We are in the middle of a mass extinction brought about by human activity.
What is mass extinction?
A vast number of species going extinct at one period in time is called mass extinction. It is also known as a biotic crisis, as it leads to a decline in the world’s biodiversity. In a mass extinction, species disappear faster than they are replaced by new species.
What are the causes of mass extinction?
Earlier extinctions took place due to natural causes like global climate change, fluctuating sea levels and catastrophic events like volcanic eruptions or asteroid impacts. However, the ongoing extinction is the result of human actions.
How many mass extinction events have occurred on the Earth?
The Ordovician-Silurian extinction, 444 million year ago; the Devonian extinction, 360 million years ago; the Permian extinction, 250 million years ago; the Triassic-Jurassic extinction, 201 million years ago; and the Cretaceous extinction, 65 million years ago. The first eliminated marine invertebrates, the second, tropical marine species. The third and the largest decimated most of the marine species and many terrestrial vertebrates, and the fourth destroyed all the Triassic reptiles. The fifth last was most likely caused by an asteroid hitting the Earth. It killed off dinosaurs of all species, including the remaining non-avian dinosaurs.
Are we facing a sixth mass extinction?
At present, we are in the middle of the sixth mass extinction, the Holocene extinction, which is entirely caused by the humans. It started 10,000 years ago with the beginning of agriculture and industrialization. Human activities like deforestation, climate change, and pollution have been major contributors.
These events wipe out numerous species, reshaping ecosystems and allowing the evolution of new species. They can disrupt habitats, biodiversity, ecological stru and food chains.
What is the impact of mass extinctions?
These events wipe out numerous species, reshaping ecosystems and allowing the evolution of new species. They can disrupt habitats, biodiversity, ecological structure and food chains.
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However, these events can also describe the weather in a certain place, but while the weather can change in just a few hours, climate changes over a longer span of time.
Most of the sea animals that can be dangerous to us are too timid to attack humans. These include the dangerous-looking reef shark and the sea snake. Although a number of fishermen die from the poisonous bite of the sea snake, the animal itself is usually too lazy to bite. Another dangerous inhabitant of the sea is the puffer fish. Some of its body parts are highly poisonous, especially the liver and ovaries. Despite this, its meat is served as a delicacy and a few people have also died because of eating incorrectly cooked puffer fish. The sea wasp is one of the most poisonous animals of the world. The stinging cells of this species of jellyfish contain a poison which can cause death within minutes. Annually, up to 70 people die because of sea wasp stings. 
The sea turtles developed from land turtles about 200 million years ago. Their legs transformed into broad fins and the armour gradually acquired a streamlined shape to generate less resistance while diving. Due to its flat armour, the animals cannot retract the head when it senses danger. Sea turtles spend their whole life in the sea and go on land only for laying eggs. The female turtles dig a hole in the sand with their flippers, lay the eggs in the whole (nest), re-fill the nest with sand and then return to the ocean. Hatching usually takes place at night, when there is a lesser chance of predators like seagulls being around. Once the young ones reach the surface, they instinctively head towards the sea. 
Fish move by flapping their fins or through wriggling movements of their body. In addition, they have an air bladder, which they can fill with gases when needed. These features make them lighter and they can float in the water like a submarine. Jellyfish usually drift. To move forward, they pull together their cap and push the water downwards—a same recoil principle that is used in jet fighters and octopuses. 
The growth rate of the frog’s population is the same in ponds near the coast or in the garden at home. Tadpoles develop from fertilized gelatinous eggs. These are larvae of frogs, which have gills and move forward with the help of a tail. Gradually, they go through a transformation to become a frog. The process is called metamorphosis. The tadpoles develop front and back legs, cartilages become bones, the tail retracts, lungs develop, and the gills regress. Now, it can move on land as well as water. Such a metamorphosis takes place in other animals too, for instance in butterflies or beetles.
Salt meadows are area at the sea coast that are not flooded daily by sea water like the tidelands or the beaches. They are flooded only by the high tide at regular intervals. The soil in these areas contains salt and is poorly ventilated; therefore, only a few plants can survive. Plants having an affinity for salt, like the beach lilac and the beach aster, grow here. Since the meadows sink completely in the strong sea at high tides, the plants of the salt meadows have resistant stalks and leaves to withstand the forces of the wine and the sea.
Frogs croak in freshwater ponds, and a few metres ahead, fish and jellyfish swim in the salt water of the sea. The narrow coastal land between the land and the sea could contain steeply falling coastal cliffs or flat coastal regions, which are flooded regularly like the sensitive tideland and salt meadows. 
The highest mountain peak of the world is Mount Everest with a height of 8850 m. Animal do not live on ice-covered mountain peaks and in the areas where even mountaineers can manage only with an oxygen flask. Animals that have adapted themselves to the rough conditions live below a height of 6000 m – goat-like animals such as Himalayan tahrs, ibexes and cashmere goats, the pikas with their small ears and the endangered snow leopard. Yaks are also found here, which the local people use for carrying loads. 
