hey everybody it's Mr smees and welcome to the unit 3 Ultimate review packet video make sure to have that study guide printed out so you can follow along and make sure you understand all the concepts we cover in this video if you're ready to think like a mountain and write like a scholar let's get started now unit 3 is an interesting one because it's going to cover populations both from a human standpoint and from a natural ecosystem standpoint the first five Topics in the unit will be focused more on natural population dynamics that we might see in a deer herd whereas the last four Topics in unit 3 will be focused more on human population Dynamics and remember we have the time stamps here for each topic on the screen in case you need to jump right to the topic that you need to review most so starting off with topic 3.1 we need to review the difference between General species and specialist species and as their name implies specialist species are species that rely on a really narrow Niche or a really unique set of habitat or food requirements so pandas would be a great example of a specialist species because of their unique Reliance on bamboo generalist species on the other hand have a really wide range of tolerance when it comes to food requirements and habitat requirements they can utilize a ton of different food sources surviv in a ton of different habitats and they're really resilient because of this that means they can easily adapt to changes or disruptions in their food supply or their habitat needs because of this generalist species are much better equipped to adapt to ecosystem disturbances raccoons are a perfect example of generalist species because well there's not a lot of food that they won't eat and there aren't a lot of places that they won't move into well at least the TV gets decent reception huh must be some kind of nature show help help get it off get off so an ecosystem is undergoing rapid change generalist species are going to be advantaged they're going to be able to Pivot or switch to a different food source or habitat Source if their preferred source is disrupted on the flip side specialist species are more advantaged in ecosystems that are stable because they're uniquely adapted to the precise conditions of that stable ecosystem so now that we've reviewed how to classify organisms based on their food and habitat needs we'll talk about how to classify them based on the reproductive strategies see when it comes to reproduction which all organisms have to do there's two basic strategies that organisms take to Maxim imize the number of their offspring that survive now the first approach is what I like to call quantity over quality these organisms produce so many offspring that at least a few of them are bound to survive we call these species R strategist or R selected species because the letter R is used to represent exponential growth in math and our strategists are capable of really high population growth rates this is because they're generally smaller organisms that reach sexual maturity early in life and offer little parental care to the large number of offspring that they produce and this is what I mean by the quantity over quality approach they have so many offspring to account for the fact that most of them won't survive to adulthood now the second strategy is the exact opposite it's quality over quantity these types of organisms have a few Offspring at a time and invest a tremendous amount of energy into caring for them and ensuring that they survive we call these organisms K strategists or k selected species because they typically live in populations that are at or near caring capacity which is represented with the letter k k selected species are generally larger organisms like mammals that reach sexual mature later in life and only produce a few Offspring at a time they typically have longer lifespans than our strategists and live in habitats where competition for resources is very high now I like to help my students remember our selected and K selected species by remembering that our selected parents run away after they have their offspring whereas K selected parents stick around and care for their offspring but even better than just memorizing the names of these two different types of species is understanding how different ecosystem conditions can actually reward the different strategies in an ecosystem where there are lots of other species and competition for resources is high K strategists are at a huge Advantage because they can actually help their offspring gain those sought-after food resources on the other hand an ecosystem where there are abundant food resources and competition isn't as Fierce favors are strategist they can produce a massive amount of offspring that can take advantage of these abundant resources we also need to think about how the reproductive strategies of our strategist and K strategist can impact other organisms in their ecosystems because of their capability to reproduce really rapidly or their High biotic potential our strategists are more likely to be invasive species when they move to a new ecosystem Cas strategists on the other hand are more likely to suffer from an invasive R selected species this is because their population growth rate is really slow due to the low number of offspring that they have making them more susceptible to competition from new R selected strategists that move in another concept we need to review as we think about reproduction is survivorship survivorship is just the percentage of a given population that's still surviving at any given point in their relative lifespan so there are three main survivorship types or curves that we need to review here type one survivorship is seen mostly in K selected species like large mammals this is where organisms provide a high degree of Parental care and ensure that almost all offsprings survive infancy and make it to adulthood at the other end of the spectrum we have type three survivorship where very few offspring are surviving infancy and making it to adulthood these are typically smaller are strategist species that receive little to no parental care so they're often prey for larger organisms then in the middle we have type two survivorship where there's a pretty consistent mortality