0:00:02.116,0:00:04.372 In our series on biology we[br]spent many weeks together, 0:00:04.372,0:00:06.293 talking about the physiology[br]of animals and plants, 0:00:06.293,0:00:08.331 and how cells work[br]together to make tissues, 0:00:08.331,0:00:10.020 to make organs, to make organ systems. 0:00:10.020,0:00:13.105 To make us the hunks of meat[br]and vegetables that we are. 0:00:13.105,0:00:14.539 Understanding the whole organism. 0:00:14.539,0:00:17.582 It's important to know what's[br]going on at all those levels. 0:00:17.889,0:00:19.957 The same is true for ecology. 0:00:20.249,0:00:23.806 Only, instead of zooming in and out on[br]different levels within a living thing, 0:00:23.806,0:00:25.870 we can zoom in and out on the earth. 0:00:26.162,0:00:27.402 Depending on the power[br]of the magnification, 0:00:27.402,0:00:30.425 we can understand a whole range[br]of things about our planet. 0:00:30.425,0:00:32.897 For instance, we can look[br]at groups within a species, 0:00:32.897,0:00:35.553 and how they live together[br]in one geographic area. 0:00:35.553,0:00:37.074 That's population ecology. 0:00:37.074,0:00:38.371 There's also community ecology, 0:00:38.371,0:00:40.951 where you look at groups of[br]different organisms living together, 0:00:40.951,0:00:42.700 and figure out how they[br]influence each other. 0:00:42.700,0:00:45.952 Then, the most zoomed out[br]we get is ecosystem ecology. 0:00:46.259,0:00:49.391 The study of how all living[br]and non-living things, 0:00:49.467,0:00:51.481 interact within an entire ecosystem. 0:00:51.696,0:00:54.621 Let's start by zooming in[br]with population ecology. 0:00:54.821,0:00:56.534 The study of groups within a species, 0:00:56.534,0:00:58.867 that interact mostly with each other. 0:00:59.313,0:01:00.613 To understand why these populations, 0:01:00.613,0:01:03.650 are different in one time and[br]place than they are in another. 0:01:03.850,0:01:07.072 How, you may be asking yourself, is[br]that in any way useful to anyone ever? 0:01:07.441,0:01:09.893 Well, it's actually super[br]useful to everybody always. 0:01:09.893,0:01:13.192 Let's look, for instance, at[br]the outbreak of West Nile Virus, 0:01:13.192,0:01:16.333 that struck Dallas, Texas[br]in the summer of 2012. 0:01:16.733,0:01:19.243 In Dallas County, twelve[br]people died from the virus, 0:01:19.243,0:01:20.573 as of the filming of this. 0:01:20.742,0:01:22.744 Nearly three hundred[br]people have been infected. 0:01:22.897,0:01:24.810 In 2011 the whole state of Texas, 0:01:24.810,0:01:28.042 reported only twenty seven cases[br]of West Nile and only two deaths. 0:01:28.457,0:01:29.340 That seems kind of significant. 0:01:29.340,0:01:30.038 So, what's up? 0:01:30.038,0:01:32.033 Turns out that this is a[br]population ecology problem. 0:01:32.033,0:01:33.911 West Nile is a mosquito born illness, 0:01:33.911,0:01:36.717 and the population of[br]mosquitoes in Dallas in 2012, 0:01:36.717,0:01:39.114 busted through brick walls[br]like the Kool-aid man, 0:01:39.114,0:01:40.599 spreading West Nile like crazy. 0:01:40.599,0:01:44.121 Why did this outbreak happen in[br]2012 and not the year before? 0:01:44.551,0:01:47.390 And why did it happen in[br]Texas and not in New Jersey? 0:01:47.390,0:01:49.382 The answer, is population ecology. 0:01:49.382,0:01:59.637 (fast lively music) 0:01:59.637,0:02:02.318 Before we start solving any[br]disease outbreak mysteries, 0:02:02.318,0:02:04.816 we got to understand the[br]fundamentals of population ecology. 