Ecology and Vegetation

Ecology and Vegetation carries 4 of the 40 Botany questions, spread evenly over four blocks: ecosystem structure and biotic interactions, the carbon and nitrogen cycles with their named bacteria, Nepal's forest belts and invasive plants, and succession. Almost nothing here needs calculation; almost everything needs a name attached to the right process. Learn the tables and the two cycle figures and this unit is free marks.

6.1 Pond and forest ecosystems: structure and function

What an ecosystem is

An ecosystem is a self-regulating unit in which a community of organisms interacts with its physical environment, energy flowing through it one way and matter cycling within it (Tansley, 1935). Its abiotic half is light, temperature, water, air, soil, pH and the inorganic nutrients; its biotic half is producers (autotrophs), consumers (heterotrophs) and decomposers (saprotrophs — bacteria and fungi, with earthworms and termites as detritivores). Structure is who is there and in what layers; function is productivity, decomposition, energy flow and nutrient cycling.

Structure of a pond ecosystem

A pond is a small freshwater standing (lentic) ecosystem. Its abiotic part is the water itself — temperature, dissolved O₂ and CO₂, pH, light penetration, nitrates and phosphates. Its biotic part is arranged in four zones:

Submerged plants (Hydrilla, Vallisneria) root in the littoral mud and reach into the limnetic zone. The decomposers sit in the mud and take everything that sinks, so a pond recycles its nutrients fast — a nutrient spill can choke it with algae within days.

Structure of a forest ecosystem

A forest is terrestrial, and its structure is vertical. A sal forest in Chitwan has five layers:

Consumers are layered too: deer and insects below, monkeys and birds in the canopy, tiger and leopard through all of them; the decomposers work in the litter.

Function: energy flow and the 10 % law

Photosynthesis is the only route by which new energy enters an ecosystem. The total fixed by the producers is the gross primary productivity (GPP); what is left after they pay for their own respiration is the net primary productivity (NPP):

Energy passes along a food chain, and at every transfer most of it is lost as heat in respiration, plus what is not eaten, not digested or excreted. Raymond Lindeman (1942) measured this and stated the 10 % law: only about 10 % of the energy of one trophic level reaches the next, the other 90 % being lost, mostly as heat.

Two consequences follow. Energy flow is unidirectional — once degraded to heat it is useless to a producer, whereas matter cycles. And food chains are short, because only a tenth survives each transfer.

Types of food chain. A grazing (predator) chain starts from a living plant: grass → grasshopper → frog → snake → hawk. A detrital (decomposer) chain starts from dead organic matter: leaf litter → earthworm → bird. In a forest most energy travels through the detrital chain. Real chains interlock into a food web, which is what makes an ecosystem stable — if one prey disappears, a predator can switch to another.

Ecological pyramids

An ecological pyramid plots the numbers, biomass or energy at each trophic level, producers at the base.

Biotic interactions

The relationship between two species is classified by its effect on each partner: + benefited, − harmed, 0 unaffected.

6.2 Carbon and nitrogen cycles; greenhouse effect, acid rain, ozone depletion and climate change

The carbon cycle

Carbon is exchanged between four reservoirs: the atmosphere (as CO₂, about 900 Gt C), living organisms and soil (about 2 000 Gt C), the oceans (about 38 000 Gt C of dissolved inorganic carbon) and fossil fuels. Carbonate rock holds far more, but exchanges only over millions of years.

• Photosynthesis removes CO₂ and fixes it into carbohydrate — the only large biological sink.
• Respiration by plants, animals and microbes returns CO₂ to the air.
• Decomposition of litter and humus is, globally, the largest single return flux.
• Feeding moves carbon from plants to animals as organic matter.
• Combustion of fossil fuel and firewood returns carbon locked away for millions of years.
• Ocean exchange — the sea absorbs roughly a quarter of the CO₂ humans release.

The nitrogen cycle

Nitrogen is 78 % of the air, but as N₂ it is almost inert: the two atoms are held by a triple bond no plant or animal can break. The nitrogen cycle is the story of how that bond is broken and how nitrogen passes from one form to another.

