Organisms and Populations: NEET notes

1. Ecology and populations

NCERT §11.1, §11.1.1

• Life can be studied at many levels of organisation: macromolecules, cells, tissues, organs, individuals, populations, communities, ecosystems and biomes.
• At every level two kinds of question can be asked. A 'how' question looks for the mechanism (how does a bulbul sing? its voice box and a vibrating bone). A 'why' question looks for the significance (why does it sing? to reach its mate in the breeding season).
• Ecology studies how organisms interact with one another and with their physical, abiotic surroundings.
• It works at four levels of organisation: organisms, populations, communities and biomes. This chapter stays at the population level.
• A population is a group of individuals of one species living in a well defined area, sharing or competing for similar resources, and able to interbreed.
• For ecological study, a group produced by asexual reproduction also counts as a population.
• Examples: all the cormorants of a wetland, rats in an abandoned house, teak trees in a forest tract, bacteria on a culture plate, lotus plants in a pond.
• An individual has to cope with a changed environment, but natural selection works at the level of the population. So population ecology connects ecology with population genetics and evolution.

2. Population attributes

NCERT §11.1.1

• A population has features that no single individual has. An individual is born and dies; a population has a birth rate and a death rate.
• These rates are per capita: the change in number divided by the number of members.
• Birth rate example: a pond had 20 lotus plants last year and 8 new ones were added, making 28. Birth rate = 8/20 = 0.4 offspring per lotus per year.
• Death rate example: 4 of 40 laboratory fruit flies die in a week. Death rate = 4/40 = 0.1 individuals per fruit fly per week.
• An individual is male or female, but a population has a sex ratio, for example 60 per cent females and 40 per cent males.
• A population contains individuals of different ages. Plotting the per cent of individuals in each age group gives an age pyramid.
• For humans the age pyramid usually shows males and females side by side.
• The shape of the pyramid tells whether the population is growing, stable or declining. A broad base of young age groups marks a growing population; a narrow base marks a declining one.

3. Population density

NCERT §11.1.1

• The size of a population reflects its status in the habitat. Competition, predation or a pesticide are all judged by the change they cause in population size.
• Sizes range from fewer than 10 (Siberian cranes at the Bharatpur wetlands in any year) to millions (Chlamydomonas in a pond).
• Population size is technically called population density and is written N. It need not be a head count.
• Total number is usually the right measure, but it can mislead. In an area with 200 carrot grass (Parthenium hysterophorus) plants and one huge banyan, counting would make the banyan look unimportant.
• In such cases per cent cover or biomass is the more meaningful measure of density.
• Counting is also impractical when a population is huge, as with a dense bacterial culture in a petri dish.
• Often relative density is enough: the number of fish caught per trap serves as a measure of the fish population in a lake.
• Most population sizes are estimated indirectly. The tiger census in national parks and tiger reserves often relies on pug marks and fecal pellets.

4. Four processes of population growth

NCERT §11.1.2

• Population size is not fixed. It changes with food availability, predation pressure, bad weather and other factors, and these changes show whether a population is flourishing or declining.
• Density in a habitat changes through four basic processes. Natality and immigration add to it; mortality and emigration take away from it.
• Natality: the number of births in the population during a given period, added to the starting density.
• Mortality: the number of deaths in the population during that period.
• Immigration: the number of individuals of the same species that move into the habitat from elsewhere during the period.
• Emigration: the number of individuals that leave the habitat for somewhere else during the period.
• If N is the density at time t, the density one step later is Nt+1 = Nt + [(B + I) − (D + E)].
• Density rises when births plus immigrants (B + I) exceed deaths plus emigrants (D + E).
• Normally births and deaths matter most. Immigration becomes important in special cases, such as when a new habitat is first being colonised.

