Organisms and Populations

In this chapter: population attributes (density, birth and death rates, sex ratio, age pyramids); measuring density; population growth, exponential and logistic models; life history variation; population interactions (predation, competition, parasitism, commensalism, mutualism, amensalism). In the rationalised NCERT, the section on the organism and its environment (abiotic factors and adaptations) has been removed; this page follows the current syllabus.

Populations and their attributes

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding. Natural selection operates at this level. A population has attributes that an individual does not:

• Birth rate (natality) and death rate (mortality), expressed per capita. Example: if 20 lotus plants last year gave 8 new plants this year, the birth rate is 8/20 = 0.4 offspring per lotus per year. If 4 of 40 fruit flies die in a week, the death rate is 4/40 = 0.1 per fly per week.
• Sex ratio: e.g. 60 per cent females and 40 per cent males.
• Age distribution: plotted as an age pyramid. For human populations, pyramids show males and females in a combined diagram.

Population density and how it is measured

Density (N) need not always be counted in numbers. Suitable measures depend on the organism:

• For a huge banyan tree next to 200 Parthenium plants, numbers mislead; percent cover or biomass is more meaningful.
• Very large populations (e.g. bacteria in a Petri dish) are measured indirectly.
• Relative density is often enough: the number of fish caught per trap.
• Indirect estimation: tiger census in national parks and tiger reserves uses pugmarks and faecal pellets.

Population growth

Density changes through four processes: natality (B) and immigration (I) increase it; mortality (D) and emigration (E) decrease it.

N_t+1 = N_t + [(B + I) − (D + E)]. Under normal conditions, births and deaths are the most important factors; immigration and emigration matter in special conditions, such as when a new habitat is being colonised.

Exponential growth (J-shaped curve)

• When resources (food and space) are unlimited, each species realises its full innate potential to grow.
• dN/dt = (b − d) × N. Let (b − d) = r, so dN/dt = rN.
• r is the intrinsic rate of natural increase, a very important parameter for assessing the impact of any biotic or abiotic factor on growth. Examples: Norway rat 0.015; flour beetle 0.12; human population of India, 1981: 0.0205.
• Integral form: N_t = N₀ e^rt, where e is the base of natural logarithms (2.71828).
• Plotting N against time gives a J-shaped curve. Any species growing exponentially under unlimited resources can reach enormous densities in a short time (Darwin showed how even a slow-growing elephant could do this).

Logistic growth (sigmoid, S-shaped curve)

• No population has unlimited resources, which leads to competition; eventually the "fittest" survive and reproduce.
• A habitat has enough resources to support a maximum possible number, beyond which no growth is possible: the carrying capacity (K).
• Growth shows a lag phase, then acceleration, then deceleration, and finally an asymptote when N reaches K. This is Verhulst-Pearl logistic growth.
• dN/dt = rN [(K − N) / K].
• Since resources are finite sooner or later, the logistic model is considered more realistic.

Life history variation

Populations evolve to maximise their reproductive fitness (Darwinian fitness, high r) in their habitat. Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo); others breed many times (most birds and mammals). Some produce a large number of small offspring (oysters, pelagic fishes); others produce a small number of large offspring (birds, mammals).

Population interactions

Predation, parasitism and commensalism share a feature: the interacting species live closely together.

Predation

• Predation is nature's way of transferring energy fixed by plants to higher trophic levels. Herbivores are, in a broad ecological context, not very different from predators.
• Predators keep prey populations under control. When the prickly pear cactus was introduced into Australia in the early 1920s, it spread rapidly over millions of hectares of rangeland; it was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced. This is the basis of biological control.
• Predators help maintain species diversity by reducing competition among prey. In the rocky intertidal communities of the American Pacific coast, the starfish Pisaster is an important predator. When all starfish were removed from an enclosed area experimentally, more than 10 species of invertebrates became extinct within a year because of interspecific competition.
• A predator that is too efficient and overexploits its prey may also become extinct for lack of food; so predators in nature are "prudent".
• Prey defences: camouflage (cryptic colouring) in some insects and frogs; distastefulness, e.g. the monarch butterfly, which is distasteful to birds because of a chemical it acquires by feeding on a poisonous weed as a caterpillar.
• Plants cannot run away, so they defend chemically and morphologically: thorns of Acacia and cactus; Calotropis in abandoned fields produces highly poisonous cardiac glycosides, so no cattle or goats browse on it. Nicotine, caffeine, quinine, strychnine and opium are chemicals plants produce as defences.
• Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts).

