Population Dynamics

populations will grow and shrink over time for many reasons one of those is the birth rate which is the number of births occurring in a period of time in this graph with number of individuals on the side and time on the bottom the overall birth rate is increasing which means the population is likely also increasing death rate or mortality rate is the number of deaths in a period of time in this diagram the death rate is dropping but what does that actually mean that means that less people are dying as time goes on this also means that the population will likely increase the simple way to find out if a population is growing is to calculate the births minus the deaths if the number is positive the population is growing if the number is negative the population is shrinking but this is perhaps oversimplified in reality there's also immigration and emigration to consider so here's a more realistic growth rate calculation the growth rate is equal to the births plus the immigration minus the deaths and the immigration you can remember that immigration means individuals coming into a population and emigration is individuals exiting the population immigration in emigration exit the distribution of individuals among different ages in a population is called age structure age structures are often represented in graphs or population pyramids let's look at a population pyramid here's a population pyramid for the United States it's fairly stable and it doesn't actually look that much like a pyramid on the left are the male's of the population and on the right are the females of the population you can see that it's more rectangular than a pyramidal here's another developed country again it has more of a square shape than a pyramid now less developed countries tend to have an actual pyramid shape because they have higher growth rates but don't have as many individuals that live to an old age a survivorship curve is a graph that shows number or proportion of individuals surviving the each age for a given species or group type one survivorship curves produce few offspring and care for the young they have high survival rates of the young and they live out most of their expected lifespan and die in old age examples of type 1 are humans or even large mammals like elephants in the type 2 survivorship curve there's a constant death rate throughout the lifespan an example of this would be coral type 3 survivorship curves have many young most of which will die at a very young age an example would be plants they make lots of seeds but very few of them will actually make it into adulthood now each of these types of survive will have their benefits and their drawbacks for example humans have higher survivorship but they don't make as many offspring if you watch a population begin to grow over time you'll find that the population size expands by ever-increasing increments during successive intervals the larger the population gets the more individuals there are to reproduce which leads to exponential growth this graph is sometimes called a J curve because of the shape of a J that it makes at this point there are unlimited resources and unlimited space but the reality is that at some point space and resources will run out as size of a population increases the rate of reproduction will actually decrease when the population reaches carrying capacity the population growth ceases and you see a flattened line carrying capacity is determined by limiting factors like space food availability and predators this logistic growth curve looks like an S that's been stretched out so this is sometimes called an S curve limiting factors can be categorized as density dependent or density independent factors density dependent factors include resource limitations such as shortages of food or nesting sites and they're triggered by increasing population density with density dependent factors an individual's chance of surviving or reproducing depends on the number of individuals in the same area density independent factors such as weather floods and fires reduce the population by the same proportion regardless of a population size both of these can affect where the care capacity is thanks for watching this episode of teacher's pet don't forget to Like and subscribe and follow me on Twitter at science pet

Channel: Teacher's Pet

Views: 123,816

Keywords: population pyramid, growth rate, survivorship curve, age structure, exponential, logistic, growth, biology, Population Ecology (Field Of Study), Population Dynamics (Field Of Study)

Id: XFpM5e_6rtM

Length: 4min 35sec (275 seconds)

Published: Tue Mar 24 2015