3.2 - K-Selected r-Selected Species

Learning Objectives

Introduction: Two Approaches to Reproduction

Biotic Potential

• What sets it: How many offspring are born at a time, how often the organism reproduces, how young it is when it starts, and how many of its young survive to reproduce themselves.

• High vs. low: r-selected species have a very high biotic potential. A house mouse can start breeding at about 6 weeks old and have a new litter every few weeks. K-selected species have a low biotic potential. An African elephant is pregnant for about 22 months and usually has one calf every 4 to 5 years.

• Where the names come from: In the population growth equation, r is the growth rate and K is the carrying capacity. r-selected species are shaped by evolution to grow fast (high r), and K-selected species to compete well in a population that stays near its carrying capacity (K).

K-Selected Species (Quality)

• Size and lifespan: Typically larger, longer lived animals, like large mammals.

• Maturity and reproduction: They take a long time to reach sexual maturity. However, they often get the chance to reproduce many times over their long lifespan.

• Population growth: They have a lower biotic potential (maximum reproductive rate) and slower population growth. Their populations are usually relatively stable and tend to exist near their carrying capacity (maximum population size).

• Energy per offspring: They put a lot of energy into each offspring: long pregnancies, nursing, and years of protection and teaching. A young chimp nurses for about 4 to 5 years and stays with its mother for years after that.

• Environment and competition: They live in fairly stable environments, where the population stays near carrying capacity. Because the habitat is usually “full,” competition for resources like food, territory, and mates is relatively high, so individuals that are large, strong, and well cared for have the edge.

• Examples: Elephants and chimps.

• Slow recovery: Their slow reproductive rate makes them slower to repopulate after an environmental change or disturbance like a disease or a forest fire.

• Double whammy: The death of a parent often means the death of their vulnerable offspring too, which hits the population size even harder.

• Extinction risk: K-selected species are less likely to adapt to rapidly changing environments because they have longer generation times and smaller population sizes, which means less genetic diversity and therefore less adaptability. As a result, they’re far more likely to go extinct than r-selected species.

• Invasive species hit them hardest: Because K-selected species replace their losses slowly, they can’t bounce back when an invasive predator or competitor shows up. On Pinzón Island in the Galápagos, introduced black rats ate nearly all giant tortoise eggs and hatchlings for more than a century, so almost no young tortoises joined the population. The tortoises held on only because adults live so long and scientists raised young tortoises in captivity and returned them. After the rats were wiped out in 2012, the first tortoises hatched in the wild there in over 100 years were found in 2014.

r-Selected Species (Quantity)

• Size and lifespan: Generally smaller, shorter lived organisms like insects, fish, and plants.

• Maturity and reproduction: They reach sexual maturity very quickly. Since they often don’t live long enough to reproduce many times, they have to make their one reproductive event count.

• Population growth: They have a very high biotic potential and experience very fast population growth. However, their population sizes are very variable, subject to dramatic increases or decreases.

• Environment and competition: They’re common in unstable or unpredictable environments, like temporary ponds, recently burned or cleared land, and places with big seasonal swings. Disturbances keep their populations well below carrying capacity much of the time, so competition for resources is usually relatively low. The species that wins is the one that reproduces fastest.

• Reproducing once: Some r-selected species put everything into one reproductive event and then die. Pacific salmon swim upstream, lay thousands of eggs, and die soon after spawning. Many weeds are annual plants that grow, make seeds once, and die within a year.

• Examples: Spiders, fish, and oysters.

• Invasive potential: r-selected species are more likely to become invasive species because their populations grow so rapidly.

• Out-competition: When they move into a new ecosystem, they can often out-compete slower K-selected species for limiting resources like food or water.

• Example: Zebra mussels reached the Great Lakes in the 1980s in ships’ ballast water. They mature quickly, and a single female can release hundreds of thousands of eggs a year, so they spread fast through rivers and lakes, crowding out native mussels.

The r-K Spectrum

• Some species, like the frog and the hare, fall somewhere in the middle.

• The hare produces far more offspring than a chimp (several litters a year), but as a mammal she still nurses her young, which most fish and insects never do. Most frogs are r-selected (many species lay hundreds or thousands of eggs and leave them), but some give real parental care. Poison dart frogs, for example, lay small clutches of eggs and carry their tadpoles on their backs to small pools of water.

• Mixed traits: Sea turtles are large and can live for decades (K traits), but each female lays around 100 eggs per nest, leaves them on the beach, and gives no care, so most hatchlings die young (r traits).

• Strategies can change with conditions: The same species can shift along the spectrum. White-tailed deer with plenty of good food breed younger (some females breed in their first year) and often have twins. In crowded, overgrazed areas, fewer young females breed and more does have only one fawn. So when resources are abundant and the population is below K, deer reproduce more like r-strategists, and near K they act more like K-strategists.