Organisms have evolved two very different strategies for life and reproduction. In this lesson, we will examine one strategy called r-selection, and provide some examples of organisms that employ this strategy.
Have you ever wondered why elephants only have one baby at a time but mice seem to have twelve every other month? Or why the dandelions in your backyard spring up overnight and release thousands of seeds into the air, while the oak tree never seems to get any bigger and only occasionally drops acorns directly beneath it?The answer lies in the life history strategies of these different species and the niches they have evolved to fill. Life history strategies can include an organism’s growth, reproduction, and survival rates.
K and R Selection
Life history strategies can be split into two broad categories: R-selection and K-selection. These letter designations come with mathematical equations used to predict and model population growth. The exact equations are not important for this lesson, but knowing the parameters these letters represent can be helpful.
- R represents the population growth rate.
- K represents the carrying capacity, which is the number of individuals of a species that the resources in a habitat can support.
The term selected refers to which of these factors the organism is trying to maximize. Possessing traits to increase the growth rate would be R-selected traits. Possessing traits to ensure that the population of a species doesn’t exceed the carrying capacity would be K-selected. Of course, the process of evolution selects these traits; the organisms themselves don’t ‘choose’ which life history they want to follow.
For this lesson, let’s focus on R-selection and some examples of R-selected species.
The best way to look at R-selected species is to use an example: the dandelion (genus: Taraxacum).I’m sure everyone has seen a dandelion, the yellow flowers that go on to produce floating, white seed heads.
Dandelions are considered weeds that some people go to great lengths to exterminate from their yards, usually only temporarily! Dandelions are only found in yards, fields, or recently disturbed areas where they can get enough light for growth without being shadowed by larger plants.Listing the life history characteristics of the dandelion might as well be the exact definition of R-selection.Dandelions are relatively small, grow very rapidly, and have a short lifespan. They reach sexual maturity rapidly, produce a large number of seeds that can disperse long distances via wind, and then die. Most of the seeds never reach a favorable place to germinate, but since the dandelion releases so many, it is inevitable that some will survive and grow. The small dandelion is unable to compete with other, larger species, making the excess of seeds a necessity.These characteristics describe R-selected species perfectly:
- Rapid development
- High reproductive rate
- Early reproductive age
- Small body size
- One reproductive cycle
- Short lifespan
- Poor competitive ability
- High mortality of offspring
- Population below the carrying capacity of the habitat
- Good offspring dispersal
- Found in disturbed habitats
- Limited or no parental care
Remember, R-selection represents the population growth rate.
The rapid development and numerous offspring of R-selected species maximizes the population growth rate. Just remember our example of a weed. The success of the weed depends on its ability to maximize the population growth rate. Weeds have evolved to grow fast, reproduce prodigiously, and die quickly. They invade disturbed areas like cleared forests, drained ponds, or areas of fire damage, and then grow and reproduce before slower developing competitors can overshadow them.
Examples of R-Selected Species
Using the checklist here, you can predict which species are R-selected. In the plant kingdom, generally any weeds like dandelions, goldenrod, poison ivy, ragweed, and sumac employ R-selected life history strategies.
There are also animals that have the R-selected characteristics. Many aquatic organisms like some species of fish, frogs, and salamanders have relatively short lifespans, reach sexual maturity rapidly, and reproduce by releasing hundreds of eggs with only a few surviving to adulthood. Arthropods like insects, spiders, and crustaceans are also R-selected. Even some small mammals employ R-selection. Mice have short lifespans relative to other mammals, and they produce frequent, large litters with only a small percentage ever reaching adulthood.Not all R-selected species must have all these traits, however. The higher vertebrates like mice, fish, and amphibians are capable of multiple litters, and some species demonstrate parental care.
In general, if a species demonstrates most of the traits, it can be classified as R-selected.
Species that employ R-selection are perfectly evolved to move quickly into an area, pass on their genes, and get out of the way when more competitive species arrive on the scene. The next time you go out to rid your yard of dandelions, stop and think for a minute about what those weeds evolved to accomplish. That minute might just be the extra time that dandelion needs to ensure its descendants a place in your yard for generations to come.
In this lesson we learned about life history strategies, specifically the R-selected traits of the dandelion. R-selected traits are those biological traits that help increase a population’s growth rate. The R in R-selected represents population growth rate. Dandelions are particularly good examples of R-selected traits because they have several of the physical characteristics associated with R-selected traits including: rapid development, small size, and short lifespan.