Population ecology: The Texas mosquito mystery

In our series on biology we spent many weeks together, talking about the physiology of animals and plants, and how cells work together to make tissues, to make organs, to make organ systems. To make us the hunks of meat and vegetables that we are. Understanding the whole organism. It's important to know what's going on at all those levels. The same is true for ecology. Only, instead of zooming in and out on different levels within a living thing, we can zoom in and out on the earth. Depending on the power of the magnification, we can understand a whole range of things about our planet. For instance, we can look at groups within a species, and how they live together in one geographic area. That's population ecology. There's also community ecology, where you look at groups of different organisms living together, and figure out how they influence each other. Then, the most zoomed out we get is ecosystem ecology. The study of how all living and non-living things, interact within an entire ecosystem. Let's start by zooming in with population ecology. The study of groups within a species, that interact mostly with each other. To understand why these populations, are different in one time and place than they are in another. How, you may be asking yourself, is that in any way useful to anyone ever? Well, it's actually super useful to everybody always. Let's look, for instance, at the outbreak of West Nile Virus, that struck Dallas, Texas in the summer of 2012. In Dallas County, twelve people died from the virus, as of the filming of this. Nearly three hundred people have been infected. In 2011 the whole state of Texas, reported only twenty seven cases of West Nile and only two deaths. That seems kind of significant. So, what's up? Turns out that this is a population ecology problem. West Nile is a mosquito born illness, and the population of mosquitoes in Dallas in 2012, busted through brick walls like the Kool-aid man, spreading West Nile like crazy. Why did this outbreak happen in 2012 and not the year before? And why did it happen in Texas and not in New Jersey? The answer, is population ecology. (fast lively music) Before we start solving any disease outbreak mysteries, we got to understand the fundamentals of population ecology. For starters, a population is just a group of individuals, of one species who interact regularly. How often organisms interact have a lot to do with geography. You're going to have a lot more face time with the folks you live near, than those who live farther away. As a result, individuals who are closer to you, will be the ones that you compete with for food and living space, mates, all that stuff. In order to understand why populations are different, from time to time and place to place, a population ecologist needs to know a few things about a population. Like, it's density. In this instance, how many mosquitoes there are, in the greater Dallas area that might come into contact with each other. A population's density changes due to a number of factors, all of which are pretty intuitive. It increases when new individuals are either, born or immigrate, that is, move in. It decreases because of deaths or emigration, or individuals moving out. Simple enough, but as a population ecologist, you also need to know about the geographic arrangement, of the individuals within the population. This is their dispersion. Like, are the mosquitoes all clumped together? Are they evenly spaced throughout the county? Is there some kind of random spacing? The answers to these questions give scientists, a snapshot of a population at any given moment. To figure out a puzzle like the West Nile outbreak, which involves studying how a population has changed over time, you have to investigate one of population ecology's central principles. Population growth. There are all kinds of factors that drive population growth, and they can vary radically from one organism to the next. Things like fecundity. How many offspring an individual can have in a lifetime, make a huge difference in the size of a population. For instance, why do mosquito populations seem to grow so quickly, while, the endangered black rhino may never recover, from a single act of poaching? For starters, mosquitoes can have two thousand offspring, in their two week lifetime. While the rhino can have like five in forty years. Still, a population doesn't usually or even ever, grow to its full potential and it can't keep growing indefinitely. To understand how fast or slow, and high or low a population actually grows, you need to focus on what's keeping growth in check. These factors are appropriately called, limiting factors. Say, you're a mosquito in Dallas in 2011, the year before the outbreak. Back then, the growth rate wasn't what it was in 2012, so something was keeping you down. To figure out what your limiting factors were, you first have to narrow down what you need as a mosquito, to live and reproduce successfully. First, you got to find your food. Now, you mosquitoes, you eat all kinds of things. But in order to reproduce, assuming you're a female, you need a blood meal. You have to find a vertebrate and suck some of its blood out. Presumably there's no shortage of vertebrates walking around Dallas, for you to suck blood out of. I have good friends who are vertebrates in Dallas. You might even be able to suck some of their blood. Next, temperature. Because you mosquitoes are ectothermic, it has to be warm, in order for you to be active. Now, Texas is pretty warm and the winter of 2011, 2012 was especially balmy. In fact, the summer of 2012 was exceptionally hot, which helps speed up the mosquito life cycle. That's one limiting factor that's been removed, for Dallas area mosquitoes. Moving on to mates. If you're a female mosquito, you need to find a nice male mosquito, with a job and preferably his own car, because Dallas is a pretty big city, to mate with. This isn't actually all that hard because the way that mosquitoes do it. Males just gather into a mosquito cloud, at dusk every night during mating season, and all the female has to do is find her local dude cloud, and fly into it in order to get mated with. Easy cheese. Finally, space. And, aha! Because here we have another important clue. Mosquitoes need to lay their eggs in stagnant water, if there's anything mosquito larva hate, it's a rainstorm flushing out the little puddle of water, they've been living in. Since Dallas saw a pretty severe drought in the summer of 2012, there were lots of pockets of stagnant, nasty mosquito water, sitting around acting as nurseries for many, many West Nile infected mosquitoes. When we look at this evidence, we find at least two limiting