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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 those 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 had been infected.

  • But in 2011, the whole state of Texas reported only 27 cases

  • of West Nile, and only 2 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-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 gotta understand the fundamental principles of population ecology.

  • For starters, a population is just a group of individuals

  • of one species who interact regularly.

  • How often organisms interact has a lot to do with geography:

  • You're gonna have a lot more face time with folks you live near

  • than those who live farther away.

  • As a result, individuals who are closer to you will be

  • the ones you'll 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 are 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 mosquitos all clumped 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 a 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 can 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, 5 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 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 keepin' ya down.

  • To figure out what your limiting factors were,

  • you have to first narrow down what you need as a mosquito,

  • to live and reproduce successfully.

  • First you've got to find your food.

  • Now, you mosquitos, 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 Texas is plenty warm,

  • and the winter of 2011 and 2012 was especially balmy.

  • In fact, the summer of 2012 was exceptionally hot,

  • which helps 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 you know Dallas

  • is a pretty big city, to mate with.

  • This isn't actually all that hard because of the way

  • that mosquitoes do it, males just gather into a mosquito cloud

  • at dusk every night during mating season, and all a 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, ah-ha!

  • Because here we have another important clue.

  • Mosquitoes need to lay their eggs in stagnant water.

  • And if there's anything mosquito larvae 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' 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 mosquitos' 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's 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 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 mosquitos.

  • 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 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.

  • 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 3 times as many reproducing mosquitoes,

  • the population grew three times as fast as when there were 1,000.

  • So then there are 9,000, at which point, it's growing three times

  • as fast as when there were 3,000. And on and on into infinity.

  • And in this scenario, the mosquitos are all,

  • "CARRYING CAPACITY MY CHITIN-COVERED BUTT! THERE'S NO STOPPING US!"

  • But you know that 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 just 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 the 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!

  • 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 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 2 weeks.

  • So our deaths, over a span of two weeks, will be 100.

  • Half of these mosquitoes are going to be female, so 50 of them.

  • And they can produce about 2,000 babies in their lifetime,

  • so that's times 2,000. [Yeesh!]

  • So 50 mommy mosquitos times 2,000 babies per mommy

  • and you get births equaling 100,000 little baby mosquitos.

  • 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.

  • 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