Chapter 9 — Adaptations in Different Ecosystems

An ecosystem is a community of living organisms together with the non-living surroundings with which they interact, functioning as one unit. A pond, a forest, a grassland, a desert, a mangrove swamp, a coral reef, even a rotting log or a paddy field is an ecosystem. The biotic components are the producers (green plants and algae), the consumers (herbivores, carnivores, omnivores) and the decomposers (bacteria and fungi). The abiotic components are sunlight, temperature, water, air, soil, minerals and the physical features of the place. The abiotic factors differ enormously between a pond and a desert, and it is these differences that demand different adaptations.

The habitat of an organism is the particular place in the ecosystem where it lives: the mud of the pond bottom for a mussel, the surface film for a water strider, the canopy for a langur, the burrow for a rat. Its niche is its role and way of life within the habitat, what it eats, when it is active, what eats it.

Adaptation is any feature of an organism, in its structure, its body functions or its behaviour, that helps it to survive and reproduce in its habitat. Adaptations are inherited; they arose over many generations because individuals that happened to possess them left more offspring than those that did not. This is natural selection, and adaptation is its product. An adaptation is always relative to a particular environment: the thick fur that suits a yak on a Himalayan pass would kill it in the Godavari delta, and the gills that suit a fish are useless on land.

Adaptations are of three kinds. Structural (morphological) adaptations are features of the body: the webbed feet of a duck, the spines of a cactus, the streamlined body of a fish, the hump of a camel. Physiological adaptations are features of body chemistry and function: the ability of a camel to tolerate a rise in body temperature, the concentrated urine of a desert rat, the antifreeze proteins in the blood of polar fish, the crassulacean acid metabolism of succulents that open their stomata only at night. Behavioural adaptations are features of what the organism does: the nocturnal habit of desert animals, the migration of birds, the hibernation of frogs, the burrowing of crabs at high tide.

Adaptations are also grouped by the factor they answer. Adaptations to water (too much or too little), to temperature (heat and cold), to light, to food supply, to predators, and to locomotion in a particular medium (swimming, flying, burrowing, climbing). This chapter takes the major ecosystems one by one and asks, in each, what the abiotic conditions are and how organisms meet them.

Two cautions. First, an adaptation is not something an organism decides to grow; it is the result of long evolution, and an individual lotus cannot become a cactus if its pond dries. Second, the more specialised the adaptation, the more the organism depends on its ecosystem; a mangrove cannot live inland, so when a mangrove swamp is cleared for shrimp ponds, its species are lost.

Water covers about 71 per cent of the Earth, and aquatic ecosystems are of two great classes: freshwater (ponds, lakes, tanks, rivers, streams, paddy fields, marshes) and marine (seas, oceans, estuaries, coral reefs, mangrove swamps). Andhra Pradesh has the Kolleru and Pulicat lakes, the Krishna and Godavari rivers with their estuaries, thousands of village tanks and 974 kilometres of coast, so aquatic ecosystems are all around us.

Life in water is very different from life on land, and the differences set the adaptations. First, water supports the body: it is about 800 times denser than air, so aquatic organisms need little skeleton or woody tissue to hold themselves up, and a whale can grow to a size that no land animal could bear. Second, water resists movement: it is far more viscous than air, so moving through it needs a streamlined body. Third, oxygen is scarce: water holds only about 1 per cent oxygen by volume, compared with 21 per cent in air, and warm or polluted water holds even less, so aquatic animals need efficient organs for taking oxygen from water, or must surface to breathe air. Fourth, light fades with depth: most photosynthesis occurs in the top few metres, the euphotic zone, and below about 200 metres in the sea there is no light at all, so plants live only near the surface and deep-sea animals depend on food sinking from above. Fifth, temperature is stable: water heats and cools slowly, so aquatic organisms rarely face the extremes that land organisms do. Sixth, salt concentration matters: freshwater is more dilute than body fluids, so freshwater organisms constantly gain water by osmosis and must pump it out; seawater is saltier than most body fluids, so marine bony fishes constantly lose water and must drink and excrete salt. Seventh, water flows: organisms in rivers and streams must hold on or be swept away, and organisms in the intertidal zone are alternately submerged and exposed.

A pond or lake has zones. The littoral zone is the shallow edge where light reaches the bottom and rooted plants grow; the limnetic zone is the open, lit surface water with floating plankton; the profundal zone is the deep, dark water where decomposers work; and the benthic zone is the bottom mud. Plants and animals are arranged according to these zones, and the next topics describe their adaptations.

The plants of water are called hydrophytes and the animals are broadly aquatic animals, divided into those that live wholly in water (fish, prawns, mussels), those that float or swim (plankton and nekton), those that live on the bottom (benthos), and those that live partly on land and partly in water (frogs, water birds, crocodiles), which are amphibious.

Hydrophytes are plants that grow in water or in soil saturated with water. According to their position in the water they are grouped into four types, and every village tank in Andhra Pradesh shows all four.

Free-floating hydrophytes float on the surface without any contact with the bottom: water hyacinth (Eichhornia), pistia (water lettuce), duckweed (Lemna), Wolffia (the smallest flowering plant), Azolla and Salvinia. Submerged hydrophytes live entirely under water, either rooted in the mud (Hydrilla, Vallisneria, Potamogeton) or free-floating below the surface (Ceratophyllum, Utricularia). Rooted hydrophytes with floating leaves are anchored in the mud but send long petioles up so that their leaves lie flat on the surface: lotus (Nelumbo), water lily (Nymphaea), Trapa (water chestnut, singhara). Emergent (amphibious) hydrophytes are rooted in the mud with their lower parts in water and their upper parts in air: typha (cattail), sagittaria, reeds, rice, Ranunculus, and many marsh plants. Some, like lotus, stand between the third and fourth groups, with both floating and aerial leaves.

