Curiosity · Chapter 12 — How Nature Works in Harmony

In parts of Odisha, Jharkhand, West Bengal, Assam and Chhattisgarh, elephants sometimes walk out of the forest and into farms and villages. Why would a forest animal do that? When rain fails, waterholes dry up and grass and leaves become scarce. When trees are cut for roads and buildings, the forest also shrinks. The elephants then go looking for food, such as bananas and sugarcane, in the fields nearby. Crops get damaged, and sometimes people and animals get hurt. To reduce this conflict, wildlife scientists have mapped elephant corridors, strips of land that join one forest to another so that animals can move safely between them.

This story shows that the parts of nature are linked: rain affects plants, plants affect elephants, and elephants affect people. To understand such links we begin with the idea of a habitat.

A habitat is the place where an organism lives and finds what it needs. A habitat can be very large, like a forest, or very small, like the bark of a single tree. Every habitat has two kinds of parts:

• Biotic components: the living things, such as plants, animals and microorganisms.
• Abiotic components: the non-living things and conditions, such as air, water, soil, sunlight and temperature.

Look at how a fish lives in a pond. It gets its food, such as small plants and tiny animals, from the biotic part of the pond. It gets oxygen, which is dissolved in the water, from the abiotic part. The pond also gives it shelter and space to grow.

Different organisms can share one habitat and still use it differently. In a forest, a rodent may be active in the warm day, while a snake comes out in the cool night. They live in the same place but meet different conditions, so they are not always fighting for the same thing. This sharing is one way in which living things manage to live together in harmony.

When you look into a pond you rarely see just one fish. You see many fish of the same kind swimming together. Scientists use special words to describe such groups, going from small to large.

• Individual: a single organism, such as one fish.
• Population: all the organisms of the same kind living in one habitat at a given time, such as all the rohu fish in a pond this month.
• Community: all the different populations living together in the same habitat, such as the fish, frogs, snails, insects, plants and microbes of the pond.
• Ecosystem: the community together with the abiotic components (water, air, soil, sunlight, temperature), and all the interactions between them.

Each level contains the one before it. A population is part of a community, and a community is part of an ecosystem. So an ecosystem is the largest of these and a single organism is the smallest.

You can count a population. Mark out a square of 1 m by 1 m in the school garden and count the plants of one kind inside it. If you find 20 plants of one kind and 5 of another, the first kind has a population of 20 and the second a population of 5 in that square, at that time. The words "at that time" matter, because populations change with seasons, births, deaths and movement.

Could a habitat hold only one kind of organism? If every organism were the same, they would all need the same food, water and space at the same time. They would compete hard, and the resources could run out. In real habitats many kinds live together, each using resources in its own way.

Ecosystems can be big or small. A forest, a grassland or a desert is a terrestrial (land) ecosystem; a pond, river, lake or sea is an aquatic (water) ecosystem. Even a single big tree can be studied as a small ecosystem. Ecosystems also overlap: a river flows through a forest and past farmland, and animals move between them. A farm is a human-made ecosystem, while a forest is a natural one.

In an ecosystem nothing lives on its own. The parts keep acting on one another. These actions are called interactions, and there are three kinds to look for:

Living things depend on non-living things. Plants need sunlight, carbon dioxide and water to make food. Soil holds plants up and gives them nutrients. Air gives oxygen for respiration to both plants and animals. But the dependence goes both ways: plants give out oxygen, their roots hold soil together and stop it being washed away, and they keep the ground moist and the air cool.

Here is a real study that shows how far one interaction can reach. Researchers compared two ponds. Pond A had fish and many flowering plants around it. Pond B had no fish and fewer flowering plants. Why should fish in the water matter to flowers on the bank? The chain of reasons goes like this:

• Fish eat the young (larvae) of dragonflies, so Pond A had fewer dragonflies.
• Adult dragonflies hunt flies, bees and butterflies, so with fewer dragonflies, more bees and butterflies survived near Pond A.
• Bees and butterflies carry pollen from flower to flower. This pollination is needed for fruits and seeds to form.
• So the plants near the pond with fish produced more seeds.

The fish affected the plants only indirectly, through two other animals. This is why scientists say that every member of a community matters. If people overfish such a pond, the effect could travel all the way to the flowers around it.

Ask of any organism: where does its food come from? The answer puts it into one of three groups.

Producers make their own food. Green plants use sunlight, carbon dioxide and water to make food by photosynthesis. They are also called autotrophs (auto = self, troph = food). Grass, trees, crops and the algae in a pond are producers. All other life depends on them.