The term ‘tree-line’ describes the height above which trees can no longer grow in the mountains. The main reason for this is the very low temperature in these places. If we climb a mountain, we find a temperature drop of about 6°C for every kilometre. In addition, the temperature fluctuations between day and night as well as summer and winter are very high. The soil cover is thin so the roots of trees do not get a firm hold. Only small pine trees, cedars, grass, and moss grow above the tree-line before the start of the rocky and icy area. 
Groundhogs live in widely branched burrows below the ground, but come out on the surface every day to collect grasses, seeds, and insects. A few species live in colonies in high mountains. They divide work amongst them—when a few animals have gone out in search of food, the others are on guard. If they sense danger from approaching enemies, they emit a shrill whistle to warn the others. In the summer, the animals must feed enough to put on a thick layer of fat so that they can survive the 6-9 months long winter sleep. 
Both animals have adapted to their mountain habitat. The chamois live in the upper mountain forests and fields and climb higher only in summers. The ibexes, on the other hand, live in icy and rocky regions and look for food in mountain meadows. They never go down to the forest. Species of ibexes are found in the mountain regions of Europe and Asia, on the Arabian Peninsula, and in north-eastern Africa. Chamois, on the other hand, are found only in the mountains of Europe: in the Alps, Carpathian Mountains, and Pyrenees as well as in Caucasus Mountains. 
Most of the forest ants build hills at sunny forest edges from needles, twigs, and moss. The well-organized huge colonies of ants are called ‘hills’. Several hundred queens live with their ‘community’ in these hills. The ants divide the work among themselves. While the queens lay eggs, the soldier ants must provide protection for the eggs and the worker ants get food for the larvae. In the month of June, the winged male and female ants hatch. The males die after the nuptial flight. The impregnated young queens look for a new ground or return to the old one to lay eggs.
It is easy to distinguish between the large fallow deer, the red deer, and the small deer. The male red deer has royal antlers with several ends, while a roebuck (male roe deer) has only one small horn. The small deer, red deer, and fallow deer belong to the biological family of the stag, which also includes the moose and the reindeers. The small deer, weighing about 15 kg, are the most frequently seen today. They live in the area between forest and arable land. The female doe can give birth to two to three fawns, which bear three long series of white spots on the back. Red deer weigh up to 150 kg and prefer to live in forest clearings. The hinds (female red deer) can give birth to a maximum of one calf in a year. 
A forest is a large area of land covered with trees. Forests can be classified into many types, some of which are the mixed temperate forests with both coniferous and deciduous trees, the temperate forests, the subtropical forests, the tropical forests, and the equatorial rainforests. The trees that make up the forest create a special environment which, in turn, affects the kinds of animals and plants that can exist in the forest. Large and small animals such as the stags, deer, wild boars, foxes, badgers, marten, and squirrels live in the forests. 
Although an oak apple looks like a fruit, it is actually a deformed leaf hanging by its stem or petiole on the underside of oak leaves. Gall wasps form oak apples. In summer, the female wasp injects its eggs into the leaves of oak trees. The larvae that hatch inside the leaf are small and round. As they grow, they cause a chemical reaction that forms a gall around the larvae. These larvae eat and grow within the gall before emerging as adult wasps. 
Many birds such as starlings, cranes, and swallows migrate to the south in autumn because they do not find enough food in winter. It is not good to feed the birds because it may disturb their regular habits of finding food for themselves. Their destination and the migratory path is the same every year. For years, scientists puzzled over the birds’ ability to take exactly the same path every year but now it is assumed that birds possess a ‘magnetic sense of direction’ and orient themselves to the magnetic field of the Earth, using the position of the sun during the day, and the stars at night. 
Moles spend a large part of their lives in a self-dug burrow system below the ground. The animals push the loose earth that is a result of digging to the earth’s surface after every 50-100 cm. The result is a typical molehill, about 20 cm high. Sometimes we also come across huge hills with air holes all around. Below these molehills the moles make a nest in which they give birth to their young ones. 
A lot of activities are carried out in our gardens by animals and insects living underground, such as moles, rats, and earthworms. Upon a closer look, we can also find animal tracks on the leaves of the spherical oak-apple tree or the fine webs with which spiders catch their prey. Besides the animals hidden underground, there are birds that chirp and fly around in the gardens. 
‘Survival of the fittest’ describes the evolution theory of Charles Darwin. In the struggle to survive, the species that are healthy, strong, and have adapted themselves best to their environment will win. They ensure their survival through healthier genetic material that is passed on to their offspring. Thus, a giraffe with a longer neck can reach leaves high on the trees to feed himself. While a giraffe with a shorter neck may stay hungry, since he has access only to the leaves lower on the tree. Eventually, the hungry giraffe may become so weak that he falls prey to lions. 
