The scientific consensus is that current global warming is primarily caused by human activities releasing greenhouse gases — especially CO₂ from burning fossil fuels (coal, oil, natural gas), with contributions from deforestation, agriculture, and industry.

Ecological restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed — reintroducing native species, removing invasive species, restoring hydrology, and allowing natural succession to proceed.

Biodiversity provides multiple benefits: ecosystem stability and resilience (more redundancy), ecosystem services (clean water, air, pollination), potential medicines (25% of pharmaceuticals derive from plants), food security (genetic diversity of crops), and cultural/aesthetic value.

Bioremediation uses microorganisms (bacteria, fungi) or plants (phytoremediation) to degrade, detoxify, or immobilise environmental pollutants in soil, water, and air. Examples include using Pseudomonas bacteria to break down oil spills.

Density-independent factors affect populations regardless of their density — volcanic eruptions, earthquakes, floods, droughts, and severe storms kill a fixed proportion of organisms irrespective of population size. Density-dependent factors (disease, competition, food) intensify with higher density.

Addressing climate change requires combined action at all levels — reducing greenhouse gas emissions through: renewable energy transition, energy efficiency, forest protection, sustainable agriculture, and international agreements like the Paris Agreement (2015).

Biomimicry is design inspired by nature's proven patterns and strategies. Designing buildings using passive ventilation based on termite mound architecture (termite mounds maintain constant temperature without energy input) is a classic example.

The rate of photosynthesis determines gross primary productivity (GPP). Net primary productivity (NPP) = GPP minus plant respiration. NPP is the energy available to consumers and determines the overall productivity and biomass of the ecosystem.

Ozone depletion in the stratosphere allows more harmful UV-B and UV-C radiation to reach Earth's surface. This increases skin cancer, cataracts, and immune suppression in humans; damages marine phytoplankton; and harms terrestrial plants and animals.

The ecological footprint concept (Rees and Wackernagel, 1990s) measures human demand on nature — the area of biologically productive land and water needed to produce resources consumed and absorb wastes generated by a person or population.

Increased plant growth absorbing more CO₂ is a negative feedback — warming stimulates plant growth (in some regions), which absorbs more CO₂, reducing the greenhouse effect and partially counteracting warming. Negative feedback reduces the original perturbation.

Deforestation is the main cause of soil erosion — tree roots bind soil, leaf litter protects the surface, and canopy intercepts rainfall. Without trees, rain directly impacts bare soil, detaching particles and runoff carries them away, causing erosion and sedimentation of waterways.

Ocean acidification occurs as oceans absorb increasing atmospheric CO₂ — forming carbonic acid and lowering ocean pH (now ~8.1, down from pre-industrial 8.2). This dissolves calcium carbonate shells of corals, molluscs, and plankton, threatening marine food webs.

Agroforestry is an integrated approach that combines growing trees with crops or livestock on the same land. It improves biodiversity, soil quality, and carbon storage while providing multiple products (timber, food, fodder) and reducing erosion.

A habitat is the physical place where an organism lives — the specific environment providing the resources (food, shelter, water, space) it needs. It is defined by abiotic conditions (temperature, rainfall, soil type). Niche describes how the organism lives.

Phosphorus enters the phosphorus cycle primarily through the weathering of phosphate-containing rocks (like apatite). Unlike carbon and nitrogen, there is no atmospheric reservoir of phosphorus — it cycles only through soil, water, and organisms.

Biomagnification is the progressive increase in concentration of persistent pollutants (DDT, PCBs, mercury) at each trophic level. Organisms at higher trophic levels accumulate higher concentrations because the pollutants are not metabolised and accumulate in fatty tissues.

Botanical gardens are ex situ conservation — they maintain living plant collections outside their natural habitats. They serve as living museums, research centres, and gene banks. Other ex situ methods include zoos, seed banks, and tissue culture banks.

Sustainable development (Brundtland Commission, 1987) is 'development that meets the needs of the present without compromising the ability of future generations to meet their own needs' — balancing economic, social, and environmental concerns.

