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r and K selection

Learn about r-selected and K-selected species, different types of survivorship curves, and why the environment matters.

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  • blobby green style avatar for user Ankit Pradhan
    Is the statement correct?

    "r- selected species have high r value and low k-value
    k- selected species have low r and high k-value"
    (1 vote)
    Default Khan Academy avatar avatar for user
    • leafers ultimate style avatar for user Travis Fisher
      Generally speaking that statement is correct. In environments that are constantly changing, meaning a usually low k-value, r-selected species will flourish due to their high fecundity. Their quick reproduction allows them to take advantage of an environment that can change very quickly. Stable environments invite k selected species which will be able to harness the environment for a longer period of time. Their slow reproduction allows the environment to not be overly exploited.

      However, it isn't always so black and white. Instead of being binary, there is a continuum between each end. Mr. Anderson notes this nearing the last minute of this video when he talks about sea turtles that reproduce like a r-species, yet have long life spans like a k-species.
      (1 vote)
  • leafers sapling style avatar for user noahfish
    i had a homework question: a lot of ecologist believe that it is a common situation that in some eco-systems (rain forests and reefs) the K , r and alpha of competitor species are the same. what is the Implications of that kind of situation, what will happen to the size of the populations and to what state the ecosystem converge
    (1 vote)
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Video transcript

it's mr. Andersen and this podcast I'm going to talk about R and K selection or R selected and K selected species right here we've got a chameleon female that's just like two eggs right here we have a frog that's just laid thousands of eggs one of these is an R selected one is a K selected if you know exactly which is which right now then you probably don't need to watch the rest of the video but for those of you that are now left let me talk about R in case selection basically we have to talk about population growth and so any population where you have a positive R and R stands for growth rate if you ever have it a positive R you're gonna follow a curve that looks kind of like this you'll first of all have exponential growth and then eventually you'll run out of space or food there's going to be competition and eventually you're going to hit what's called logistic growth and so we call this the J shaped curve the exponential or the sigmoid alure the S shaped curve which is logarithmic and so what is n and is going to be the number of individuals in the population and so right here if we put n on this side and we put time on the x-axis as time goes on the numbers are going to increase so this would be a low n value right here and this would be a higher end value and this would be a really high end value and so it's important that you know what n is that's the number in the population R is the growth rate remember if it's ever positive and how do we calculate growth rate it's basically the number of people being born minus the number of people who died if we ever have a positive one then we're gonna get this curve now it's not always radical like this sometimes it's going to be really gradual like this sometimes it's gonna be incredibly quick depending on what the R value is and then the last thing is K K stands for carrying let's try to spell that right capacity and so basically that means in an area for a certain group of pop a certain population there's like a maximum amount that they can support and so we call that the carrying capacity and that's where the limit is it's eventually gonna level out on that and so if I were to talk about in the real world cuz this is just a math formula basically in the real world we put Wolves back in Yellowstone Park and so there was tons of food they're not a lot of competition and so basically their growth rate went up and then it's dropped and it's gone up and it's dropped and it's kind of reached this carrying capacity a maximum number of wolves that you're gonna have in Yellowstone Park and so it's not as smooth as that but it certainly has an r-value the N value is increased and then it's reached kind of a K so it's important that you know what n R and K are another thing that's important is that you understand the way life exists this is the story of life and death is gonna vary and we really have three different types so you happen to be a type 1 survivorship curve and what that means is think about all the kids in your class or all the kids who are born on the same day that you were basically most of you are gonna survive that first day and most of you are gonna survive now a few people will die right away but most you're gonna survive you're gonna make it through your teenage years and your middle ages and eventually you're going to drop off and all of you are gonna die in these last 10 15 20 years that's because we're type 1 we're humans and we give a lot of you could see this right here parental care to our offspring now if we look at a frog here's some frog eggs down here they show what's called a type 3 survivorship curve on day one thousands of little frogs almost all of them die but a few of them survive and then they're gonna level off and then they're eventually all going to die now we also have something in the middle type too we call that songbirds are an example of that and so they're gonna die off at a linear rate throughout their whole lifecycle and so Robins are an example of that Robin's on average live about a year but some live a few months and some live 3 years but on average is just kind of drop off until they're all dead the ones the cohort the ones that are born on that one specific day remember our environments are going to differ sometimes we're gonna live in environments that are relatively stable like a desert and sometimes not so much like a rain forest or even a coral reef especially right now they're changing radically almost every day and so basically let's get to what and our selected species is are selected are going to be those that base their life on our and remember our is growth rate and so these are species that want to quickly grow and exploit a niche as as fast as they can and so these are things that normally in an unstable environment they have - daddy what does that mean fecundity basically means is that they can make a lot of babies really really quickly in general these things are small they don't live that long and their offspring just kind of spread to the ends of the earth and so an example of this would be a dandelion they show up really early in the spring they reproduce quickly if you've ever seen a dandelion when it goes to seeds the Caesar just spread out given very little parental care bacteria insects diatoms which are algae or type of this or squid babies are essentially laid by the females males put sperm all over that you have these packets and then the parents just head away they just leave and then eventually all the squid are born most of them are food for fish but some of them are able to survive and so that's the are selected species you're putting your life in growth rate so what's K selected species well these things are going to put their lives basically around carrying capacity they're in it for the long haul then it they're in it for exponential and then that logistic growth they want to live a long time generally they live in a more stable environment they're larger as far as body size they're gonna live a lot longer there's a lot of parental care so you as a human are going to be a K selected species if we look at plants compare this to the dandelion a tree like this some trees will live hundreds if not thousands of years or whales are an example of K selected species but you don't necessarily have to be big this is an Arctic Tern that lives decades and they have a lot of parental care and so they're k-selected species know this that in life it's not always one or the other there are some species that actually show a continuum and so a sea turtle you might look at a sea turtle it's big lives a really long time you might say well that's maybe a k-selected species but then if you remember of how they're born they're just gonna lay a bunch of eggs cover them up in a hole and then just kind of swim away and all of these eggs are gonna hatch these little baby sea turtles are kind of on their own and so that would be more of an are selected and so even within a lifetime you can be R and K selected and even by gender so if you think about it not not humans per se but in in animals males tend to live more than our selected species produce as many as they can spread as much of their sperm as far as they can whereas females are more taste selected investing a lot in those offspring in that long haul so let's go back and look at that first slide the chameleon is it R or K selected well it's investing in just these two eggs and so it's in it for the long haul where this would be an R selected species and I hope that cell phone