The greenhouse effect

Short-wave radiation from the Sun passes almost unchanged through the atmosphere and is absorbed by the surface, which warms. The warm surface re-emits energy as long-wave infrared, and greenhouse gases absorb this infrared and re-radiate it in all directions, so part of it returns to the surface instead of escaping. The surface therefore settles at about +15 °C instead of −18 °C. That natural greenhouse effect makes life possible; adding gases to it — the enhanced greenhouse effect — drives current climate change.

Acid rain

Normal rain is already slightly acidic, pH ≈ 5.6, because it dissolves atmospheric CO₂ to form weak carbonic acid. Rain is called acid rain when its pH falls below about 5.0, which happens when it dissolves sulphur dioxide and nitrogen oxides:

• SO₂ + H₂O → H₂SO₃, which oxidises to H₂SO₄ (sulphuric acid)
• 2NO₂ + H₂O → HNO₃ + HNO₂, giving HNO₃ (nitric acid)

The sources are coal and oil burning (SO₂) and vehicle and power-station exhausts (NOₓ). The damage is indirect as well as direct: acid water dissolves aluminium from the soil, and the Al³⁺ reaching a lake is what kills fish. It also leaches calcium and magnesium from soil and eats into limestone and marble. Nepal's emissions are small, but Kathmandu Valley brick kilns and traffic make local acid aerosols, and Himalayan lakes are poorly buffered.

Depletion of the ozone layer

Ozone (O₃) forms in the stratosphere (15–35 km up) when UV light splits O₂ and the free oxygen atoms recombine with O₂. This layer absorbs most of the Sun's UV-B, which would otherwise damage DNA and cause skin cancer and cataracts. Its thickness is measured in Dobson units (DU); the normal column is about 300 DU.

The CFC mechanism, step by step:

1. Chlorofluorocarbons (CFCl₃, CF₂Cl₂) — refrigerants, propellants, foam-blowing agents — are chemically inert, so they drift up into the stratosphere without being destroyed.
2. There UV-C light breaks the C–Cl bond, releasing a chlorine radical: CF₂Cl₂ --UV--> CF₂Cl• + Cl•.
3. The chlorine radical attacks ozone: Cl• + O₃ → ClO• + O₂.
4. The chlorine monoxide then takes an oxygen atom from another ozone molecule: ClO• + O₃ → Cl• + 2O₂ — and the chlorine atom is regenerated.
5. Because it is not used up, one atom can destroy about 100 000 ozone molecules before it is removed. Chlorine is a catalyst.

The Antarctic "ozone hole" is a spring phenomenon. In the extreme cold of the polar night, polar stratospheric clouds of ice form, and reactions on their surfaces convert the inactive reservoir forms (HCl, ClONO₂) into active chlorine; when sunlight returns, ozone is destroyed very fast. The Montreal Protocol (1987) phased out CFCs, and stratospheric chlorine is now falling, though full recovery will take decades.

Climate change and Nepal's exposure

The enhanced greenhouse effect has raised the global mean surface temperature by about 1.1–1.2 °C above the pre-industrial value. Nepal is exposed in four ways:

• Glacier retreat. Meltwater first increases, then falls away, so glacier-fed rivers swell and later run low in the dry season.
• Glacial lake outburst floods (GLOF). Meltwater collects behind a loose moraine dam; when it breaks, a wall of water and debris runs down the valley. Imja and Tsho Rolpa are the classic examples, and Thame in Solukhumbu was damaged in 2024.
• A shifting monsoon. Longer dry spells inside the season, then very heavy rain in a few days — drought at transplanting, floods at harvest.
• Upward migration of the belts. As the tree line climbs, alpine species lose the ground above them and the highest ones are lost first.

The consequences are already visible: more landslides, winter water shortage in mid-hill towns and a longer dengue season in the Terai. Adaptation means community forest management, GLOF early-warning systems and check dams on fragile slopes.

6.3 Forest types of Nepal; biological invasion

Forest belts by altitude

Nepal spans only 147 000 km², but rises from about 60 m in the Terai to 8 848 m at Sagarmatha, so a day's walk uphill passes the belts of a whole continent. About 45 % of the land is forest or other wooded land. Temperature falls by roughly 0.6 °C for every 100 m of ascent, and the monsoon is heaviest in the east.