Water vapour is the largest contributor to the natural greenhouse effect — responsible for about 50% of the greenhouse warming. CO₂ contributes about 20%, with the rest from methane, nitrous oxide, and ozone. However, human activities primarily affect CO₂.

Demography is the statistical study of populations — their size, density, distribution, age structure, birth rates, death rates, and changes over time. It applies to both human populations and wildlife populations in ecology.

A keystone species has a disproportionately large impact on ecosystem structure and function relative to its abundance. Removing a keystone species causes dramatic changes in community composition — like removing the keystone from an arch causing it to collapse.

The oxygen and carbon cycles are intimately linked through photosynthesis (consumes CO₂, releases O₂) and cellular respiration (consumes O₂, releases CO₂). These two processes maintain atmospheric balance of both gases.

Density-dependent factors have greater effects as population density increases — food availability, disease transmission, predation rates, and competition all intensify with higher population density. Temperature, rainfall, and floods are density-independent factors.

Intraspecific competition intensifies as population density increases because more individuals compete for the same limited resources. This is the mechanism of density-dependent regulation — increasing density leads to increased competition and reduced per-capita resource availability.

Carbon footprint can be reduced by: using renewable energy (solar, wind), planting trees (carbon sequestration), improving energy efficiency, reducing meat consumption, using public transport, and reducing consumption and waste.

Biotic potential (r_max) is the theoretical maximum rate of population increase under ideal conditions (unlimited resources, no predators, no disease, optimal conditions). Actual growth rate is always less due to environmental resistance.

Elephants are keystone species in African savanna — they shape the landscape by uprooting trees (preventing forest encroachment), creating water holes by digging, and dispersing seeds. Their removal would transform the ecosystem dramatically.

Photochemical smog forms when nitrogen oxides (NOₓ from vehicle exhaust) and volatile organic compounds (hydrocarbons) react in the presence of sunlight to produce ozone (at ground level — a pollutant), PAN, and other oxidants.

Global warming effects include: rising sea levels (from ice melt and thermal expansion), more frequent and intense extreme weather (heat waves, storms, droughts, floods), shifts in species ranges, ocean acidification, and disruption of ecosystems.

Bioremediation is the use of microorganisms (bacteria, fungi, algae) or plants (phytoremediation) to degrade, detoxify, or immobilise environmental pollutants — including oil spills, heavy metals, pesticides, and organic solvents.

Ecological efficiency is the percentage of energy at one trophic level that is incorporated into the next trophic level (typically about 10%). It determines how much energy is available at higher trophic levels and limits food chain length.

A biome is a large geographic area with a distinctive climate and characteristic community of plants and animals. Tropical rainforest, desert, tundra, taiga, and grassland are biomes. Ponds, coral reefs, and lakes are aquatic ecosystems, not biomes.

Primary productivity is the rate of energy fixation by producers (autotrophs) through photosynthesis or chemosynthesis. Gross primary productivity (GPP) is total fixation; net primary productivity (NPP) = GPP minus producer respiration.

Thermal pollution is caused primarily by industrial cooling water (especially from power plants and factories) discharged into water bodies at elevated temperatures. Higher temperatures reduce oxygen solubility, harming aquatic organisms, especially fish.

Stable ecosystems typically have high species diversity — more species means more complex food webs with redundancy (multiple species performing similar roles). If one species is lost, others can fill its role, making the ecosystem more resilient.

Nitrification is the two-step process: (1) Nitrosomonas bacteria oxidise ammonia (NH₃) to nitrite (NO₂⁻); (2) Nitrobacter bacteria oxidise nitrite to nitrate (NO₃⁻). Nitrates are the main form of nitrogen absorbed by plant roots.

The ozone layer is located in the stratosphere (15-40 km altitude). Ozone (O₃) absorbs most UV-B and UV-C radiation. CFCs, halons, and other chemicals catalytically destroy ozone, creating the 'ozone hole' observed over Antarctica.

A food chain is a linear sequence showing who eats whom — from producer (grass) → primary consumer (rabbit) → secondary consumer (fox) → tertiary consumer (eagle). It represents one pathway of energy flow through an ecosystem.