factors, for Dallas' mosquito population growth, that were removed in 2011. The constraints of temperature and space. It was plenty hot and there were lots, of egg-laying locations so the bugs were free to go nuts. Population ecologists group limiting factors like these, into two different categories. Density dependent and density independent. They do it this way because we need to know, whether a population's growth rate is being controlled, by how many individuals are in it, or whether it's being controlled by something else. The reason these limitations matter, is because they affect what's known as the carrying capacity, of the mosquitoes' habitat. That's the number of individuals that a habitat can sustain, with the resources that it has available. So, density dependent limitations are factors that inhibit growth, because of the environmental stress caused by a population size. For example, there may simply not be enough, food, water, and space to accommodate everyone. Or maybe because there are so many individuals, a nearby predator population explodes, which helps keep the population in check. Things like disease can also be a density dependent limitation. Lots of individuals living in close quarters, can make infections spread like crazy. Now, I don't think that the Dallas mosquitoes, are going to run out of vertebrates to dine on any time soon, but let's say hypothetically, that the explosion of local mosquito populations, caused a similar explosion, in the number of Mexican free tailed bats, the official flying mammal of the state of Texas. They eat mosquitoes. That would be a limiting factor that was density dependent. More mosquitoes leads to more bats, which leads to fewer mosquitoes. It's pretty simple. When density dependent limitations start to kick in, and start to limit a population's growth, that means that the habitat's carrying capacity has been reached. The other type of limiting factor, the density independent ones, have nothing to do with how many individuals there are, or how dense the population is. A lot of times, these limitations are described, in terms of some catastrophe. A volcanic eruption, a monsoon, a Chernobyl. In any case, some crucial aspect of the population's lifestyle, changes enough that it makes it harder to get by. These factors don't have to be super dramatic. Going back to mosquitoes, say, in 2013 there's a huge thunderstorm. A really gully washer in Dallas every day for three months. That's going to disturb the clutches of mosquito eggs, hanging out in the stagnant water. So the number born that year would be substantially smaller. By the same token, if the temperature swung the other way, and it was unseasonably cold all summer, the bugs' growth rate would drop. Now, the truth is, there are a billion and a half situations, both big and small that could lead to a population, either reaching its carrying capacity, or collapsing because of external factors. It's a population ecologist's job to figure out what those factors are. That is what math is for. Our friend math says that any population of anything, anything, will grow exponentially, unless there's some reason that it can't. Exponential growth means that the population grows, at a rate proportional to the size of the population. Here at the beginning of 2012, we might only have had a thousand mosquitoes in Dallas, but then after, say, one month we got three thousand. Now, with three times as many reproducing mosquitoes, the population grew three times as fast, as when there were a thousand. Then there are nine thousand, at which point it's growing three times as fast, as when there were three thousand. And on and on into infinity. And in this scenario, the mosquitoes are all, carrying capacity my chitin-covered butt! There's no stopping us! But you know what doesn't really happen? I mean, it can happen for a while. Humans have been on an exponential growth curve, since the Industrial Revolution, for example. Eventually something always knocks the population size back down. That thing might be a density dependent factor, like food scarcity or an epidemic. Or a density independent one, like an asteroid that takes out the whole continent. Regardless, this exponential growth curve can't go up forever. When those factors come into play, a population experiences only logistic growth. This means that the population is limited, to the carrying capacity of its habitat, which, when you think about it, ain't too much to ask. See how this graph flattens up at the top? The factor that creates that plateau is almost always, a density dependent limitation. As you add mosquitoes, eventually the rate of population growth, is going to slow down because they run out of food or space. When we get to where that number levels off, that number is the carrying capacity, of the mosquito population in that particular habitat. Now, let's apply all of these ideas, using a simple equation that will allow us, to calculate the population growth of anything we feel like. I know it's math, but wake up because this is important. The city of Dallas is depending on you! So, let's calculate the growth of Dallas' mosquito population, over a span of two weeks. All we have to do to get the rate of growth, that's R, is take the number of births. Births minus the number of deaths. Then divide that all by the initial population size. Which we generally just call N. So, let's say we start with an initial population, of a hundred mosquitoes. Each of those mosquitoes lives an average of two weeks. Our deaths, over a span of two weeks, will be one hundred. Half of these mosquitoes are going to be female, so fifty of them. They can produce about two thousand babies in their lifetime, so that's times two thousand. Ugh! Fifty mommy mosquitoes times two thousand babies per mommy. You get births equaling one hundred thousand little baby mosquitoes. Once we plug in all the numbers into this equation, even though this is totally a hypothetical, we will see the true scope of Dallas' mosquito problem. Blink, in two weeks the population had a hundred thousand babies, and only a hundred of them died. This is a population growth rate, if you do the math, of nine hundred and ninety nine. This means, that for every mosquito out there, at the beginning of two weeks, there will be ninety hundred and ninety nine more, at the end of two weeks. That is a ninety nine thousand, eight hundred percent increase. By Thor's hammer! Again, these are hypothetical numbers, but it gives you a sense of how a population, can just go out of control, when all the factors we talk about go in its favor. You guys haven't even seen trouble until you see, what the graph of human population looks like, over the last couple millennia.