Ecology and Ecosystem

Ecology is the study of how living things relate to each other and to the world around them. The word was coined in 1866 by the German biologist Ernst Haeckel, from the idea of 'house': the Earth is one big house, and ecology asks who lives in it, how they share it and what happens when one family moves or disappears. Once you see ecology as a story of relationships and not a list of definitions, every term in this chapter falls into place.

Remember the direction of the ladder: each step contains the one below it. The term Ecosystem was given by the British botanist A. G. Tansley in 1935. The term Biosphere was introduced by Eduard Suess; it is the thin zone of Earth, a few kilometres above and below sea level, where life exists.

An Ecotone is the transition zone where two ecosystems meet, such as the meeting of forest and grassland, land and sea (mangroves) or river and sea (estuary). Because the zone borrows species from both sides and adds a few species of its own, it often has more variety and higher density than either neighbour. This richness at the border is called the Edge Effect, and the special species that live mostly in the ecotone are called Edge Species.

Populations do not grow for ever. When food, space and water are plentiful, a population grows faster and faster, giving a J-shaped (exponential) curve. Soon resources start to run short and growth slows down, finally flattening at the level the environment can support, giving an S-shaped (logistic) curve. That ceiling is the Carrying Capacity: the maximum number of individuals of a species that an environment can sustain indefinitely.

An ecosystem is a working unit of nature: living things (biotic) and non-living things (abiotic) of one place, linked together by the flow of energy and the cycling of nutrients. A garden counts as an ecosystem not because it has plants and insects, but because those living things keep interacting with sunlight, water, soil and temperature. Take away either side and the 'system' stops working.

Producers (autotrophs) are the only members who bring new energy into the living world: they trap sunlight by photosynthesis and make food from carbon dioxide and water. Everyone else is a heterotroph and depends on them. Consumers eat other organisms. Decomposers break down dead plants and animals, return minerals to the soil and keep the cycle going; without them the world would be buried under dead matter and the soil would run out of nutrients.

Ecosystems are of two broad kinds. Natural ecosystems run on their own, without human help: forests, grasslands, deserts, ponds, lakes, rivers, seas and estuaries. Artificial (man-made) ecosystems are built and managed by people, such as crop fields, gardens, aquariums and reservoirs; they need constant human care, like water, fertiliser or feed, and break down quickly if the care stops.

Productivity is the rate at which producers make organic matter. Gross Primary Productivity (GPP) is the total food made by photosynthesis; Net Primary Productivity (NPP) is what is left after plants use some for their own respiration, so NPP = GPP - respiration. NPP is the food actually available for consumers. On land, tropical rainforests are among the most productive; in water, estuaries and coral reefs are among the most productive.

Think of energy as money that enters the ecosystem only once, through the producers, and is then spent at every step. A food chain shows who eats whom: the arrow runs from the food to the eater. Grass is eaten by a grasshopper, the grasshopper by a frog, the frog by a snake and the snake by an eagle. Every stage is a trophic level, and at every transfer most of the energy is lost, which is why the chain cannot go on for ever.

In nature, a grasshopper is eaten by frogs, lizards and birds, and a snake eats frogs, rats and birds. So single chains cross each other and form a net. This net of interlinked food chains is a food web. A web is a safer arrangement than a single chain: if one food source disappears, the consumer can switch to another. This is why a rich, complex food web makes an ecosystem more stable.

Now the key concept of the chapter. Only about 10 per cent of the energy at one trophic level is passed to the next; the other 90 per cent is used up in breathing, movement and growth, lost as heat, or left in undigested waste. This is Lindeman's 10 per cent law (1942). If producers hold 10,000 units, herbivores get about 1,000, small carnivores 100 and top carnivores only 10. That is why a food chain rarely has more than four or five links, why top carnivores are few in number, and why eating lower on the chain feeds more people.

Energy flows in one direction only: Sun to producers to consumers, and finally to heat. It is never recycled. Nutrients (carbon, nitrogen, water) go round and round. The first law of thermodynamics says energy is conserved; the second says some is always lost as heat in every transfer, which is why the flow is one-way.

An ecological pyramid draws the trophic levels as bars stacked one over another, producers at the base. The idea was introduced by the British ecologist Charles Elton. There are three kinds: the pyramid of numbers (how many individuals), the pyramid of biomass (total dry weight of living matter) and the pyramid of energy (energy per unit area per unit time).

A harmful twist: some poisons, such as DDT and mercury, are not broken down and are not excreted. They are stored in the body and passed to the eater of every link. Since each top animal eats many animals below it, the poison gets more and more concentrated as we go up the chain. This is biomagnification. Eagles and ospreys at the top got so much DDT that their egg shells became thin and broke. The same poison can be passed to humans through fish and milk.