0:02:04.816,0:02:08.286 For starters, a population is[br]just a group of individuals, 0:02:08.286,0:02:10.989 of one species who interact regularly. 0:02:10.989,0:02:14.240 How often organisms interact[br]have a lot to do with geography. 0:02:14.240,0:02:16.541 You're going to have a lot more face[br]time with the folks you live near, 0:02:16.541,0:02:18.259 than those who live farther away. 0:02:18.259,0:02:20.597 As a result, individuals[br]who are closer to you, 0:02:20.597,0:02:22.888 will be the ones that you compete[br]with for food and living space, 0:02:22.888,0:02:24.353 mates, all that stuff. 0:02:24.353,0:02:26.349 In order to understand why[br]populations are different, 0:02:26.349,0:02:27.879 from time to time and place to place, 0:02:27.879,0:02:31.105 a population ecologist needs to know[br]a few things about a population. 0:02:31.105,0:02:32.425 Like, it's density. 0:02:32.425,0:02:34.603 In this instance, how[br]many mosquitoes there are, 0:02:34.603,0:02:37.742 in the greater Dallas area that might[br]come into contact with each other. 0:02:37.742,0:02:40.161 A population's density changes[br]due to a number of factors, 0:02:40.161,0:02:41.787 all of which are pretty intuitive. 0:02:41.787,0:02:43.739 It increases when new[br]individuals are either, 0:02:43.739,0:02:46.022 born or immigrate, that is, move in. 0:02:46.468,0:02:49.750 It decreases because of[br]deaths or emigration, 0:02:49.750,0:02:50.956 or individuals moving out. 0:02:50.956,0:02:53.370 Simple enough, but as[br]a population ecologist, 0:02:53.370,0:02:55.996 you also need to know about[br]the geographic arrangement, 0:02:55.996,0:02:58.200 of the individuals within the population. 0:02:58.707,0:03:00.122 This is their dispersion. 0:03:00.122,0:03:01.839 Like, are the mosquitoes[br]all clumped together? 0:03:01.839,0:03:03.691 Are they evenly spaced[br]throughout the county? 0:03:03.691,0:03:05.633 Is there some kind of random spacing? 0:03:05.633,0:03:07.408 The answers to these[br]questions give scientists, 0:03:07.408,0:03:09.788 a snapshot of a population[br]at any given moment. 0:03:10.080,0:03:12.513 To figure out a puzzle like[br]the West Nile outbreak, 0:03:12.574,0:03:15.339 which involves studying how a[br]population has changed over time, 0:03:15.339,0:03:19.082 you have to investigate one of[br]population ecology's central principles. 0:03:19.512,0:03:20.817 Population growth. 0:03:20.817,0:03:22.784 There are all kinds of factors[br]that drive population growth, 0:03:22.784,0:03:25.308 and they can vary radically[br]from one organism to the next. 0:03:25.308,0:03:26.551 Things like fecundity. 0:03:26.551,0:03:28.993 How many offspring an individual[br]can have in a lifetime, 0:03:28.993,0:03:31.518 make a huge difference in[br]the size of a population. 0:03:32.010,0:03:35.596 For instance, why do mosquito[br]populations seem to grow so quickly, 0:03:35.596,0:03:38.164 while, the endangered black[br]rhino may never recover, 0:03:38.164,0:03:39.944 from a single act of poaching? 0:03:39.944,0:03:42.467 For starters, mosquitoes can[br]have two thousand offspring, 0:03:42.467,0:03:44.115 in their two week lifetime. 0:03:44.484,0:03:47.287 While the rhino can have[br]like five in forty years. 0:03:47.456,0:03:50.034 Still, a population doesn't[br]usually or even ever, 0:03:50.034,0:03:53.096 grow to its full potential and it[br]can't keep growing indefinitely. 