate is proportional to the number already present, each generation adds more individuals than the last, so a plot of N against time gives a J-shaped curve that rises ever more steeply. In a jar with a fixed amount of flour, resources are limited. Growth is slow at first, then accelerates, and then slows as competition for food and space intensifies, finally levelling off when the population reaches the carrying capacity K that the jar can support. This is logistic growth, described by dN/dt = rN (K − N)/K, and it gives a sigmoid or S-shaped curve. Because no habitat has truly unlimited resources, the logistic model is the more realistic description of a natural population.
What is meant by resource partitioning, and how does it allow two competing species to coexist?
Answer
Resource partitioning is the situation in which two species that would otherwise compete for the same resource use it in different ways, at different times, or in different parts of the habitat, so that the overlap between them is reduced. Gause's competitive exclusion principle holds that two species competing for the same limiting resource cannot coexist indefinitely, because the competitively superior one eventually eliminates the other. Resource partitioning avoids this outcome by ensuring that each species has a portion of the resource largely to itself. A classic illustration is a group of closely related warbler species living in the same tree, in which each species forages in a different zone of the canopy, so all of them can persist together.
Describe the different ways in which organisms cope with a stressful or unfavourable environment, giving a named example of each strategy.
Answer
Regulate: the organism maintains homeostasis by keeping its internal body temperature and osmotic concentration constant despite changes outside. All birds and mammals, and a few lower vertebrates and invertebrates, do this. In summer, when the outside temperature exceeds body temperature, a human sweats so that evaporative cooling brings the temperature down, and in winter shivering generates heat by muscular activity. Regulation is energetically expensive, which is why very small animals rarely adopt it in extreme climates — they have a large surface area relative to volume and would lose heat too fast to afford it. Conform: the great majority of animals and nearly all plants let their internal environment change with the external one. For a small-bodied aquatic animal, the cost of thermoregulation would outweigh the benefit, so it simply conforms and tolerates the change. Migrate: the organism moves away temporarily from the stressful habitat to a more hospitable one and returns when conditions improve. Every winter the Keoladeo National Park in Bharatpur receives large numbers of migratory birds that have travelled from distant colder regions. Suspend: the organism reduces or halts its activity until conditions improve. Bacteria, fungi, and lower plants form thick-walled spores that germinate when conditions are favourable, and seeds and other vegetative reproductive structures enter dormancy. Among animals, bears undergo hibernation to escape the winter cold, some snails and fish undergo aestivation to escape the heat and desiccation of summer, and many zooplankton species in lakes and ponds enter diapause, a suspended stage of development, when conditions are unfavourable. Adapt: over evolutionary time, populations acquire genetically fixed features that suit them to their habitat. The kangaroo rat of the North American deserts never drinks water, meeting its needs from internal fat oxidation and concentrating its urine; desert plants have a thick cuticle, sunken stomata, and use the CAM pathway so their stomata open at night; and mammals of colder climates have shorter ears and limbs to reduce heat loss, which is Allen's Rule.
Explain the main types of interspecific interaction with named examples, and state the effect each has on the two species involved.
Answer
Interactions between two species are classified by whether each species is benefited (+), harmed (−), or unaffected (0). Mutualism (+ +): both species benefit. The lichen is a close association of a fungus and a photosynthesising alga or cyanobacterium, in which the fungus provides shelter and mineral absorption and the partner supplies food. Mycorrhizae are associations between fungi and the roots of higher plants, in which the fungus helps the plant absorb soil nutrients and receives sugars in return. The fig and its pollinating wasp species show one of the most tightly bound examples, the wasp pollinating the fig while using some of its developing seeds for its own larvae. Competition (− −): both species are harmed, since each reduces the resource available to the other. It occurs when two species require the same limiting resource. Gause's competitive exclusion principle holds that two species competing for the same limiting resource cannot coexist indefinitely and the inferior competitor is eliminated, though resource partitioning — as when related warbler species forage in different zones of the same tree — can allow coexistence. Predation (+ −): the predator benefits and the prey is killed. Besides being a means of transferring energy from one trophic level to the next, predation keeps prey populations under control — the prickly pear cactus introduced into Australia was brought under control only after a predatory moth was introduced. Prey species evolve defences in response, such as camouflage in insects and the poisonous or distasteful chemicals of many plants that deter herbivores. Parasitism (+ −): the parasite benefits at the expense of the host, but usually does not kill it outright. Parasites tend to show loss of unnecessary sense organs, adhesive organs or suckers, a simplified digestive system, and high reproductive capacity. Examples include the human liver fluke and Cuscuta, a parasitic plant that lacks chlorophyll and draws nutrition from the host it grows on. Brood parasitism, in which the koel lays its eggs in a crow's nest for the host to rear, is a special case. Commensalism (+ 0): one species benefits and the other is neither helped nor harmed, as when an orchid grows as an epiphyte on the branch of a mango tree, or a cattle egret feeds on insects flushed out by a grazing cattle. Amensalism (− 0): one species is harmed while the other is unaffected, as when one organism secretes a chemical that inhibits the growth of another nearby.
An ecology class studies a pond over one season. In March they count 500 water beetles; by the end of April the count is 800, and by the end of May it is 1,250. In June, with the pond partly dried and food scarce, the count rises only to 1,400 and by July it stays near 1,450. The students also note that a species of small fish in the pond eats the beetle larvae, that a floating fern and an algal species both compete for surface light, and that a tiny snail rides on the shells of the beetles without affecting them. (a) Which growth model describes the beetle population over the whole season, and what is the approximate carrying capacity? (b) Write the equation for this growth model and explain why the growth slowed after May. (c) Name the type of interaction between the fish and the beetle larvae, and between the fern and the alga, with their symbols. (d) Name the interaction between the snail and the beetle and justify your answer.
Answer
(a) The population shows logistic growth, giving a sigmoid or S-shaped curve — rapid increase at first, then a slowing, and finally a plateau. The carrying capacity K of this pond is about 1,450 beetles, since the population levels off near that value. (b) The model is dN/dt = rN (K − N)/K, where N is the population size, r the intrinsic rate of natural increase, and K the carrying capacity. Growth slowed after May because resources became limiting — the pond partly dried and food became scarce. As N approaches K, the term (K − N)/K falls towards zero, so the rate of increase falls even though individuals continue to reproduce. In practical terms, competition for the shrinking food and space raised the death rate and lowered the birth rate until the two roughly balanced. (c) The fish eating the beetle larvae is predation, (+ −), since the fish benefits and the larvae are killed. The floating fern and the alga both requiring surface light is competition, (− −), since each reduces the light available to the other and both are harmed. (d) The snail and the beetle show commensalism, (+ 0). The snail benefits by gaining transport and a place to live, while the beetle is neither helped nor harmed by carrying it. It is not parasitism, because the snail takes nothing from the beetle's body and causes it no damage.
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