Organisms and Populations

2.1 Core concepts

• Ecology defined. Ecology studies the interactions among organisms and between an organism and its physical (abiotic) environment. It is basically concerned with four levels of biological organisation — organisms, populations, communities and biomes — and this chapter explores ecology at the population level (NCERT §11 intro, p. 191).
• Population defined. In nature individuals rarely live alone; majority of them live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed, and constitute a population. Even groups arising via asexual reproduction are treated as populations for ecological purposes. Cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond are all examples. Selection operates at the population level, so population ecology links ecology to population genetics and evolution (NCERT §11.1.1, p. 191).
• Population attributes. An individual has births and deaths; a population has birth rates and death rates that are per capita — change in numbers with respect to members of the population. Example: 20 lotus plants gave 8 new plants this year, so the per-capita birth rate = 8/20 = 0.4 offspring per lotus per year. In a lab population of 40 fruitflies, 4 died in a week → per-capita death rate = 4/40 = 0.1 per fruitfly per week. Other population attributes: sex ratio (e.g. 60% females, 40% males) and age distribution (NCERT §11.1.1, p. 191).
• Age pyramids. When the per-cent of individuals of a given age/age group is plotted, the resulting structure is called an age pyramid. For human populations age pyramids show age distribution of males and females in a diagram. The shape indicates growth status — (a) expanding (broad base), (b) stable (more rectangular), (c) declining (narrow base) (NCERT §11.1.1, Fig. 11.1, p. 192).
• Population density (N). Population size (technically called population density, designated N) need not be measured in numbers only. It can be as low as <10 Siberian cranes at Bharatpur wetlands in any year or run into millions (Chlamydomonas in a pond). Total number is generally the most appropriate measure but can be meaningless or impossible — e.g. **200 Parthenium hysterophorus plants vs a single huge banyan tree where stating banyan density as 1 underestimates its role; here per cent cover or biomass is more meaningful. Relative densities (fish caught per trap) serve some purposes. Some populations must be estimated indirectly — the tiger census in national parks/tiger reserves is often based on pug marks and faecal pellets** (NCERT §11.1.1, p. 192).
• Four basic processes affecting density. Density at time t+1 follows N(t+1) = N(t) + [(B + I) − (D + E)] where natality (B) = births, immigration (I) = individuals entering, mortality (D) = deaths, emigration (E) = individuals leaving. Under normal conditions B and D dominate, but in a newly-colonised habitat immigration may contribute more than birth rate (NCERT §11.1.2, p. 193, Fig. 11.2).
• Exponential growth. When resources (food and space) are unlimited, each species realises its innate potential to grow — Darwin's natural-selection observation. dN/dt = (b − d) × N = rN, where r = intrinsic rate of natural increase, a very important parameter for assessing impacts of any biotic or abiotic factor on population growth. The integral form is N(t) = N(0) e^(rt) producing a J-shaped curve (NCERT §11.1.2, p. 194, Fig. 11.3).
• Sample r values. Norway rat r = 0.015, flour beetle r = 0.12, human population in India 1981, r = 0.0205 — NCERT explicitly asks students to find the current value using current birth and death rates (NCERT §11.1.2, p. 194).
• Darwinian J-curve illustration. NCERT narrates the chess-grain anecdote where wheat doubles on each square; by half-way the entire kingdom's wheat is inadequate — used to dramatize how fast even slow-growing organisms (elephant, Paramecium doubling daily) build up under unlimited resources (NCERT §11.1.2, p. 195).
• Logistic (Verhulst-Pearl) growth. No real population has unlimited resources; competition occurs and only the 'fittest' individual survives and reproduces. A given habitat has enough resources to support a maximum population beyond which no further growth is possible — this limit is the carrying capacity (K). A population growing under limited resources shows a lag phase → acceleration → deceleration → asymptote at K, producing a sigmoid curve described by dN/dt = rN[(K − N)/K] — the Verhulst-Pearl Logistic Growth. Since resources for growth for most animal populations are finite and become limiting sooner or later, the logistic model is considered the more realistic one (NCERT §11.1.2, pp. 195–196, Fig. 11.3).
• Life-history variation. Populations evolve to maximise reproductive fitness (Darwinian fitness — high r) in the habitat in which they live. Under particular selection pressures, organisms evolve the most efficient reproductive strategy: breed only once in a lifetime — Pacific salmon fish, bamboo; or many times — most birds and mammals; many small offspring — Oysters, pelagic fishes; or few large offspring — birds, mammals. Life-history traits evolve in relation to constraints imposed by abiotic and biotic components of the habitat (NCERT §11.1.3, p. 196).
• Population interactions exist always. No natural habitat is inhabited by a single species; the minimal requirement is one more species on which it can feed; even plants need soil microbes and pollinators. Hence many interactive linkages exist even in minimal communities (NCERT §11.1.4 intro, p. 196).
• Sign convention (Table 11.1, p. 197). Interspecific interactions of two species use '+' (beneficial), '−' (detrimental), '0' (neutral). Mutualism (+/+), Competition (−/−), Predation (+/−), Parasitism (+/−), Commensalism (+/0), Amensalism (−/0). Predation, parasitism and commensalism share a common characteristic — the interacting species live closely together.
• Predation — roles. Predators are nature's way of transferring to higher trophic levels the energy fixed by plants; herbivores (sparrow eating seeds, deer eating grass) are also predators in a broad ecological sense. Predators (i) act as conduits for energy transfer across trophic levels, (ii) keep prey populations under control, (iii) help maintain species diversity. Invasive species spread fast in the absence of their natural predators — prickly pear cactus introduced into Australia in the early 1920s caused havoc on millions of hectares of rangeland; it was brought under control only after a cactus-feeding moth predator was introduced. Biological control in agricultural pest management is based on this ability (NCERT §11.1.4(i), p. 197).
• Pisaster experiment. In rocky intertidal communities of the American Pacific Coast, the starfish Pisaster is an important predator. When all starfish were experimentally removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year because of inter-specific competition — showing predators reduce intensity of competition among competing prey species and thereby maintain diversity (NCERT §11.1.4(i), pp. 197–198).
• Prey defences. Predators are 'prudent' because over-exploitation drives the prey extinct. Prey have evolved defences: cryptic colouration (camouflage) in some insects and frogs; chemical defence — the Monarch butterfly is highly distasteful to predator birds because of a special chemical, acquired during its caterpillar stage by feeding on a poisonous weed. For plants, herbivores are the predators; ~25 per cent of all insects are phytophagous. Plants evolved **thorns (Acacia, Cactus) as morphological defence; chemical defences include the highly poisonous cardiac glycosides of Calotropis (no cattle/goats browse it), and the commercial plant chemicals nicotine, caffeine, quinine, strychnine, opium** — all evolved as anti-herbivore defences (NCERT §11.1.4(i), p. 198).