0:03:53.281,0:03:55.031 To understand how fast or slow, 0:03:55.031,0:03:57.712 and high or low a[br]population actually grows, 0:03:57.773,0:03:59.882 you need to focus on what's[br]keeping growth in check. 0:04:00.220,0:04:01.916 These factors are appropriately called, 0:04:02.531,0:04:03.854 limiting factors. 0:04:03.854,0:04:05.941 Say, you're a mosquito in Dallas in 2011, 0:04:05.941,0:04:07.112 the year before the outbreak. 0:04:07.388,0:04:10.432 Back then, the growth rate[br]wasn't what it was in 2012, 0:04:10.432,0:04:12.344 so something was keeping you down. 0:04:12.774,0:04:14.925 To figure out what your[br]limiting factors were, 0:04:14.956,0:04:17.140 you first have to narrow down[br]what you need as a mosquito, 0:04:17.140,0:04:19.360 to live and reproduce successfully. 0:04:19.589,0:04:20.741 First, you got to find your food. 0:04:20.741,0:04:22.830 Now, you mosquitoes, you[br]eat all kinds of things. 0:04:22.830,0:04:25.326 But in order to reproduce,[br]assuming you're a female, 0:04:25.326,0:04:26.727 you need a blood meal. 0:04:27.311,0:04:29.733 You have to find a vertebrate[br]and suck some of its blood out. 0:04:29.733,0:04:32.432 Presumably there's no shortage of[br]vertebrates walking around Dallas, 0:04:32.432,0:04:33.875 for you to suck blood out of. 0:04:33.875,0:04:36.121 I have good friends who[br]are vertebrates in Dallas. 0:04:36.613,0:04:38.566 You might even be able to[br]suck some of their blood. 0:04:38.781,0:04:39.498 Next, temperature. 0:04:39.498,0:04:42.129 Because you mosquitoes are[br]ectothermic, it has to be warm, 0:04:42.129,0:04:43.975 in order for you to be active. 0:04:44.014,0:04:48.297 Now, Texas is pretty warm and the winter[br]of 2011, 2012 was especially balmy. 0:04:48.297,0:04:51.261 In fact, the summer of[br]2012 was exceptionally hot, 0:04:51.261,0:04:53.546 which helps speed up[br]the mosquito life cycle. 0:04:53.546,0:04:55.540 That's one limiting factor[br]that's been removed, 0:04:55.540,0:04:57.148 for Dallas area mosquitoes. 0:04:57.148,0:04:58.618 Moving on to mates. 0:04:58.618,0:05:01.321 If you're a female mosquito, you[br]need to find a nice male mosquito, 0:05:01.321,0:05:03.155 with a job and preferably his own car, 0:05:03.155,0:05:05.676 because Dallas is a pretty[br]big city, to mate with. 0:05:05.676,0:05:07.499 This isn't actually all that hard[br]because the way that mosquitoes do it. 0:05:07.945,0:05:10.321 Males just gather into a mosquito cloud, 0:05:10.321,0:05:12.387 at dusk every night during mating season, 0:05:12.387,0:05:15.105 and all the female has to do[br]is find her local dude cloud, 0:05:15.105,0:05:17.057 and fly into it in[br]order to get mated with. 0:05:17.657,0:05:18.267 Easy cheese. 0:05:18.267,0:05:19.511 Finally, space. 0:05:19.511,0:05:20.460 And, aha! 0:05:20.460,0:05:22.711 Because here we have[br]another important clue. 0:05:22.711,0:05:25.182 Mosquitoes need to lay their[br]eggs in stagnant water, 0:05:25.582,0:05:26.970 if there's anything mosquito larva hate, 0:05:26.970,0:05:29.198 it's a rainstorm flushing out[br]the little puddle of water, 0:05:29.198,0:05:30.107 they've been living in. 0:05:30.107,0:05:32.943 Since Dallas saw a pretty severe[br]drought in the summer of 2012, 0:05:32.943,0:05:35.970 there were lots of pockets of[br]stagnant, nasty mosquito water, 0:05:35.970,0:05:38.182 sitting around acting[br]as nurseries for many, 0:05:38.182,0:05:40.376 many West Nile infected mosquitoes. 0:05:40.883,0:05:42.026 When we look at this evidence, 0:05:42.026,0:05:43.479 we find at least two limiting factors, 0:05:43.479,0:05:45.878 for Dallas' mosquito population growth, 0:05:45.878,0:05:48.076 that were removed in 2011. 