Biological invasion

An invasive alien species is a plant or animal moved by humans outside its natural range that then spreads on its own and causes harm. Nepal's roads and disturbed riverbanks give invaders the bare, sunny ground they need, and without their natural enemies they outgrow the native flora.

Control is manual removal before flowering, biological control with host-specific insects against Mikania and water hyacinth, and above all keeping the ground covered so invaders cannot establish.

6.4 Ecological succession: hydrosere and xerosere; ecological adaptation

The idea of succession

Ecological succession is the directional change in species composition at one place over time, until a stable community — the climax — is reached. The sequence of communities is a sere; each intermediate community is a seral stage; the first is the pioneer community.

• Primary succession starts on a surface that has never carried vegetation and has no soil — bare rock, a new lava flow, a retreating glacier's moraine, a fresh landslide scar.
• Secondary succession starts where a community has been destroyed but soil and seeds remain — an abandoned field (khoriya), a burnt forest, a cleared hillside. It is much faster because the soil is already made.

A succession that begins in water is a hydrosere; one that begins on dry rock is a xerosere.

The hydrosere, stage by stage

The xerosere, stage by stage

Trends during succession, and the climax

The climax community is the end point: a community in equilibrium with the regional climate, whose species composition stays roughly constant from year to year. F. E. Clements held that every region has one climax set by climate (the monoclimax theory); A. G. Tansley held that soil, fire and grazing can produce several stable end points in one climate (polyclimax). Climate sets the potential; local factors decide which climax is reached.

Ecological adaptation

An adaptation is an inherited feature that helps an organism survive in its environment. The examinable sets are the ones named by their habitat:

The Himalayan belt adds dwarf and cushion forms that stay close to the warm ground, hairy or waxy surfaces that cut water loss in the dry winter, and early flowering in the brief snow-free window. Betula utilis at the tree line grows as a stunted, twisted shrub — the same species, a different shape, because the environment sets the form.

Formula and fact sheet

Practice questions

Set A — Recall (30 questions)

1. The law that only about 10 % of the energy at one trophic level passes to the next was proposed byaE. P. OdumbA. G. TansleycR. L. LindemandF. E. Clements
2. Organisms that break down dead organic matter into inorganic nutrients are calledadecomposersbprimary consumerscproducersdtertiary consumers
3. Which ecological pyramid can never be inverted?apyramid of numbersbpyramid of biomasscpyramid of numbers in a grasslanddpyramid of energy
4. The association between the fungus and the alga in a lichen is an example ofacommensalismbmutualismcparasitismdamensalism
5. Cattle egrets that follow grazing buffalo in the Terai to catch the insects they disturb showacommensalismbmutualismcpredationdamensalism
6. Cuscuta (amarbel) growing over a host shrub is an example ofaepiphytismbsaprophytismcparasitismdcommensalism
7. The free-living aerobic nitrogen-fixing bacterium of soil isaNitrosomonasbAzotobactercNitrobacterdPseudomonas
8. The conversion of ammonium to nitrite in the nitrogen cycle is carried out byaNitrobacterbRhizobiumcAzotobacterdNitrosomonas
9. Nitrate is reduced step by step to nitrogen gas mainly byaNitrosomonasbPseudomonascRhizobiumdNitrobacter
10. Among the reservoirs that exchange carbon quickly with the atmosphere, the largest isathe atmospherebliving organismscsoil organic matterdthe ocean
11. The gas that contributes the largest share of the natural greenhouse effect isawater vapourbcarbon dioxidecmethanedCFC-12
12. Clean rain is slightly acidic, with a pH of about 5.6, mainly because it dissolvesasulphur dioxidebammoniaccarbon dioxidednitrogen oxides
13. The thickness of the ozone layer is expressed inapascalsbsievertscdecibelsdDobson units
14. Chlorofluorocarbons destroy stratospheric ozone because ultraviolet light releases from themafluorine atomsbchlorine atomsccarbon atomsdhydrogen atoms
15. The Montreal Protocol (1987) is an international agreement toareduce greenhouse gas emissions from industrybprotect wetlands of international importancecphase out ozone-depleting substancesdcontrol trade in endangered species
16. In Nepal, sal (Shorea robusta) forests are characteristic of theatropical zone below about 1 000 mbtemperate zone at 2 000–3 000 mcalpine zone above 4 000 mdsubalpine zone at 3 000–4 000 m
17. Chir pine (Pinus roxburghii) is a characteristic tree of Nepal'saalpine zonebtropical Teraicsubtropical zonednival zone
18. Water hyacinth (Eichhornia crassipes), which chokes Nepali lakes, was introduced fromaAfricabSouth AmericacAustraliadEurope
19. The pioneer community of a hydrosere isareed swampbsedge meadowcsubmerged plantsdphytoplankton
20. The pioneer organisms of a xerosere on bare rock areacrustose lichensbmossescannual herbsdshrubs
21. The final stable community of a succession, in equilibrium with the climate, is called theaseral communitybclimax communitycpioneer communitydedaphic community
22. Succession that begins on a bare surface with no soil and no seeds isasecondary successionbautogenic successioncprimary successiondallogenic succession
23. Lantana camara, called banmara in Nepali, is an example ofaan invasive alien speciesban endemic speciesca rare medicinal plantda nitrogen-fixing tree
24. The decomposers of a pond ecosystem are chieflyaphytoplanktonbzooplanktoncfishdbacteria and fungi
25. In a forest food chain, the herbivores form theafirst trophic levelbthird trophic levelcsecond trophic leveldfourth trophic level
26. The "mile-a-minute" weed that smothers regeneration in Chitwan's forests isaLantana camarabMikania micranthacParthenium hysterophorusdEichhornia crassipes
27. Nepal's alpine zone, above the tree line, is dominated byasal and sissoobchir pine and chilaunecoak and spruceddwarf shrubs, cushion plants and grasses
28. The greenhouse gas with the highest global warming potential per molecule among these isaCFC-12bcarbon dioxidecmethanednitrous oxide
29. Amensalism is a relationship in whichaboth species benefitbone species is harmed and the other is unaffectedcboth species are harmeddone benefits and the other is unaffected
30. In a food chain, the proportion of energy transferred from one trophic level to the next is abouta1 %b50 %c10 %d90 %