Because water supports them, supplies water and dissolved minerals all over the body, and offers little oxygen, hydrophytes share a set of adaptations. Roots are poorly developed, since absorption is not their main job; in free-floating plants they hang loose and serve mainly for balance, root hairs are few or absent, and root caps are replaced in some (pistia, water hyacinth) by root pockets that protect the tip. Submerged plants such as Ceratophyllum have no roots at all. Stems are soft, slender and flexible, with little mechanical or woody tissue, so that they bend with currents instead of breaking; in rooted plants the stem is often a rhizome buried in the mud (lotus). Aerenchyma, a spongy tissue with large air spaces, fills the petioles, stems and leaves; it stores oxygen released in photosynthesis, carries it down to the roots in the airless mud, and makes the plant buoyant. The petiole of the lotus and the swollen petiole of water hyacinth are full of it. Leaves of submerged plants are thin, finely divided or ribbon-like (Vallisneria, Hydrilla, Ceratophyllum), giving a large surface for absorbing gases and light and offering little resistance to water; they have no stomata, no cuticle and a thin epidermis with chloroplasts, and they absorb carbon dioxide and minerals directly from the water. Floating leaves are large, flat, circular and entire, with a waxy, water-repellent upper surface that keeps the stomata dry, and stomata only on the upper surface, which faces the air; the lower surface, touching water, has none. Emergent leaves are like those of land plants, with stomata on both sides. Vascular tissue is reduced, with xylem often a mere cavity, since water need not be pulled up from below. Reproduction is largely vegetative, by fragments, offsets and runners (which is why water hyacinth chokes a tank within a season); flowers are borne above water for pollination by insects and wind, except in Vallisneria, whose pollination happens on the surface film, and in Ceratophyllum, which is pollinated under water.

Fish are the most completely aquatic vertebrates, and their bodies answer each condition of life in water. The body is streamlined, spindle-shaped, tapering at both ends, so that it slips through water with least resistance; the head merges with the trunk without a neck, and there are no projecting ears or limbs. The skin is covered with overlapping scales that lie backwards and with slippery mucus from skin glands, which reduces friction, keeps out parasites and seals the body against osmosis. Fins are the organs of locomotion: the tail (caudal) fin provides the push, the paired pectoral and pelvic fins steer, brake and balance, and the median dorsal and anal fins keep the fish upright. The muscles of the trunk are arranged in W-shaped blocks (myotomes) that bend the body from side to side in waves that drive it forward.

Gills take oxygen from water. Each gill has a bony arch bearing rows of thin filaments with thousands of tiny plates (lamellae) full of blood capillaries, giving a huge surface. Water is drawn in through the mouth and pushed out over the gills through the gill slits, covered in bony fish by the operculum; blood in the lamellae flows in the direction opposite to the water (counter-current flow), which extracts as much as 80 per cent of the dissolved oxygen. Fish in stagnant, oxygen-poor water have additional accessory respiratory organs: the climbing perch (Anabas) has a folded labyrinth organ above the gills, the catfishes Clarias and Heteropneustes have air sacs, and the lungfish has true lungs, so these fish can gulp air and survive in drying ponds.

Bony fish have a gas-filled swim bladder (air bladder) whose volume they adjust to hover at any depth without swimming; sharks, which lack it, must keep swimming or sink. The lateral line, a canal along each side with sensory cells, detects vibrations and pressure changes, letting a fish sense a wall, a predator or a shoal-mate in the dark. Eyes have no eyelids (the water washes them), a spherical lens for seeing in water, and no tear glands. Freshwater fish excrete large volumes of dilute urine and take up salt through the gills; marine fish drink seawater and excrete salt through gill cells. Fish are cold-blooded, so their body temperature follows the water and they need less food than a mammal of the same size. Most lay large numbers of eggs in water where fertilisation is external.

Other aquatic animals show parallel adaptations. Whales and dolphins, though mammals, have a streamlined body, forelimbs modified into flippers, a horizontal tail fluke, a layer of blubber for warmth and buoyancy, nostrils moved to the top of the head as a blowhole, and the ability to hold their breath for long dives. Frogs have webbed hind feet and a smooth moist skin through which they breathe under water. Water birds such as ducks have webbed feet, waterproof plumage oiled from the preen gland, and a flat bill for sieving. Crocodiles have eyes and nostrils on the top of the head so that they can lie almost submerged, a valve that closes the throat under water, and a powerful laterally flattened tail. Prawns and crabs breathe by gills under the shell; mussels filter food and oxygen through gills; plankton such as Daphnia stay afloat with spines, oil droplets and constant beating of appendages. The water strider walks on the surface film on water-repellent hairs, and the mosquito larva hangs from the film breathing through a siphon.

A desert is a region receiving less than about 250 mm of rain a year, where evaporation exceeds rainfall. Deserts may be hot (Thar in Rajasthan, Sahara, Arabian) or cold (Ladakh, Gobi). Rain is scarce and irregular, the sky is cloudless, so days are burning hot (over 45 °C in summer) and nights, with nothing to hold the heat, are cold; the wind is dry and strong; the soil is sandy or rocky, poor in humus, and holds little water; and the surface is often salty. Parts of Rayalaseema, though not true desert, are semi-arid with 500 mm of rain, and show many of the same plants and animals. The problem for every desert organism is the same: to obtain water, to keep it, and to avoid overheating.

