Opportunist or r-selected species
Each species has a characteristic mode of reproduction. At one extreme are species that reproduces early and
put most of their energy into reproduction. They:
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
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have many (usually small) offspring each time they reproduce,
reach reproductive age rapidly
have short generation time
give their offspring little or no parental care of protection to help them survive,
are short-lived (usually with a life span of less than a year)
Species with this reproductive pattern overcome the massive loss of their offspring by producing so many
unprotected young that a few will survive to reproduce many offspring to begin the cycle again.
Species with such a capacity for a high intrinsic rate of increase (r) are called r-selected species. Algae, bacteria,
rodents, annual plants (such as dandelions) and most insects are examples. Such species tend to be
opportunists. They reproduce and disperse rapidly when conditions are favorable or when a disturbance
opens up a new habitat or niche for invasion, as in the early stages of ecological succession.
Changed environmental conditions from disturbances can allow opportunist species to gain a foothold.
However, once established, their populations may crash because of changing or unfavorable environmental
conditions or invasion by more competitive species. Therefore, most r-selected species or opportunist species
go through irregular and unstable boom-burst cycles in their population size. To survive, opportunists must
continually invade new areas to compensate for being displaced by more competitive species.
Opportunist or r-selected species
Each species has a characteristic mode of reproduction. At one extreme are species that reproduces early and
put most of their energy into reproduction. They:





have many (usually small) offspring each time they reproduce,
reach reproductive age rapidly
have short generation time
give their offspring little or no parental care of protection to help them survive,
are short-lived (usually with a life span of less than a year)
Species with this reproductive pattern overcome the massive loss of their offspring by producing so many
unprotected young that a few will survive to reproduce many offspring to begin the cycle again.
Species with such a capacity for a high intrinsic rate of increase (r) are called r-selected species. Algae, bacteria,
rodents, annual plants (such as dandelions) and most insects are examples. Such species tend to be
opportunists. They reproduce and disperse rapidly when conditions are favorable or when a disturbance
opens up a new habitat or niche for invasion, as in the early stages of ecological succession.
Changed environmental conditions from disturbances can allow opportunist species to gain a foothold.
However, once established, their populations may crash because of changing or unfavorable environmental
conditions or invasion by more competitive species. Therefore, most r-selected species or opportunist species
go through irregular and unstable boom-burst cycles in their population size. To survive, opportunists must
continually invade new areas to compensate for being displaced by more competitive species.
Competitor or K-selected species
At the other extreme are Competitor or K-selected species. These species




put fairly little energy into reproduction.
tend to reproduce late in life
have few offspring with long generation times,
put most of their energy into nurturing and protecting their young until they reach reproductive age.
Typically these offspring




develop inside their mothers (where they are safe),
are fairly large
mature slowly
are cared for and protected by one or both parents until they reach reproductive age.
This reproductive pattern results in a few big and strong individuals that can compete for resources and
reproduce a few young to begin the cycle again.
Such species are called K-selected species because they tend to do well in competitive conditions when their
population size is near the carrying capacity (K) of their environment. Their populations typically follow a
logistic growth curve. Examples are most large mammals, birds and large and long-lived plants (such as the
saguaro cactus, oak trees, redwood trees, and most tropical rain forest trees). Many K-selected species,
especially those with long generation times and low reproductive rates (such as elephant, shark, and
rhinoceroses), are prone to extinction. Most competitor or K-selected species thrive best in ecosystems with
fairly constant environmental conditions.
Competitor or K-selected species
At the other extreme are Competitor or K-selected species. These species




put fairly little energy into reproduction.
tend to reproduce late in life
have few offspring with long generation times,
put most of their energy into nurturing and protecting their young until they reach reproductive age.
Typically these offspring




develop inside their mothers (where they are safe),
are fairly large
mature slowly
are cared for and protected by one or both parents until they reach reproductive age.
This reproductive pattern results in a few big and strong individuals that can compete for resources and
reproduce a few young to begin the cycle again.
Such species are called K-selected species because they tend to do well in competitive conditions when their
population size is near the carrying capacity (K) of their environment. Their populations typically follow a
logistic growth curve. Examples are most large mammals, birds and large and long-lived plants (such as the
saguaro cactus, oak trees, redwood trees, and most tropical rain forest trees). Many K-selected species,
especially those with long generation times and low reproductive rates (such as elephant, shark, and
rhinoceroses), are prone to extinction. Most competitor or K-selected species thrive best in ecosystems with
fairly constant environmental conditions.
Survivorship curves
Individuals of species with different reproductive strategies tend to have different life expectancies, or expected
lengths of life. One way to represent the age structure of a population is with a survivorship curve, which
shows the percentages of the members of a population surviving at different ages. There are three generalized
types of survivorship curves: I, II, and III (sometimes known as late loss, early loss, and constant loss
respectively).
In populations with a type 1 survivorship
curve, most individuals survive until old age.
In populations with a type 2 survivorship
curve, individuals have a constant chance of
dying throughout their lives.
In populations with a type 3 survivorship
curve, most individuals die young.
Type I individuals survive well early in life and
generally live many years. At an advanced age,
the death rate increases dramatically. Examples
include large mammals.
Type II individuals have a death rate that is
relatively constant at any age. Examples include
lizards, hydra, and some small mammals.
Type III individuals initially have a rather low
chance of survival. Those that do survive may
live to an advanced age. Examples include many
fish and other marine organisms.