over the last century, the human population size and growth rate has exploded. We went from a world where there was never more than two billion humans ever to now a world where there's nearly eight billion people on the planet, and that is rising. In fact, if you're curious, here are some estimates from the U. N As to population increases in the coming century. Now it's important to remember that, just like there's a carrying capacity in nature, there's a carrying capacity for humans, and there is a limit to global carrying capacity for the human population. Although estimates for what this is very and additionally that could change based on technological advances. However, the point is we need to be conscious about our ecological footprint, which is the amount of land needed to sustain an organism or populations. Use of resource is so if we think about it in terms of an individual, there's gonna be a certain amount of land that person needs. Thio provide them with the necessary nutrients, right food and whatnot. Also, that individual is going to generate waste. There needs to be a certain amount of space for that waste to go so that that person doesn't kill themselves with their own waste. I mean, these are just two examples, but the point is we need to think about are use of this earth and how sustainable our lifestyles are because this is unsustainable, as we saw with our, uh, our logistic growth model. Previously, there is a carrying capacity. We just haven't hit it yet. Now, I don't mean to fear monger anything, but those are important things to think about. However, moving on, I'm gonna talk about demographic transitions. This is maybe getting a little more into sociological territory. But this is a transition from high birth rate and high death rates, toe low birth rates and low death rates. So, essentially, this is why you see, you know what are often referred to as developing nations have many more young people than older people because their birth rates are high and their death rates are high. In more developed nations, we have lower birth and death rates. And so our age pyramids, instead of looking like that, look mawr like this because there's just a higher survival for a large range of ages. Now the fertility rate is the average number of surviving Children each woman has in her lifetime. And this factors into what we call the replacement rate, which is the fertility rate required for women to give birth to enough Children to sustain the population. And this is something we're thinking about in developed nations now because, as we can see, for example, in places like Japan, their population is decreasing because their replacement rate is, well, not high enough. They're not sustaining their population. Their fertility rate is too low. Generally speaking, I should say replacement rate is about 2.1, because if you think about it, you have. And this is for developed nations. I should say this is like us. Uh, you know, if you think about it, a woman would have to give birth to 2.1 Children. Basically, that's saying that most women need to give birth to two Children, which will replace their them Selves. And you know, there, partner. And you know this 0.1 is basically because there's, uh you know, ah, chance that not all of these people are going to make it to the age where they can reproduce. You know, they're they're not gonna make it all the way through that generation time. So that's why it's 2.1. But basically you could almost just think of it as two to replace each parent. All right with that. Let's call it a day. It's all I have for this lesson. See you guys next time.