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Population ecology: The Texas mosquito mystery

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- 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. In understanding the whole organism it's important to know what's going on at all those levels. And 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. And then the most zoomed out we get is ecosystem ecology, the study of how all living and nonliving things interact within an entire ecosystem. So 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, 12 people died from the virus as of the filming of this, and nearly 300 people have been infected. But in 2011 the whole state of Texas reported only 27 cases of West Nile and only two deaths. That seems kind of significant, so what's up? It 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. So 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. (upbeat music) Before we start solving any disease outbreak mysteries, we gotta 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 gonna 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. But 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 its 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, and 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 come together? Are they evenly spaced across the county? Is there some kind of random spacing? The answer to these questions give scientists a snapshot of a population at any given moment, and 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 could have in a lifetime make a huge difference in the size of a population. So 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 2,000 offspring in their two-week lifetime, while the rhino can have like five in 40 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 gotta 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. So you have to find a vertebrate and suck some of its blood out. Presumably there is no shortage of vertebrates walking around Dallas for you to suck blood out, if 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 and has to be warm in order for you to be active. And in Texas it's pretty warm and the winter of 2011 and 2012 was especially bombing. In fact the summer of 2012 was exceptionally hot which helped speed up the mosquito life cycle so 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 the 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. And if there's anything mosquito larva hate, it's a rainstorm flushing out the little puddle of water they've been living in. And 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. So when we look at this evidence, we find at least two limiting factors for Dallas's 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. And the reason these limitations matter is because they affect what's known as the carrying capacity of the mosquito's 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 and 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 gonna 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, and 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. But 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. But these factors don't have to be super dramatic. Going back to mosquitoes, say in 2013, there's a huge thunderstorm, a real gully washer in Dallas everyday for three months. That's gonna 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 spoken if the temperature swung the other way and it was unseasonably cold all summer, the bug's growth rate would drop. Now the truth is there are 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. And 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. So here at the beginning of 2012 we might only have had 1,000 mosquitoes in Dallas, but then after say one month, we got 3,000. Now with three times as many reproducing mosquitoes, the population grew three times as fast as when they were at 1,000, so then there are 9,000, at which point it's growing three times as fast as when they were 3,000 and on and on into infinity. And in this scenario the mosquitoes are all carrying capacity my cotton-covered butt, there's no stopping us. But you know it 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, but 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. And 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. And 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's 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, and 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 100 mosquitoes, and each of those mosquitoes lives an average of two weeks, so our deaths over a span of two weeks will be 100. Half of these mosquitoes are gonna be female, so 50 of them, and they can produce about 2,000 babies in their lifetime, so that's times 2,000. Ooh so 50 mommy mosquitoes times 2,000 babies per mommy, and you get births equaling 100,000 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's mosquito problem. So blink, in two weeks the population had 100,000 babies and only 100 of them died. So this is a population growth rate, if you do the math, of 999. This means that for every mosquito out there at the beginning of two weeks, there will be 999 more at the end of two weeks. That is a 99,800% 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 that we talk about go in its favor. And you guys haven't even seen trouble until you see what the graph of human population looks like over the last couple of millennia.
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