0:05:48.076,0:05:50.167 The constraints of temperature and space. 0:05:50.351,0:05:51.924 It was plenty hot and there were lots, 0:05:51.924,0:05:55.910 of egg-laying locations so[br]the bugs were free to go nuts. 0:05:55.910,0:05:58.147 Population ecologists group[br]limiting factors like these, 0:05:58.147,0:05:59.672 into two different categories. 0:05:59.672,0:06:02.441 Density dependent and density independent. 0:06:02.441,0:06:03.682 They do it this way[br]because we need to know, 0:06:03.682,0:06:05.559 whether a population's growth[br]rate is being controlled, 0:06:05.559,0:06:07.046 by how many individuals are in it, 0:06:07.046,0:06:08.912 or whether it's being[br]controlled by something else. 0:06:08.912,0:06:10.206 The reason these limitations matter, 0:06:10.206,0:06:12.831 is because they affect what's[br]known as the carrying capacity, 0:06:12.831,0:06:14.523 of the mosquitoes' habitat. 0:06:14.969,0:06:17.780 That's the number of individuals[br]that a habitat can sustain, 0:06:17.780,0:06:19.027 with the resources that it has available. 0:06:19.027,0:06:22.131 So, density dependent limitations[br]are factors that inhibit growth, 0:06:22.131,0:06:25.263 because of the environmental[br]stress caused by a population size. 0:06:25.724,0:06:27.824 For example, there may[br]simply not be enough, 0:06:27.824,0:06:29.968 food, water, and space[br]to accommodate everyone. 0:06:29.968,0:06:31.824 Or maybe because there[br]are so many individuals, 0:06:31.824,0:06:34.491 a nearby predator population explodes, 0:06:34.491,0:06:36.687 which helps keep the population in check. 0:06:36.687,0:06:39.176 Things like disease can also be[br]a density dependent limitation. 0:06:39.437,0:06:41.081 Lots of individuals[br]living in close quarters, 0:06:41.081,0:06:42.912 can make infections spread like crazy. 0:06:42.912,0:06:44.275 Now, I don't think that[br]the Dallas mosquitoes, 0:06:44.275,0:06:46.265 are going to run out of vertebrates[br]to dine on any time soon, 0:06:46.265,0:06:50.972 but let's say hypothetically, that the[br]explosion of local mosquito populations, 0:06:50.972,0:06:52.261 caused a similar explosion, 0:06:52.261,0:06:54.154 in the number of Mexican free tailed bats, 0:06:54.154,0:06:56.676 the official flying mammal[br]of the state of Texas. 0:06:56.676,0:06:58.607 They eat mosquitoes. 0:06:58.607,0:07:01.396 That would be a limiting factor[br]that was density dependent. 0:07:01.396,0:07:03.254 More mosquitoes leads to more bats, 0:07:03.254,0:07:04.931 which leads to fewer mosquitoes. 0:07:04.931,0:07:05.686 It's pretty simple. 0:07:05.686,0:07:07.542 When density dependent[br]limitations start to kick in, 0:07:07.542,0:07:09.510 and start to limit a population's growth, 0:07:09.510,0:07:12.373 that means that the habitat's[br]carrying capacity has been reached. 0:07:12.373,0:07:15.413 The other type of limiting factor,[br]the density independent ones, 0:07:15.413,0:07:17.441 have nothing to do with how[br]many individuals there are, 0:07:17.441,0:07:19.460 or how dense the population is. 0:07:19.460,0:07:21.487 A lot of times, these[br]limitations are described, 0:07:21.487,0:07:22.842 in terms of some catastrophe. 0:07:23.242,0:07:25.435 A volcanic eruption, a[br]monsoon, a Chernobyl. 0:07:25.435,0:07:28.681 In any case, some crucial aspect[br]of the population's lifestyle, 0:07:28.681,0:07:31.222 changes enough that it[br]makes it harder to get by. 0:07:31.222,0:07:32.976 These factors don't have[br]to be super dramatic. 