Plants adapted to dry conditions are xerophytes. Botanists recognise three strategies. Ephemerals (drought escapers) are annuals such as Tribulus and many desert grasses that survive the dry months as seeds, germinate within days of a shower, flower and set seed in a few weeks, and die; they escape drought rather than resist it. Succulents (drought resisters) store water in fleshy stems (cactus, Euphorbia, Opuntia or prickly pear, Cereus) or leaves (Aloe, Agave, Bryophyllum); the stem of a cactus is a green, ribbed, water-storing cylinder that shrinks in drought and swells after rain, and it does all the photosynthesis while the leaves are reduced to spines. Non-succulent perennials (true xerophytes, drought endurers) such as Acacia (babool), Prosopis (kikar), Calotropis (madar), Zizyphus (ber), Capparis and Nerium (oleander) survive drought with tough, adapted tissues and very deep roots.

The adaptations of xerophytes reduce water loss, increase water uptake and store water. Roots are very extensive: either very deep (the tap root of Prosopis reaches 30 metres to the water table) or very wide and shallow (cactus roots spread metres in the top few centimetres to catch every shower); they grow fast and have many root hairs. Stems are either fleshy and green (succulents) or hard, woody and covered with thick bark; many are reduced or spiny. Leaves are small or absent (cactus spines, the tiny leaflets of Acacia), or shed in the dry season (Zizyphus), or needle-like (Casuarina), or rolled (grasses), or leathery and shiny; all these cut the surface through which water is lost. The cuticle is thick and waxy, sometimes with a coat of hairs or white powder that reflects sunlight (Calotropis). Stomata are few, small, and sunken in pits or grooves (oleander) where still, moist air slows transpiration; in succulents they open only at night (crassulacean acid metabolism, CAM), taking in carbon dioxide when it is cool and storing it as acid for use in daylight. The cells have a high osmotic pressure that pulls water in strongly, and tissues can survive severe drying. Many xerophytes have thorns and bitter latex (Calotropis, Euphorbia) against browsing animals, and seeds with hard coats that lie dormant for years until enough rain falls.

Desert ecosystems are fragile: the soil takes centuries to form and a few years of overgrazing turns semi-arid land into true desert, a process called desertification that threatens parts of Rayalaseema.

Desert animals face the same problems as desert plants, getting water and keeping cool, but they can also move, and most of their adaptations combine body features with behaviour.

The camel, the ship of the desert, is the classic example, and every one of its features has a purpose. It can drink up to 100 litres in ten minutes and then go for a week or more without water, because it can lose a quarter of its body weight as water without harm (a human dies at about 12 per cent) and its blood does not thicken, since its oval red cells swell and shrink without bursting. The hump stores fat, not water; the fat is a food reserve for lean times, and it is concentrated on top of the body so that the rest of the body has no insulating layer and can lose heat. The camel's body temperature is allowed to rise from about 34 °C in the morning to over 40 °C by afternoon, so that it does not need to sweat during the day and loses the heat to the cool night air; a human, by contrast, sweats to hold a steady 37 °C. It sweats little, its dung is dry, and its urine is thick and scanty. Its nostrils can close against blowing sand and their moist lining traps water vapour from each breath; long eyelashes, bushy eyebrows and a third eyelid protect the eyes from sand; the ears are small and hairy. The feet have two broad, padded toes that spread on soft sand like snowshoes and are protected from the hot surface by thick soles; horny pads on the knees and chest let it kneel on burning ground. Its long legs keep the body a metre above the hottest air near the sand, and its thick coat reflects sunlight and insulates against heat. Its tough lips and mouth let it browse thorny Acacia and Calotropis that other animals refuse, and it can drink brackish water.

Small desert animals cannot carry such reserves, and most solve the problem by behaviour: they are nocturnal, spending the day in cool, humid burrows a metre below the surface where the temperature is a steady 25 to 30 °C, and coming out at night to feed. Desert rats and gerbils, the kangaroo rat of the American deserts, desert foxes, hedgehogs, scorpions, beetles and many snakes and lizards do this. The kangaroo rat never drinks; it lives entirely on the metabolic water made by oxidising the dry seeds it eats, and it saves that water with kidneys that make urine four times as concentrated as a human's, with a nose that cools each exhaled breath so that its moisture condenses, and by having no sweat glands. The desert fox and the desert hare have huge ears that radiate heat. Lizards raise their body off the hot sand, run on two legs, or 'swim' under it; the sand fish and the spiny-tailed lizard Uromastyx retreat to deep burrows. Snakes such as the saw-scaled viper move by sidewinding, touching the sand at only two points. Insects have a waterproof waxy cuticle and excrete almost dry uric acid; the desert beetle drinks fog condensing on its back. Many desert animals are sandy-coloured, which both camouflages them and reflects heat. Some, like the desert toad, aestivate, lying dormant in mud through the dry months and emerging to breed in the brief pools after rain.

Desert birds such as the sand grouse fly long distances to water at dawn, and the male soaks his belly feathers to carry water back to the chicks. The larger animals, the chinkara gazelle and the blackbuck of the Rayalaseema plains, get most of their water from the dew on grass and from succulent plants, and can survive on very little drinking water.

Where the Godavari and Krishna rivers meet the sea, in the Coringa sanctuary near Kakinada and the estuaries of Krishna district, stands a strange forest that grows in salt water and mud, flooded twice a day by the tide. This is the mangrove ecosystem, found on sheltered tropical coasts, estuaries and deltas; the Sundarbans of Bengal and Bangladesh is the largest in the world, and Coringa is the second largest in India. The trees are called mangroves, and the chief genera on our coast are Rhizophora, Avicennia, Sonneratia, Bruguiera, Ceriops and Excoecaria.

