Population ecology: The Texas mosquito mystery

and 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 and 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 turns out that this is a population ecology problem West Nile is a mosquito-borne 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 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 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 D colleges 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 populations density changes due to a number of factors all of which are pretty intuitive it increases when new individuals are either born or emigrate that is moving 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 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 well 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 and forty years still the 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 is 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 so 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 and on Texas is pretty warm in the winter of 2011 2012 was especially bombing in fact the summer of 2012 was exceptionally hot which helps speed up the mosquito lifecycle 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 the job and preferably his own car because you know 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 larvae 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 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 ecologist 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 populations 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 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 infection spread like crazy now I don't think that the Valles mosquitoes are going to run out of vertebrates to dine on anytime 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 populations growth that means that the habitats 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 Chernobyl in any case some crucial aspect of the populations 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 real 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 small by the same token if the temperature swung the other way and it was unseasonally 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 ecologists 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 a thousand 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 there were a thousand so then there are 9,000 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 the mosquitoes are all carry capacity my chitin covered but there's no stopping us but you know it doesn't really happen I mean it can't happen for a while humans have been on an exponential growth curve since the Industrial Revolution for example but eventually something always knocked 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 wear that number levels off that number is the carrying capacity of the mosquito population into 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 thousands mosquito population over a span of two weeks all we have to do to get the rate of growth that's our 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 a hundred mosquitoes and each of those mosquitoes lives an average of two weeks so our deaths over a span of two weeks will be 100 have these mosquitoes are going to be female so fifty of them and they can produce about 2,000 babies in their lifetime so that's times 2,000 so fifty money 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 a hundred thousand babies and only hundred 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 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 that we talked 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