Competition

• Darwin was convinced that interspecific competition is a potent force in organic evolution. It is commonly believed to occur between closely related species competing for the same limited resources, but this is not always so.
• Unrelated species can compete: in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton.
• Resources need not be limiting: in interference competition, the feeding efficiency of one species may be reduced by the interfering and inhibitory presence of the other.
• On the Galapagos Islands, the Abingdon tortoise became extinct within a decade after goats were introduced, apparently due to the goats' greater browsing efficiency.
• Competitive release: a species whose distribution is restricted to a small area by a superior competitor expands dramatically when the competitor is removed. Connell's field experiments on the rocky sea coasts of Scotland: the larger, competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chthamalus from that zone.
• Gause's competitive exclusion principle: two closely related species competing for the same resources cannot co-exist indefinitely; the competitively inferior one will eventually be eliminated.
• Resource partitioning: species facing competition may evolve mechanisms that promote co-existence. MacArthur showed that five closely related species of warblers living on the same tree avoided competition through behavioural differences in their foraging activities.

Parasitism

• Many parasites are host-specific (they can parasitise only a single species of host), so host and parasite tend to co-evolve.
• Adaptations: 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, with one or two intermediate hosts. The human liver fluke (a trematode) depends on two intermediate hosts: a snail and a fish. The malarial parasite needs a vector (mosquito).
• Parasites harm the host: they may reduce survival, growth and reproduction, reduce population density, and make the host physically weak and more vulnerable to predation.
• Ectoparasites feed on the external surface: lice on humans, ticks on dogs, many marine fish infested with ectoparasitic copepods. Cuscuta, a parasitic plant commonly found on hedge plants, has lost its chlorophyll and leaves during evolution and derives nutrition from the host.
• Endoparasites live inside the host body at different sites (liver, kidney, lungs, red blood cells); their life cycles are more complex because of extreme specialisation, and they have simple morphological features with high reproductive potential.
• Brood parasitism in birds: the parasitic bird lays its eggs in the nest of its host and lets the host incubate them. The eggs have evolved to resemble the host's in size and colour. Example: the cuckoo (koel) laying eggs in the crow's nest.

Commensalism

• An orchid growing as an epiphyte on a mango branch.
• Barnacles growing on the back of a whale.
• Cattle egret foraging close to grazing cattle: the cattle stir up insects from the vegetation, which the egrets catch.
• Sea anemone (with stinging tentacles) and clown fish: the fish gets protection from predators, which stay away from the stinging tentacles; the anemone does not appear to gain anything.

Mutualism

• Lichens: an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
• Mycorrhizae: fungi and roots of higher plants; the fungi help absorb nutrients from the soil, the plant gives the fungi carbohydrates.
• Plant-animal mutualism involves pollination and seed dispersal; plants offer rewards such as pollen and nectar, or juicy, nutritious fruits.
• Fig and wasp: a given fig species can be pollinated only by its "partner" wasp species. The female wasp uses the fruit as an oviposition (egg-laying) site and pollinates the fig inflorescence while searching for suitable sites; the fig offers the wasp some of its developing seeds as food for the larvae.
• Sexual deceit: the Mediterranean orchid Ophrys has one petal that bears an uncanny resemblance to the female bee in size, colour and markings. The male bee is attracted to what it perceives as a female, "pseudocopulates" with the flower, and is dusted with pollen. If the female bee's colour patterns change even slightly during evolution, pollination success will be reduced unless the orchid flower co-evolves.

Amensalism

One species is harmed while the other is unaffected (−/0). It is not discussed in detail in the textbook but appears in match-the-column questions.

Common traps: (1) Urn-shaped = declining, bell-shaped = stable. (2) In dN/dt = rN(K − N)/K, growth stops when N = K, not when N = 0. (3) Flamingoes and fishes are unrelated species competing, a favourite "exception". (4) Balanus excludes Chthamalus, not the other way round. (5) Liver fluke: snail and fish are intermediate hosts. (6) Clown fish and sea anemone is commensalism, not mutualism, in NCERT. (7) The koel lays eggs in the crow's nest: brood parasitism.

NEET focus

• Birth and death rate calculations; age pyramid shapes.
• N_t+1 equation; exponential and logistic equations; r values.
• Sign table for interactions; matching interaction to example.
• Prickly pear, Pisaster, monarch butterfly, Calotropis, Abingdon tortoise, Connell, Gause, MacArthur, fig-wasp, Ophrys.

Practice questions