Life here is doubly difficult. The water is salty, so the plants face the osmotic problem of a desert while standing in water: the salt outside pulls water out of their cells. The soil is waterlogged, soft mud with no oxygen, so roots cannot breathe. The tide floods and exposes the plants every twelve hours, and cyclones and waves batter them. Seeds falling into moving salt water would be swept away or would fail to germinate in the mud. Mangroves answer each problem.

Against salt, some mangroves (Rhizophora) are salt excluders, with root membranes that filter out most of the salt as water enters; others (Avicennia) are salt excreters, with salt glands on the leaves that pump out salt, which dries as white crystals you can taste; and some store salt in old leaves and shed them. The leaves are thick, leathery and succulent, with a thick cuticle and sunken stomata to conserve the water that is so costly to obtain, exactly like xerophytes; mangroves are therefore called physiological xerophytes, plants standing in water yet short of it.

Against airless mud, mangroves have breathing roots. Avicennia and Sonneratia send up pneumatophores, pencil- or cone-like vertical roots that rise from horizontal roots in the mud and stand above it in thousands, like a bed of nails; their surface has pores (lenticels) through which air enters and is carried down through spongy tissue to the buried roots. Bruguiera has knee roots that loop up and down above the mud. Against soft mud and tides, Rhizophora has stilt (prop) roots that arch out from the trunk and branches into the mud like the legs of a tripod, anchoring the tree and breaking the force of waves; some species have plank-like buttress roots. The tangle of roots traps silt and builds new land.

Against the loss of seeds, many mangroves show vivipary: the seed germinates while the fruit is still on the parent tree, growing a long green seedling (propagule) up to 30 centimetres long with a heavy, pointed root end. When it drops, it either sticks upright in the mud below and roots at once, or floats away and, after drifting for weeks, lodges in mud elsewhere and takes root. The seedling thus avoids germinating in salt water and is planted ready-made.

The mangrove is a nursery for the sea: the roots shelter the young of prawns, crabs, mullet and other fishes on which the coastal fishery depends; mud skippers, fiddler crabs, oysters, water birds, otters and, in the Sundarbans, tigers live here. Mangroves protect the coast from cyclone waves and erosion, as villages behind the Coringa mangroves found during the 1996 cyclone, and they store large amounts of carbon. Yet they are cleared for shrimp farms, salt pans and ports, and their loss destroys both the fishery and the coast's shield.

A forest is an ecosystem dominated by trees. Its conditions depend on rainfall and temperature: the tropical evergreen (rain) forests of the Western Ghats, the Andamans and the north-east receive over 200 cm of rain and stay green all year; the tropical deciduous forests of the Eastern Ghats, the Nallamala hills and most of Andhra Pradesh receive 100 to 200 cm with a long dry season, and their teak, sal, terminalia and bamboo shed their leaves in the hot months; thorn forests of acacia and prosopis grow in the drier districts; and temperate and coniferous forests of oak, pine, deodar and fir grow in the Himalaya.

Inside a dense forest the abiotic conditions change from top to bottom. The canopy receives full sun, wind and rain; the forest floor receives only 1 to 2 per cent of the light, is humid, still and shaded, and is covered with decaying litter. The forest is therefore arranged in layers (stratification): the emergent giants, the canopy of tree crowns, the understorey of small trees and shrubs, the herb layer, and the floor of mosses and fungi. Each layer has its own community.

Forest plants are adapted to competition for light. Trees grow tall and straight with the crown at the top and few low branches; rain-forest trees have buttress roots, plank-like flanges at the base that support the great height on shallow soil. Leaves of the canopy are thick, and many have drip tips, pointed ends that shed rain quickly so that the leaf dries and fungi cannot grow. Plants of the floor have large, thin, dark green leaves to catch the little light; many, such as ferns and mosses, need the constant humidity. Because light is at the top, many plants reach it without building a trunk: climbers and lianas use tendrils, hooks and twining stems to climb trees; epiphytes such as orchids, ferns and mosses perch on branches. Deciduous trees shed their leaves in the dry season to stop water loss, flower and fruit before the monsoon, and store food in thick roots; bamboos and grasses regrow from underground stems after fire.

Forest animals are adapted to arboreal (tree) life or to the floor. Monkeys (langur, bonnet macaque) have grasping hands and feet with opposable thumbs, a long tail for balance, and forward-facing eyes for judging distance; the slender loris has huge eyes for the night; the giant squirrel of the Eastern Ghats leaps with a bushy tail as rudder; the flying lizard Draco and flying squirrels glide on skin flaps; tree frogs and geckos have adhesive pads on their toes; chameleons have gripping feet, a prehensile tail, independently moving eyes, a shooting tongue and colour change; woodpeckers have chisel bills, stiff tail props and two toes forward and two back; hornbills nest in tree holes; snakes such as the vine snake are green, slender and arboreal. On the floor, deer and gaur are browsers and grazers with keen hearing and alarm calls; the tiger and leopard are striped or spotted to break up their outline in dappled light and hunt by stealth; the sloth bear digs termites with long claws; wild boar root in the litter; and millions of ants, termites, beetles, earthworms and fungi decompose the leaf fall, returning nutrients to the trees. Many forest animals are nocturnal or crepuscular, and the forest is loud with calls because sight is limited among the trees.

