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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