Consumers cannot make their own food and eat other organisms. They are also called heterotrophs (hetero = other). Consumers are sorted by what they eat:

• Herbivores eat only plants or plant parts: deer, hare, cow, grasshopper.
• Carnivores eat only animals: leopard, tiger, the hawk called shikra.
• Omnivores eat both plants and animals: crow, fox, mouse, and also humans.

Decomposers feed on dead plants, dead animals and animal waste. Fungi such as mushrooms and many bacteria break the complex substances in dead matter into simpler ones. They are called saprotrophs (sapro = rotten). After the rains you may see mushrooms growing on a fallen log; they are slowly breaking it down. Beetles and flies on animal dung also help break waste into smaller pieces.

Decomposition returns nutrients to the soil, where plants take them up again. The same nutrients go round and round: from soil to plant, from plant to animal, from dead plant or animal back to soil. In nature nothing is truly wasted. If all decomposers vanished from a forest, dead leaves and bodies would pile up, the soil would run short of nutrients, and plants, and then animals, would suffer. This is why a compost pit in a school garden works: decomposers turn kitchen and garden waste into rich, dark manure.

In a grassland, grass is eaten by a hare, and the hare may be eaten by a leopard. Writing this as a line gives a food chain: a simple sequence that shows who eats whom.

We draw an arrow from the food to the eater, so the arrow shows the direction in which food (and the energy in it) moves:

grass → grasshopper → frog → snake → eagle

Every food chain starts with a producer, because only producers can capture the Sun’s energy as food.

Each step of a food chain is called a trophic level (troph = food):

• First trophic level: producers, such as grass and millet.
• Second trophic level: herbivores, such as hares, deer and grasshoppers.
• Third trophic level: small carnivores, such as frogs.
• Higher levels: larger carnivores, such as snakes, eagles, tigers.

Now count the organisms at each level. In a millet field you might find many hundreds of millet plants, a smaller number of mice that feed on them, and only one or two eagles hunting the mice. If you draw a bar for each level, with the largest number at the bottom, you get a shape like a pyramid: wide at the base and narrow at the top.

Why does it narrow? A mouse uses most of the food it eats to stay alive, move and keep warm. Only a small part becomes its body, which is all an eagle can get by eating it. So each higher level has less food available, and can support fewer animals. That is why there are always more plants than plant-eaters, and very few top predators.

Food chains also help us predict. In the chain grass, grasshopper, frog, snake, if frogs disappear, grasshoppers lose a predator and increase, eating more grass, while snakes lose their food and decrease.

A real grassland has more than one food chain. A mouse is eaten by a snake, an owl and a hawk. Grass is eaten by a hare, a grasshopper and a mouse. When we join all the food chains of an ecosystem, they cross and connect into a network called a food web.

A food web is safer for the animals in it than a single chain. If one kind of food becomes scarce, a hawk that eats mice, snakes and small birds can switch to another. But the links also mean that a change in one place can spread through the whole web, sometimes in surprising ways.

Think of a pond that becomes polluted:

• Many water plants die.
• With fewer plants, less oxygen is added to the water.
• Fish die for lack of oxygen.
• With fewer fish to eat them, insects multiply.
• The insects spread to nearby farms and damage the crops.
• Farmers spray more pesticides, which pollute soil and water further.

One change led to a whole chain of changes. This is called a cascade.

India has seen this happen. In the 1980s, India exported large numbers of frog legs, mostly from the Indian bullfrog. As frogs were caught in huge numbers, their populations fell. Frogs eat insects, so crop pests increased. Farmers then used more chemical pesticides, which harmed soil, water and human health. The Government of India banned the export of frog legs to stop the damage. The lesson is that a frog is worth far more alive in a rice field than on a dinner plate abroad.

An ecosystem is in balance when the interactions keep populations and resources fairly steady. This balance is not frozen; numbers rise and fall with the seasons. But a large disturbance, natural like a drought or human-made like overhunting, can push it out of balance.

Besides eating one another, organisms in an ecosystem are linked in other ways. One of the most important is competition. Organisms compete when they need the same thing and there is not enough for all: food, water, space, or sunlight. Trees in a forest compete for light, so they grow tall. Two kinds of birds may compete for the same fruit; over time one may start feeding at a different height or time of day. Competition keeps any one population from growing too large, which helps keep the ecosystem in balance.