Forests recycle water and nutrients, hold the soil on hillsides, feed the rivers of Andhra Pradesh from the Eastern Ghats, and shelter most of the land's species. Their loss to timber, mining and cultivation is the greatest threat to the animals adapted to them, which cannot live in the fields that replace the trees.

Grasslands occur where rainfall (25 to 75 cm) is too little for forest but too much for desert, and where grazing and fire keep trees out. The savannas of Africa, the prairies of North America, the steppes of Asia and the grasslands of the Deccan plateau are examples. Grasses are adapted to being eaten, trodden and burnt: they grow from the base of the leaf and from buds at or below the soil surface, so cutting the top does no harm; their fibrous roots form a dense mat that holds the soil and survives fire and drought; their narrow leaves roll up in dry weather; and they spread by underground stems and by seeds carried by wind. Grassland animals are adapted to open country where there is nowhere to hide. Grazers such as blackbuck, chinkara, nilgai, wild ass and the great herds of Africa have long legs and hooves for speed, eyes on the sides of the head for a wide view, keen hearing and smell, and live in herds in which many eyes watch for predators; their high-crowned, continuously growing teeth cope with the silica-rich grass. Predators such as the cheetah (once found in India), wolf and jackal rely on speed or on hunting in packs. Many small animals, the mongoose, gerbil, rat, and the great Indian bustard's prey, live in burrows, which protect them from fire, heat and predators, and their sandy colours match the dry grass. The great Indian bustard, the lesser florican and the lark are ground-nesting grassland birds now threatened because grasslands are ploughed or planted with trees.

Mountain (alpine) ecosystems above the tree line, in the Himalaya above about 3,500 metres, have thin air with less oxygen, intense ultraviolet light, cold, strong wind, a short summer and snow for much of the year. Plants are dwarf and cushion-shaped or creeping (rhododendron, juniper, saxifrages, edelweiss), hugging the ground where wind is weaker and the air is warmer; they have small, thick, hairy leaves that reduce water loss and screen ultraviolet light, deep roots, and bright flowers that open within days of the snow melting to complete their cycle in the short summer; many are perennials with food stored underground. Animals of the high mountains, the yak, the Himalayan tahr, the snow leopard, the bharal, the marmot, the Tibetan gazelle, have thick fur or dense woolly undercoats, compact bodies with short ears and tails to lose less heat, large lungs and hearts and blood rich in haemoglobin to use the thin air, broad hooves or padded paws for snow and rock, and they migrate downhill in winter or, like the marmot, hibernate. Mountain birds such as the lammergeier soar on the updraughts; the bar-headed goose flies over the Himalaya with blood that binds oxygen unusually well.

Polar ecosystems, the Arctic tundra and Antarctica, have long dark winters, temperatures below minus 40 °C, permanently frozen subsoil (permafrost) and a summer of a few weeks. Tundra plants are mosses, lichens, sedges and dwarf willows a few centimetres tall, growing and flowering in the brief summer. Polar animals show the classic cold adaptations: a thick layer of fat (blubber) under the skin in seals, whales, walruses and polar bears, which insulates and stores energy; thick fur with an air-trapping undercoat, or dense waterproof feathers in penguins; white colour in the polar bear, arctic fox, arctic hare and snowy owl for camouflage in snow, with the fox and hare turning brown in summer; small ears, short tails and rounded bodies to reduce the surface from which heat is lost; hairy soles and broad paws for walking on ice; counter-current heat exchange in the legs of penguins and the flippers of seals, where warm arterial blood warms the returning cold venous blood so that little heat is lost from the feet; huddling in emperor penguins; hibernation in the female polar bear during her winter denning; antifreeze proteins in the blood of Antarctic fish that stop ice crystals forming; and migration of the caribou, arctic tern and geese away from the winter. Polar animals also breed in the short summer when food, the plankton bloom and the insects, is abundant.

Some plants have found ways of living that free them from the ground, or from making their own food, and each of these special modes of life has its own adaptations.

Epiphytes are plants that grow perched on other plants, usually on the branches and trunks of trees, using them only for support and not for food. They are common in humid forests where the tree canopy is the only place with light: orchids (Vanda, Dendrobium), ferns (bird's nest fern), mosses, lichens, some aroids and, in South India, the wild pepper's relatives. An epiphyte has no contact with the soil, so its problems are water and minerals. Orchids have two kinds of root: clinging roots that grip the bark, and hanging aerial roots covered with a spongy, silvery tissue called velamen, made of dead cells, which soaks up rain and dew like blotting paper and absorbs water vapour from humid air; the green tips of the roots even photosynthesise. Minerals come from the dust, decaying bark and bird droppings that collect in the plant. Water is stored in swollen stem bases (pseudobulbs) and thick, leathery, waxy leaves with sunken stomata, for the branch dries quickly between showers; epiphytes are thus xerophytic in leaf though they live in a rain forest. The bird's nest fern forms a funnel of leaves that collects litter and water, making its own pocket of soil; the staghorn fern does the same with shield-shaped basal fronds. Orchid seeds are dust-like, carried by wind to high branches, and germinate only with the help of a fungus.

Climbers are plants with weak stems that reach light by climbing other plants or supports rather than building a strong trunk of their own. They save the cost of wood and grow very fast. Twiners such as bean, morning glory, Dioscorea and the woody lianas of the forest coil their whole stem around the support. Tendril climbers such as cucumber, gourd, pea, passion flower and grape have thread-like tendrils (modified leaves, leaflets, stipules or branches) that are sensitive to touch and coil round anything they meet, then shorten into a spring that pulls the plant up. Root climbers such as betel vine, pepper, money plant and ivy grip with short adventitious roots along the stem. Hook and thorn climbers such as Bougainvillea, rose, rattan cane and Ziziphus hang on with curved thorns. Leaf climbers such as Gloriosa use the coiled tips of their leaves. The stems of climbers have wide vessels to carry water fast to distant leaves and are often flattened or grooved to bend without breaking.

