Questions: Life History Strategies: r- and K-Selection

Question 1

A forest fire destroys 80% of a temperate forest ecosystem. After 5 years, which organisms would you expect to have recovered most fully?

• A. Old-growth conifers — K-selected species have deep root systems that survive fire
• B. Annual weeds and fast-reproducing insects — r-selected species rapidly colonize disturbed habitat
• C. Large mammals like deer — they have high mobility and can quickly recolonize from surrounding areas
• D. Neither — severe disturbance eliminates all life history strategies equally

Answer: (B)

r-selected species are specifically adapted to exploit disturbed, resource-abundant environments. After a fire, there are open resources, low competition, and high-quality habitat for rapid colonizers — exactly the conditions that favor high reproductive rate, small offspring, and short generation time. Old-growth conifers may eventually return, but on a timescale of decades to centuries. This asymmetry in recovery time is why conservation biology is particularly concerned about K-selected species: once lost, they cannot quickly recolonize.

Question 2

What is the fundamental mechanism that prevents any organism from being both maximally r-selected (many small offspring, rapid reproduction) and maximally K-selected (few large offspring, high parental investment) simultaneously?

• A. Natural selection has historically pushed all lineages toward one extreme or the other
• B. Energetic and physiological trade-offs mean energy spent on reproduction cannot also be spent on offspring quality or parental survival
• C. r and K selection occur in different environments, so no single organism experiences selection pressure for both
• D. Brain size limits the cognitive capacity needed for parental care in r-selected species

Answer: (B)

The r/K continuum exists because of real biophysical constraints: energy and time are finite. Resources invested in producing many small eggs cannot simultaneously be invested in nourishing each egg or in maintaining the parent's own survival and future reproduction. A salmon pouring all energy into a single massive spawning event dies immediately after; it cannot also invest in each offspring. An albatross investing two years in raising one chick lives decades longer. These are not evolutionary choices — they are enforced trade-offs between current reproduction, offspring quality, and parental self-maintenance.

Question 3

K-selected species are evolutionarily superior to r-selected species because investing more in each offspring produces better-adapted individuals.

• T. True
• F. False

Answer: False

Neither strategy is 'superior' — each is optimal in its ecological context. K-selected strategies evolve under intense competition near carrying capacity, where offspring quality determines survival. r-selected strategies evolve in unpredictable or frequently disturbed environments where producing many offspring quickly maximizes the chance that some survive. A dandelion is exquisitely adapted to exploit disturbed habitats; a blue whale is exquisitely adapted to an ancient, stable ocean. Neither is 'better' — both are fit solutions to different evolutionary problems.

Question 4

K-selected species are more vulnerable to extinction than r-selected species when human activities cause population declines.

• T. True
• F. False

Answer: True

K-selected species have low reproductive rates, long generation times, and produce few offspring — they cannot quickly replace individuals lost to hunting, habitat destruction, or bycatch. A population of blue whales or condors that drops to 50 individuals may require decades to recover even under perfect protection. An r-selected species like a mouse or dandelion can recover from a similarly severe reduction in weeks or months. This is why wildlife conservation prioritizes K-selected megafauna: they are the least resilient to the rapid population declines that humans cause.

Question 5

Explain the fundamental trade-off that underlies all life history variation, and why it prevents any species from being both maximally reproductive and maximally long-lived.

Model answer: All organisms have a finite energy budget that must be allocated among growth, survival, and reproduction. Energy committed to reproduction — producing gametes, gestating young, providing parental care — is unavailable for somatic maintenance and repair. Organisms that invest heavily in current reproduction age faster and die sooner; those that invest in self-maintenance survive longer but reproduce less in any given period. This reproduction-vs-survival trade-off is enforced by physiology: the same cellular machinery cannot simultaneously maximize reproductive output and longevity.

This trade-off is documented experimentally: artificially increasing reproductive output in birds (adding eggs to clutches) reduces parental lifespan. Reducing reproductive effort extends lifespan in lab organisms from fruit flies to nematodes. The evolutionary reason is that in environments where adult mortality is high, current reproduction is more valuable than investing in uncertain future survival; where adult mortality is low, investing in maintenance and future reproduction is the better strategy. The r/K continuum is simply the ecological expression of this underlying physiological trade-off.