5. Exponential growth

NCERT §11.1.2 (i)

• Unimpeded growth needs food and space. When a habitat's resources are unlimited, a species can realise its full innate potential to multiply, as Darwin noted, and the population grows exponentially (geometrically).
• With per capita birth rate b and per capita death rate d, the change in N per unit time is dN/dt = (b − d) × N.
• Writing r = b − d gives dN/dt = rN. Here r is the intrinsic rate of natural increase, a key measure for judging how any biotic or abiotic factor affects growth.
• Some r values: Norway rat 0.015, flour beetle 0.12, and the human population of India in 1981, 0.0205.
• Plotting N against time for this model gives a J-shaped curve.
• In integral form, Nt = N0 e^(rt), where Nt is the density after time t, N0 the density at time zero, and e = 2.71828, the base of natural logarithms.
• Under unlimited resources, even a slow breeder can reach huge numbers quickly. Darwin showed this for the elephant.
• The chessboard story makes the point: one grain on square 1, doubling on each of the 64 squares. By the halfway mark the king saw that all the wheat of his kingdom would not be enough.
• In the same way, one Paramecium dividing by binary fission once a day would reach an astonishing number in 64 days, if food and space never ran out.

6. Logistic growth and life history

NCERT §11.1.2 (ii), §11.1.3

• No population in nature has unlimited resources. Individuals compete for what there is, and the 'fittest' survive and reproduce.
• A habitat can support only a maximum number of a species, beyond which the population cannot grow. This limit is the carrying capacity, K.
• With limited resources a population shows a lag phase, then acceleration, then deceleration, and finally an asymptote when density reaches K.
• N plotted against time then gives a sigmoid (S-shaped) curve. This is Verhulst-Pearl logistic growth: dN/dt = rN (K − N)/K.
• Because resources for most animal populations are finite and sooner or later limiting, the logistic model is considered the more realistic one.
• Populations evolve to maximise reproductive fitness, also called Darwinian fitness (a high r value), in their habitat.
• Some organisms breed only once in a lifetime (Pacific salmon, bamboo); others breed many times (most birds and mammals).
• Some produce many small offspring (oysters, pelagic fishes); others produce a few large ones (birds, mammals).
• Ecologists suggest these life history traits evolved under the limits set by the abiotic and biotic parts of each habitat.

7. Population interactions

NCERT §11.1.4

• No natural habitat holds just one species. Every species needs at least one other species to feed on.
• Even a plant that makes its own food needs soil microbes to break down organic matter and return inorganic nutrients, and often an animal to pollinate it.
• Plants, animals and microbes therefore interact and together form a biological community.
• Interspecific interactions are those between populations of two different species. For each species the effect can be beneficial (+), detrimental (−) or neutral (0).
• Mutualism (+, +): both species gain. Competition (−, −): both lose.
• Predation and parasitism (+, −): one gains (predator, parasite) and the other is harmed (prey, host).
• Commensalism (+, 0): one gains and the other is neither helped nor harmed. Amensalism (−, 0): one is harmed and the other is unaffected.
• Predation, parasitism and commensalism have one feature in common: the two species live closely together.

8. Predation

NCERT §11.1.4 (i)

• Predation is nature's way of passing the energy fixed by plants to higher trophic levels. A sparrow eating a seed is as much a predator as a tiger eating a deer; herbivores are, broadly, predators too.
• Predators keep prey populations in check; without them prey could reach very high densities and destabilise the ecosystem.
• Exotic species often turn invasive because the new land lacks their natural predators. Prickly pear cactus brought to Australia in the early 1920s spread over millions of hectares of rangeland until a cactus-feeding moth from its native range was introduced.
• Biological control of crop pests is based on this ability of a predator to regulate its prey.
• Predators also maintain species diversity by easing competition among prey species. When the starfish Pisaster was removed from an enclosed intertidal area on the American Pacific coast, more than 10 invertebrate species went extinct within a year through interspecific competition.
• A predator that overexploits its prey drives the prey, and then itself, to extinction; so predators in nature are 'prudent'.
• Prey defences: some insects and frogs are camouflaged (cryptically coloured); some are poisonous. The Monarch butterfly tastes very bad to birds because of a chemical it takes up as a caterpillar from a poisonous weed.
• Nearly 25 per cent of all insects are phytophagous (feed on plant sap and other parts). Plants cannot run away, so they defend themselves.
• Morphological defence: thorns, as in Acacia and cactus, are the commonest.
• Chemical defence: Calotropis makes highly poisonous cardiac glycosides, so cattle and goats never browse it. Nicotine, caffeine, quinine, strychnine and opium are also made by plants as defences against grazers and browsers.