Some pairs of organisms live closely together for a long time. Scientists name three kinds of such relationships according to who benefits:

A simple way to remember: in mutualism both win; in commensalism one wins and the other does not notice; in parasitism one wins at the other’s cost. The organism that lives on and harms another is the parasite, and the one it lives on is the host.

Predator and prey also keep each other in check. The wildlife biologist A.J.T. Johnsingh, who studied animals in Bandipur National Park in Karnataka, showed that tigers and leopards depend on prey such as deer and wild boar, and that a healthy prey population is needed for predators to survive. Protecting a tiger therefore means protecting the deer and the grasslands they eat from.

All these relationships, feeding, competing, helping and harming, together form the web of life that keeps an ecosystem working. Remove one partner, and the other partner, and many more, may feel the change.

Humans are part of ecosystems and depend on them every day. Forests give us fresh air, fertile soil, food, fibre, timber and medicines. Rivers, lakes and seas give water and fish. Ecosystems also store rainwater, control floods and give us beautiful places to rest and enjoy. These gifts are often called the benefits of ecosystems.

The Sundarbans show how valuable, and how fragile, an ecosystem can be. It lies where the Ganga and Brahmaputra rivers meet the sea, shared between India and Bangladesh, and has the largest mangrove forest in the world. Mangroves are trees that grow in salty, muddy coastal water. Their dense roots slow down storm winds and waves and protect villages behind them. In 2004, a villager in Cuddalore district of Tamil Nadu found that his village, sheltered by mangroves, suffered less from the tsunami than nearby villages. The Sundarbans are also home to many endangered animals and plants, and UNESCO declared them a World Heritage Site in 1987.

Yet the Sundarbans are under threat:

• mangroves are cut for fuelwood and to make farms,
• animals are hunted illegally and forest resources are overused,
• factory waste and untreated sewage pollute the rivers.

Other ecosystems across India face similar problems: deforestation, overuse of resources, pollution, careless use of land, and invasive species (plants or animals brought from elsewhere that spread quickly and crowd out local ones).

How do we protect them? One way is to set aside protected areas, parts of land or water kept mainly for wildlife. India has national parks, wildlife sanctuaries, biosphere reserves and community conserved areas. Examples include Jim Corbett National Park (Uttarakhand), Manas National Park (Assam), the Nilgiri Biosphere Reserve, Chilika Lake (Odisha), Keibul Lamjao National Park (Manipur) and Hemis National Park (Ladakh). Many communities also protect nature as part of their culture; some tribes in Arunachal Pradesh and Chhattisgarh treat the tiger as sacred.

Humans have also built ecosystems, such as farms, fish ponds and parks. Unlike natural ecosystems, these need regular human care. Well designed, they can support wildlife and reduce pollution too.

Farming feeds India, and a farm is itself an ecosystem. Between 1950 and 1965 India faced food shortages because crop yields were low. From the mid-1960s, tractors, machines, high-yield seeds, synthetic fertilisers and pesticides helped increase food production greatly. This period is called the Green Revolution, and it helped India become food secure.

But using these methods without care for many years has caused problems:

• Too much synthetic fertiliser lowers soil fertility over time by reducing friendly soil microbes and humus, the dark organic matter that binds soil particles. Soil without enough humus is easily eroded.
• Pesticides also kill the natural predators of pests, such as beetles, spiders and frogs, so pests can come back in larger numbers. Some pests become resistant to pesticides.
• Heavy irrigation and repeated ploughing disturb earthworms and other soil animals, and pumping too much groundwater lowers the water table.
• Growing the same crop again and again on the same land is called monoculture. It reduces the variety of living things and harms pollinators, which many crops need.

Understanding ecosystems shows a better path. Sustainable farming means growing food in a way that keeps the soil, water and living things healthy for the future. Practices include:

• adding compost and manure to build humus;
• growing different crops in turn (crop rotation) or together (mixed cropping);
• protecting natural pest-eaters and pollinators instead of spraying heavily;
• saving water with careful irrigation;
• organic and natural farming, which use few or no synthetic chemicals.

Farmers who use these methods often find that the soil holds water better and that birds, frogs and earthworms return to their fields. These ideas are old as well as new. The ancient Indian text Vrikshayurveda stresses feeding the soil with organic manures, such as Kunapa Jala, a liquid made by fermenting plant and animal waste. Fermentation breaks complex substances into simpler ones that plants can use, just as decomposers do in a forest.