⏳ Estimated Reading Time: 6 - 8 minutes

Identify differences between K- and r-selected species.

K- and r-selected species

Different organisms have different reproductive strategies. You may have noticed this before. Frogs like to lay thousands of eggs and then hop away, hoping that some of them will survive. While mammals tend to have fewer offspring at a time but take care of them.

K- and r-selected species are two types of species that differ in their reproductive strategies and population dynamics.

K-selected Species

Organisms that have a much lower reproductive rate but care for and protect their young until they can compete with adults for resources are known as K-strategists. K-selected species are like the helicopter parents of the animal kingdom. They have few offspring, but they invest a lot of time and energy in raising them. Think of elephants, whales, or humans (well, some humans).

Choose one accordion tab that interests you the most and open it. Open another accordion tab that you have not explored yet. Compare and contrast it with the previous one. Finish by checking the other tabs you have not yet explored:

🐼 A Preference for Stable Environments (click to reveal)

K-strategists survive best in stable environments because they have adaptations that allow them to compete for resources, avoid predation, and invest in their offspring.

🐘 Large Size (click to reveal)

K-strategists are usually larger, stronger, or better protected than r-strategists, which makes them less vulnerable to predators and environmental stresses.

🦧 Long-Lived (click to reveal)

They have longer life spans, which means they can reproduce multiple times and maintain their population size. They have fewer but larger and higher quality offspring each time they reproduce.

🐊 Late Maturity (click to reveal)

K-strategists are late to mature because they have adaptations that allow them to compete for resources, avoid predation, and invest in their offspring.

🐋 Parental Care (click to reveal)

K-strategists have parental care because they invest heavily in their offspring to ensure their survival and reproduction in environments where resources are limited, and competition is high. Parental care can include providing food, shelter, protection, and teaching to the offspring. Parental care can also increase the offspring’s chances of finding mates and passing on their genes.

These species tend to become rarer, endangered, or extinct when humans alter environments and species tend to exhibit logistic growth (we will discuss this in a future lesson in this module). They also have a larger ecological footprint, which means that they use more resources and produce more waste per individual.

Essential Knowledge

K-selected species tend to be large, have few offspring per reproduction event, live in stable environments, expend significant energy for each offspring, mature after many years of extended youth and parental care, have long life spans/life expectancy, and reproduce more than once in their lifetime. Competition for resources in K-selected species' habitats is usually relatively high.

K-selected species are typically more adversely affected by invasive species than r-selected species.

r-selected Species

Organisms that produce massive amounts of young but leave their offsprings' survival to nature are known as r-strategists. R-selected species are like the party animals of the animal kingdom. They have many offspring, but they don’t care much about them. Think of insects, bacteria, or rabbits (well, some rabbits). These organisms tend to exhibit the following characteristics:

Choose one accordion tab that interests you the most and open it. Open another accordion tab that you have not explored yet. How are these characteristics different from the K-selected species? Finish by checking the other tabs you have not yet explored:

🪳 Adapted to Unstable and Unpredictable Environments (click to reveal)

r-strategists are adapted to unstable and unpredictable environments, where they can quickly exploit new resources and produce many offspring.

🦎 Small Body Size (click to reveal)

r-strategists have a small body size because they do not face competition and unpredictable environmental conditions often cause mass mortality, so smaller body size is preferred over larger body size among r-strategists.

PS - can you imagine an elephant having 100 babies at once?

🦐 Short Lifespan (click to reveal)

A short lifespan is beneficial for r-strategists because it allows them to reproduce quickly and exploit new resources before they are depleted or become too competitive. A short lifespan also reduces the risk of predation, disease, and senescence (getting old).

🐞 Many Offspring (click to reveal)

By having many offspring in a short time, r-strategists increase their chances of survival and adaptation in unpredictable environments.

🪼 Low Recruitment (click to reveal)

Low recruitment is beneficial for r-strategists because it reduces the competition and predation pressure on their offspring. By producing many offspring and leaving their survival to nature, r-strategists can ensure that some of them will find suitable habitats and resources to grow and reproduce. Low recruitment also allows r-strategists to maintain a high population growth rate and adapt to changing environmental conditions.

These species are often known as weedy or opportunistic species and are also often some of the first species to colonize an area after a disturbance. These species also tend to exhibit exponential growth (we will discuss this in a future lesson in this module). They also have a smaller ecological footprint but can cause ecological problems if they become too abundant and overexploit their resources.

Essential Knowledge

r-selected species tend to be small, have many offspring, expend or invest minimal energy for each offspring, mature early, have short life spans, and may reproduce only once in their lifetime. Competition for resources in r-selected species' habitats is typically relatively low.

r-selected species are minimally affected by invasive species. Most invasive species are r-selected species.

Can a Species be BOTH r- and K-selected?

a photograph of daphnia on a black background — ***When is Daphnia r-selected? K-selected?***

Many species have reproductive strategies that are not uniquely r-selected or K-selected, or they change in different conditions at different times. This means that they can exhibit traits of both r- and K-selection, depending on the environmental factors that affect their survival and

Some species may switch from r-selection to K-selection as they grow older, or as the population density increases. Some species may also have a mixed strategy, where they produce both many small offspring and few large offspring, or where they vary their parental care and maturation time. These species are more flexible and adaptable than strictly r- or K-selected species, and they can cope with changing environments better. Daphnia, shown in the image to the right, make clones in stable environments and in unpredictable environments, they reproduce sexually and produce fewer but more diverse offspring.

Interestingly, humans don't fit a particular profile. They have many traits of K-strategists, such as large size, longer-lived, and taking care of their young. However, the human population also exhibits characteristics of r-selected species, such as high reproductive potential, high adaptability, and low mortality rates.

Essential Knowledge

Many species have reproductive strategies that are not uniquely r-selected or K-selected, or they change in different conditions at different times.

Biotic Potential

a black and white photograph of a large number of rabbits in Australia — ***What are some of the factors that give rabbits such a high biotic potential***

In the 1800s, rabbits were introduced to Australia.

The biotic potential of a population are abiotic and biotic factors that cause populations to grow or improve the rate at which members of a species reproduce. The rabbits in Australia had a high biotic potential because they can grow quickly and reproduce rapidly when resources are abundant.

While this was beneficial for the rabbit population, it meant that rabbits are considered invasive species in Australia, destroying crops and land by overgrazing and digging, competing with native herbivores for food and habitat, and providing a prey source for introduced predators like cats and foxes, increasing their numbers and threatening native wildlife.

For example, favorable light for plants or favorable temperatures for animals are abiotic factors that help populations grow. A generalized niche or few predators are examples of biotic factors that help populations grow.

Biotic potential refers to the maximum reproductive rate of a population in ideal conditions.

Because r- and K-selected are human constructs given to natural populations, they aren't always a good fit for all populations - natural populations don't know they are supposed to fit into one of these two categories.

You should know the difference between a specialist (K-selected species) and a generalist (r-selected species) and how each type of species fares during and after a disturbance.

Check for Understanding

Complete the crossword puzzle below to check your understanding of k-selected, r-selected, generalist, and specialist species.

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