9. Competition

NCERT §11.1.4 (ii)

• Darwin saw interspecific competition as a strong force in organic evolution.
• Competition is not only between closely related species. Unrelated species can compete too: flamingoes that visit certain shallow lakes in South America compete with the fishes living there for zooplankton.
• Resources need not be limiting. In interference competition, one species feeds less efficiently because the other is present, even when food and space are plentiful.
• So competition is best defined by its effect: one species' fitness, measured as its r, drops significantly when the other species is around.
• In laboratory experiments such as Gause's, the superior competitor eventually eliminates the other when resources are limited; evidence of this in nature is not always conclusive.
• Circumstantial evidence: the Abingdon tortoise of the Galapagos Islands went extinct within a decade of goats being introduced, apparently because the goats browsed more efficiently.
• Competitive release: a species confined to a small area by a superior competitor spreads widely when that competitor is removed. Connell showed that on the rocky coasts of Scotland the larger barnacle Balanus excludes the smaller Chthamalus from the intertidal zone.
• Herbivores and plants seem to suffer more from competition than carnivores do.
• Gause's Competitive Exclusion Principle: when two closely related species depend on the same resources, they cannot live together for ever, and the weaker competitor is eliminated. This may hold when resources are limiting, but not otherwise.
• Species may instead evolve ways to co-exist, such as resource partitioning (feeding at different times or foraging differently). MacArthur showed five closely related warbler species co-existing on one tree through differences in foraging behaviour.

10. Parasitism

NCERT §11.1.4 (iii)

• Parasitism offers free lodging and food, so it has evolved in many groups, from plants to higher vertebrates.
• Many parasites are host-specific, able to live on only one host species, so host and parasite co-evolve: each new host defence is countered by the parasite.
• Adaptations of parasites: loss of unnecessary sense organs, adhesive organs or suckers to cling to the host, loss of the digestive system, and high reproductive capacity.
• Life cycles are often complex. The human liver fluke (a trematode) needs two intermediate hosts, a snail and a fish; the malarial parasite needs a mosquito as vector.
• Most parasites harm the host: they lower its survival, growth and reproduction, reduce its population density, and can weaken it enough to make it easier prey.
• Ectoparasites feed on the host's outer surface: lice on humans, ticks on dogs, and copepods on many marine fish.
• Cuscuta, a parasitic plant on hedge plants, has lost its chlorophyll and leaves and draws its food from the host.
• The female mosquito needs human blood for reproduction but is not considered a parasite.
• Endoparasites live inside the host (liver, kidney, lungs, red blood cells). Being extremely specialised, they have more complex life cycles and simplified body structure, with the emphasis on reproduction.
• Brood parasitism: the parasitic bird lays eggs in the host's nest for the host to incubate. The cuckoo (koel) lays in the crow's nest, and its eggs have evolved to resemble the host's in size and colour.

11. Commensalism

NCERT §11.1.4 (iv)

• In commensalism one partner gains while the other is left neither better nor worse off.
• An epiphytic orchid perched on a mango branch gains support and light; the mango tree gains nothing apparent.
• Barnacles growing on the back of a whale benefit; the whale gains nothing apparent.
• Cattle egrets forage close to grazing cattle, because the moving cattle stir up insects from the vegetation that the egrets would otherwise struggle to find.
• The clown fish lives among the stinging tentacles of a sea anemone and is protected from predators, which avoid the tentacles. The anemone does not appear to gain anything.