Human activities are the primary driver of current biodiversity loss — habitat destruction, overexploitation (hunting, fishing), invasive species introduction, pollution, and climate change. This has triggered the 'sixth mass extinction' with rates 100-1000x the background rate.

Interspecific competition is between different species competing for the same resource. Lions and hyenas competing for prey, and plants of different species competing for sunlight, are interspecific. Male deer fighting for mates is intraspecific competition.

Hydrosere is succession that begins in a freshwater habitat (pond, lake) — starting with phytoplankton, then submerged plants, floating-leaved plants, emergent plants (reeds, sedges), marsh, wet woodland, and eventually climax forest as the water body fills.

Rainfall is an abiotic (non-living) factor that limits population size — insufficient rainfall reduces food availability and habitat quality. Predation, disease, and competition are biotic (living) factors affecting population size.

Methane (CH₄) traps about 25 times more heat per molecule than CO₂ over a 100-year period (Global Warming Potential of 25). However, CO₂ is more significant overall because of its much higher atmospheric concentration.

Global ecology (or macroecology) studies ecological processes and patterns at the largest spatial scales — the entire biosphere. It examines how global processes (climate change, biogeochemical cycles, biodiversity patterns) function at planetary level.

Timber from sustainably managed forests is a renewable resource — trees can regrow within human timescales if harvesting rates do not exceed growth rates. Natural gas, petroleum, and nuclear fuel (uranium) are non-renewable resources.

Approximately 90% of energy is lost as heat at each trophic level (through metabolism, maintaining body temperature, and movement). Only about 10% is incorporated into the biomass of organisms at the next level (the 10% rule of ecological efficiency).

K-strategists have: slow development, long lifespan, low reproductive rate, large body size, few offspring, extensive parental care, and stable populations near carrying capacity. They are adapted to stable environments where competition is intense.

Liebig's Law of the Minimum (1840) states that the growth of an organism is controlled by the scarcest essential resource (limiting factor), not the total amount of resources available. Plant growth is often limited by whichever nutrient is most deficient.

A population where births exceed deaths (positive natural increase) will grow in size. If births equal deaths, the population is stable. If deaths exceed births (negative natural increase), the population will decline (assuming no net migration).

Ecotourism generates revenue from tourists visiting natural areas, which can fund conservation efforts, support local communities as an alternative to destructive activities, and raise public awareness of the value of biodiversity.

Acid rain (pH 4-5) causes acidification of lakes and streams, killing fish and aquatic invertebrates that cannot tolerate low pH. It also damages forests (leaches nutrients from leaves, releases toxic aluminium from soil), and corrodes stone buildings.

In a grassland food chain: Grass (producer) → Grasshopper (1° consumer) → Frog (2° consumer) → Snake (3° consumer) → Eagle (4° consumer). The frog eats grasshoppers (primary consumers) making it a secondary consumer.

The number of organisms in an ecosystem is limited by: limiting factors (the scarcest essential resource — water, food, light, nutrients) and primary productivity (energy entering the system). Liebig's Law of the Minimum states growth is limited by the scarcest factor.

Methane (CH₄) is produced by methanogenic archaea during anaerobic decomposition of organic matter in waterlogged soils, wetlands, rice paddies, and animal digestive systems. Methane is a potent greenhouse gas (25x more powerful than CO₂ over 100 years).

Producers (autotrophs — plants, algae, cyanobacteria) convert light energy (or chemical energy in chemosynthesis) into chemical energy stored in organic molecules through photosynthesis. They form the base of food chains and are the primary energy source for ecosystems.

An ecotone is the transition/boundary zone between two adjacent ecosystems or biomes (e.g., forest edge, estuary between river and sea). Ecotones often have characteristics of both adjacent ecosystems and may support unique species.

Habitat fragmentation divides continuous habitats into isolated patches, causing: isolation of populations (reducing gene flow and increasing inbreeding), edge effects, vulnerability to extinction (small populations), and barriers to migration.

