K and r Reproductive Strategies Each species on the earth employs

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K and r Reproductive Strategies
Each species on the earth employs a different strategy to ensure that they leave offspring behind before they die.
The organism’s genes usually reinforce this strategy and so it is difficult for most organisms to deviate much from
their inherited strategy. There are two fundamental types of reproductive strategies and they can be described as
antagonistic – meaning that when you are reliant on one you normally use less of the other. These two strategies
are the K strategy and the r strategy. K strategists are often said to be “K- selected” and r strategists are said to be
Biologists often refer to a “spectrum” or continuum that helps us distinguish one organism’s reproductive strategy
from another. All living organisms are supposed to fall somewhere on this continuum between the two extremes (r
and K). Also, organisms can be differentiated from one another in terms of their relative reliance on one strategy
over the other.
r ____._________._________.______.________.__________._________._________._______.____ K
mollusks insects fish
amphibians reptiles mammals apes humans
Those organisms described as r-strategists typically live in unstable, unpredictable environments. Here the ability to
reproduce rapidly (exponentially) is important. Such organisms have high fecundity and relatively little investment in
any one progeny individual; they are typically weak and subject to predation and the vicissitudes of their
environment. The “strategic intent” is to flood the habitat with progeny so that, regardless of predation or mortality,
at least some of the progeny will survive to reproduce. Organisms that are r-selected have short life spans, are
generally small, quick to mature and waste a lot of energy.
K-strategists, on the other hand occupy more stable environments. They are larger in size and have longer life
expectancies. They are stronger or are better protected and generally are more energy efficient. They produce,
during their life spans, fewer progeny, but place a greater investment in each. Their reproductive strategy is to grow
slowly, live close to the carrying capacity of their habitat and produce a few progeny each with a high probability of
survival. Typical K-selected organisms are elephants, and humans. The table below summarizes some of the
differences between r-organisms and K-organisms.
Life Span
Energy Usage
Litter Size
Age at Reproduction
Maturation Speed
Investment in Offspring
Size at Birth
An organism’s behavior determines its chances for reproductive success and therefore the r-strategy is by far the
more robust strategy. The behavior of an r strategist is dictated by its genes, which are naturally selected over
geological time. This means that r strategists are self-sufficient and their behavior rarely goes wrong. The behavior
of a K strategist on the other hand is completely dependent on the lessons from its parents and so extreme K
strategy is very risky. It is not surprising that many organisms cannot be categorized neatly into this r vs. K scheme.
Many organisms adopt an intermediate strategy or even adopt different strategies depending on local conditions at
any given time. In fact, an organism capable of alternating between an r-strategy and a K-strategy might well be the
best-fit organism over all because its adaptability permits survival under a broader range of conditions.
Adapted from http://www.cs.montana.edu/webworks/projects/stevesbook/contents/chapters/chapter002/section004/blue/page003.html
1. Evaluate the strengths and limitations of r-strategists and K-strategists.
2. Compare and contrast the type of environment to which r-selected and K-selected species are adapted.
3. Explain whether K- or r- strategists would be regulated by density-independent factors.
4. Explain whether K- or r- strategists would be regulated by density-dependent factors.
5. Describe and explain the shape of a survivorship curve for K-selected species.
6. Look at the survivorship curves below and describe a species that would fit each of the curves.
Adapted from http://www.cs.montana.edu/webworks/projects/stevesbook/contents/chapters/chapter002/section004/blue/page003.html

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