12. Mutualism

NCERT §11.1.4 (v)

• In mutualism both interacting species benefit.
• Lichens are an intimate partnership between a fungus and photosynthesising algae or cyanobacteria.
• In mycorrhizae, fungi live in partnership with the roots of higher plants: the fungus helps absorb nutrients from the soil, and the plant supplies energy-rich carbohydrates.
• Plant-animal mutualisms are the most striking. Plants need animals to pollinate flowers and disperse seeds, and pay them with pollen and nectar or with juicy, nutritious fruit.
• The system must also guard against 'cheaters', such as animals that steal nectar without pollinating. So flower and pollinator often co-evolve.
• Many fig species have a strict one-to-one partnership with a wasp species: only the partner wasp can pollinate that fig.
• The female wasp pollinates the fig inflorescence while looking for egg-laying sites; it lays eggs in the fruit, and the fig gives some developing seeds as food for the wasp larvae.
• Many orchids have flower patterns that attract a particular pollinator, such as bees and bumblebees, but not all orchids offer a reward.
• The Mediterranean orchid Ophrys uses 'sexual deceit': one petal resembles a female bee in size, colour and markings. The male bee 'pseudocopulates' with it, picks up pollen, and carries it to the next flower.
• If the female bee's colour pattern changed during evolution, the orchid would have to co-evolve its petal to keep pollination success.

Must-know facts

1. Ecology works at four levels: organisms, populations, communities, biomes.
2. Natural selection acts at the population level, which links population ecology to genetics and evolution.
3. Population attributes: birth rate, death rate, sex ratio, age distribution; rates are per capita.
4. Lotus: 8 new on 20, birth rate 0.4 per lotus per year. Fruit flies: 4 of 40 die, death rate 0.1 per fly per week.
5. Age pyramid shapes show growing, stable or declining populations.
6. Population density N can be a number, per cent cover or biomass; relative density (fish per trap) often suffices.
7. Tiger census uses pug marks and fecal pellets.
8. Nt+1 = Nt + [(B + I) − (D + E)]; natality and immigration add, mortality and emigration subtract.
9. Exponential: dN/dt = rN, r = b − d; Nt = N0 e^(rt); J-shaped curve.
10. r values: Norway rat 0.015, flour beetle 0.12, India (1981) 0.0205.
11. Logistic: dN/dt = rN (K − N)/K; sigmoid curve with lag, acceleration, deceleration, asymptote at K.
12. Logistic growth (Verhulst-Pearl) is the more realistic model.
13. Breed once: Pacific salmon, bamboo. Many small offspring: oysters, pelagic fishes.
14. Signs: mutualism +/+, competition −/−, predation and parasitism +/−, commensalism +/0, amensalism −/0.
15. Prickly pear in Australia controlled by a cactus-feeding moth; Pisaster removal led to over 10 invertebrate species going extinct.
16. Nearly 25 per cent of insects are phytophagous; Calotropis makes cardiac glycosides.
17. Gause: Competitive Exclusion Principle; MacArthur: five warbler species co-exist by resource partitioning.
18. Connell: Balanus excludes Chthamalus on the Scottish coast (competitive release when removed).
19. Human liver fluke needs a snail and a fish; the koel lays eggs in the crow's nest (brood parasitism).
20. Commensalism: orchid on mango, barnacles on whale, cattle egret, clown fish; mutualism: lichen, mycorrhiza, fig-wasp, Ophrys.

Common traps

Key terms

Population

Individuals of one species in a defined area that share or compete for resources and can interbreed.

Birth rate

Births per member of the population in a given period.

Sex ratio

The proportion of females and males in a population.

Age pyramid

A plot of the per cent of individuals in each age group, showing growth status.

Population density (N)

Population size, measured as numbers, per cent cover or biomass.

Natality

Number of births added to a population in a given period.

Emigration

Individuals leaving the habitat for elsewhere in a given period.

Intrinsic rate of natural increase (r)

r = b − d, per capita birth rate minus per capita death rate.

Carrying capacity (K)

The maximum population a habitat can support for a species.

Logistic growth

Growth that slows as N approaches K, giving a sigmoid curve.

Darwinian fitness

Reproductive fitness, a high r value.

Mutualism

An interaction in which both species benefit.

Commensalism

One species benefits, the other is unaffected.

Amensalism

One species is harmed, the other is unaffected.

Competitive release

A species expands its range when a superior competitor is removed.

Resource partitioning

Competing species co-exist by using a resource at different times or in different ways.

Brood parasitism

A bird laying eggs in another species' nest for it to incubate, like the koel in a crow's nest.

Co-evolution

Linked evolution of two interacting species, such as a flower and its pollinator.