Man and His Environment.

No organism lives alone. Every plant, animal and microbe is woven into a web of feeding relationships and chemical cycles, all powered by the Sun. Ecology is the study of those relationships — how living things interact with one another and with the soil, water and air around them — and of how human beings, more than any other species, are now reshaping that whole system.

1 · The levels of ecological organisation

Ecology is the branch of biology that studies the interactions between organisms and their environment. The environment of an organism is everything that surrounds and affects it — both the other living things and the non-living surroundings. Ecologists study life at rising levels of organisation:

• Organism — a single living individual (one rohu fish, one acacia tree).
• Population — all the individuals of one species living in the same area at the same time (all the rohu in a pond).
• Community — all the populations of different species living and interacting in one area (the fish, plants, insects and microbes of the pond together).
• Ecosystem — a community plus its non-living surroundings, working as one functional unit (the pond community + its water, mud, dissolved gases and sunlight).
• Biosphere — the thin global layer of land, water and air in which all life exists; the sum of every ecosystem on Earth.

2 · The ecosystem: biotic & abiotic components

An ecosystem is a unit in which living organisms interact with each other and with their physical surroundings, with a continuous flow of energy and cycling of materials between them. Every ecosystem has two kinds of components.

The abiotic factors set the physical limits — how much light, water and warmth there is decides which organisms can survive. The biotic factors are the living interactions: feeding, competition, predation and decay.

Habitat and niche

The habitat is the place where an organism lives — its "address" (a desert, a freshwater pond, the bark of a tree). The ecological niche is the organism's role or way of life in that habitat — its "profession": what it eats, what eats it, when it is active, and how it affects its surroundings.

3 · Energy flow: who feeds whom

Almost all energy in an ecosystem comes from the Sun. It enters the living world only through one group and then passes from organism to organism as food. By their feeding role, organisms fall into three groups.

• Producers (autotrophs) — green plants, algae and some bacteria that capture sunlight and build food by photosynthesis. They are the only doorway for energy to enter the ecosystem.
• Consumers (heterotrophs) — organisms that cannot make their own food and must eat others:
  • Primary consumers — herbivores that eat producers (grasshopper, goat, deer).
  • Secondary consumers — carnivores (or omnivores) that eat primary consumers (frog, small snake).
  • Tertiary consumers — top carnivores that eat secondary consumers (hawk, big cat); they have few or no natural predators.
• Decomposers — bacteria and fungi that break down dead bodies and waste, releasing the locked-up minerals back into the soil and air. Without them nutrients would never be recycled and the cycles would stop.

4 · Food chains, food webs & trophic levels

A food chain is a single straight line showing how energy passes as one organism eats the next:

Each step in a food chain is a trophic (feeding) level: producers are the 1st trophic level, primary consumers the 2nd, and so on. The arrow always points in the direction the energy flows — from the eaten to the eater.

In nature few animals eat only one thing. When many food chains cross and link, they form a food web — a realistic network in which one organism belongs to several chains. A food web is more stable than a single chain: if one species is lost, predators can switch to alternative prey.

5 · Ecological pyramids & the 10% rule

The amount of energy, biomass or number of organisms at each trophic level can be drawn as a bar, stacked to make an ecological pyramid — widest at the producers, narrowing upward.

• Pyramid of numbers — the number of organisms at each level. Usually narrows upward, but can be inverted (one large tree feeding many insects).
• Pyramid of biomass — the total dry mass of living material at each level. Almost always upright on land.
• Pyramid of energy — the energy stored at each level per unit time. This is always upright, never inverted, because energy is lost at every step.

6 · Biogeochemical cycles

Unlike energy, the chemical elements of life are recycled: they pass from the abiotic environment into organisms and back again, in biogeochemical cycles. Two are essential for exams.

The carbon cycle

Carbon moves between the air (as CO₂) and living things:

• Photosynthesis removes CO₂ from the air and fixes it into glucose and other organic compounds in producers.
• Respiration by all organisms returns CO₂ to the air as they release energy from food.
• Decomposition of dead bodies and waste by bacteria and fungi releases CO₂ back to the air (and carbon to the soil).
• Combustion — burning fossil fuels (coal, oil, gas) and wood returns ancient carbon to the air as CO₂, raising its level fast.

