Organisms and Populations

🎯 Key Points

• Eurythermal/euryhaline = tolerate WIDE range (temperature/salinity); Stenothermal/stenohaline = tolerate only NARROW range — prefix "eury-"=wide, "steno-"=narrow
• Stress responses: Regulate (homeostasis, e.g. endotherms) → Conform (body matches environment, ~99% of species, e.g. ectotherms) → Migrate (temporarily relocate) → Suspend (dormancy: Hibernation=winter, Aestivation=summer, Diapause=insects/zooplankton)
• Exponential growth: dN/dt=rN (unlimited resources, J-curve); Logistic growth: dN/dt=rN(K−N)/K (limited resources, S-curve, growth→0 as N→K) — logistic is the realistic model
• Interactions by who benefits/harmed: Mutualism(+,+) Competition(−,−) Predation/Parasitism(+,−) Commensalism(+,0) Amensalism(0,−) — signs tell you the relationship type instantly
• Gause's competitive exclusion principle: two species competing for the IDENTICAL limiting resource cannot coexist indefinitely — one eventually excludes the other (demonstrated by Connell's barnacle studies)

Organism and Its Environment

• Ecology is the study of organisms in relation to their environment; it operates at the level of the organism, population, community, ecosystem, and biome.
• Organisms are adapted to their habitat's prevailing climate and other conditions through physiological, morphological, and behavioural adaptations evolved over generations.
• Species distribution on Earth is largely determined by climate and topography, and locally by edaphic (soil) factors.

Major Abiotic Factors

• Temperature: ecologically the most important abiotic factor; affects enzyme kinetics, metabolic rate, and is responsible for major adaptations. Organisms that can tolerate and thrive in a wide range of temperatures are called eurythermal; those restricted to a narrow temperature range are stenothermal.
• Water: essential for life; availability is a major limiting factor in terrestrial habitats (deserts) and salinity is critical in aquatic habitats. Organisms tolerating a wide range of salinity are euryhaline; those tolerant of only a narrow range are stenohaline.
• Light: required by green plants for photosynthesis; many small plants in forests are adapted to photosynthesise optimally under very low light (shade adaptation). Light also affects the biological clock/photoperiod of organisms (flowering, migration, breeding). Ultraviolet (UV) component can damage cells, though it is largely screened out by the ozone layer.
• Soil: composition, grain size, and aggregation determine soil porosity, which affects water-holding capacity and aeration; soil pH, mineral composition, and topography also influence the type of vegetation and organisms present in an area.

Responses to Abiotic Stress

• Regulate: maintain a constant internal environment (homeostasis) through physiological (sometimes behavioural) means; e.g., mammals and birds are homeotherms / endotherms, regulating body temperature by mechanisms like sweating and panting (cooling) or shivering (heat generation).
• Conform: most organisms (about 99%) cannot self-regulate and their body temperature/osmotic concentration changes with the ambient environment; these are conformers (e.g., poikilotherms or ectotherms such as most invertebrates, fish, amphibians, and reptiles).
• Migrate: organisms move away temporarily from a stressful habitat to a more favourable area and return when stressful period ends; e.g., Siberian birds migrating to Indian wetlands such as Keoladeo National Park (Bharatpur) during winter.
• Suspend: under unfavourable conditions, some organisms enter a state of dormancy:
  • Hibernation: dormancy during winter to escape cold (e.g., bears).
  • Aestivation: dormancy during summer to escape heat and desiccation (e.g., snails, fish in deserts/dry regions).
  • Diapause: a stage of suspended development seen in zooplankton and many insects during unfavourable conditions.
  • Many plant seeds and spores of bacteria/fungi/lower plants also become dormant to survive extreme conditions, germinating again when conditions become favourable.

Population Attributes

• A population is a group of interbreeding individuals of the same species occupying a given area at a particular time; populations have unique attributes not shown by individuals.
• Population density: the number of individuals per unit area or volume at a given time; usually expressed relative to total space (e.g., number per hectare) since absolute counting is often impractical (in such cases, indirect measures like percent cover or pug marks may be used).
• Natality (birth rate): number of births during a specified time in the population relative to population size; can exceed 1.
• Mortality (death rate): number of deaths in the population during a specified time.
• Population size increases if natality and immigration are higher than mortality and emigration, and decreases if the reverse is true.
• Age pyramid (age distribution): a plot of the percentage or proportion of individuals of a given age (or age group) relative to the total population, giving a pictorial representation of the age structure; its shape reflects whether a population is growing, stable, or declining.

Population Growth

• Population density is not static; it changes depending on food availability, predators, and the weather.
• Exponential growth: occurs when resources (food and space) are unlimited; growth rate is proportional to population size and follows the equation dN/dt = rN, where N is population size, t is time, and r is the intrinsic rate of natural increase. Integral form: Nt = N0 * e^(rt). This produces a J-shaped growth curve.
• Logistic growth: in nature, resources are finite and limiting, so populations grow exponentially only initially, then growth slows as the population approaches the carrying capacity (K), producing an S-shaped (sigmoid) curve. The logistic growth equation (Verhulst-Pearl logistic growth model) is: dN/dt = rN * (K - N) / K, where K is the carrying capacity.
• The logistic model is considered more realistic, since no population can increase exponentially indefinitely as resources are always finite, eventually limiting growth.

