Ecology

Ecology is concerned with four levels of biological organisation: organisms, populations, communities and biomes. At the population and community level, organisms interact with each other in various ways that determine the structure and functioning of the biological community. **Population:** A group of individuals of a given species sharing or competing for similar resources in a defined geographical area, potentially interbreeding. A population has attributes that individual organisms do not — birth rates, death rates, sex ratio, and age distribution. --- ### Abiotic Factors and Adaptations Major abiotic environmental factors affecting organisms: **Temperature:** Most ecologically relevant environmental factor. Affects enzyme kinetics, metabolism, activity of organisms. Range: sub-zero (polar/high altitude) to >50°C (tropical deserts). Some microbes (archaebacteria) flourish in hot springs and deep-sea hydrothermal vents where temperatures far exceed 100°C. - **Eurythermal:** Organisms tolerant to a wide range of temperatures (few species) - **Stenothermal:** Restricted to a narrow range of temperatures (vast majority) **Water:** After temperature, the most important factor. Productivity and distribution of plants depends heavily on water. Aquatic organisms care about quality (chemical composition, pH) and salinity. - Salinity: <5 ppt in inland waters; 30-35 ppt in the sea; >100 ppt in hypersaline lagoons - **Euryhaline:** Tolerant to wide range of salinity - **Stenohaline:** Restricted to narrow salinity range **Light:** Essential for photosynthesis (autotrophs). Many animals use photoperiod (diurnal/seasonal light variation) as cue for foraging, reproductive, and migratory activities. **Soil:** Nature and properties vary; depend on climate, weathering process, soil development. Composition, grain size, aggregation determine percolation and water holding capacity. pH, mineral composition and topography determine vegetation type. **Responses to abiotic stress — four strategies:** 1. **Regulate:** Maintain homeostasis through physiological (or behavioural) means. All birds and mammals can maintain constant body temperature (thermoregulation). Mammals maintain constant internal temperature of 37°C. 2. **Conform:** 99% of animals and nearly all plants cannot maintain constant internal environment — body temperature changes with ambient temperature. Energetically too expensive for small animals to thermoregulate. 3. **Migrate:** Move away temporarily to more hospitable area. Migratory birds fly from Siberia to Keoladeo National Park (Bharatpur, Rajasthan) every winter. 4. **Suspend:** Thick-walled spores (bacteria, fungi, lower plants); seeds and vegetative structures (higher plants). In animals: **hibernation** (bears in winter), **aestivation** (snails, fish to avoid summer heat/desiccation), **diapause** (many zooplankton in lakes/ponds — stage of suspended development). **Key adaptations:** - Kangaroo rat (N. American desert): meets all water requirements through internal fat oxidation; concentrates urine - Desert plants: thick cuticle; stomata in deep pits; CAM photosynthesis pathway (stomata closed during day); plants like Opuntia have no leaves (reduced to spines) - High altitude: body compensates low oxygen by increasing red blood cell production, decreasing binding capacity of hemoglobin, increasing breathing rate (**altitude sickness** at >3,500m) - **Allen's Rule:** Mammals from colder climates generally have shorter ears and limbs to minimise heat loss --- ### Population Attributes Population density (N): Total number of individuals per unit area. Can also be measured as percent cover or biomass. Tiger census in national parks based on pug marks and fecal pellets. **Four basic processes changing population density:** - **Natality (B):** Number of births added to initial density - **Mortality (D):** Number of deaths during given period - **Immigration (I):** Individuals that have come into the habitat from elsewhere - **Emigration (E):** Individuals that left the habitat Equation: N(t+1) = Nt + [(B+I) − (D+E)] **Age pyramid:** Shows age distribution (per cent individuals of a given age group) of a population graphically. Reflects whether population is: - **Expanding:** Broad base (many pre-reproductive individuals) - **Stable:** Near equal proportions - **Declining:** Narrow base (fewer pre-reproductive individuals) --- ### Population Growth Models **Exponential Growth:** When resources are unlimited (ideally), population grows exponentially. The equation: dN/dt = (b − d) × N; let (b − d) = r, then **dN/dt = rN** where r = intrinsic rate of natural increase — the most important parameter for assessing impacts of biotic/abiotic factors on population growth. Integral form: Nt = N₀e^(rt) Results in **J-shaped curve** when N is plotted against time. r values: Norway rat = 0.015; flour beetle = 0.12; human population in India (1981) = 0.0205. **Logistic Growth (more realistic):** No population has unlimited resources in nature. Resources become limiting → competition for resources → 'fittest' individual survives and reproduces. Every habitat has a maximum possible number beyond which no further growth = **carrying capacity (K)**. Equation (Verhulst-Pearl Logistic Growth): **dN/dt = rN[(K−N)/K]** Population shows: lag phase → acceleration → deceleration → asymptote (at carrying capacity). Results in **sigmoid (S-shaped) curve**. Logistic growth model is more realistic. --- ### Population Interactions (Interspecific) Populations of different species do not live in isolation. Interactions are classified by their effects (+, −, 0): | Species A | Species B | Interaction | |-----------|-----------|-------------| | + | + | Mutualism | | − | − | Competition | | + | − | Predation | | + | − | Parasitism | | + | 0 | Commensalism | | − | 0 | Amensalism | **(i) Predation (+/−):** Predators transfer energy fixed by autotrophs to higher trophic levels. Key roles: - Keep prey populations under control - Help maintain species diversity by reducing intensity of competition among prey species - The prickly pear cactus introduced into Australia (1920s) spread rapidly — brought under control only after a cactus-feeding predator moth from its natural habitat was introduced → demonstrates biological control - Starfish Pisaster is a keystone predator on American Pacific Coast — removing it caused >10 invertebrate species to become extinct within a year due to interspecific competition Prey defences against predation: - Cryptic colouration (camouflage): insects and frogs - Chemical toxins: plants store nicotine, caffeine, quinine, strychnine as defence against grazers - Monarch butterfly: highly distasteful to predators due to chemical acquired from poisonous weed during caterpillar stage - Plants: thorns (Acacia, Cactus); Calotropis produces cardiac glycosides **(ii) Competition (−/−):** Both species are negatively affected. **Gause's Competitive Exclusion Principle:** Two closely related species competing for same resources cannot coexist indefinitely; competitively inferior one will be eliminated eventually. However, competition need not always lead to exclusion: - **Resource partitioning:** Species competing for same resource avoid competition by using it at different times, different foraging patterns. MacArthur showed five species of warblers living on same tree could coexist due to behavioural differences in foraging. - **Competitive release:** A species restricted to a small geographical area because of competitively superior species expands dramatically when competing species is removed (Connell's barnacle experiments on Scottish rocky sea coasts: Balanus vs Chthamalus) - Abingdon tortoise (Galapagos) became extinct within a decade after goats were introduced — evidence of competitive exclusion in nature **(iii) Parasitism (+/−):** Parasite benefits; host is harmed. Parasites are usually host-specific (co-evolution of host

Frogs play a significant role in ecosystems, contributing to ecological balance and offering benefits to humankind. They are known to consume insects, thereby protecting crops from pest damage. In their capacity as insectivores, frogs serve as an important link within food chains and food webs, facilitating the transfer of energy and nutrients through the ecosystem. Beyond their ecological contributions, frogs are also utilized by humans, with their muscular legs being consumed as food in certain countries.

An **ecosystem** is a functional unit of nature where living organisms interact among themselves and with their surrounding physical environment. Ecosystems vary from a small pond to a large forest or a sea. The entire biosphere can be regarded as a global ecosystem — a composite of all local ecosystems on Earth. Two basic categories: **terrestrial** (forest, grassland, desert) and **aquatic** (pond, lake, wetland, river, estuary). Crop fields and aquariums are man-made ecosystems. The four components of an ecosystem's function: 1. Productivity 2. Decomposition 3. Energy flow 4. Nutrient cycling **Vertical distribution** of different species in an ecosystem gives its species composition — this stratification is seen when, for example, trees occupy the top stratum of a forest, shrubs the second, herbs and grasses the bottom layers. --- ### Productivity **Primary production** = amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. Expressed in weight (g m⁻²) or energy (kcal m⁻²) per year. - **Gross Primary Productivity (GPP):** Rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration (R). - **Net Primary Productivity (NPP):** GPP minus respiration losses. NPP = GPP − R. This is the available biomass for consumption by heterotrophs (herbivores and decomposers). - **Secondary Productivity:** Rate of formation of new organic matter by consumers. Annual net primary productivity of the whole biosphere: approximately **170 billion tons** (dry weight) of organic matter. Of this, despite occupying 70% of the surface, the productivity of the oceans is only 55 billion tons. Rest is on land. Plants capture only **2-10 per cent** of PAR (Photosynthetically Active Radiation — less than 50% of incident solar radiation). --- ### Decomposition **Decomposers** (mainly fungi and bacteria) break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients. This process is called **decomposition**. Dead plant remains (leaves, bark, flowers) and dead animal remains including fecal matter = **detritus** — the raw material for decomposition. **Steps in decomposition:** 1. **Fragmentation:** Detritivores (e.g., earthworm) break detritus into smaller particles 2. **Leaching:** Water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts 3. **Catabolism:** Bacterial and fungal enzymes degrade detritus into simpler inorganic