Set B — Understanding (18 questions)

1. A pyramid of numbers is inverted inaa grasslandba single large tree carrying thousands of insectsca pond full of phytoplanktonda maize field
2. Energy flow in an ecosystem is described as unidirectional becauseaenergy is recycled by decomposersbenergy passes only through herbivorescenergy is stored as biomassdheat lost at each level cannot be used again by producers
3. Decomposers are not placed at the top of an energy pyramid because theyatake energy from every level and release it as heat by a detrital pathwaybare too small to countcare producersddo not respire
4. Two grass species in one field need the same light, water and nitrogen. Their relationship isamutualismbcommensalismccompetitiondparasitism
5. A Penicillium mould releases a substance that kills bacteria around it while the mould is unaffected. This isaparasitismbpredationccommensalismdamensalism
6. Lentil and chickpea enrich the soil because Rhizobium in their root nodulesadecomposes dead rootsbreduces atmospheric N₂ to ammonia using nitrogenasecoxidises nitrate to nitrogen gasdconverts ammonia to nitrite
7. A farmer keeps a paddy field flooded instead of draining it. This reduces nitrogen loss because floodingaincreases nitrogen fixationbprevents ammonificationcstops the conversion of ammonium to nitrate, which would then be denitrifieddstops nitrate uptake by rice
8. Greenhouse gases warm the surface because theyaabsorb outgoing long-wave infrared radiation and re-radiate part of it backbabsorb incoming ultraviolet radiationcreflect short-wave radiation from the Sundstop conduction from the ground
9. Acid rain kills fish in poorly buffered mountain lakes mainly byaheating the waterbadding nutrients that cause algal bloomsclowering the pH and releasing toxic aluminium ions from the soildraising the dissolved oxygen
10. One chlorine atom can destroy thousands of ozone molecules because chlorine acts asaa reactant that is used upba catalyst that is regenerated in the cycleca solventdan oxidising agent that is consumed
11. The Antarctic ozone hole appears in spring becauseathe Sun is brightest over the pole thenbmost CFCs are made in Antarcticacvolcanic ash destroys ozonedice crystals in polar clouds turn stored chlorine into active forms
12. A glacier-fed river in Nepal now peaks earlier in summer than it did fifty years ago. The most likely cause isaearlier snowmelt and glacier retreat under a warming climateban increase in winter snowfallca decrease in monsoon rainfalldtectonic uplift of the Himalaya
13. After a landslide strips a Himalayan slope to bare rock, the first colonists are likely to beasal seedlingsblichens and mossesctall grassesdoak saplings
14. During succession from an open pond to a forest, total biomass and species diversity generallyaboth decreasebstay constantcboth increasedrise and then fall to zero
15. Mikania micrantha threatens Chitwan's sal forest mainly because itagrows over and smothers native shrubs and regenerationbis a root parasite of salcpoisons the soil with heavy metalsdattracts elephants that fell trees
16. The biomass of phytoplankton in a pond is often less than that of the zooplankton feeding on it. This is possible becauseazooplankton also photosynthesisebphytoplankton are decomposerscthe energy pyramid is inverteddphytoplankton divide fast, so a small crop supports a large consumer biomass
17. Forests matter in the carbon cycle mainly because theyarelease CO₂ during photosynthesisbprevent nitrogen fixationcstore carbon in wood and soil and remove CO₂ from the airdconvert carbon into nitrogen
18. A xerosere differs from a hydrosere in that a xerosereastarts dry and soil-less, and depends on lichens to make the first soilbstarts in open waterchas no pioneer stagedalways ends in a grassland