• Competition. Darwin saw interspecific competition as a potent force in organic evolution. Competition is NOT restricted to closely-related species and need NOT require limiting resources — totally unrelated flamingoes vs resident fishes compete for zooplankton in some South American lakes; in interference competition the feeding efficiency of one species is reduced by the inhibitory presence of another even when food/space are abundant. Best definition: a process in which the fitness (r) of one species is significantly lower in the presence of another species (NCERT §11.1.4(ii), pp. 198–199).
• Evidence for competition. Lab evidence (Gause and others) is clear — competitively superior species eliminates the inferior; field evidence is "not always conclusive". Strong circumstantial cases: (a) Abingdon tortoise (Galapagos) became extinct within a decade after goats were introduced, apparently due to the greater browsing efficiency of goats; (b) competitive release — a species restricted to a small range expands its distribution dramatically when the competing superior species is experimentally removed; (c) Connell's elegant field experiments on rocky sea coasts of Scotland showed the **larger and competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone. Herbivores and plants appear more adversely affected by competition than carnivores** (NCERT §11.1.4(ii), p. 199).
• Competitive Exclusion Principle. Gause's principle states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior will be eliminated eventually — true if resources are limiting. Recent studies suggest co-existence rather than exclusion is also possible via resource partitioning — MacArthur showed five closely related warbler species living on the same tree avoided competition by behavioural differences in foraging activities (timing/pattern) (NCERT §11.1.4(ii), p. 199).
• Parasitism — evolution and adaptations. Parasitism offers free lodging and meals, hence has evolved in many taxa from plants to higher vertebrates. Many parasites are host-specific — host and parasite tend to co-evolve (host evolves resistance, parasite counteracts). Adaptations include: loss of unnecessary sense organs, presence of adhesive organs or suckers, loss of digestive system, high reproductive capacity. Life cycles are complex with intermediate hosts / vectors — the human liver fluke (a trematode) needs two intermediate hosts (a snail and a fish); the malarial parasite needs a vector — mosquito (NCERT §11.1.4(iii), pp. 199–200).
• Effects on host & types. Majority of parasites harm the host — reduce survival, growth, reproduction, density and make the host vulnerable to predation. Ectoparasites feed on the external surface — **lice on humans, ticks on dogs, marine copepods on fish, Cuscuta (a parasitic plant on hedge plants that has lost chlorophyll and leaves). Female mosquito is not considered a parasite although it needs blood for reproduction. Endoparasites** live inside the host (liver, kidney, lungs, RBCs); life cycles are more complex due to extreme specialisation; morphological/anatomical features are simplified while reproductive potential is emphasised (NCERT §11.1.4(iii), p. 200).
• Brood parasitism. A fascinating example in birds — the parasitic bird lays its eggs in the nest of its host and lets the host incubate them. Eggs of the parasitic bird have co-evolved to resemble the host's eggs in size and colour to reduce detection — observe the cuckoo (koel) and the crow during breeding season (spring–summer) (NCERT §11.1.4(iii), pp. 200–201).
• Commensalism examples. One benefits, the other neither harmed nor benefited: (i) orchid as epiphyte on mango branch; (ii) barnacles growing on the back of a whale; (iii) cattle egret and grazing cattle — the egret forages close to cattle because the cattle stir up insects that would otherwise be hard to catch; (iv) sea anemone (with stinging tentacles) and clownfish — the fish gets protection from predators, the anemone gets no apparent benefit (NCERT §11.1.4(iv), p. 201).
• Mutualism examples. Both species benefit. Lichens = intimate mutualism between a fungus and photosynthesising algae or cyanobacteria. Mycorrhizae = association between fungi and the roots of higher plants — fungi help nutrient absorption from soil; plant provides energy-yielding carbohydrates. Plant–animal relationships are spectacular — pollination and seed dispersal; plants offer pollen and nectar for pollinators and juicy nutritious fruits for seed dispersers; the system is safeguarded against 'cheaters'. Hence many plant–animal interactions involve co-evolution (NCERT §11.1.4(v), pp. 201–202).
• Fig–wasp mutualism. In many fig species there is a tight one-to-one relationship with the pollinator wasp species. The female wasp uses the fruit as an oviposition site and uses the developing seeds within the fruit for nourishing its larvae. The wasp pollinates the fig inflorescence while searching for suitable egg-laying sites; in return for pollination, the fig offers some of its developing seeds as food for the developing wasp larvae (NCERT §11.1.4(v), p. 202, Fig. 11.4).
• ***Ophrys* — sexual deceit. Not all orchids offer rewards. The Mediterranean orchid Ophrys employs 'sexual deceit' — one petal of its flower bears an uncanny resemblance to the female of a bee species in size, colour and markings. The male bee is attracted, 'pseudocopulates' with the flower** and is dusted with pollen; on pseudocopulating with another flower, it transfers the pollen and pollinates. If the female bee's colour pattern changes during evolution, the orchid co-evolves to maintain the resemblance (NCERT §11.1.4(v), p. 202, Fig. 11.5).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 11.1, p. 192 — Age pyramids for human population. Three pyramids labelled left-to-right: Expanding (broad base, narrow top), Stable (more rectangular columns), Declining (narrow base, wide top). Three horizontal age strata stacked: Pre-reproductive (bottom), Reproductive (middle), Post-reproductive (top). Each pyramid splits males and females.
• Figure 11.2, p. 193 — Population density flow. Central oval Population Density (N) with arrows: Natality (B) and Immigration (I) flow in (+); Mortality (D) and Emigration (E) flow out (−). Embodies N(t+1) = N(t) + [(B + I) − (D + E)].
• Figure 11.3, p. 194 — Growth curves. Same axes (population density N vs time t). Curve (a) — exponential, dN/dt = rN, J-shaped, unrestricted. Curve (b) — logistic, dN/dt = rN[(K − N)/K], sigmoid with lag → acceleration → deceleration → asymptote at horizontal dashed line K (carrying capacity).
• Table 11.1, p. 197 — Six interspecific interactions. Species A/B sign columns: Mutualism +/+; Competition −/−; Predation +/−; Parasitism +/−; Commensalism +/0; Amensalism −/0.
• Figure 11.4, p. 201 — Fig–wasp mutualism. Two panels: (a) Fig flower pollinated by wasp; (b) Wasp laying eggs in a fig fruit (oviposition). Reinforces one-to-one mutualism.
• **Figure 11.5, p. 202 — Ophrys + bee.** Single photograph of bee acting as a pollinator on an orchid flower; petal mimics female bee shape/colour → pseudocopulation → pollen transfer.

