Organisms and Populations

Competition is a -/- interaction that occurs when individuals of different species compete for a resource that limits the survival and reproduction of each species, e.g., a leopard and a lion in a forest or grassland. Brood parasitism or the laying of one's eggs in the nest of another individual, is a reproductive strategy whereby parasites foist the cost of rearing their offspring onto another individual, the host, e.g., a cuckoo laying eggs in a crow's nest. Mutualism is an interspecific interaction that benefits both species (+/+), e.g., fungi and roots of a higher plant in mycorrhizae. Commensalism is the interaction in which one species benefits and the other is neither harmed nor benefited, e.g., a cattle egret and a cattle in a field.

Population A is stable at first and then declines sharply, while population B rises gradually and then rapidly.
Both are herbivores feeding on the same resource, so population B outcompeted population A for food.

A true parasite lives on or in the host, obtains its food from the host and usually completes its life cycle there, harming the host.
The head louse lives on the human scalp, feeds on human blood and also lays its eggs on the hair; it is an ectoparasite in the true sense.
The female $Anopheles$ needs blood only for reproduction and visits the host just to feed; NCERT states that the female mosquito is not considered a parasite.
The koel laying eggs in the crow's nest is brood parasitism, in which only parental care is exploited.
A foetus drawing nourishment from its mother is a normal part of reproduction of the same species, not parasitism.
Hence the true parasite is the head louse.

(a) is correct: both the lion (a carnivore) and the sparrow feeding on grain (a herbivore) depend on other organisms for food, so ecologically both are consumers.
(b) is correct: in the rocky intertidal communities of the American Pacific coast, the starfish $Pisaster$ is an important predator; when it was removed experimentally, more than 10 species of invertebrates became extinct within a year because of interspecific competition. So it maintains species diversity.
(c) is wrong: predators in nature are 'prudent'; if a predator over-exploited its prey, the prey would become extinct and the predator would follow. Prey species have also evolved defences.
(d) is wrong: nicotine, strychnine, caffeine, quinine and opium are chemicals produced by plants as defences against grazers and browsers; they are not metabolic disorders.
Hence the two correct statements are (a) and (b).

In the logistic growth equation $\frac{dN}{dt} = rN\left[\frac{K-N}{K}\right]$, N is population density, r is the intrinsic rate of natural increase and K is the carrying capacity. Carrying capacity is the maximum population size that the environment can support with its available resources.

Gause's competitive exclusion principle states that two closely related species competing for the same resources cannot co-exist indefinitely. During competition, the competitively inferior species will be eliminated; this may be true if resources are limiting. As per the solution in the source, both statements are treated as true.

Gause's 'Competitive Exclusion Principle' states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually. In general, herbivores and plants appear to be more adversely affected by competition than carnivores.

G. F. Gause, working with two species of Paramecium, showed that two closely related species competing for the same resources cannot coexist indefinitely; the inferior competitor is eliminated.
This is called the competitive exclusion principle (it is also known as Gause's principle).
Allen's rule relates to shorter extremities in mammals of colder climates; Weismann's theory concerns the continuity of germplasm.
So, as per the answer key, this law is the competitive exclusion principle.

Logistic growth – A population growing in a habitat with limited resources. Exponential growth – Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time. Expanding age pyramids – The percent individuals of pre-reproductive age is largest followed by reproductive and post reproductive age groups. Stable age pyramid – The percent individuals of pre-reproductives and reproductive age group are same.

An age pyramid with a very broad base has a large proportion of young (pre-reproductive) individuals and represents a rapidly growing population.
When measures to reduce population growth take effect, fewer children are born, so the base of the pyramid becomes narrower in relation to the bars above it.
In the figure, pyramid B has the broader base (a steeply triangular shape), so it is the earlier pyramid of the fast-growing population.
Pyramid A has a slightly narrower base, showing that the youngest age groups have shrunk a little; it is therefore the more recent pyramid and shows a slight reduction in the growth rate.
Hence the correct interpretation is that 'A' is more recent and shows a slight reduction in the growth rate.

Population density at time $t+1$: $N_{t+1}=N_t+[(B+I)-(D+E)]$. Population density increases when births plus immigrants (B+I) exceed deaths plus emigrants (D+E).

Predator is exemplified by Pisaster, a keystone predator (A-II); Mutualism is exemplified by the female wasp and fig relationship (B-III); Parasitism is exemplified by Plasmodium (C-IV); Sexual deceit is exemplified by Ophrys, an orchid that mimics a female insect (D-I).

Migratory flamingoes and resident fish in South American lakes is an example of interspecific competition (A-IV); Abingdon tortoise becoming extinct after introduction of goats is an example of interference competition (B-I); Chathamalus expanding its distributional range in the absence of Balanus is an example of competitive release (C-II); Five closely related species of Warblers feeding in different locations on the same tree is an example of resource partitioning (D-III).

The equation $\dfrac{dN}{dt}=rN$ describes exponential (J-shaped) growth, occurring when resources are not limiting. The equation $\dfrac{dN}{dt}=rN\left(\dfrac{K-N}{K}\right)$ describes logistic (S-shaped) growth, occurring when resources become limiting as the population approaches the carrying capacity K.

Immigration adds individuals to a population, so it has a positive impact on population density (increasing it), unlike emigration which decreases it.

Herbivores-Plants is an example of predation (a-iii); Mycorrhiza-Plants is an example of mutualism (b-ii); Sheep-Cattle is an example of competition (c-iv); Orchid-Tree is an example of commensalism (d-i).