Energy is spent and lost, but matter is only borrowed. The atoms of carbon, nitrogen, oxygen and water in your body have been used by countless living things before you, and will be used by others after you. The movement of these elements between the living world (bio), the air, water and rocks (geo) through chemical changes (chemical) is called a biogeochemical cycle. Each element has a reservoir, the large store from which it enters the living world, and an exchange pool, the smaller, faster-moving part.

The Water Cycle (hydrological cycle) runs on solar energy, which is the answer examiners usually want. The sun heats oceans, lakes, rivers and soil; water evaporates, and plants also release it through transpiration (together called evapotranspiration). In the cooler upper air the vapour condenses into clouds, falls as precipitation (rain, snow, hail), and then flows as runoff into rivers and the sea, seeps into the ground as groundwater or is stored as ice. About 97 per cent of Earth's water is in the oceans, and only about 3 per cent is fresh water, most of it frozen in glaciers and ice caps.

The Carbon Cycle: carbon is the backbone of every organic molecule. The air holds only about 0.04 per cent carbon dioxide, yet it is the doorway of carbon into life. Green plants and algae take it in by photosynthesis; animals get it by eating plants; and everyone returns it by respiration. Dead matter returns it by decomposition, volcanoes release some, and burning of wood and fossil fuels (coal, petroleum) releases carbon that was locked away for millions of years. The oceans dissolve a large amount of CO2, and sedimentary rocks such as limestone hold the largest stock of carbon.

The Nitrogen Cycle: air is about 78 per cent nitrogen gas (N2), yet plants and animals cannot use it, because the two atoms are held together by a very strong triple bond. Nitrogen has to be 'fixed', that is converted into ammonia or nitrates, before plants can absorb it. Nature does this in three ways: lightning (physical fixation), nitrogen-fixing microbes (biological fixation, the most important) and, today, fertiliser factories (industrial fixation, Haber-Bosch process).

Why do farmers sow pulses after wheat or rice? Legume roots leave nitrogen compounds in the soil, so the next crop needs less urea. This is the logic behind crop rotation and green manuring. Inside the nodule the enzyme nitrogenase does the fixing, and a pink pigment, leghaemoglobin, mops up oxygen because oxygen would damage the enzyme.

The Phosphorus Cycle is a sedimentary cycle with no gaseous phase. Phosphate is released slowly from rocks by weathering, taken up by plants, passed to animals along the food chain, and returned by decomposition and excretion (bird droppings, or guano, are rich in phosphate). A lot of phosphate eventually washes to the sea floor and becomes rock again over millions of years. Because the stock in soil is small, farmers add phosphate fertilisers, and the run-off from fields can cause algal blooms in lakes. The Sulphur Cycle is partly sedimentary and partly gaseous: burning coal releases SO2, which causes acid rain.

Ecosystems are not frozen pictures; they change like a film. Ecological succession is the slow, orderly and fairly predictable replacement of one community by another, until a stable, mature community is reached. Walk past an abandoned field: first weeds appear, then grasses, then shrubs and finally trees. Each stage (a sere) changes the soil, light and moisture in a way that suits the next, not itself, and so prepares its own replacement.

Succession is driven first by the community itself (autogenic) and sometimes by outside forces such as fire, flood or climate change (allogenic). As succession proceeds, the number of species, total biomass, soil depth and the complexity of food webs increase, and the ecosystem becomes more stable. The final stage is the climax community, which stays more or less the same as long as the climate stays the same. Eutrophication speeds up the filling of a lake and thereby hastens its succession towards dry land.

Now the relationships between species. Ecologists write each as a pair of signs: plus (+) benefits, minus (-) harmed, zero (0) neither. Once you know the sign pair, you can name the interaction in any example the examiner gives.

Competition has a famous principle: Gause's Competitive Exclusion Principle says that two closely related species competing for the same limiting resource cannot live together for ever; the better competitor wins and the other is eliminated or shifts to a different niche. This explains resource partitioning, where several similar species coexist by feeding on different parts of the same tree or at different times.

A biome is a very large ecological region where climate, mainly temperature and rainfall, decides the type of vegetation, and the vegetation in turn decides which animals live there. If you know the climate, you can almost guess the plants and animals; this is why the same biome, such as a desert, looks alike in Africa, Arabia and Rajasthan.

India has most of these living systems in miniature. Tropical wet evergreen forests in the Western Ghats, the north-east and the Andaman-Nicobar islands; tropical deciduous (monsoon) forests across the peninsula; thorn forests and desert in the Thar; Terai grasslands at the Himalayan foothills; alpine meadows and cold desert in Ladakh; mangroves in the Sundarbans; and coral reefs in the Gulf of Mannar, Gulf of Kutch, Lakshadweep and Andaman-Nicobar.

Shola forests are pockets of stunted evergreen forest found in the folds and depressions of valleys of the high Western Ghats (Nilgiri, Anamalai, Palani hills), surrounded by rolling montane grassland. They are not conifer forests, and they are not found in arid zones. The forest-grassland mosaic acts like a sponge that feeds the streams of South India.