or death rate throughout the relative lifespan of the species these are often Cas selected speci species like small rodents or Birds who have a handful of Offspring and provide some parental care to them but also may lose Offspring to Predators or just lack of ability to provide enough food to them as we continue to review natural populations we have to review caring capacity caring capacity is the maximum number of individuals of a given species that an ecosystem can support competition for resources is a big part of what establishes the caring capacity for a given species in a given ecosystem and we call the specific resources that organisms compete for limiting resources these are things like food water water and shelter that limit the number of individuals that can survive in a given ecosystem you can think of caring capacity for an ecosystem as the maximum capacity sign that you might see in a restaurant there's a limited number of chairs and tables in the restaurant and that determines how many diners can eat there at a given time now the trickled natural ecosystems is they don't have the maximum carrying capacity printed on the side of the tree so that the deer know how many fonds they can bring into the ecosystem each spring so what happens is a repetitive cycle of overshooting the carrying capacity and the population dying back or dipping back below the caring capacity this happens as you exceed the amount of food resources that organisms have and then some of the organisms starve because of the lack of resources in Apes we call this cycle overshoot and die off or dieback now sometimes the die off is really severe because the overg grazing that occurred during overshoot degrades the plant Community or degrades the soil nutrients so severely that it also decreases the carrying capacity for plants in the ecosystem there could also be a disease outbreak that spreads more rapidly because organisms are so tightly clustered together so both of these examples competition to resources and dis disase are great instances of what we call density dependent factors that limit population growth this means that their effects are more pronounced the more densely packed population gets density independent factors of population growth on the other hand impact populations regardless of their density so something like a tsunami or a forest fire that rages through an ecosystem is going to kill organisms limit their population size and it doesn't really matter how big it was to begin with now the actual resources that limit the carrying capacity are different based on different populations or different species so seabird populations might have their carrying capacity set by the number of available Nest sites whereas a mountain line may have its carrying capacity set by the total territory since they need huge hunting ranges and typically defend a central layer now the resources that species need can also determine their distribution or how they're spread out in the space that they occupy if we were to look at an aerial view of a population like seabirds we would see they have a uniform distribution since there's roughly equal space between each individual in the population population of trees on the other hand is going to be limited by very different resources they're going to be limited by the amount of nitrogen and phosphorus in the soil and the amount of canopy space or the amount of sunlight they can get access to because of this tree populations typically exhibit what we call random distribution because their growth depends largely on where their seeds fall and where the soil conditions and the sunlight availability are right then we also have populations that show a clumped distribution these are going to be species that rely on group or herd protection from predators and so they tend to stick very closely together so what happens when the conditions for growth are perfect there are abundant resources and no limiting factors for a population well the answer is this doesn't really happen in natural ecosystems but when we create these conditions in Laboratories we see something called exponential growth on a graph this exponential growth model makes a j-shaped curve now should also look familiar from graphing exponential functions in your math classes the problem is this type of growth isn't possible for long what happens eventually is the limiting factors like food or space become scarce so the growth slows down and approaches a caring capacity we call this more realistic model of growth the logistic growth model but remember it's just a model real population have carrying capacities but they typically oscillate around them in this overshoot dieback pattern that we talked about in topic 3.4 so now that we've covered ecological or natural populations we need to turn our attention to human populations we'll start topic 3.6 with something called age structure diagrams these are a form of graph used to visualize the proportion of a country's population that's in each age cohort and remember that cohort is just a fancy name for a group so each bar on the graph that we're about to see represents all the number of individuals in the same age range now the key to age structure diagrams is to remember that you need to break the diagram into three basic groups and look at those groups population sizes relative to each other those groups are the pre-reproductive age cohort from 0 to 14 the reproductive age cohort from 15 to 44 and the post reproductive age cohort from age 45 on up now the number one mistake ape students make in unit 3 is just memorizing the shapes of the age structure diagrams and not actually understanding what the different age cohort sizes mean so let's take a minute to review what the relative size of each age cohort can tell us about a country's population growth rate if the pre-reproductive age cohort of the population is significantly larger than any of the other age cohorts it tells you that the country has a really high birth rate this means that the population also likely has a high growth rate and that the country is probably still industrializing this means they still have an agrarian or a farming based economy where child labor is valuable and it's a means of family income it also likely tells us