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

Any four: early maturity, many offspring, small offspring, little or no parental care, short life span, small body size, Type III survivorship, populations fluctuate, often limited by density-independent factors.

Populations stay near K, so competition is intense. Large, well-cared-for offspring that can compete successfully leave more descendants than many small ones.

Sockeye: semelparous (spawns once, then dies). Black bear: iteroparous (breeds several times).

Type I.

The aphids rebound quickly: they mature in days and produce many young, so their high rₘₐₓ lets survivors restore the population within weeks.

Its low rₘₐₓ (late maturity, few offspring) means it replaces losses slowly, so even moderate harvests can exceed its growth and the population takes decades to recover.

From the logistic growth model: r is the intrinsic rate of increase, K is carrying capacity.

Any one: dandelion, mosquito, house mouse, aphid, many weeds and insects.

Any one: black bear, moose, whale, elephant, human.

A compromise in which resources spent on one trait cannot be spent on another.

Reproducing several times in a lifetime.

A vole matures in weeks and has several litters a year, so it is r-selected compared with a moose. But it has fewer offspring and more parental care (nursing) than a mosquito, so it is K-selected compared with a mosquito.

It is mixed. Many eggs and no care are r-like (Type III survivorship). Late maturity, long life and repeated breeding (iteroparity) are K-like. Many long-lived fish show this mixed pattern, and lake sturgeon (which mature even later and spawn only every few years) show it in an extreme form; their low adult turnover makes them vulnerable to overfishing despite their huge egg numbers.

t = ln 81 ÷ 0.25 = 4.394 ÷ 0.25 = 17.6 years.

Bare, sunny ground is a new, unstable habitat with few competitors. Species that disperse widely (wind-blown seeds) and reproduce fast reach it first and grow quickly before slower competitors arrive. Fireweed, for example, colonizes burned areas in Canada rapidly.

After spawning, the dead salmon (and the parts bears and eagles drag into the forest) decompose, releasing nitrogen and phosphorus carried from the ocean. Trees and other plants near the stream absorb these nutrients, so a semelparous strategy also links ocean and forest ecosystems.

In ponds that dry up unpredictably, many tadpoles die regardless of their quality (density-independent mortality). Frogs that breed early and often and produce many eggs are more likely to have at least some offspring metamorphose before the pond dries, so their alleles become more common. In a permanent lake, populations stay near K and competition and predation dominate, so frogs that produce fewer, larger, better-provisioned eggs (or that grow to a larger adult size) have offspring that survive competition better, and these traits are favoured.

Reducing adult deaths is usually more effective. Turtles have Type III survivorship among young but rely on long-lived adults breeding for decades. Protecting eggs raises early survival, but most extra hatchlings still die before maturing (which takes many years). Saving one adult female preserves many years of future reproduction. Population models for long-lived turtles have repeatedly shown that adult survival has the largest effect on λ. (A strong answer may note that both together are best.)

Insect (r): a boom-and-bust graph that becomes more extreme; density-independent weather dominates; risks include pest outbreaks in good years (for example forest insects) as well as local crashes, but quick recovery. Mammal (K): a logistic-type graph near K; more variable weather may lower K (less food) or cause unusual die-offs; because of low rₘₐₓ, each crash takes years to decades to recover from, so repeated shocks can drive a long-term decline.