Dinosaurs existed on Earth 65 million years ago. Today, the scientists believe that their mass extinction was triggered by a huge meteorite hit to the east of Mexico. The meteorite impact caused blasts and flood waves. Huge quantities of poisonous gases filled the Earth’s atmosphere. Then there was acid rain that polluted the seas and the plants died. Huge dust clouds blocked the heat from the sun and it became cold. The cold-blooded dinosaurs became sluggish and starved. Young ones did not hatch from their eggs. Only the small animals, which adapted themselves quickly and did not need a lot of food, survived.
The grey whale lived around 300 years ago in the Atlantic and Pacific Oceans. The giant animals were hunted extensively for their meat and oil. They became extinct in the Atlantic by the end of the 17th century, but can still be found in the Pacific Ocean. Around 20,000 animals migrate every year in October from Alaska in the north to the coast of Mexico, where they give birth to their young ones in winter. In west pacific, there are at most 200 grey whales at present and they are listed as critically endangered. These animals are sensitive to noise and are being displaced from their habitat because of the construction of drilling rigs for the crude oil industry.
Since the time animals have evolved species continue to become extinct. The most well-known example of extinct species is that of the dinosaurs. Often the species cannot adjust fast enough to the changing environmental conditions, or new species arise and displace the existing ones. Today, many species are becoming extinct because of the actions of humans. Elephants, big cats, and rhinos are hunted ruthlessly, forest areas are cleared, and the grey whales are endangered. These are only a few of the species mentioned in the ‘Red List’. On a positive note, new animal species are also being discovered.
Insects have the largest number of species among animals on the Earth. Out of ten different animal species, eight are of insects. They can survive in almost all types of habitats regardless of temperature. Many can survive only in specific environmental conditions, while others are found in all parts of the world. The reason for this difference within the species is not known. Insects have six legs and a body divided into three parts with a solid outer covering called the armour. Many insects are useful (bees which pollinate fruit tree flowers), while others are harmful (plant louse on indoor plants). 
‘Mammal’ is derived from the word ‘mammary’. The young ones of mammals suckle milk from the teats of the mother. This milk contains all the essential nutrients for growth. Except for the monotremes, all mammals are born as young ones and are not hatched from eggs. Contrary to almost all other animal species, mammals nurture and train their young ones for a considerable time. Mammals have a constant body temperature of 36-40°C, so they need a hide (like an elephant) or a thick layer of fat (like a whale) as protection against extreme heat or cold.
There are more than 1.5 million animal species. Ranging from simple animals like sponges and jellyfish, the invertebrates include the arthropod phylum with over 1 million species of insects. The higher- level vertebrates include fish with about 35,000 species. Around 350 million years ago, when aquatic animals moved to land, they adapted to the new environment and amphibians were born. They adapted some more and became reptiles. From reptiles came the birds and around 4000 known species of mammals. 
Below a certain depth all the porous spaces are saturated with water. This upper limit of the ground water is called the water table.
Water cycle or hydrological cycle is the natural evaluations of water through biosphere. Water is lost from earth’s surface to the atmosphere by evaporation from rivers, oceans, and seas etc. This atmospheric water forms clouds and creates rainfall. The water that collects on land, again flows to oceans, seas and rivers etc., and completes the cycle. 
Rain, snow, rivers, streams, brooks, lakes, ponds, wells, springs, underground water, ice sheets, etc. are the main sources of water. 
Pure water is colourless, odourless and tasteless. It can exist as solid (ice), liquid (water) and gas (steam). Water freezes at 0°C or 32° F and boils at 100°C or 212°F. It is a peculiar liquid in that it expands when it freezes. That is why ice floats on water. 
Water makes up 60-70% of the human body or about 40 liters of which 25 are inside the cells, 15 outside. People can’t survive normally more than five or six days without water or two or three days in a hot environment. 
To scientists, water is a compound of hydrogen and oxygen. One molecule of water contains two atoms of hydrogen and one atom of oxygen. There is about 1.4 billion Cubic kilometer of water on the earth which covers about 71% of the earth’s surface. About 97% of the water on the earth is in the oceans. 
Biosphere is a term used to describe the parts of the earth and the thin layer of air above its surface that supports life. It is limited to the waters of the earth, a fraction of its crust and the lower regions of the atmosphere. It is also the part where solar energy is used to bring about photosynthesis and other chemical and physical charges. 
It is a broad natural assemblage of plants and animals shaped by common patterns of vegetations and climate. Some examples of biomes are forest biomes, desert biomes, grass land biomes or tundra biomes etc. 
The biotic factor concerns the organic variable affecting an ecosystem. For example, the changing population of elephants and its effect on the African Savana.
