R-Selected Species: Examples & Definition

Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

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.

Reproduction

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: K-selection and r-selection. These letter designations come from 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 simply 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.

r Selection

The best way to look at r-selected species is to use an example: the Dandelion (Genus: Taraxacum).

Dandelions and Seed Head
Photo of Dandelions

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 distance 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 area like cleared forests, drained ponds, or areas of fire damage, grow and reproduce before slower developing competitors can overshadow them.

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