The nitrogen cycle

Nitrogen makes up ~78% of the air, but plants cannot use N₂ gas directly. It must be turned into usable compounds and later returned:

• Nitrogen fixation — N₂ gas is converted to ammonia / nitrates. Done by nitrogen-fixing bacteria (e.g. Rhizobium in the root nodules of legumes such as beans and gram, and Azotobacter in soil), by lightning, and industrially in fertiliser factories.
• Nitrification — nitrifying bacteria oxidise ammonia → nitrites (Nitrosomonas) → nitrates (Nitrobacter). Plants absorb these nitrates to make proteins.
• Assimilation & ammonification — animals get nitrogen by eating plants; when organisms die, decomposers return the nitrogen to the soil as ammonia (ammonification).
• Denitrification — denitrifying bacteria convert nitrates back to N₂ gas, returning it to the air and completing the cycle.

7 · Population growth & the factors that limit it

A population grows when births and immigration exceed deaths and emigration. With unlimited resources it grows exponentially — a steep J-shaped curve. But no environment is unlimited. As numbers rise, food, space, water and oxygen run short and disease and predators increase, so growth slows and levels off at the carrying capacity (K) — the maximum the environment can sustain — giving an S-shaped (sigmoid) curve.

The forces that hold a population in check are environmental resistance:

• Density-dependent factors get stronger as the population grows denser — competition for food and space, disease, predation, accumulation of waste.
• Density-independent factors act regardless of size — floods, droughts, fire, severe cold, earthquakes.

The human population is unusual: medicine, sanitation and farming have pushed our carrying capacity up so the world population is still rising steeply, placing huge pressure on resources and the environment.

8 · Pollution

Pollution is the addition to the environment of harmful substances (pollutants) — or of excess heat, noise or radiation — that damage living things. It is classed by where it acts.

Air pollution

Causes: burning fossil fuels in vehicles, factories and power stations; brick kilns; crop-residue and rubbish burning. Main pollutants: carbon monoxide (CO), sulphur dioxide (SO₂), oxides of nitrogen (NOₓ), smoke and dust (particulates), and CO₂.
Effects: respiratory disease and smog (a serious problem in winter in Lahore and Karachi); acid rain (SO₂ and NOₓ form acids that damage crops, forests, lakes and buildings).

Water pollution

Causes: untreated sewage, industrial effluents, fertiliser and pesticide run-off, oil spills.
Effects: water-borne disease (cholera, typhoid); eutrophication — fertiliser run-off feeds an algal bloom which, when it decays, uses up the dissolved oxygen and suffocates fish; biomagnification — toxins like pesticides become more concentrated up the food chain.

Land (soil) pollution

Causes: solid waste and plastics, over-use of pesticides and fertilisers, industrial dumping.
Effects: loss of soil fertility, poisoning of crops and groundwater, and non-biodegradable plastic that persists for centuries.

The greenhouse effect & global warming

Some atmospheric gases — chiefly CO₂, methane (CH₄) and water vapour — let the Sun's short-wave radiation through but absorb the long-wave infra-red (heat) the Earth re-radiates, trapping it like the glass of a greenhouse. This natural warming keeps Earth habitable; but burning fossil fuels and deforestation are raising CO₂, intensifying the effect and causing global warming — rising temperatures, melting glaciers (a direct threat to the Indus river system and to coastal Karachi), and erratic, extreme weather.

Deforestation

Clearing forests for timber, fuel and farmland removes the trees that absorb CO₂ and release O₂, so it worsens global warming, causes soil erosion and flooding, and destroys habitats — a pressing issue in Pakistan's northern forests.

9 · Conservation of resources & biodiversity

Conservation is the wise, planned management of natural resources so that they are used without being destroyed and remain available for the future (sustainable use). Biodiversity — the variety of all living species and their genes — must be protected because each species has a role, and many are sources of food and medicine yet to be discovered.

• Protect species & habitats — national parks and wildlife reserves; protecting endangered Pakistani species such as the snow leopard, markhor, Indus river dolphin and marine turtles.
• Manage resources sustainably — reforestation and afforestation; controlled fishing and grazing; soil and water conservation.
• Reduce pollution & waste — treat sewage and effluent, follow the 3 Rs — reduce, reuse, recycle, and shift to clean renewable energy (solar, wind, hydel).
• Restore the cycles — plant trees to lock up CO₂ and use biological nitrogen fixation instead of excess fertiliser.