The living components of biosphere such as plants, animals etc. are called biotic components. The non-living components of biosphere such as water, air, stones, etc. are called abiotic components. 
Habitat is defined as a localized environment in which an organism lives and which provides for all or almost all of its needs. 
The term food pyramid is used to describe the quantities of materials that pass from one kind of eater to another. The word pyramid is used because the amount of energy associated with the food material becomes smaller at every link or upward step.
All food chains begin with green plants because they are the only living things that can produce their own food. Green plants produce food by the process of photosynthesis.
Animals living in trees are called arboreal such as monkey.
Plants that grow on other plants using them only for support are called epiphytes. They have no roots in the soil. Epiphytes do not parasitize the plants they grow on.
Camouflage is the disguise used by animals to hide from their enemies. Men also use camouflage, particularly in wars. Most animals use camouflage to hide from predators or other animals that prey on them. Camouflaging is done by adopting matching colour to the surroundings, so that one looks like a part of it.
An animal which obtains food by hunting other animals is called a predator. 
It is an opposite process of hibernation. Some animals go under the ground during the dry season of summer. In zoology it is a state of inactivity and reduced metabolic activity that occurs during the dry season in species such as lungfish and snails. 
Animals and plants which live on the bottom of the sea or lake are called benthos. 
Very small plants and animals living in water are called planktons. They are an important source of food for larger water animals.
Parasites are those animals or plants which live on the living plants and in the bodies of living animals. Parasites that live inside the host such as liver flukes and tapeworms are called entroparasites; those that live on exterior, such as fleas and lice are called ectoparasites. They are harmful for the host.
The living beings that feed on dead or decayed organic matter are called saprophytes. Most saprophytes are fungi and bacteria which contain no chlorophyll. Many other animals eat dead plants and animal matter. They are useful scavengers and in sewage farms and refuse dumps break down organic matter into nutrients and easily assimilable by green plants. 
When food chains are interconnected in an ecosystem they are called food webs. Sometimes one type of plant or animal is eaten by more than one kind of animal. In this way, two or more than two food chains get interconnected and form a food web. The more complex food web shows a greater variety of relationships. 
In ecology, a sequence showing feeding relationship between organisms in particular ecosystem is known as food chain. Each organism depends on the next lowest member of the chain for its food. For example, grass is eaten by a deer and deer is eaten by a lion. Other examples of food chains are algae-duck-otter and flower-butterfly-lizard-snake-peacock.
Living beings which eat both plants and animals are called omnivores. Omnivores have digestive adaptions intermediate between those of herbivores and carnivores. They can digest variety of food stuffs. Human beings are omnivorous, because they eat meat, fish, vegetables and fruits. Other omnivorous animals are rats, pigs, bear and certain birds. 
The livings being which eat other animals are called carnivores such as lion bear, cat, dog, wolf, weasel etc. Carnivores have special teeth for tearing and chewing flesh. In other words all flesh eating animals are carnivores. They also produce special enzymes for digesting meat. 
Apart from animals, there are some plants that capture insects for food. They are called carnivorous plants. Since they cannot prepare their own protein, they get it by eating worms and insects. Venus fly trap, pitcher plant, sundew plant etc. are some examples of carnivorous plants. Similarly a plant called ‘Indian pipe’ is found in the hills of Shimla also eats insects. 
Living beings which consume only plants are called herbivores. We sometimes call them vegetarians. Well-known herbivores are rabbits, deer, cows, sparrows, squirrels and honeybees. They have specially designed teeth and produce special enzymes to digest plant matter.
The four elements of an ecosystem are: (a) The non-living environment, (b) Producers (green plants), (c) Consumers and (d) Decomposers. The non-living environment of an ecosystem includes sunlight, water, oxygen, minerals, dead plants and animal matter. The producers of an ecosystem are green plants. Animals, both herbivorous and carnivorous are the consumers of an ecosystem. Decomposers are bacteria, fungi and insects that decompose dead plants and animals.
The ability of nature to remain unchanged is called the balance of nature or ecological balance. The whole earth is an ecosystem, a system of give and take among plants, animals and their surroundings. In theory the ecosystem is a closed cycle but in practice the ecosystem is seldom in a state of balance. Gradual readjustment to the composition of a balanced community is continuously taking place. Man is often responsible for causing sudden ecological imbalance through activities like deforestation, polluting the atmosphere, destroying habitats, seas, lake and by rapid industrialization or sometimes through the out break of new disease. 
Ecosystem is a functional unit consisting of interacting organisms and all aspects of the environment in any area. It contains both the non-living (abiotic) and living (biotic) components.
Ecology is the branch of science that is concerned with the relationships of life forms with each other and with their surroundings. The word ecology was coined in 1866 by Ernst Haeckel. It is derived from two ancient Greek words meaning “study of the home”. The word ecology comes from the Greek word Oikos meaning ‘household’.