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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
Have you ever taken a walk through a hardwood forest? You probably noticed the many large ash, oak, and maple trees, reaching up into the sky and shading the understory. On the ground, you might have seen a few ferns, tree saplings, and other plants adapted to this lower light environment. You could leave this forest today, return in ten years, and notice that everything looks nearly identical.
Now, consider a walk through a field filled with grasses, dandelions, goldenrod, and sumac trees. If you leave this field and return even a few months later, you might be surprised how much things have changed. Suddenly, there are five-foot-tall sumac trees where there was only goldenrod. And all the dandelions are gone, replaced by ragweed.
So, why does the forest plant population seem perpetually stable, while the field population is constantly changing? The answer lies in the life history strategies of these different species and the niches they have evolved to fill.
K-selection and r-selection are the two broad categories of life history strategies. The life history strategy of a species incorporates aspects of how the organism reproduces, its strategies for survival, how the organism interacts with its habitat, and how it's able to compete with other organisms within the habitat.
The K and r come from mathematical equations used to predict and model population growth. We don't need to know the exact equations for this lesson, but knowing the parameters these letters represent can be helpful.
K represents the carrying capacity, which is simply the number of individuals of a species that the resources in a habitat can support. r represents the population growth rate, which measures how fast a given population grows in relation to the initial population over a set period of time.
The term 'selected' refers to traits employed by the organism to optimize either the carrying capacity or population growth rate. To clarify, traits that ensure the population doesn't exceed the carrying capacity would be K-selected traits. Traits that maximize the growth rate would be r-selected traits. Of course, the process of evolution 'selects' these traits; the organisms themselves don't choose which life history strategy they want to follow.
For this lesson, let's focus on K-selection and some examples of K-selected species.
Elephants are the classic example for examining K-selected traits. Native to Africa and Asia, these large mammals grow very slowly. It can take them upwards of 20 years to reach sexual maturity, and the female elephants, called cows, carry one offspring for nearly two years. The herd then cares for the baby elephant for several more years, to ensure that the infant survives to adulthood.
These traits are perfect to optimize the population size at or near the carrying capacity. Few offspring that take a long time to develop ensure that the population doesn't grow too rapidly, exceed the carrying capacity, or outstrip the resources of the environment. Thus, the elephant is considered a K-selected species.
There are numerous K-selected traits, including:
Considering the aforementioned traits, you should be able to predict which species are K-selected. In our forest example, the large trees (maple, ash, oak, and others) are K-selected. They live a long time, grow very slowly, and can get large enough to outcompete smaller species.
Like the elephant, most birds and mammals develop slowly, care for only a few young over multiple births, and are strong competitors. Some reptiles, like alligators and crocodiles, are also K-selected, living for a long time and laying eggs multiple times. Some even provide early parental care over their eggs and offspring.
There are always exceptions, however. Mice are mammals that are better described as r-selected because they are small, able to reproduce early, and have many offspring with a high mortality rate. It's also important to note that not all K-selected species will have all the traits. Sea turtles, for example, have long lifespans and slow development but produce and abandon many offspring, of which few survive. In general, if a species demonstrates most of the traits, it can be classified as K-selected.
One final important point about K-selected species: the same traits that ensure populations near the carrying capacity also put them at higher risk of extinction. Low reproductive rate and advanced age of sexual maturity make K-selected species unable to quickly replenish numbers in the event of a large number of deaths, be they from disease, habitat destruction, or any other natural or unnatural cause.
Let's pay one final visit to our forest.
The K-selected species there are perfectly evolved to outcompete and survive by forcing out smaller, less competitive members of the ecosystem. There was a time when those maple, ash, and oak trees were small, struggling to grow above the goldenrod and sumac. However, once those trees surpassed them, the r-selected field plants had to find greener pastures, while the K-selected trees have persisted year after year, forming a mature hardwood forest.
K-selection and r-selection are the two broad categories of life history strategies. K represents the carrying capacity, r represents the population growth rate, and selected refers to traits employed by the organism to optimize either the carrying capacity or population growth rate.
Traits that ensure the population doesn't exceed the carrying capacity are known as K-selected traits. This includes characteristics like slow development, low reproductive rate, strong competitive ability, and good parental care, among others. Elephants are the classic example of a K-selected species and are joined in this classification by most other mammals, as well as most birds, some reptiles, and various hardwood trees.
It's important to remember that not all K-selected species will have all the traits noted in this lesson. However, in general, if a species demonstrates most of the traits, it can be classified as K-selected. And while K-selected traits ensure populations near the carrying capacity, they also put those populations at higher risk of extinction.
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Back To CourseGED Science: Tutoring Solution
34 chapters | 724 lessons