Parasitic plants draw water, minerals or food from a host plant through special absorbing organs called haustoria that penetrate the host's tissues. Total parasites have no chlorophyll and take everything from the host: Cuscuta (dodder, amarbel), a leafless yellow thread that twines over hedges and takes its food from the host's phloem; Rafflesia of Malaysia, which lives inside its host vine and appears only as the largest flower in the world, a metre across; and Orobanche (broomrape) on tobacco and brinjal roots. Partial parasites have green leaves and make their own sugar but take water and minerals from the host: Loranthus and Viscum (mistletoe) on the branches of mango, neem and other trees, spread by birds that wipe the sticky seeds on the bark, and Striga (witchweed) on the roots of sorghum and sugarcane, a serious pest. Parasites have reduced roots and leaves, rapid growth, and enormous numbers of tiny seeds that germinate only when they sense the host.

Related to these are saprophytes such as the ghost orchid Monotropa and many fungi, which have no chlorophyll and feed on dead organic matter with the help of fungal partners, and the symbiotic lichens, in which a fungus and an alga live as one body, able to colonise bare rock and tree bark where neither could live alone.

In bogs, marshes and wet, acidic, sandy soils where nitrogen is scarce because decomposition is slow, a few plants have evolved a remarkable adaptation: they trap and digest insects and other small animals to obtain nitrogen and other minerals. These are the insectivorous (carnivorous) plants. They are green and make their own sugar by photosynthesis; the insects supply only the mineral nutrients that the soil cannot. About 600 species are known, and several grow in India.

The pitcher plant (Nepenthes), found wild in India only in the Khasi and Garo hills of Meghalaya, modifies the tip of its leaf into a hanging jug or pitcher up to 20 centimetres deep, with a lid that keeps out rain. The rim of the pitcher is brightly coloured and secretes nectar; insects attracted to it slip on the waxy inner wall and fall into the pool of digestive fluid at the bottom, where enzymes secreted by glands break down their soft parts and the products are absorbed. Downward-pointing hairs inside prevent escape. The American Sarracenia and the Australian Cephalotus have pitchers formed from whole leaves.

The Venus flytrap (Dionaea) of the Carolina bogs has leaves whose blade is hinged along the midrib into two lobes edged with teeth, with three sensitive trigger hairs on each lobe. When an insect touches a hair twice within about twenty seconds, the lobes snap shut in a tenth of a second, the teeth interlock, and glands pour out enzymes that digest the prey over a week; then the trap reopens. The double touch prevents the trap from closing on a falling leaf or raindrop.

The sundew (Drosera), which grows in wet places in India including the Eastern Ghats and the Western Ghats, has small round or spoon-shaped leaves covered with hundreds of red stalked glands, each tipped with a glistening drop of sticky fluid that looks like dew and attracts insects. An insect that lands is held fast; the surrounding tentacles slowly bend over it, the leaf may fold, and enzymes digest the prey. The butterwort (Pinguicula) of the Himalaya has greasy leaves that work in the same way, rolling their edges over a caught insect.

The bladderwort (Utricularia), a common submerged hydrophyte in the tanks and paddy fields of Andhra Pradesh, carries on its finely divided leaves hundreds of tiny bladders, each about 2 millimetres across, with a trapdoor and trigger bristles. The plant pumps water out of the bladder so that it is under negative pressure; when a water flea or mosquito larva touches the bristles the door springs open, the animal is sucked in with a rush of water in a fraction of a millisecond, the door closes, and the prey is digested. It is the fastest trap in the plant kingdom. Bladderworts have no roots and are rootless free-floating or slightly anchored plants, flowering above the water with small yellow or purple blooms.

Insectivorous plants show that adaptation can cross the ordinary boundary between plant and animal ways of feeding when the environment demands it. They are also fragile: draining wetlands and bogs, and collection for sale, threaten many of them, and the Indian pitcher plant is protected by law.

Birds live in the air as fish live in water, and their whole body is shaped by the demands of flight: to be light, to be strong, to generate lift and thrust, and to supply the enormous energy that flying needs. The aerial habitat is the same over every ecosystem, so these adaptations are common to almost all birds, from the sparrow to the eagle.

Lightness. The bones are hollow, thin-walled and strutted inside like an aircraft's wing (pneumatic bones), and many are fused or absent; the skull is paper-thin; the teeth are replaced by a light horny beak; there is no urinary bladder and the nitrogenous waste is excreted as semi-solid uric acid; the ovary and oviduct of the female are single, on the left side only, and the reproductive organs shrink outside the breeding season; the digestion is rapid so that undigested food is not carried. A frigate bird with a two-metre wingspan has a skeleton weighing about 100 grams, less than its feathers.

Strength and streamlining. The body is boat-shaped and tapered, with feathers lying smoothly backwards to cut drag. The bones of the trunk are fused into a rigid box, and the breastbone (sternum) bears a deep keel to which the great flight muscles attach; the pectoralis, which pulls the wing down, may be a quarter of the body weight. The forelimbs are modified into wings, whose bones are fused and reduced for strength, and the wing has the curved cross-section of an aerofoil, thicker in front and arched above, so that air flowing faster over the top produces lift. The tail feathers steer and brake. The neck is long and flexible, letting the beak do what the forelimbs no longer can.