This is a classic example of mutualism, where both species benefit: plants get pollinated, and animals receive food rewards (pollen/nectar).

Calotropis produces cardiac glycosides as a chemical defence mechanism, which are toxic/distasteful to most herbivores, deterring them from feeding on it.

Increase in population $=2500-250=2250$. Birth rate per capita $=\dfrac{2250}{250}=9$ per snail per year.

When resources are abundant, a population grows in an exponential or geometric fashion; the numbers increase slowly at first and then explosively.
The curve obtained by plotting the numbers against time is J-shaped.
In such growth the population overshoots the carrying capacity of the environment and then crashes suddenly, when an environmental factor (such as the end of the favourable season or a shortage of food) becomes limiting.
The explosive increase of the insect population in the rainy season, followed by its sudden disappearance at the end of the season, is exactly this pattern.
Similar J-shaped curves are seen in algal blooms and in a reindeer population allowed to grow in a predator-free environment.
In S-shaped (sigmoid) growth the population would level off at the carrying capacity and remain stable instead of crashing.
So this shows that its population growth curve is of J-type.

A population growing in a habitat with limited resources shows a logistic growth curve. For logistic growth: $\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$ If K = N then $\frac{K - N}{K} = 0$, so $\frac{dN}{dt} = 0$ and the population reaches the asymptote.

$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$, where N = population density, r = intrinsic rate of natural increase, K = carrying capacity
When $\frac{N}{K} = 1$, $\frac{K - N}{K} = 0$, therefore $\frac{dN}{dt} = 0$.

The metabolic rate of an organism at rest at normal temperature is its basal metabolic rate (BMR). BMR is inversely proportional to body weight.
Small animals have a higher BMR, so they get more energy and can run uphill more easily than larger animals.

At high altitude the atmospheric pressure is low, so less oxygen is available.
Over time the body acclimatises: it increases red blood cell production, decreases the binding affinity of haemoglobin (so oxygen is released more easily to tissues) and increases the breathing rate.
After six months these people are acclimatised, so they have more RBCs and haemoglobin with a lower binding affinity; altitude sickness occurs only in the first few days.

The graph plots the internal level of a factor (e.g., body temperature or osmotic concentration) against the external level.
Line a is horizontal: the internal level stays constant whatever the external level, so the organism maintains homeostasis. It is a regulator (e.g., birds and mammals).
Line b is a straight diagonal: the internal level changes in direct proportion to the external level. It is a conformer (about 99% of animals and nearly all plants).
Line c stays constant over a limited range of the external level and beyond that range changes with it. It is a partial regulator.
Hence a is regulator, b is conformer and c is partial regulator.

(1) and (2): bears go into hibernation (winter sleep) during winter to escape the cold weather. Aestivation is summer sleep, used to avoid heat and desiccation.
(3): a conical (triangular) age pyramid with a broad base has a large pre-reproductive group, so it represents an expanding (growing) population; a stable population has a bell-shaped pyramid.
(4): the fig and its pollinating wasp both benefit (the wasp gets a site for egg laying and food for its larvae, the fig gets pollinated), so this is mutualism, not commensalism.
(5): an area with a high level of species richness and endemism is called a biodiversity hot spot.
The only option in which every fill-up is correct is (1) – hibernation, (2) – escape, (3) – expanding, (5) – hot spot.

The kangaroo rat of the North American deserts is a classic example of adaptation to scarcity of water.
(iii) is correct: it feeds on dry seeds and does not need to drink water. It meets all its water requirements through the internal oxidation of fat, in which water is a by-product (metabolic water).
(iv) is correct: it has the ability to concentrate its urine, so that only a minimal volume of water is used to remove excretory products. It also does not use water for cooling the body (no sweating or panting); it avoids heat by staying in burrows during the day.
(i) is not correct: it is not dark coloured, and it excretes highly concentrated liquid urine, not solid urine.
(ii) is not correct: a covering of thick hairs and a slow breathing rate are not its adaptations for conserving water.
So the two correct statements are (iii) and (iv).

Prey animals have evolved various defences to escape predation.
Colour change in the chameleon is camouflage: it blends with the background and is not easily seen by predators.
Melanism in moths (as in the peppered moth on soot-covered tree trunks) is also camouflage against predatory birds.
The puffer fish swallows water or air and inflates its body to a large, spiny ball that predators find difficult to swallow; this is a defence against predators.
Poison fangs in snakes are mainly a means of capturing and killing prey; they are an offensive adaptation of a predator, not an innate means of escaping from predation.
So the incorrect example is poison fangs in snakes.
Note: the answer key of the source gives the puffer fish option; that is changed here because inflation of the puffer fish is a classic anti-predator defence.

In developing countries like India, the birth rate (fertility) remains high.
At the same time, the death rate (mortality) has fallen rapidly because of better medical facilities, vaccination, sanitation and food supply.
High birth rate together with low or falling death rate results in rapid population growth (population explosion).
Because many children are born every year, a large proportion of the population is young. The age pyramid is broad at the base (expanding, triangular shape).
So the characteristic features are high fertility, low or rapidly falling mortality rate, rapid population growth and a very young age distribution.

In the growth phase, the rate of increase of bacteria is proportional to their number: $\dfrac{dN}{dt} = \mu N$
Integrating: $N = N_0e^{\mu t}$
Taking logarithms: $\log N = \log N_0 + \mu t$ (in natural log form)
This has the form $y = mx + c$ with $y = \log N$, $m = \mu$ (positive) and $x = t$.

So a semilog plot (log of number against time) of growing bacteria is an ascending straight line.