 Do you know we have the maximum species diversity near the equator and as we move further from the equator either towards north or towards south, this diversity decreases. Why is that? Let's discuss all about it in this video. And we will begin by talking about the different zones we can divide our planet into. These are the climatic zones. So the area near the equator, we call it the tropics. Then adjacent to tropics, we have the temperate areas and then the poles. And as we move from tropics to poles, the species diversity decrease. That means the diversity of species is not evenly distributed. Well, are there any particular species or all the species on this planet follow this pattern? Well, with few exception, almost all the life forms that we know of follow this pattern. The plants and animals that we see on a regular basis, or if we talk about the world under water or about the world which we can see through a microscope, all these different life forms show a huge variety near the tropics and not in the poles. So what is so special about the tropics? What do you think? Well, to some, it might seem obvious or even like a silly question because it is quite prominent even from the picture that the tropic has a very huge area. It occupies almost 40% of the total area of our planet. So more the area means more number of species, right? Simple as that. Well, let's check if that's true. For that, let's take an example. Let's take an example of a frog species. Let's just call it the green frog. Well, this is not the one which you can kiss and it will turn into a prince charming. This is just an ordinary neighborhood frog, okay? Now let's assume that they leave near the equator or in the tropics and if they have more area and more resources, they will reproduce more and their population will increase. But here we are not talking about the abundance or the population of a particular species. Instead, we are talking about the diversity of species. Why are there more different kinds of frog in the tropics? And more land area alone is not an explanation for more number of species, right? So the more you think about it, the weirder it gets, right? Because life has shown that it can grow pretty much everywhere from the poles to the tropics. Yet, it's most biodiverse places are in the tropics. Interesting, right? Now let me bring to you another map that shows species diversity. Here as you can see, as the color moves from red to blue, the diversity decreases. And from the color you can tell that it depicts exactly what we just discussed, right? But let me give you certain data. So let's say Colombia, which is somewhere here, it has 1400 species of birds, 1400 different kinds of birds are seen in Colombia. Now moving a little more towards poles from Colombia, we have our own country in India here. So India has 1200 species of birds. Let me write India here. We know major portion of India is in the tropics. So we also have a good diversity. We have 1200 different species of birds. Now moving a little more towards the poles, we have the famous New York here and New York has just 105 species of birds. And moving further towards poles, let's talk about Greenland. We have Greenland here and Greenland, poor Greenland only has 56 different species of birds. So you just saw, we moved from tropics to poles and the species diversity decreased. And since we see this decrease and this gradient as we move along the latitude, we call it the latitudinal gradient in diversity. Alright, now let's get back to the prime question of this video. Why the tropics? Why are tropics this chosen area for more biodiversity? Well, a lot of hypothesis has been proposed explaining as to why biodiversity is so rich in the tropics. And in this video, we will discuss about three most important hypothesis or the hypothesis that is widely accepted. And talking about the first one, the first one says that the tropics are older than the temperate areas. How can we call one part of the planet older than the other? Well, by this they meant that the tropics remained comparatively stable than the temperate areas and the temperate areas were exposed to frequent glaciation and it also went through the last ice age where most species went extinct and the newly found species there are believed to have newly colonized those areas. So compared to the biodiversity there, the tropical biodiversity is older. The species there got more time to evolve, to specialize, right? And time is the main factor for spaciation. We all know that new species take millions of years to form to evolve from its ancestral species, right? And the tropics were fortunate enough to have had that evolutionary time for species diversification and that is why tropics have more biodiversity compared to other parts. Now moving ahead to the next hypothesis which is my personal favorite one is niche specialization. Now niche has a number of definition but in simple words it simply means the conditions inhabited that is required for an organism to survive. Okay, now how is this niche specialization helping in having more diversity in the tropics because that is what we are interested in, right? The tropical environments are less seasonal and relatively more constant and predictable and if situation like these prevail, organism can take the risk to specialize themselves into one particular niche. Let me explain this to you with the help of an example. So tropics have almost equal hours of day and night and it is constant throughout the year with slight changes of course but it is mostly constant and since it has equal hours of day and night, birds which are active during the night get equal time to feed and to be active just like the birds that are active during the day and since these hours of day and night have remained constant over millions of years, birds have slowly evolved into nocturnal and diurnal birds and this is their niche and this niche specialization is possible because the conditions in the tropics are stable. But this is not true when we talk about the poles. There are seasons and months where sun do not rise. Let's take the example of animals at the poles, let's talk about the bears there. So one season there will be all snow-caped mountains with no food. The other season can be the happiest one with food and fruits and trees everywhere enough resources to survive. Now talking about niche specialization, just imagine if this bear specialized itself to stay in conditions where there is abundance of resources, what would happen when the season change? They will probably go extinct right? And again specialization takes millions of years of constant environment and here the environment is never constant. So in the poles or in the temperate areas, rather than specializing the animals there has to spend their time and energy into adapting to new conditions every month or every changing season. So animals there have to be more adaptable and therefore spaciation is much slower. On the other hand, as we discussed earlier, due to constant condition in the tropics niche specialization is possible that finally leads to formation of new species, right? And that again explains why there is more bad diversity in the tropics. Now let's move on to the third and another famous hypothesis which says that the position of the earth and the sun is such that the tropics receive higher solar energy than the other parts. And higher solar energy means higher productivity. Now let's think of it this way. If the productivity in an area is more, the growth of the plants and animals there will be faster. Now how is that helping form new species? You ask? Okay. We will take the example of microorganism spectator. It is very easy to see their generations, right? And a slight change in one particular generation might give rise to new species fast because their growth is faster. They give more generation in less time, right? Same happens in the tropics. For us it may seem slow but actually compared to rest of the world due to higher solar energy the growth is faster. And if the growth is faster there are more generations in less time. And that helps in speciation and not just that. Solar energy indirectly affects all other hypothesis that we discussed about. So higher solar energy, higher productivity in the tropics have helped in formation of new species over millions of years. So these are the three famous hypothesis that tells us why there is more biodiversity in the tropics. But again you can go look up for more hypothesis that explains the biodiversity through a different perspective. So this was all about the diversity of species as we move along the latitude.