Organisms and Populations

Ecology studies organisms in relation to their environment, at increasing levels of organisation: organism → population → community → ecosystem → biome → biosphere. At the organism level, life is shaped by major abiotic (physical) factors: temperature, water, light and soil. Temperature is the most important ecological factor; organisms tolerant of a wide temperature range are eurythermal, while those tolerant of a narrow range are stenothermal. Similarly, for salinity, euryhaline organisms tolerate a wide range and stenohaline a narrow range.

When the environment becomes stressful (too hot, cold or dry), organisms respond in one of four ways — a guaranteed NEET set. The first is to regulate: maintain a constant internal environment (homeostasis) despite changes outside. Birds and mammals are the main regulators — they keep a constant body temperature (thermoregulation) and osmotic concentration. This is energetically expensive but very successful.

The second response is to conform. Conformers do not maintain a constant internal state; instead their body temperature and osmotic concentration change with the surroundings. The vast majority of animals (about 99%) and nearly all plants are conformers, because regulation costs too much energy for small organisms (which lose heat fast due to a high surface-area-to-volume ratio).

The third response is to migrate — to move away temporarily from the stressful habitat to a more favourable one, as migratory birds do in winter (e.g. to Keoladeo National Park, Bharatpur). The fourth is to suspend activities — to become dormant: bacteria and fungi form thick-walled spores; animals undergo hibernation (winter sleep, e.g. bears) or aestivation (summer dormancy, e.g. snails) and zooplankton show diapause. For NEET, fix the ecology levels, that temperature is the chief abiotic factor (eury-/steno- terms), and the four responses with examples (regulate = birds/mammals; conform = most organisms; migrate; suspend = hibernation/aestivation/diapause/spores).

An adaptation is any attribute — morphological, physiological or behavioural — that helps an organism survive and reproduce in its habitat. Adaptations are products of long evolution, and the textbook examples are commonly asked.

The most famous physiological adaptation is the kangaroo rat of the North American deserts: it can meet all its water requirement through the internal oxidation of fats (so it can live without ever drinking water) and it produces highly concentrated urine to minimise water loss. Desert plants show clear adaptations to conserve water: a thick waxy cuticle, sunken stomata, and the special CAM (Crassulacean Acid Metabolism) photosynthetic pathway that lets them keep stomata closed in the day. In Opuntia (a cactus), the leaves are reduced to spines and the flattened green stem carries out photosynthesis.

Several adaptations relate to temperature and size. Allen's Rule states that mammals of colder climates have shorter ears and limbs (extremities) to reduce heat loss. By a related principle, mammals in colder regions tend to be larger in body size (a smaller surface-area-to-volume ratio conserves heat). In cold seas, animals like seals have a thick layer of blubber (fat) for insulation.

Adaptations can also be behavioural and biochemical. Desert lizards bask in the sun when cold and move into shade or burrows when hot to keep their temperature suitable. People going to high altitude face low oxygen and may suffer altitude sickness; the body acclimatises by producing more red blood cells and increasing breathing rate. Some microbes (archaebacteria) even live in hot springs and deep-sea vents at extreme temperatures. For NEET, fix the kangaroo rat (water from fat oxidation, concentrated urine), desert-plant adaptations (CAM, sunken stomata, spines/Opuntia), Allen's Rule (short extremities in cold) and altitude acclimatisation (more RBCs).

A population is a group of individuals of the same species living in a given area. Unlike an individual, a population has attributes that only a group can have. Population density is the number of individuals per unit area (or volume). Natality is the birth rate and mortality the death rate (per capita). Sex ratio is the proportion of males to females. The age distribution is shown as an age pyramid: a broad-based expanding (growing) pyramid, a bell-shaped stable one, or an urn-shaped declining one.

Population size changes through four processes: natality (B) and immigration (I) increase it, while mortality (D) and emigration (E) decrease it. So the population at the next time is N(t+1) = N(t) + [(B + I) − (D + E)]. When births and immigration exceed deaths and emigration, the population grows.

Population growth follows one of two models. Exponential growth occurs when resources (food, space) are unlimited: the population grows faster and faster, giving a J-shaped curve, described by dN/dt = rN (where r is the intrinsic rate of natural increase). Such unlimited growth cannot continue for long in nature.

The more realistic model is logistic growth, because every habitat has a finite carrying capacity (K) — the maximum population the environment can support. Here growth is fast at first, then slows as the population nears K, giving an S-shaped (sigmoid) curve, described by dN/dt = rN[(K − N)/K]. As N approaches K the growth rate falls to zero and the population levels off. For NEET, fix the population attributes (density, natality/mortality, age pyramids), the growth equation N(t+1), and the contrast between exponential (J-shaped, unlimited, dN/dt = rN) and logistic (S-shaped, carrying capacity K) growth.

No population lives alone — species interact, and these population interactions are classified by their effect (+ benefit, − harm, 0 neutral) on each of the two species. Knowing the sign-pattern and a textbook example for each is core NEET recall.

Mutualism (+/+) benefits both species. Classic examples are lichens (a mutualism between a fungus and an alga), mycorrhiza (fungus + plant roots) and the remarkable fig–wasp relationship (the wasp pollinates the fig and uses it to lay eggs). Competition (−/−) harms both species, which fight over the same limited resource; Gause's competitive exclusion principle states that two species competing for the same resources cannot coexist indefinitely — the inferior one is eliminated.

Two interactions benefit one and harm the other (+/−). In predation, the predator (+) kills and eats the prey (−); predators keep prey populations in check and are important for energy transfer and controlling invasive species. In parasitism, the parasite (+) lives on/in the host (−) and harms it (usually without killing it quickly); parasites are often host-specific and may have special adaptations (loss of sense organs, presence of suckers/hooks).

The remaining two involve a neutral partner. Commensalism (+/0) benefits one species while the other is unaffected — examples are an orchid growing on a tree branch, barnacles on the back of a whale, and the cattle egret that feeds on insects stirred up by grazing cattle. Amensalism (−/0) harms one species while the other is unaffected. For NEET, memorise the sign-pattern + example for each: mutualism (+/+, lichen/mycorrhiza/fig-wasp), competition (−/−, Gause's principle), predation and parasitism (+/−), commensalism (+/0, orchid/barnacle/cattle egret) and amensalism (−/0).