0:07:32.976,0:07:36.755 Going back to mosquitoes, say, in[br]2013 there's a huge thunderstorm. 0:07:36.785,0:07:39.980 A really gully washer in Dallas[br]every day for three months. 0:07:39.980,0:07:41.903 That's going to disturb the[br]clutches of mosquito eggs, 0:07:41.903,0:07:43.354 hanging out in the stagnant water. 0:07:43.354,0:07:46.434 So the number born that year[br]would be substantially smaller. 0:07:46.434,0:07:48.170 By the same token, if the[br]temperature swung the other way, 0:07:48.170,0:07:49.939 and it was unseasonably cold all summer, 0:07:49.939,0:07:51.668 the bugs' growth rate would drop. 0:07:51.668,0:07:54.335 Now, the truth is, there are a[br]billion and a half situations, 0:07:54.335,0:07:57.079 both big and small that[br]could lead to a population, 0:07:57.079,0:07:58.726 either reaching its carrying capacity, 0:07:58.726,0:08:00.693 or collapsing because of external factors. 0:08:01.139,0:08:04.800 It's a population ecologist's job to[br]figure out what those factors are. 0:08:05.261,0:08:07.120 That is what math is for. 0:08:07.258,0:08:10.413 Our friend math says that[br]any population of anything, 0:08:10.413,0:08:12.383 anything, will grow exponentially, 0:08:12.383,0:08:13.970 unless there's some reason that it can't. 0:08:14.385,0:08:16.404 Exponential growth means[br]that the population grows, 0:08:16.404,0:08:19.310 at a rate proportional to[br]the size of the population. 0:08:19.710,0:08:21.117 Here at the beginning of 2012, 0:08:21.117,0:08:23.265 we might only have had a[br]thousand mosquitoes in Dallas, 0:08:23.295,0:08:25.799 but then after, say, one[br]month we got three thousand. 0:08:25.799,0:08:27.560 Now, with three times as[br]many reproducing mosquitoes, 0:08:27.560,0:08:29.597 the population grew three times as fast, 0:08:29.597,0:08:30.899 as when there were a thousand. 0:08:30.899,0:08:32.243 Then there are nine thousand, 0:08:32.243,0:08:34.433 at which point it's growing[br]three times as fast, 0:08:34.433,0:08:35.874 as when there were three thousand. 0:08:35.874,0:08:37.537 And on and on into infinity. 0:08:37.537,0:08:39.734 And in this scenario,[br]the mosquitoes are all, 0:08:39.734,0:08:41.924 carrying capacity my chitin-covered butt! 0:08:41.924,0:08:43.631 There's no stopping us! 0:08:43.631,0:08:44.657 But you know what doesn't really happen? 0:08:44.672,0:08:46.464 I mean, it can happen for a while. 0:08:46.464,0:08:48.832 Humans have been on an[br]exponential growth curve, 0:08:48.832,0:08:51.438 since the Industrial[br]Revolution, for example. 0:08:51.438,0:08:54.918 Eventually something always knocks[br]the population size back down. 0:08:54.918,0:08:56.941 That thing might be a[br]density dependent factor, 0:08:56.941,0:08:58.836 like food scarcity or an epidemic. 0:08:58.836,0:09:00.273 Or a density independent one, 0:09:00.273,0:09:02.656 like an asteroid that takes[br]out the whole continent. 0:09:03.086,0:09:06.745 Regardless, this exponential[br]growth curve can't go up forever. 0:09:06.745,0:09:08.023 When those factors come into play, 0:09:08.023,0:09:11.070 a population experiences[br]only logistic growth. 0:09:11.070,0:09:12.634 This means that the population is limited, 0:09:12.634,0:09:14.938 to the carrying capacity[br]of its habitat, which, 0:09:14.938,0:09:17.428 when you think about it,[br]ain't too much to ask. 0:09:17.428,0:09:19.401 See how this graph flattens up at the top? 0:09:19.401,0:09:22.583 The factor that creates that[br]plateau is almost always, 0:09:22.598,0:09:24.255 a density dependent limitation. 