The pyramid of biomass shows the total dry mass (biomass) of organisms at each trophic level. It usually decreases from producers to top consumers. Occasionally it can be inverted (e.g., in aquatic ecosystems where phytoplankton biomass is small but has high turnover).

Population density is the number of individuals per unit area (or volume) — individuals per km² for terrestrial species, or individuals per litre for aquatic species. It is distinct from population size (total number) and population growth rate.

Decomposers (bacteria, fungi) release nutrients locked in dead organic matter by breaking it down into inorganic compounds (CO₂, water, ammonia, phosphates). These nutrients are then available for producers to absorb, completing nutrient cycles.

The boreal forest (taiga) is dominated by coniferous (cone-bearing) trees — spruces, pines, firs, and larches. It is the world's largest terrestrial biome, stretching across northern North America, Europe, and Asia.

Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) minus plant respiration (R). NPP represents the energy actually available to primary consumers. NPP = GPP - R. Secondary productivity is the energy stored by consumers.

Rhizobium bacteria in legume root nodules is mutualism — the bacteria fix atmospheric nitrogen providing the plant with a usable nitrogen source, while the plant provides carbohydrates and a protected microhabitat for the bacteria. Both benefit.

An ecotone is the transition zone between two adjacent ecosystems (e.g., forest-grassland edge). Ecotones often have higher species diversity than either adjacent ecosystem — this is called the edge effect.

Invasive species are typically characterised by: rapid spread, lack of natural predators (not option B which says many), outcompeting native species, and threatening local biodiversity. Having many natural predators would actually limit their invasiveness.

Nitrogen in amino acids (and all biological molecules) ultimately comes from atmospheric N₂ fixed by nitrogen-fixing bacteria (Rhizobium, Azotobacter, cyanobacteria). This fixed nitrogen enters the soil and is absorbed by plants as nitrates.

Ex situ conservation preserves species outside their natural habitat. Examples include seed banks (Royal Botanic Gardens Kew Millennium Seed Bank), zoos, botanical gardens, aquaria, and tissue culture banks. In situ conservation protects species in their natural habitat.

In a grassland biome, the climax community is grassland itself — the stable end-point of succession determined by the regional climate (insufficient rainfall for forest). Grassland persists until climate changes or major disturbance occurs.

Habitat destruction is the leading cause of species extinction today — deforestation, wetland drainage, coral reef bleaching, and urbanisation destroy the ecosystems species need to survive. The current extinction rate is 100-1000 times the background rate.

Weather is short-term atmospheric conditions (today's temperature, today's rain) while climate is the long-term pattern of weather (average temperature, rainfall, seasons) in a region over decades. 'Climate is what you expect, weather is what you get.'

Permafrost is the permanently frozen layer of soil or subsoil found beneath the surface in arctic, subarctic, and alpine regions. Only the active layer (surface) thaws seasonally. Permafrost stores vast amounts of organic carbon.

An endemic species is found exclusively in a particular geographic area and nowhere else. Island species are often endemic. High endemism is a criterion for biodiversity hotspots. Endemic species are especially vulnerable to extinction.

Ecological footprint is a measure of the area of biologically productive land and water an individual, city, or country requires to produce the resources it consumes and absorb the wastes it generates. Developed countries have much larger footprints.

Biomass is the total mass of living (or recently living) organisms in a given area or volume, usually measured as dry weight per unit area. It represents stored biological energy and is used to measure productivity at each trophic level.

Major carbon sinks (that absorb more CO₂ than they release) include: forests (photosynthesis fixes CO₂ into wood), oceans (CO₂ dissolves in seawater, used by marine phytoplankton), and soil. Industrial factories and respiration are carbon sources.

The hermit crab using an empty shell is commensalism — the crab benefits (protection, mobility) while the dead shell's original owner is unaffected. The crab neither helps nor harms any living organism.

Hydrosere is ecological succession in an aquatic environment (pond, lake). It progresses from open water → submerged plants → floating plants → emergent plants (reeds) → marsh/swamp → wet woodland → climax forest as sedimentation fills the water body.

