3.2 K-Selected r-Selected Species

K-selected species

K-selected species thrive near their environment's carrying capacity (K). They have long lifespans, mature slowly, produce few offspring, and invest heavy parental care. Populations grow via a logistic growth curve, stabilizing near K. Examples: elephants, whales, humans. They're vulnerable to extinction because low reproductive rates make population recovery slow after disturbance.

r-selected species

r-selected species maximize reproductive rate (r), producing many offspring with little to no parental care. They mature quickly, have short lifespans, and populations grow exponentially, often crashing in boom-and-bust cycles. Examples: insects, rodents, dandelions, bacteria. They thrive in unstable or unpredictable environments where rapid colonization is favored over survival investment per offspring.

Spectrum, not categories

Most species fall along a continuum between pure r- and K-selected traits rather than fitting one extreme. Environmental stability influences which strategy is favored: stable environments favor K-selected traits (competition-based survival), while variable/disturbed environments favor r-selected traits (fast colonization). Understanding this helps predict population responses to environmental change or human disturbance.

Key terms

K-selected species

Species with slow maturity, long lifespans, few offspring, and high parental care; thrive near carrying capacity.

r-selected species

Species with rapid maturity, short lifespans, many offspring, and little parental care; population grows exponentially.

Carrying capacity (K)

The maximum population size an environment can sustain given available resources.

Boom-and-bust cycle

Rapid population growth followed by a sudden crash, common in r-selected species.

r/K continuum

A spectrum along which species fall based on their mix of r-selected and K-selected traits, rather than strict categories.

Try an AP-style question

Which trait is characteristic of r-selected species?

1. Many offspring with little parental care
2. Populations near carrying capacity
3. Long lifespans
4. Few offspring with much parental care