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.
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|
|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 rhinocroses), are prone to extinction.
Most competitor or K-selected species thrive best in
ecosystems with fairly constant environmental conditions.
In contrast, opportunists thrive in habitats that have experienced
disturbances such as a tree falling, a forest fire, or the clearing of a forest
or grassland for raising crops.
Many organisms have reproductive patterns between the extremes of r-selected species and K-selected species, or they change from one extreme to the other under certain environmental conditions. In agriculture we raise both r-selected species (crops) and K-selected species (livestock). The reproductive pattern of a species may give it a temporary advantage, but the availability of suitable habitat for individuals of a population in a particular area is what determines its ultimate population size. Regardless of how fast a species can reproduce, there can be no more dandelions than there is dandelion habitat and no more zebras than there is zebra habitat in a particular area.