that there are limited educational and economic opportunities for women now if the pre-reproductive portion of the population is just a little bit bit larger than the reproductive age cohort it tells you that the country is still growing but that the growth rate is slowing down so the economy is likely becoming a little more industrialized and Healthcare and educational access are increasing for the population as the pre-reproductive and reproductive age cohorts become equal or nearly equal it tells you that the population growth rate is very slow and that the country is nearing a stable population size the economy is likely industrialized and Healthcare and education are widely accessible especially to women this in turn causes them to have fewer children as they gain e iomic Independence and delay child birth to pursue education or career opportunities and finally when a country's economy becomes really highly developed we actually see a growth rate slow down so much that it can become negative this is because people are having fewer children than it takes to replace themselves these age structure diagrams have larger portions of the population in reproductive and post-reproductive age than in pre-reproductive age this is both because birth rate is so low and because their life expectancy has increased with this widespread access to Quality Healthcare so we we've touched on birth rate a bit as we've talked about age structure diagrams but now we'll take a deeper look at the factors that influence a country's birth rate first we have to remember that total fertility rate or TFR is the average number of children that a woman in a given population will give birth to in her lifetime so thinking back to age structure diagrams those countries with really wide bases or really large proportions of pre-reproductive age cohorts those countries will have a really high TFR or total fertility rate but what are the factors in a given country that can influence the TFR well one of the biggest factors is the average age of a woman when she has her first child the earlier in life a woman has her first child the more children she'll have over the span of her life now if we go deeper into the factors that influence the age of first child birth we'll see a mix of economic social and cultural factors at play so if a country is still undergoing industrialization and has a large portion of the population living in an agrarian or farming Society having more children means more labor on the Family Farm And therefore more income these countries also often lack an education system to provide women with knowledge about academic or career skills or even knowledge of their own reproductive systems and family planning this limits their ability to be financially independent and to make decisions about when to have children and how many children to have now while these are objective truths of demography or of the study of human populations we need to be careful that we don't make the assumption that women in developing nations are just having large numbers of children because there's no other option and many developing nations cultural and religious values look really favorably on having many children women and families may be intentionally choosing to have larger families for economic religious or cultural reasons so what factors explain why developed Nations have such a lower TFR than developing nations well one of the big reasons is that as countries develop economically they also develop from a healthcare and an educational standpoint so people are beginning to leave behind the agrarian lifestyle for more manufacturing or service sector jobs these are Industries where having a large family isn't as necessary economically women are also starting to go to school longer in these countries which often leads to delayed childbirth as they pursue educational or career opportuni unities in fact this is one of the strongest factors that decreases TFR we see a huge inverse relationship between the number of years women spend in school and the average number of children they have now with this educational and Economic Opportunity for women also comes increased knowledge of and access to family planning resources these are things such as contraceptives like oral birth control or condoms in fact sometimes governments even try to intervene to lower the TFR in their countries by conducting public education campaigns about family planning or offering women economic opport unities such as a microc credit loan if they wait until later in life to have children or there's the China route of essentially mandating lower birth rates by forcing families with more than one child to pay an extremely heavy tax rate although we should note that China's growth slowed so significantly that that number has now been increased to two Beyond just TFR there's a whole host of other demographic characteristics that we need to review when we think about human populations the first two we'll take a look at are two of the most basic and that's CBR crude birth rate and CDR crude death rate this is the number of births or deaths per 1,000 people in a population now since these numbers are out of 1,000 we can easily turn them into a percent which means out of 100 to figure out the percent growth rate of a country we subtract CDR from CBR and then divide that number by 10 because crude birth rate and death rates are per 1,000 and we want a percent which means per 100 so let's try a practice problem here quickly pause the video and see if you can find the percent growth rate of the global population using the global CBR and CDR data from 2019 all right let's see how you did we're just going to plug these numbers into the formula and so that means we'll subtract 7.52 or the CDR from 17.89% and that gives us a global population growth rate of 1.37% another type of calculation we need to do with populations is to determine the doubling time now there's a fun little Apes trick for this called the rule of 70 and all it requires is that you divide 70 by the growth rate as a % of the population so if we wanted to figure out how many years it will take for the Earth's human population to double we would just divide 70 by 1.37 and we'd find that it should take about 67.5 years now this brings