Set C — Application (12 questions)

1. In a grassland the producers fix 20 000 kJ of energy in a year. If the 10 % law holds at each step, the energy available to the secondary consumers isa2 000 kJb1 000 kJc100 kJd200 kJ
2. A food chain is grass → grasshopper → frog → snake → hawk. If the hawk receives 10 kJ, the energy fixed by the grass was abouta10 000 kJb100 000 kJc1 000 kJd1 000 000 kJ
3. A lake beside farmland receives heavy fertiliser runoff. Algae bloom, then die, and fish die. The oxygen sequence isaoxygen rises in the bloom by day, then falls as decomposers use itboxygen falls at once and stays lowcoxygen is unchangeddoxygen rises permanently
4. A student counts 1 000 000 phytoplankton, 200 000 zooplankton, 2 000 small fish and 5 large fish in a pond. The pyramid of numbers from these data isainverted at the basebspindle-shapedcuprightdinverted at the top
5. In a legume field Rhizobium fixes 20 kg of N₂ per hectare per year; 5 kg is lost by denitrification and 3 kg by leaching. The nitrogen added to the soil isa28 kgb12 kgc20 kgd8 kg
6. A forest is cleared and then abandoned. Forty years later a young forest stands on the site. This isaprimary successionba hydrosereca xeroseredsecondary succession
7. In a pond the sequence phytoplankton → zooplankton → small fish → kingfisher is aagrazing food chainbdetrital food chaincfood webdparasitic chain
8. The Terai lies below 300 m, the mid-hills reach 2 500 m and the tree line is near 4 000 m. A forest of oak, rhododendron and spruce at 2 600 m belongs to theatropical zonebalpine zonectemperate zonedsubtropical zone
9. Using \Delta F = 5.35\ln(C/C_0), the radiative forcing when CO₂ rises from 280 ppm to 560 ppm isa5.35 W m⁻²b1.50 W m⁻²c7.40 W m⁻²d3.71 W m⁻²
10. A 1 000 ha forest stores 120 t of carbon per hectare. If a fire destroys half of it, the CO₂ released is about (1 t C = 3.67 t CO₂)a220 000 tb120 000 tc60 000 td440 000 t
11. A river receives effluent at pH 3; downstream its pH is 5. The hydrogen-ion concentration hasaincreased 100 timesbdecreased 2 timescdecreased 100 timesddecreased 1 000 times
12. A crop field is abandoned. Annual weeds appear in year 1, grasses dominate by year 3, shrubs by year 10 and a sal forest by year 40. Biomass and soil organic matteraboth fallbboth risecbiomass rises while soil organic matter fallsdbiomass falls while soil organic matter rises