Detritus is dead particulate organic matter — fragments of dead plants and animals at various stages of decomposition. Detritivores (earthworms, millipedes) and decomposers (bacteria, fungi) process detritus, recycling nutrients.

Tropical rainforests have the highest net primary productivity (NPP) of any terrestrial biome (~2000 g C/m²/year) due to year-round high temperature, intense sunlight, and abundant rainfall. Open oceans have low NPP despite their vast area.

Biomagnification: DDT concentration increases at each trophic level because it is stored in fat and not metabolised. Top predators (eagles, killer whales) accumulate the highest concentrations — causing reproductive failure in raptors (thinning eggshells).

Gause's competitive exclusion principle states that two species competing for exactly the same resources cannot coexist indefinitely — one will eventually outcompete the other to local extinction. Species that do coexist must have slight niche differences.

In a grassland ecosystem: grass (producer) → grasshopper (primary consumer/herbivore) → frog (secondary consumer) → snake (tertiary consumer) → hawk (quaternary consumer). Grasshoppers eat grass and are primary consumers.

Interspecific competition occurs between individuals of different species competing for the same limited resources (food, territory, nesting sites). The competitive exclusion principle states that two species cannot occupy exactly the same niche indefinitely.

Protecting natural habitats (in situ conservation) is the most effective measure for biodiversity conservation — it preserves entire ecosystems with all their species and ecological processes. Zoos (ex situ) help but cannot replace habitat protection.

The phosphorus cycle has no significant atmospheric component — phosphorus does not exist as a gas under normal conditions. It cycles between rocks (weathering releases phosphate), soil, water, and organisms without passing through the atmosphere.

The tundra biome is characterised by permafrost (permanently frozen subsoil), low biodiversity, very short growing season (6-10 weeks), treelessness (only low shrubs, mosses, lichens), and extreme cold (-40°C in winter).

Agricultural runoff carrying nitrogen and phosphorus fertilisers is the primary cause of eutrophication — excessive nutrient input causes explosive algal growth (algal bloom), blocking light and depleting oxygen when algae die and decompose.

Biotic potential (r_max) is the theoretical maximum rate of population increase under ideal conditions (unlimited resources, no predators, no disease). In reality, environmental resistance prevents populations from reaching their biotic potential.

Mycorrhizae (fungus-root associations) are mutualistic — fungi increase plant's mineral (especially phosphate) and water absorption while the plant provides carbohydrates to the fungus. About 90% of all plant species have mycorrhizal associations.

The ice-albedo feedback is a positive feedback loop: warming melts Arctic ice → exposed dark ocean absorbs more heat (less reflected by white ice) → more warming → more melting. This amplifies the original warming signal.

Carrying capacity (K) is the maximum population size that an environment can indefinitely sustain given the available resources. When population size exceeds K, resource depletion causes population decline back toward K.

Secondary succession occurs in areas where a community previously existed but was disturbed (fire, flood, logging). Soil and some organisms remain, allowing faster recovery. Pioneer species are typically hardy plants, not lichens as in primary succession.

Nitrogen is often the limiting factor for plant growth in terrestrial ecosystems — plants need nitrogen for protein and nucleic acid synthesis but cannot use atmospheric N₂ directly. Nitrogen availability limits primary productivity in many ecosystems.

Intraspecific competition occurs between individuals of the same species competing for the same limited resources (food, territory, mates). Interspecific competition is between individuals of different species.

Deep-sea hydrothermal vent ecosystems are based on chemosynthesis — bacteria oxidise hydrogen sulphide (H₂S) released from vents to obtain energy for fixing CO₂. These ecosystems are completely independent of solar energy.

Biodiversity hotspots (concept by Norman Myers) are regions with exceptionally high species richness and endemism (species found nowhere else) that are under severe threat from human activities. 36 hotspots contain more than 50% of all endemic plant species.

The tundra biome is characterised by permafrost — a permanently frozen layer of soil beneath the surface. The Arctic tundra has a very short growing season, few plant species (mosses, lichens, sedges), and extreme cold winters.