2.4 Common confusions / NTA trap points

• Predation vs parasitism share the (+/−) sign in Table 11.1; distinguish by closeness/duration of association — parasites live in/on the host; predators do not.
• dN/dt = rN does NOT contain K. Only the logistic equation dN/dt = rN[(K − N)/K] does. NTA frequently swaps these.
• "Intrinsic rate of natural increase" is r = b − d (per-capita birth minus per-capita death), NOT total births minus total deaths.
• Commensalism (+/0) vs Amensalism (−/0) — easy to mix up; the sign on the affected partner flips.
• Koel–crow is brood parasitism (a form of parasitism), NOT commensalism or mutualism — a recurring distractor.
• In Connell's experiment, Balanus is the dominant (larger, superior) species; Chathamalus is excluded — students sometimes reverse the two.
• The fig–wasp system is mutualism even though wasp larvae eat some developing seeds — both partners benefit overall; do not call it parasitism.
• Female mosquito is NOT a parasite despite needing blood for reproduction — explicit NCERT statement.
• Cuscuta has lost chlorophyll and leaves during evolution — it is an ectoparasitic plant; the whole plant is parasitic, not the seeds.
• K vs r confusion — K is the upper limit (carrying capacity) of population size; r is the per-capita growth rate.
• Logistic curve phases — lag, acceleration, deceleration, asymptote (4 phases) — NCERT does not call it "log phase, stationary phase" (avoid microbial culture terminology).
• Monarch butterfly's poison is acquired during the caterpillar stage by feeding on a poisonous weed, not synthesised by the butterfly.

2.5 Key processes / classifications