Feathers are the bird's unique adaptation: light, strong, flexible and waterproof, made of keratin. Flight feathers on the wing (primaries and secondaries) have a stiff shaft and interlocking barbs forming a continuous vane that pushes against the air; contour feathers cover the body smoothly; down feathers trap air for warmth. The oil from the preen gland keeps them waterproof.

Energy. Flight needs ten to fifteen times the energy of resting. Birds are warm-blooded, with a body temperature of 40 to 42 °C, a four-chambered heart that beats up to 1,000 times a minute in a hummingbird, and a unique respiratory system in which the lungs are connected to air sacs throughout the body and even inside the bones; air flows through the lungs in one direction during both inhalation and exhalation, giving a continuous supply of oxygen that a mammal's in-and-out lungs cannot match, and the air sacs also lighten the body and cool the working muscles. Birds eat energy-rich food (seeds, fruit, insects, flesh) and digest it rapidly in a crop, a glandular stomach and a muscular gizzard that grinds it with swallowed grit.

Senses and behaviour. The eyes are large, with sharp vision (an eagle sees a rabbit from two kilometres), and the brain's cerebellum, which coordinates balance and movement, is large. Birds of different ecosystems differ in the details: the broad, slotted wings of vultures and eagles for soaring on thermals over grassland; the long, narrow wings of the albatross and swift for gliding and speed; the short rounded wings of jungle fowl and partridges for a quick burst out of cover; the webbed feet of ducks; the long legs of storks and cranes for wading; the talons of owls and hawks; and the flightless ostrich, emu and penguin, which have lost flight in habitats where running or swimming served better and their wings have become balancing organs or flippers.

Every animal is food for something, and every predator must catch food, so adaptations for not being seen, for not being eaten and for catching prey run through all ecosystems.

Camouflage (cryptic colouration) makes an animal hard to see against its background. Colour matching: the grasshopper is green, the desert lizard sandy, the polar bear white, the arctic fox white in winter and brown in summer, and the tiger's orange with black stripes dissolves among dry grass and shadows. Disruptive colouration, bold patches like those of the leopard, the zebra, the lapwing's chick or the painted stork, breaks up the animal's outline so that its shape is not recognised. Counter-shading: fish, deer and many birds are dark above and pale below, which cancels the shadow on the belly and makes the body look flat; a shark seen from above merges with the dark depths and from below with the bright surface. Colour change: the chameleon, cuttlefish, octopus and some frogs and flat fish change colour to match where they sit, by expanding and contracting pigment cells (chromatophores) in the skin. Resemblance to objects (mimesis): the stick insect looks like a twig, the leaf insect and the dead-leaf butterfly like leaves with veins and rot spots, the flower mantis like an orchid, the bittern like reeds when it points its bill upward, and the nightjar on the forest floor like a heap of dead leaves. Camouflage works both ways: the tiger stalks unseen and the chital is grazed unseen.

Mimicry is the resemblance of one species (the mimic) to another (the model) that is dangerous or distasteful, so that predators that have learned to avoid the model avoid the mimic too. The harmless wolf snake mimics the banded krait's black and white bands; hoverflies mimic wasps; the palatable Danaid eggfly butterfly of India mimics the poisonous plain tiger butterfly. This is Batesian mimicry. In Mullerian mimicry several distasteful species share one warning pattern, so that a predator learns it faster. Warning colouration (aposematism) is the opposite of camouflage: bright, unmistakable colours, red, yellow and black, that advertise poison or a sting, as in wasps, the monarch and plain tiger butterflies whose caterpillars feed on poisonous milkweed (Calotropis), coral snakes, poison-dart frogs and ladybirds.

Other protective adaptations include armour (the shell of the tortoise and snail, the scales of the pangolin, which rolls into a ball, the spines of the porcupine and hedgehog); chemical defence (the skunk's spray, the stink bug, the poison glands of the toad's skin, the sting of the bee, the venom of snakes, the ink of the squid); autotomy, the shedding of the tail by lizards, which wriggles and distracts the predator while the lizard escapes and later regrows it; feigning death by the opossum and some beetles; eye-spots on the wings of moths and the tail of the peacock that startle or misdirect an attacker; alarm calls and herding; and speed and burrowing. Predators for their part have keen senses (the owl's hearing, the snake's heat-sensing pits, the shark's smell), weapons (claws, talons, fangs, the mantis's forelegs), stealth (the soft-edged feathers of the owl that make its flight silent, the padded paws of the cat), and lures (the anglerfish's glowing bait, the alligator snapping turtle's worm-like tongue).

All these show that adaptation is shaped not only by climate and soil but by other organisms: predator and prey evolve together in a race that never ends.

The soil is an ecosystem in its own right, and a surprisingly crowded one: a square metre of good soil may hold hundreds of earthworms, thousands of insects and mites, millions of nematodes and billions of bacteria and fungi, as well as the roots that feed on it. Above the soil animals also burrow into it for shelter. Life underground has its own conditions: darkness, a narrow, resisting medium, less oxygen and more carbon dioxide than the air above, stable temperature and humidity, and safety from most predators and from weather.