Photosynthesis (fixes CO₂) and respiration (releases CO₂) are the main biological processes of the carbon cycle. Others include decomposition, combustion, and weathering of carbonate rocks. Nitrogen fixation belongs to the nitrogen cycle.

The photic (euphotic) zone is the sunlit upper layer of the ocean (0-200 m depth) where enough light penetrates for photosynthesis to occur. Below this is the aphotic zone where no photosynthesis is possible.

A keystone species has a disproportionately large effect on its ecosystem relative to its abundance. Sea otters are a classic keystone species — they control sea urchin populations that would otherwise destroy kelp forests, maintaining ecosystem structure.

Sunlight is the primary energy source for most ecosystems — producers capture solar energy through photosynthesis and convert it to chemical energy stored in organic molecules. Exceptions include deep-sea hydrothermal vent communities (chemosynthesis).

Nutrient cycling (biogeochemical cycles) ensures continuous availability of essential nutrients (carbon, nitrogen, phosphorus, water) for living organisms. Unlike energy (which is lost as heat), nutrients are recycled between biotic and abiotic components.

K-strategists (equilibrium species) have: large body size, few but large offspring with extensive parental care, long lifespan, late maturity, and stable populations near carrying capacity. Examples: elephants, whales, humans, eagles.

r-strategists (opportunistic species) have: high reproductive rate, many small offspring with little parental care, short lifespan, small body size, early maturity, and thrive in unstable environments. Examples: mice, insects, weeds.

Ammonification is the decomposition of organic nitrogen compounds (proteins, nucleic acids from dead organisms) into ammonia (NH₃) by ammonifying bacteria and fungi. This makes nitrogen available for nitrification.

Tubifex worms (sludge worms) are bioindicators of severe organic water pollution — they tolerate very low oxygen levels and are found in heavily polluted water. Mayfly larvae indicate clean water; tubifex indicates polluted water.

Primary consumers (herbivores) directly feed on producers (plants/algae). They occupy the second trophic level. Examples include rabbits (eat grass), caterpillars (eat leaves), and zooplankton (eat phytoplankton).

Photosynthesis removes CO₂ from the atmosphere — producers use CO₂ and water with light energy to produce glucose and oxygen. Respiration, decomposition, and combustion all release CO₂ into the atmosphere.

Tropical rainforests have the greatest species diversity of any biome — about 50-80% of all terrestrial species live in them despite covering only ~6% of Earth's surface. High rainfall, stable temperature, and solar energy support this diversity.

Chlorofluorocarbons (CFCs) — used in refrigerants, aerosols, and foam packaging — are the primary cause of stratospheric ozone depletion. UV radiation breaks CFCs releasing chlorine atoms that catalytically destroy ozone molecules.

A niche is the ecological role of an organism in its community — encompassing its diet, habitat preferences, activity patterns, interactions with other species, and how it uses resources. Habitat is where it lives; niche is how it lives.

Deforestation causes: soil erosion (tree roots hold soil), loss of biodiversity (habitat destruction), increased CO₂ (trees store carbon; burning/decomposing releases it), reduced rainfall (trees recycle water through transpiration), and loss of watershed protection.

Decomposers (bacteria, fungi) break down dead organic matter (detritus) into inorganic nutrients, completing nutrient cycles. Without decomposers, nutrients would be locked in dead organisms and ecosystems would collapse.

Lichens are pioneer species in primary succession on bare rock — they are symbioses of fungi and algae/cyanobacteria that can colonise bare rock. They secrete acids that weather rock into proto-soil, allowing mosses and then higher plants to establish.

The climax community is the final, stable community reached at the end of ecological succession. It is in equilibrium with the local climate and remains relatively unchanged until a major disturbance resets succession.

Primary succession occurs in areas where no soil or community previously existed — like bare rock after a volcanic eruption, or sand dunes. Pioneer species (lichens, mosses) gradually build soil, allowing more complex communities to develop.

Carrying capacity (K) is the maximum population size that an environment can sustainably support given the available resources (food, water, shelter, space). When a population reaches K, growth rate declines to zero.

Logistic growth (S-shaped/sigmoidal curve) occurs when resources are limited. Population grows exponentially at first, then slows as it approaches the carrying capacity (K) — the maximum population size the environment can sustain.