substances 4. **Humification:** Leads to accumulation of dark-coloured amorphous substance called **humus** — highly resistant to microbial action, undergoes decomposition at extremely slow rate; colloidal in nature, serves as reservoir of nutrients 5. **Mineralisation:** Humus further degraded by some microbes to release inorganic nutrients Decomposition is largely **oxygen-requiring** (aerobic). Rate controlled by: - Chemical composition of detritus: **slower** if rich in lignin and chitin; **quicker** if rich in nitrogen and water-soluble substances like sugars - **Climatic factors:** Warm and moist conditions favour decomposition; low temperature and anaerobiosis inhibit it --- ### Energy Flow Sun is the only source of energy for all ecosystems (except deep-sea hydrothermal vents). Energy flow is **unidirectional** — from sun to producers to consumers. **Grazing Food Chain (GFC):** Begins with living green plants (producers). Example: Grass → Goat → Man **Detritus Food Chain (DFC):** Begins with dead organic matter. Made up of decomposers (fungi and bacteria) = **saprotrophs** (sapro: to decompose). In terrestrial ecosystems, a much larger fraction of energy flows through DFC than GFC. In aquatic ecosystems, GFC is the major conduit. **Trophic levels:** - First trophic level: Producers (plants, phytoplankton) - Second trophic level: Primary consumers/herbivores (zooplankton, grasshopper, cow) - Third trophic level: Secondary consumers/carnivores (birds, fishes, wolf) - Fourth trophic level: Tertiary consumers/top carnivores (man, lion) **10 Per Cent Law (Lindeman's Law):** Only **10 per cent** of the energy is transferred to each successive trophic level from the lower trophic level. This limits the number of trophic levels in the grazing food chain. **Standing crop:** The mass of living material at a particular trophic level at a given time. Measured as **biomass** (mass of living organisms) or number per unit area. Biomass measured in terms of dry weight. **Food web:** Natural interconnection of food chains. Some organisms (cockroaches, crows) are omnivores and connect GFC and DFC. --- ### Ecological Pyramids Relationship between trophic levels expressed in terms of number, biomass or energy: 1. **Pyramid of number:** Shows number of individuals at each trophic level. Usually upright (more plants than herbivores than carnivores). Can be inverted (e.g., single large tree supports many insects) 2. **Pyramid of biomass:** Usually upright in terrestrial ecosystems. **Inverted** in aquatic ecosystems (small standing crop of phytoplankton supports large standing crop of zooplankton — biomass of fishes far exceeds that of phytoplankton) 3. **Pyramid of energy:** Always **upright** — can NEVER be inverted. Energy always decreases at higher trophic levels because some energy is lost as heat at each step. The base shows the amount of energy at each trophic level in a given time per unit area. **Limitations of ecological pyramids:** Does not account for species belonging to two or more trophic levels simultaneously; assumes simple food chain (never exists in nature); saprophytes not given any place. --- ### Ecological Succession The gradual and fairly predictable change in the species composition of a given area is called **ecological succession**. Changes are orderly and sequential. Eventually leads to a **climax community** — in near equilibrium with the environment. - Entire sequence of communities changing in a given area = **sere(s)** - Individual transitional communities = **seral stages/seral communities** - During succession: increase in diversity of species, increase in number of organisms and biomass **Primary succession:** Starts where no living organisms exist — bare rock, newly cooled lava, newly created pond. Slow process (several hundred to several thousand years to produce fertile soil on bare rock). **Secondary succession:** In areas where natural biotic communities destroyed (abandoned farms, burned or cut forests, flooded lands). Faster than primary succession since soil or sediment is already present. **Succession of plants — two types:** 1. **Hydrarch succession:** In wetter areas; series progresses from hydric to mesic conditions. In water: phytoplankton → submerged free-floating plants → submerged plant stage → reed-swamp stage → marsh-meadow stage → scrub stage → forest (climax). Water body ultimately gets converted into land. 2. **Xerarch succession:** In dry areas; series progresses from xeric to mesic conditions. Pioneers on bare rock are lichens (secrete acids to dissolve rock, helping weathering and soil formation) → bryophytes → bigger plants → stable climax forest community Both hydrarch and xerarch successions lead to medium water conditions (mesic) — neither too dry nor too wet. All succession proceeds to a similar climax community — **the mesic**. **Pioneer species:** Species that invade bare areas first. On bare rock = lichens. --- ### Nutrient Cycling (Biogeochemical Cycles) The movement of nutrient elements through the various components of an ecosystem = **nutrient cycling** (also called biogeochemical cycles: bio = living organism, geo = rocks, air, water). Nutrients are never lost from ecosystems — recycled indefinitely. Amount of nutrients (C, N, P, Ca, etc.) present in soil at a given time = **standing state**. Two types: 1. **Gaseous cycles** (e.g., nitrogen cycle, carbon cycl