Burrowing animals share a set of adaptations. The body is cylindrical or spindle-shaped, without projecting parts that would catch on the soil: the earthworm, the caecilian (a legless burrowing amphibian of the Western Ghats), the blind snake, and the mole, whose body is a velvet-furred cylinder with no visible neck. Digging organs are strong: the mole's forelimbs are short, broad and turned outwards like spades, with heavy claws and a sixth 'thumb'; the mole cricket has the same spade-like forelegs; the rat, rabbit, mongoose, bandicoot, pangolin and armadillo have powerful claws; the earthworm pushes with the pressure of its body fluid and anchors with tiny bristles (setae) on each segment, and swallows soil as it goes; the crab digs with its legs and claws; the sand boa and the shield-tailed snakes of the Eastern Ghats have a pointed, hardened snout and a strong flattened tail for pushing. Eyes are small or hidden, since there is nothing to see: the mole's eyes are minute and covered by skin, the blind snake's are mere dots, the caecilian's are under the skin and the earthworm has only light-sensitive cells; in their place touch and smell are acute, the mole's snout bristling with sensory whiskers and the star-nosed mole's with fleshy tentacles. Ears lack external flaps that would fill with soil. Fur is short, dense and lies flat in either direction so that the animal can back up in its tunnel. Burrowers tolerate low oxygen and high carbon dioxide, and breathe through the skin (earthworm, caecilian) or with lungs adapted to stale air. Burrows are of many kinds: the vertical shaft of the mud crab on the shore; the branched tunnels of the mole with a nest chamber; the rabbit's warren with many entrances; the gerbil's plugged burrow that keeps out the desert heat; the paddy-field crab's hole that lets water leak from bunds; and the termite mound, a city of galleries with ventilation chimneys that keep the fungus gardens at a steady temperature and humidity.

Soil organisms proper, the decomposers and their community, are adapted to the pore spaces. Springtails and mites are tiny; nematodes are slender and swim in the water film between soil particles; earthworms, the 'intestines of the earth', eat their way through, mixing, aerating and draining the soil and casting nutrient-rich pellets; fungi spread thread-like hyphae between particles and, as mycorrhizae, into roots; bacteria live in the water films and on particle surfaces. Many soil animals move up and down with the moisture, retreating deeper in drought and rising after rain, which is why earthworms surface in the monsoon. Roots too are adapted to the soil: root hairs greatly increase surface area, the root cap protects the growing tip as it pushes between particles, and lateral roots branch to explore every layer.

Burrowing links the ecosystems above and below. Burrows aerate and mix the soil, let rain in, and become homes for other animals; the termite and earthworm are among the most important animals in any tropical ecosystem. But burrowers are also sensitive to what we do to the soil: pesticides, compaction and the loss of organic matter empty the ground of life, and a soil without earthworms is a poorer soil.

Having seen adaptation in ecosystem after ecosystem, we can now ask where adaptations come from and what happens when an ecosystem changes.

Adaptation is the product of evolution by natural selection. Within any population, individuals vary, and some of the variation is inherited. In a given environment, individuals whose inherited features suit the conditions better survive longer and leave more offspring, so those features become commoner in the next generation. Over thousands of generations the population becomes well fitted, adapted, to its environment. The camel did not decide to grow a hump; among ancestral camels, those with more fat storage and greater tolerance of dehydration survived the dry years, and their descendants inherited those features. The same process, acting on different ancestors in different ecosystems, produced the lotus and the cactus, the fish and the bird. Where unrelated organisms meet the same problem they often evolve similar answers: the streamlined shape of the fish, the dolphin and the penguin, the wing of the bird and the bat, the succulent stems of the cactus of America and the Euphorbia of India. This is convergent evolution, and it is the strongest evidence that adaptation is a response to the environment.

Adaptation has limits. Because it is built by slow selection, an organism is adapted to the conditions of the past, and if the environment changes faster than selection can follow, the organism perishes. The specialised are most at risk: an animal that eats one plant, a plant that grows in one kind of soil, a fish that spawns in one kind of stream. The highly adapted are also the least able to move: a mangrove cannot walk inland, a coral cannot swim to cooler water. This is why the destruction of ecosystems is the chief cause of extinction today.

Ecosystems are being changed everywhere in Andhra Pradesh and India. Wetlands and tanks are drained, filled or choked with water hyacinth and sewage, and the hydrophytes, fish, frogs and water birds adapted to them disappear; Kolleru lake was reduced to fish ponds before its restoration began. Mangroves are cut for shrimp farms and ports, and the coast loses its nursery and its shield. Forests are cleared for mines, dams, roads and fields, and the arboreal animals, the epiphytes and the floor community go with them. Grasslands are ploughed or planted with exotic trees, and the blackbuck, the bustard and the florican lose their open country. Deserts and semi-arid lands are overgrazed and turn to bare sand. Mountains are warmed, and the alpine plants and the snow leopard are pushed higher until there is no higher. Polar ice melts under the polar bear. Climate change is the largest of these changes, shifting every ecosystem at once, faster than most species can adapt or move. Pollution adds poisons for which no organism has an adaptation, and invasive species such as water hyacinth, parthenium, lantana and prosopis, brought from other continents, overrun native plants that never evolved to compete with them.

The response is conservation: protecting whole ecosystems rather than single species, since the species cannot live without their ecosystem. India has national parks and sanctuaries (Coringa for mangroves, Nagarjunasagar-Srisailam for the tiger, Kolleru and Pulicat for water birds, Papikonda for the forests of the Eastern Ghats), biosphere reserves, wetland protection under the Ramsar Convention, and laws against the trade in wild species. Restoration is possible: mangroves have been replanted on the Godavari coast, tanks have been desilted and reborn, and the Ganges and other rivers are being cleaned. Each of us adds to it by saving water, refusing plastic that chokes ponds and turtles, planting native trees and not exotic ones, and by knowing, as this chapter has tried to show, that every organism is a precise answer to the place where it lives, and that when the place is lost the answer is lost with it.