Population growth rate = (Birth rate + Immigration) - (Death rate + Emigration). All four factors determine whether a population grows, declines, or stays stable. Natural increase = Birth rate - Death rate.

The cattle egret-buffalo relationship is commensalism — the egret benefits by catching insects disturbed by the grazing buffalo, while the buffalo is neither helped nor harmed. Only one organism (the egret) benefits from the association.

Solar energy is a renewable resource — it comes from the sun which provides virtually limitless energy. Coal, natural gas, and petroleum are fossil fuels — non-renewable resources formed over millions of years from ancient organisms.

Current global warming is primarily caused by human activities — burning fossil fuels (releasing CO₂), deforestation (reducing CO₂ absorption and releasing stored carbon), and agriculture (methane and nitrous oxide emissions).

The ozone layer in the stratosphere absorbs most of the sun's harmful ultraviolet (UV) radiation — particularly UV-B (315-280 nm) and UV-C (<280 nm). UV exposure causes skin cancer, cataracts, and DNA damage in living organisms.

A food web is a complex network of interconnected food chains showing feeding relationships in an ecosystem. It is more realistic than a simple food chain because most organisms eat multiple food sources and are eaten by multiple predators.

Denitrification is the conversion of nitrates (NO₃⁻) back to nitrogen gas (N₂) by anaerobic denitrifying bacteria (Pseudomonas, Thiobacillus) in waterlogged soils. It completes the nitrogen cycle by returning N₂ to the atmosphere.

Tropical rainforests have high temperatures (25-30°C year-round) and very high rainfall (>2000 mm/year). They cover about 6% of Earth's surface but contain more than 50% of all species, making them the most biodiverse terrestrial biome.

Deserts receive less than 250 mm of rainfall per year — the least of any biome. Hot deserts (Sahara, Arabian) and cold deserts (Gobi, Antarctic) share low precipitation but differ in temperature extremes.

Habitat destruction is the primary cause of biodiversity loss globally — deforestation, wetland drainage, urbanisation, and conversion of natural land to agriculture destroy the ecosystems where species live.

Biodiversity refers to the variety of life on Earth at all levels — genetic diversity (variation within species), species diversity (number and abundance of species), and ecosystem diversity (variety of habitats and ecological processes).

Parasitism is a relationship where the parasite benefits at the expense of the host. Tapeworms live in the human intestine, absorbing digested nutrients (benefit) while causing nutritional deficiency to the host (harm).

Commensalism is a relationship where one organism benefits while the other is neither significantly harmed nor helped. Example: barnacles on whales (barnacles benefit from transport, whale is unaffected).

Mutualism is a symbiotic relationship where both organisms benefit. Examples include: bee-flower (bee gets nectar, flower gets pollinated), cleaner fish-shark (fish gets food, shark gets cleaned), and Rhizobium-legume (bacteria get carbs, plant gets nitrogen).

Predation is a relationship where one organism (predator — the shark) kills and eats another (prey — the fish). It is a form of interspecific interaction that affects population sizes and drives coevolution of predator and prey traits.

Photosynthesis (CO₂ fixation) and respiration (CO₂ release) are the central processes of the carbon cycle. Photosynthesis removes CO₂ from the atmosphere; respiration returns it. The carbon cycle also includes combustion and decomposition.

Temperature is an abiotic (non-living) factor that profoundly affects metabolic rates, distribution of organisms, and seasonal behaviour. Competition, predation, and symbiosis are biotic (living) factors.

Lindemann's 10% law (1942) states that only about 10% of the energy at one trophic level is transferred to the next level. The remaining 90% is lost as heat through metabolism. This limits food chains to 4-5 trophic levels.

Eutrophication is the excessive enrichment of water bodies with nutrients (mainly nitrogen and phosphorus from agricultural runoff and sewage). It causes algal blooms that deplete oxygen when they die and decompose, creating hypoxic 'dead zones'.

