In this video we are going to look at the difference between r and K Selection.
r and K selection refer to two different strategies adopted by species in relation to parental
investment and offspring survival.
This is the trade off of quantity versus quality.
The terms r-selected and K-selected are based on the model of population dynamics.
In population dynamics we look at the number of the population N over time t.
In this relationship we can look at the rate of change in the population r.
We can also look at where the population reaches its maximum capacity also known as the carrying
capacity K. K-selected species or K strategists get their
name because they live in stable environments and have populations that lie close to the
carrying capacity K. R-selected species or r-strategists on the
other hand tend to live in unstable environments, have low survivability of offspring and short
lifespans.
This means their population is usually much smaller than the carrying capacity.
They are governed by high rates of population growth, that then drop back down due to the
unstable conditions and low survivability.
There are several traits that help determine whether a species is an r selected species
or a K-selected species.
r selected species adopt the strategy of producing large amounts of offspring, whilst providing
little or no parental care or investment to these offspring.
In this strategy each individual offspring has less chance of surviving into adulthood,
but a few survive.
They are often living in unstable environments.
They are usually small in size.
They usually reach maturity early, and often only reproduce once in their lifetime.
R-selected species also have short life expectancies and show a type III survivorship pattern,
where most die quickly but a few survive longer.
An example of an r-selected species is oysters.
Oysters produce hundreds of millions of fertilized eggs out into the ocean, where they provide
no parental care, they are small, and most don’t survive very long due to the unstable
rough waters and high levels of consumption by consumers.
K selected species on the other hand adopt the strategy of producing only a small number
of offspring but providing a large amount of parental care and investment to these offspring.
In this strategy most offspring survive into adulthood.
They are usually living in a stable environment.
They are usually large in size.
They reach maturity and reproductive age much later in life, and spend a lot of energy to
create their offspring, producing just a few of these high energy offspring.
They also usually reproduce more than once in their adult lives.
They have much longer life expectancies and show a type I or II survivorship pattern where
individuals usually survive most of their potential lifespan.
An example of a K-selected species are the elephants.
They are large, produce usually just a single large offspring which takes 18-22 months depending
on the species to gestate inside the mother.
Then it also requires 5-10 years of weaning and post birth parental care, often getting
care for up to 16 years.
They live long lives, and have multiple offspring throughout this long lifespan.
These two strategies are the two extremes, most species fall somewhere on a spectrum
between the two.
Survivorship curves are a useful tool to illustrate the differences in survivability for each
strategy and those that fall somewhere between a these strategies.
In this graph we plot the percentage of surviving organisms on the Y-axis, against the percentage
of their potential lifespan on the x-axis.
Strict r-selected species such as the oyster show a Type III survivorship curve.
The numbers of individuals surviving decreases rapidly and leaves only a few remaining to
the end of their lifespan.
Strict K-selected species such as elephants show a Type I survivorship curve.
Most survive into adulthood and until the end of their lifespan.
Those in between such as in many songbirds, show a Type II survivorship curve
They have a more steady and linear level of decline of survivorship over their potential
lifespan.
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