up a great question can Earth's current human population even double could we reach 15.8 billion people or whatever double the population is when you're watching this video don't humans have a caring capacity eventually and the answer is depends who you ask if you asked Thomas mathus this question in 1978 he would have told you that the Earth's population will reach a caring capacity and experience a massive dieback due to famine as he predicted that Earth's population would outpace its ability to grow food for those people now malus idea which we call malthusian theory was based on the fact that Earth population was growing exponentially while food supply was growing linearly but what Malthus didn't account for were technological advances in agriculture like synthetic fertilizers or pesticides these agricultural advancements ushered in by the Green Revolution increase Earth's food supply and effectively increase Earth caring capacity for humans so in addition to understanding how birth rate and death rate influence population growth we need to take a look at a few other demographic characteristics the first and maybe most important is access to clean drinking water when countries don't have access to clean drinking water they tend to have really high infant mortality rates this is because children are especially susceptible to waterborne diseases like dissenter or chalera and this might seem counterintuitive but a high infant mortality rate actually increases a country's TFR because TFR is just the total number of children born in a country and so mothers are actually having replacement children to replace the number of children that died due to these waterborne diseases and this brings up another Apes term we need to know which is replacement level fertility replacement level fertility refers to the average number of children that a woman would need to have for the population that she's a part of to remain stable in developed Nations that number is close to 2.1 so a mother would need to have two children one to replace herself one to replace the father and the point one because there are still some children that die in developed Nations and not all couples have children in developing nations on the other hand replacement level fertility will typically be much higher than 2.1 a family may need to have three or four children total in order to have two survive to replace them and the final population characteristic we need to review is GDP or gross domestic product this refers to the total market value of all finished goods and services a country produces divided by the total number of individuals in that country GDP is basically a way of looking at how wealthy a country is the higher GDP the more productive its economy is the more wages its citizens are earning and typically the more advanced it is from both a healthcare and educational standpoint so now that we've covered all these different human population characteristics we have to tie them all together with the theory of demographic transition now this theory has a bunch of different terminology and many different visual representations but the core idea Remains the Same over time countries undergo industrialization which leads to a falling death rate a rapidly growing population size and eventually a falling birth rate as well now demographers break this transition down down into five stages but in Apes the College Board really only focuses on the first four stages so that's what we'll review in this video in Phase One countries are considered pre-industrialized so they're typically still relying on agriculture we call this an agrarian economy and child labor is still an important source of family income we can see in the graphic that birth rates are high but death rates are also High leading to very little population growth if we look at the age structure diagram of these countries they're pyramid shaped but with an extremely wide base that quickly tapers off with each older age cohort in phase two countries are industrializing or transitioning from this agrarian economy to more manufacturing and service-based economy we typically refer to stage one and two countries as developing countries population growth is very high in these countries because the death rate Falls dramatically industrialization brings clean water and access to health care which is increasing the life expectancy of the population but we want to note here that birth rate remains high for a couple reasons one is just cultural momentum it takes time often a generation or two for for people to adjust to the idea of having fewer children there's often still a shortage of educational or career opportunities for women or lack of access to contraceptives in these stage two countries all of these factors contribute to this High birth rate in stage three countries the process of industrialization is considered complete and we often call these developed or industrialized countries population growth rate slows because the birth rate is now declining closer to the death rate women are likely receiving increased educational and career opportunities and both men and women are spending longer in school which which causes them to delay having children until later in life these stage three countries generally have a TFR that is approaching the replacement level of 2.1 stage four countries are considered highly developed or sometimes even post-industrial this is because citizens have such a high access to educational and career opportunities as well as family planning resources bringing TFR well below replacement level this means the country May begin to experience a negative growth rate or population decline now you can spot this in the age structure diagram because the base or the pre-reproductive age cohort will be smaller than the reproductive age cohort so there you have it ape Scholars unit three is in the books we covered everything from raccoons and panda bears to GDP and Family Planning and remember to make sure you're actually printing out and filling out the ultimate review packet that goes with each unit especially in unit 3 where you want to practice these population growth rate and rule of 70 problems thanks for watching today and as always think like a mountain and write like a scholar