The hydrological (water) cycle describes the continuous movement of water through the Earth system — evaporation from oceans/lakes, transpiration from plants, condensation forming clouds, precipitation, and runoff back to water bodies.

Decomposers (saprotrophs) break down dead organic matter into inorganic nutrients, completing nutrient cycles. Fungi and bacteria are the primary decomposers. They release nutrients back into the soil for producers to use.

Biomagnification (bioaccumulation) is the progressive increase in concentration of toxic substances (DDT, PCBs, heavy metals) at each successive trophic level, because organisms cannot excrete them and they accumulate in fatty tissues.

Secondary consumers (carnivores) eat primary consumers (herbivores). They occupy the third trophic level. Examples include frogs (eat insects), small fish (eat zooplankton), and foxes (eat rabbits).

Arthur Tansley coined the term 'ecosystem' in 1935, defining it as the complete system including not just the biotic community but also all the physical and chemical factors of the environment in a given area.

An ecosystem is a community of living organisms (biotic component) interacting with each other and with their non-living environment (abiotic component — soil, water, temperature, light) as a functional system.

The pyramid of energy shows that energy decreases at each successive trophic level because 90% of energy is lost as heat at each transfer. Only about 10% is incorporated into biomass (the 10% rule/ecological efficiency).

Coal is a non-renewable resource — it takes millions of years to form from compressed organic matter and is being consumed much faster than it can be replenished. Solar, wind, and hydroelectric are renewable energy sources.

Chlorofluorocarbons (CFCs) release chlorine atoms when broken down by UV radiation in the stratosphere. Each chlorine atom can catalytically destroy up to 100,000 ozone molecules before becoming inactive.

The ozone layer is located in the stratosphere, about 15-40 km above Earth's surface. It absorbs most of the sun's harmful UV-B and UV-C radiation, protecting life on Earth from DNA damage and skin cancer.

Acid rain is caused primarily by sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) released from burning fossil fuels and vehicle emissions. These dissolve in rainwater to form sulphuric and nitric acids, lowering pH to 4-5.

Carbon dioxide (CO₂) is the most important anthropogenic greenhouse gas — its concentration has risen from 280 ppm (pre-industrial) to over 420 ppm today due to burning of fossil fuels, deforestation, and cement production.

The greenhouse effect is caused by greenhouse gases — CO₂, methane (CH₄), nitrous oxide (N₂O), water vapour, and CFCs — that trap outgoing infrared radiation from Earth's surface, warming the planet.

Nitrogen fixation is the conversion of atmospheric nitrogen (N₂) to ammonia (NH₃) by nitrogen-fixing bacteria (Rhizobium, Azotobacter, cyanobacteria). This makes nitrogen available to plants for protein synthesis.

Food chains always start with producers (autotrophs — green plants, algae, cyanobacteria) that fix energy from sunlight. Energy flows from producers to primary consumers, then secondary consumers, and so on.

Biotic factors are living components of the environment — including predation, competition, parasitism, mutualism, and decomposition. Temperature, rainfall, and soil pH are abiotic (non-living) factors.

Primary productivity is the rate at which producers (plants, algae, cyanobacteria) fix energy from the sun through photosynthesis (or chemosynthesis). Gross primary productivity (GPP) is total fixation; net primary productivity (NPP) = GPP minus respiration.

An ecological niche is the role or 'profession' of an organism in its community — including its trophic level, habitat requirements, activity patterns, and interactions with other species. It is distinguished from habitat (where it lives).

The biosphere encompasses parts of three Earth spheres: the lithosphere (soil and rock surface — up to a few metres deep), hydrosphere (all water bodies), and atmosphere (up to about 10 km altitude).

Ecology is the scientific study of the relationships between organisms and their environment — including both biotic (living) and abiotic (non-living) components, and the interactions among them.

The term 'ecology' (from Greek: oikos = house, logos = study) was coined by Ernst Haeckel in 1869. He defined it as the study of the relationships between organisms and their environment.

The biosphere extends from the outer surface (crust) of the Earth to a height of about 10 km into the atmosphere. It includes all ecosystems — terrestrial, aquatic, and aerial — where living organisms exist.