0:09:24.255,0:09:27.143 As you add mosquitoes, eventually[br]the rate of population growth, 0:09:27.143,0:09:30.040 is going to slow down because[br]they run out of food or space. 0:09:30.286,0:09:31.801 When we get to where[br]that number levels off, 0:09:31.801,0:09:33.773 that number is the carrying capacity, 0:09:33.773,0:09:36.124 of the mosquito population[br]in that particular habitat. 0:09:36.524,0:09:38.102 Now, let's apply all of these ideas, 0:09:38.102,0:09:40.580 using a simple equation[br]that will allow us, 0:09:40.580,0:09:43.451 to calculate the population[br]growth of anything we feel like. 0:09:43.635,0:09:46.955 I know it's math, but wake[br]up because this is important. 0:09:46.955,0:09:48.932 The city of Dallas is depending on you! 0:09:49.362,0:09:52.706 So, let's calculate the growth[br]of Dallas' mosquito population, 0:09:52.706,0:09:54.083 over a span of two weeks. 0:09:54.329,0:09:57.369 All we have to do to get the[br]rate of growth, that's R, 0:09:57.676,0:09:59.995 is take the number of births. 0:10:00.995,0:10:03.944 Births minus the number of deaths. 0:10:05.468,0:10:08.878 Then divide that all by the[br]initial population size. 0:10:11.525,0:10:14.326 Which we generally just call N. 0:10:14.326,0:10:16.165 So, let's say we start[br]with an initial population, 0:10:16.165,0:10:18.302 of a hundred mosquitoes. 0:10:18.302,0:10:20.776 Each of those mosquitoes[br]lives an average of two weeks. 0:10:20.776,0:10:24.403 Our deaths, over a span of two[br]weeks, will be one hundred. 0:10:24.403,0:10:27.131 Half of these mosquitoes are going[br]to be female, so fifty of them. 0:10:27.623,0:10:30.122 They can produce about two[br]thousand babies in their lifetime, 0:10:30.122,0:10:31.952 so that's times two thousand. 0:10:31.952,0:10:32.777 Ugh! 0:10:32.777,0:10:36.283 Fifty mommy mosquitoes times[br]two thousand babies per mommy. 0:10:36.283,0:10:41.712 You get births equaling one hundred[br]thousand little baby mosquitoes. 0:10:41.712,0:10:43.361 Once we plug in all the[br]numbers into this equation, 0:10:43.361,0:10:44.756 even though this is[br]totally a hypothetical, 0:10:44.756,0:10:47.705 we will see the true scope[br]of Dallas' mosquito problem. 0:10:48.228,0:10:51.118 Blink, in two weeks the population[br]had a hundred thousand babies, 0:10:51.118,0:10:53.355 and only a hundred of them died. 0:10:53.355,0:10:55.836 This is a population growth[br]rate, if you do the math, 0:10:55.836,0:10:57.851 of nine hundred and ninety nine. 0:10:57.851,0:10:59.486 This means, that for[br]every mosquito out there, 0:10:59.486,0:11:00.365 at the beginning of two weeks, 0:11:00.365,0:11:03.221 there will be ninety hundred[br]and ninety nine more, 0:11:03.221,0:11:04.354 at the end of two weeks. 0:11:04.354,0:11:08.001 That is a ninety nine thousand,[br]eight hundred percent increase. 0:11:08.001,0:11:09.503 By Thor's hammer! 0:11:09.503,0:11:10.935 Again, these are hypothetical numbers, 0:11:10.935,0:11:13.677 but it gives you a sense[br]of how a population, 0:11:13.677,0:11:15.094 can just go out of control, 0:11:15.094,0:11:17.689 when all the factors we[br]talk about go in its favor. 0:11:17.689,0:11:19.621 You guys haven't even seen[br]trouble until you see, 0:11:19.621,0:11:22.205 what the graph of human[br]population looks like, 0:11:22.205,0:11:24.000 over the last couple millennia.