Topic 3.2 Notes – K-Selected r-Selected Species

What K-Selected and r-Selected Species Are

Every species faces the same problem. It has limited energy. Energy spent on one thing, like producing lots of offspring, cannot also be spent on making each offspring larger or protecting it.

That trade-off creates two general patterns.

• r means intrinsic capacity for increase. It connects to how fast a population could grow.
• K means carrying capacity, which is the maximum population size an environment can support over time.

So the big contrast is this:

• K-selected species put more energy into fewer offspring.
• r-selected species put less energy into many offspring.

These are ends of a continuum, not strict boxes. A species is classified by a cluster of traits, not by one detail alone. A large body size can hint at K-selection, for example, but size by itself is never enough. Some species are intermediate, and some shift depending on conditions.

Traits of K-Selected and r-Selected Species

Here’s the comparison APES wants you to know.

K-selected species

Examples you should recognize are elephants, humans, whales, bears, bison, eagles, and parrots. These species usually live where resources are already being used, so competition is intense.

r-selected species

Examples include insects, bacteria, small rodents, annual grasses, and dandelions. These species do well when open space or newly available resources appear.

What to use to classify a species

Use several traits together:

• number of offspring
• investment per offspring
• age at maturity
• life span
• frequency of reproduction
• environmental stability
• degree of competition
• maximum potential reproductive rate

A related pattern you may also see is a survivorship curve. K-selected species often resemble Type I, with high survival until old age, and many r-selected species resemble Type III, with heavy early mortality.

Survivorship curves: Types I, II, and III

Why These Strategies Work in Different Environments

In stable environments, populations often stay near carrying capacity. That means space, food, light, or nesting sites are already occupied. Offspring need to be competitive, so K-selected traits are favored. Fewer offspring survive better because each gets more investment.

In disturbed or unpredictable environments, there may suddenly be open habitat and available resources after fire, flood, clearing, or other change. In that setting, early reproduction and many offspring help a species spread quickly. That favors r-selected traits.

The trade-off to lock in is simple:

• K-selected populations grow slowly but invest in offspring quality.
• r-selected populations can grow fast but many offspring die early.

There’s also a survivorship connection:

• K-selected species usually show Type I or Type II survivorship.
• r-selected species usually show Type III survivorship.

That pattern helps support classification, but survivorship does not define it.

Biotic Potential and Population Recovery

Biotic potential is the maximum reproductive rate of a population under ideal conditions, meaning lots of resources and few limiting factors.

It is not the same as actual growth in nature. Real ecosystems have predation, disease, competition, and limited resources.

Traits linked to high biotic potential include:

• early maturity
• short generation time
• frequent or rapid reproduction
• many offspring

So r-selected species usually have higher biotic potential than K-selected species.

On graphs or in data, watch for these patterns:

• more offspring usually means less parental investment per offspring
• later maturity usually means lower maximum reproductive rate
• after disturbance, the more r-selected species usually rebounds faster
• long generation time, low reproductive output, and slow recovery point to K-selection

Invasive Species and Exceptions to the Pattern

Most invasive species are r-selected because they mature quickly, reproduce a lot, disperse widely, and colonize disturbed habitat fast. A classic example is the zebra mussel.

K-selected species are usually harmed more by invasive species because they cannot replace lost individuals quickly. Late maturity and low reproductive output slow recovery. r-selected species are generally less affected because they can replace individuals faster.

Still, don’t force every organism into a box.

• not every invasive species is r-selected
• not every mammal is K-selected
• not every plant or insect is r-selected

When the evidence is mixed, call the species intermediate on the continuum.