Population Interactions

• Mutualism: an interaction in which both interacting species benefit; examples include lichens (fungus and photosynthetic algae/cyanobacteria), mycorrhizae (fungal association with roots that helps plant absorb nutrients), and the highly specific plant-pollinator mutualisms (figs and wasps, orchids with elaborate flower structures matching specific pollinators).
• Competition: a negative interaction where closely related (or unrelated but similar) species compete for the same limited resources. Gause's competitive exclusion principle states that two closely related species competing for the same limited resources cannot coexist indefinitely; the competitively superior species will eventually eliminate the other.
• Connell's intertidal studies on two barnacle species (Chthamalus and Balanus) on rocky sea coasts of Scotland provided experimental field evidence that competition can determine species distribution; Balanus excluded Chthamalus from the lower zones of rocks through competition for space.
• Predation: an interaction in which a predator kills and consumes the prey it feeds on; predators help keep prey populations in check and can help maintain species diversity by reducing the intensity of competition among competing prey species (keystone role). Some predators are also useful in biological control (e.g., the cactus moth used to control the prickly pear cactus population in Australia).
• Parasitism: the parasite depends on the host for nutrition, often causing harm to it; many parasites have evolved special adaptations such as the loss of unnecessary sense organs and the presence of adhesive organs/suckers to cling to the host (e.g., tapeworm in human intestine, plant parasites like Cuscuta).
• Brood parasitism: a special form of parasitism in birds, where the parasitic bird lays its eggs in the nest of its host and lets the host incubate them, as seen in the cuckoo (koel) and crow.
• Commensalism: an interaction in which one species benefits and the other is neither harmed nor benefited; e.g., an orchid growing as an epiphyte on a mango branch, or the cattle egret foraging close to grazing cattle.
• Amensalism: an interaction where one species is harmed or inhibited while the other is unaffected; e.g., a large tree shading out a small plant beneath it without itself being affected.

Adaptations

• An adaptation is any attribute (morphological, physiological or behavioural) that enables an organism to survive and reproduce in its habitat; many adaptations are genetically fixed, having evolved over long periods of time
• Kangaroo rat (North American deserts): meets nearly all its water needs from internal oxidation of fat (metabolic water) and can concentrate its urine to minimise water loss
• Desert plants: have thick cuticles and sunken stomata to reduce transpiration; many (e.g. Opuntia) have no leaves at all — reduced to spines — with photosynthesis carried out by flattened green stems, and many use the special CAM (Crassulacean Acid Metabolism) pathway to keep stomata closed during the day
• Allen's Rule: mammals of colder climates generally have shorter ears and limbs to minimise heat loss (reducing surface area relative to volume)
• Cold/aquatic adaptation: polar seals have a thick layer of subcutaneous fat (blubber) that insulates the body against cold
• Physiological adaptation (acclimatisation): at high altitudes with low oxygen, the body compensates by producing more red blood cells, decreasing the binding affinity of haemoglobin, and increasing the breathing rate
• Behavioural adaptation: desert lizards bask in the sun to absorb heat when cold and move into shade or burrows when it becomes too hot, thereby thermoregulating through behaviour

Sex Ratio and Age-Pyramid Shapes

• Alongside density, natality and mortality, a population also has a sex ratio — an individual is either male or female, but a population is characterised by its proportion of males to females (e.g. 60% females and 40% males)
• When the percentage of individuals of different ages is plotted, the resulting age pyramid takes a shape that reflects the population's growth status
• Expanding (growing) population: a broad-based, triangular pyramid with a large proportion of young, pre-reproductive individuals
• Stable population: a bell-shaped pyramid, with the pre-reproductive and reproductive age groups roughly equal
• Declining population: an urn-shaped pyramid with a narrow base, showing fewer young individuals than reproductive and post-reproductive ones

Life History Variation

• Populations evolve towards a life history that maximises reproductive fitness (the number of surviving offspring) in the particular habitat they occupy, shaped by natural selection
• Semelparity (big-bang reproduction): the organism breeds only once in its lifetime, investing all its resources into a single large reproductive event — e.g. Pacific salmon and many bamboo species, which die soon after breeding
• Iteroparity: the organism breeds many times during its lifetime — e.g. most birds and mammals
• There is also a trade-off in offspring investment: some organisms produce a large number of small-sized offspring (e.g. oysters, pelagic fishes), while others produce a small number of large-sized offspring (e.g. birds, mammals)

Prey Defences and Plant Chemical Defences

• Because predation exerts strong selection pressure, prey species have evolved varied defences against being eaten: camouflage (cryptic colouration) to avoid detection, and being distasteful or poisonous to predators
• Monarch butterfly: it is highly distasteful to its bird predators because it stores a special toxic chemical acquired during its caterpillar stage from a poisonous milkweed plant it fed on
• Plant defences against herbivores: unable to run away, plants have evolved morphological defences (thorns, e.g. in Acacia and Cactus) and an array of chemical defences — many secondary metabolites deter or harm herbivores, e.g. nicotine, caffeine, quinine, strychnine and opium
• The weed Calotropis produces highly poisonous cardiac glycosides, which is why cattle and goats do not browse on it

🚀 NEET Advanced Edge

Why predation can INCREASE prey species diversity rather than reduce it: A predator that disproportionately consumes the most competitively dominant prey species prevents that species from monopolising resources, indirectly allowing weaker competitors to persist — this "keystone predator" effect is why removing a top predator from an ecosystem can paradoxically cause a crash in overall prey diversity, not an increase.

Why r and K are NOT independent of population density in the logistic model: In dN/dt = rN(K−N)/K, the term (K−N)/K acts as a damping factor that approaches 1 when N is small (growth is nearly exponential) and approaches 0 as N nears K (growth nearly stops) — this single equation smoothly captures the transition from exponential to saturated growth without needing two separate formulas.

Worked problem: A population has intrinsic growth rate r = 0.5/year and carrying capacity K = 1000. At population size N = 800, calculate dN/dt. Approach: dN/dt = rN(K−N)/K = 0.5 × 800 × (1000−800)/1000 = 0.5 × 800 × 0.2 = 80 individuals/year.