Environmental Engineering Reference
In-Depth Information
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R-selected:
R-selected species are small organisms that have short gestation periods and produce thou-
sands of offspring at one time; therefore, they have high biotic potential. Energy and resources are put
into producing many offspring and not into raising the young. This strategy means that the young are left
to their own survival, and survival depends on chance. R-selected species are short-lived and have popu-
lation sizes that vary, usually not remaining near carrying capacity but well below it. Examples include
spiders, fish, and frogs.
Not all species fit into one of these two strategies. Some organisms fall in between.
Survivorship
Survivorship curves
(like the one shown on the next page) represent the number of individuals surviving at
each age for a given species. The
y
-axis shows the number of individuals and the
x
-axis reflects time or age.
The three types of survivorship curves are
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Type I:
Organisms that reproduce at a relatively young age, have a small number of deaths at young ages,
have a long lifespan, and experience mortality mostly at an older age. Examples include humans and
most large mammals.
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Type II:
Organisms that mature quickly and have even mortality rates at all ages. Examples include ro-
dents, many reptiles, and most birds.
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Type III:
Organisms that have many offspring, and reproduce often. Many individuals die at an early
age, and there is less mortality later in life. Examples include sea turtles, parasites, and most insects.
Human Population
