Sec 4 Biology Organisms & the Environment notes

Almost all the energy moving through an ecosystem began as sunlight. Producers — green plants and algae — trap it by photosynthesis and store it in the chemical bonds of glucose. Consumers harvest that stored energy by eating: primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers top the chain. Decomposers — bacteria and fungi — break down dead bodies and wastes, releasing nutrients back to the soil and air.

Here is the asymmetry that shapes whole ecosystems: matter cycles (carbon, nitrogen, water return again and again), but energy flows once. Organisms use energy for life processes and lose it as heat, which disperses to the surroundings and can never be re-captured by photosynthesis. Every ecosystem therefore needs a continuous fresh supply of sunlight, and no food chain can recycle its energy.

At every link in a food chain, energy escapes the flow: large parts of a plant (roots, wood) are never eaten; much of what is eaten is egested undigested or excreted; and the lion’s share — typically most of the energy an animal absorbs — is released by respiration as heat for movement and warmth. What remains for growth and reproduction is the only part available to the next trophic level, commonly around 10%.

Pyramids of numbers simply count organisms per level; they can invert — one huge tree feeds thousands of insects, so the "producer" bar is a single unit. Pyramids of biomass measure the mass of living tissue at each level instead, which is why they are almost never inverted: the tree’s enormous biomass dwarfs its many small grazers. When asked why energy pyramids always narrow, the answer is the losses above — each level holds less energy than the one below.

Carbon moves through four processes. Photosynthesis removes carbon dioxide from the air, locking carbon into glucose. Respiration — in plants, animals and decomposers alike — returns it. Combustion of wood, coal, oil and gas releases carbon stored in fuel, and decomposition by bacteria and fungi returns the carbon of dead matter to the air and soil. The cycle stays balanced only while removals match additions.

Two giant stores buffer the cycle: forests hold carbon in trillions of tonnes of wood and leaf, and oceans dissolve vast quantities of carbon dioxide (and store more in shells and sediments). These carbon sinks absorb roughly half of what human activity emits — but they are slow, and not limitless.

Deforestation attacks the carbon budget from both ends: fewer trees means less photosynthesis removing CO₂, and cleared forest is often burned, releasing its stored carbon in one plume. Combustion of fossil fuels — coal, oil, natural gas — adds carbon that was buried for millions of years, far faster than sinks can absorb it. Atmospheric CO₂ climbs, thickening the layer of gases that traps heat radiating from Earth: the enhanced greenhouse effect, and with it global warming — rising average temperatures, melting ice, rising seas and shifts in weather patterns.

The levers humans can pull: switch to renewable energy (solar, wind, hydro) instead of fossil fuels; reforest and protect existing forests so photosynthesis scales back up; improve energy efficiency in buildings, vehicles and industry; and shift to public transport and cleaner vehicles. Every tonne of CO₂ avoided or reabsorbed thins the blanket.

Sewage released into water carries nutrients and organic waste. Decomposing bacteria multiply to consume it, and their respiration drains the oxygen from the water; fish and other aquatic life suffocate. The damage is bacterial respiration, not the sewage itself, that kills — a sequence worth writing in that order.

Plastic wastes in the sea persist for decades: large pieces entangle and choke wildlife (turtles, seabirds), and fragments grind down into microplastics that are swallowed throughout food webs. Insecticides sprayed on crops wash into waterways and are absorbed by producers — and here the food chain turns traitor: each consumer eats many producers, so the chemical’s concentration multiplies at every trophic level. This biomagnification leaves top carnivores — the fish eagle, the apex fish — carrying the heaviest doses, causing eggshell thinning and population crashes even when the water itself tests nearly clean.

Conservation is the protection and management of ecosystems so their species — their biodiversity — survive for the future. Three ecosystems matter disproportionately. Coral reefs shelter a quarter of marine species; they need clean water and protection from destructive fishing, with tourism and fishing managed at sustainable yields. Tropical rainforests hold the planet’s richest terrestrial biodiversity and act as carbon sinks; sustainable logging (harvesting some trees while the forest regrows), protected reserves and reforestation keep both roles alive. Mangroves buffer coastlines, nurse young fish and store carbon; conserving them means limiting clearance for shrimp ponds and development, and replanting what is lost.

Sustainable use is the balancing principle: take what the ecosystem can regenerate, leave the structure intact. Biodiversity is not sentiment — each species is an irreplaceable gene bank, a link in food webs and a buffer that keeps ecosystems stable under change.