 Imagine you're driving a car and all of a sudden you see a new big huge wall in front of you. You have no clue where that wall has come from but you need to go to the other side of the wall because your friends and family and everyone you know in your life is on the other side of the wall. But again you see no way to go past this wall. How would you feel it? Your life will turn upside down right? Probably your life will never be as normal as it was before. But have you ever imagined that this road that take us to different places is also made by cutting through different habitats and that road is no less than this big wall that we are talking about for the animals there. Right? The road feels like a dangerous patch of land which the animal fears to cross because that road no longer has the protective forest cover it had before. And we humans for our selfish need have created so many road and railway networks that just 10% of the world can be called remote today. And when we clear a forest and construct roads we create fragments of the habitats of those animals. Now what happens when we create fragments? What does it lead to? Well it's a very dangerous thing because many animals and plants require huge territories for their survival. They need resources from a very large area to survive and when that area is reduced for whatever reason their survival becomes difficult their population decrease and gradually they also may get extinct from this world. And that is actually happening we are losing a lot of different species faster than ever. Now you may argue that hey come on this is how nature works. Spatiation will take place and then few species will go extinct. This is how nature works you don't need to bother about it. But wait let me tell you that we are losing species 100,000 times faster than the pre-human times and this is happening because of human activities we need to admit that right? We are crowding our planet with humans. But this planet also belongs to all other life forms that are present here right? We are decreasing the biodiversity and I need not tell how important biodiversity is for the survival or the well-being of our planet. Now imagine a world with only human beings and then imagine a world full of all different life forms which one looks better? The second one right? Obviously with human beings in it the second one looks better and is also better for the planet. But as we just discussed our activities are declining this biodiversity. So in this video let's discuss about few human activities that needs to be checked in order to stop the loss of biodiversity. And the very first one is habitat loss and fragmentation. We discussed about it at the beginning of the video. But here a special mention needs to be of the tropical rainforest the place that is the hub or the habitat of the maximum biodiversity on this planet. And when we destroy tropical rainforest a lot of habitat is lost. Now this tropical rainforest once covered more than 14% of the Earth's surface and now it has just reduced to 6%. We have already done a lot of damage. And talking about rainforest how can we not talk about the lungs of the planet? The Amazon rainforest. Well it was so huge that it was named as the lungs of the planet but at present it is not as huge as it was before. People there are cutting down trees to cultivate soya beans and to rear beef cattle. And Brazil where this forest is has been one of the top most exporter of soya beans and beef. Well people there are enhancing their economy at the cost of the lungs of the planet. Imagine how tragic it is. So this is how we are causing habitat loss and losing biodiversity. Now apart from habitat loss and fragmentation another evil activity of human that is causing the loss of biodiversity is over exploitation. Let's understand it this way. Imagine you have a room full of hungry people and we have rice, a bowl of rice, bananas and some cookies as food options there. And we get to see that everyone goes for a cookie and no one for bananas or rice. So what happens then? Eventually there will be no cookies left on the plate. Same is the case with the resources of nature. Humans often take too many of one particular species from its natural habitat. Typically the species which is used as food source. Now it's not that we have nothing else to eat just like the rice bowl and the bananas on the table. But somehow few species are considered as delicacy and are sold for very high rates. And we humans end up harvesting them way too much. And when one species is harvested or taken at a rate faster than the population could compensate it is over exploitation. And over exploitation of any particular species whether it be a plant, animal or a fish it reduces their population and gradually leads to their extinction. And that is how again we lose biodiversity. A very recent and classic examples are passenger pigeon and the stellar cows. We humans for our greed and for our taste buds have hunted so much of them that we have bought down their population from millions to zero. Imagine. Now apart from over exploitation we have also bought in invasive alien species. Now what is that? Let's take the example of an ecosystem. So here you can see birds, trees, fishes and it is a stable ecosystem. Now due to human activities a new species is introduced into this ecosystem. Let's just call that species alien species. And it is a kind of fish. Now this alien species might have two fate. First the environment will not be suitable for this alien species and it will die. That is the end of the story. And the second possibility is that it will survive in this environment, in this new ecosystem. And if that happens then it will compete with other fishes there for resources. And somehow if this fish is stronger than most fishes there it might use up their resources so much that it will be scarce for the native fishes there and their population will decline. And sometimes it is seen that the alien species breeds faster than the native species leading to the population explosion of the alien species. And this in turn reduces the population of whatever species they feed on whether it be a fish or a plant. And that is when we call an alien species an invasive species. Now a very famous example of alien species invasion is the Nile perch. This huge fish you see is the Nile perch and it was introduced into Lake Victoria in East Africa which led to the extinction of more than 200 ecologically important species in that lake. And India today is facing similar problem with the African catfish which was illegally introduced into our lakes and rivers in the name of aquaculture but it is causing a serious threat to the indigenous catfishes of our river. So we humans need to be mindful about releasing any species into any ecosystem. So the next time you have the thought of releasing that big bright goldfish from your aquarium into your neighborhood pond think again that would not be a good thing to do. You might end up destroying the native biodiversity right? And this invasive species is not just limited to fishes or animals. Plants can also be invasive. And the invasive species here in India that we see every day are water hyacinth and carrot grass which are sucking up the native biodiversity. And that is how the species richness of an area decreases. Alright now moving ahead let's talk about the fourth point which is co-extinction. Now what is that? Let's say we have two species species A and species B. Now species A and B are related to each other in terms of food, in terms of reproduction, in terms of habitat. Now without A B cannot survive and without B A cannot survive right? Now let's say due to human activities species A went extinct. Species B won't survive right? That is what is co-extinction. In short species A and B are in obligate relationship with each other. For example the parasite that leaves on a fish. The fish would be the host and if the host go extinct the parasites that leave on it will also go extinct right? Another example would be the plant and the pollinator that co-evolved. This pollinator feeds on the nectar from this plant and this plant cannot survive without this pollinator it would propagate. So if any one of these two go extinct due to human activities the fate of the other is decided. It would also go extinct. This is what co-extinction is. So sometimes human do not understand why one particular species went extinct because we have not done any direct damage to that particular species. But there is something called co-extinction. Maybe we have done damage to such a species on which the other is directly dependent. And that is again how we lose a lot of biodiversity. So in this video we learned about the four most deadliest human activities that leads to loss of biodiversity. It is habitat loss and fragmentation, over exploitation, alien species invasion and co-extinction. And since this four is the evilest of all the activities we call it the evil quartet or the evil four.