Graphing Population Growth of R-Selected & K-Selected Species

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Every organism faces trade-offs in life, especially when it comes to how they are going to allocate their energy. In this lesson, we're going to talk about two different strategies for growth and reproduction and how they relate to population growth.

Different Life Strategies

Imagine for a minute that you are a cockroach. You are small and nimble, but you live in a very unstable environment. Overall, your chances of living a long, fulfilling life are very small. Now imagine that you are an elephant. You are an enormous creature that lives in a very stable environment, and you fear very little. You can expect to have a life that extends well beyond that of a cockroach. Now think about how this difference in life expectancy might influence your reproductive strategy and population growth. Living organisms reproduce with one specific goal - to produce offspring that will carry on genetic information to the next generation. And because there is such a broad range of living organisms on Earth, there are different ways that they reproduce and grow.

R- versus K-Selected Species

In terms of population growth, organisms may be described as either r-selected or K-selected. R-selected species are things like the cockroach. They are generally small and have short lives, so their best bet is to put their energy into growth. They grow quickly and produce as many offspring as possible to carry on genetic information. If they don't grow and mature quickly enough they may not be able to reproduce. And since mom and dad may or may not be around to provide a lot of parental care, r-selected species produce a whole bunch of cheap or low-quality offspring very quickly, sometimes thousands of them at a time! Other examples of r-selected species include frogs, bacteria, and dandelions.

R-selected species tend to produce a large number of offspring and include creatures such as frogs (eggs) or cockroaches.
r-selected species

On the other end of the spectrum are K-selected species, which are creatures like the elephant. Since they are larger and have longer life expectancies they don't need to put all of their energy into growing and maturing up front. They instead devote their energy to one or two expensive or high-quality offspring that require significantly more care. R-selected species produce so many offspring because it is not likely that many of them will survive. But, K-selected species don't have this worry, and therefore focus more attention on a smaller number of offspring. Other examples of K-selected species include humans, marine mammals, and oak trees.

K-selected species produce only a few offspring.
k-selected species

Graphing Population Growth

It's important to keep in mind that these are two extremes of a continuum. There are small organisms that are K-selected, as well as large organisms that are r-selected. There are also many organisms that fall somewhere in between.

The r and K that we've been using to describe these species actually come from somewhere. When we talk about population growth we use three things:

  • r - the growth rate of the population (how quickly or slowly it changes size)
  • N - the number of individuals in that population
  • K - the carrying capacity, the maximum number of individuals that can be supported by the resources in a given area

Since r-selected species grow very quickly and produce a lot of offspring at once, when we graph this growth it looks like a J. The population increases very rapidly over a short period of time, giving us exponential growth. Remember when you used exponents in math and you got a very large number very quickly? The same principle applies here. Think of the r as meaning 'rapid' growth.

The exponential growth of r-selected species
exponential growth

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