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A short class with Sarah and her students. The other ways are under More.
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In population models, r is the maximum rate at which a population can grow when resources are plentiful. What kind of species would make that number as large as possible?
One that has lots of babies, very quickly.
Yes. And how could it start breeding sooner?
By growing up fast. If you mature in days instead of years, you can have many generations in one summer.
Exactly. Houseflies can complete their life cycle in about a week in warm weather. What is the cost of making so many offspring?
Each one gets very little food or care, so most of them die.
That trade-off, quantity over quality, is the heart of r-selection.
Aphids on a rose bush in early summer: their numbers shoot up almost overnight. What shape is that graph?
A J-shape, like exponential growth.
Right. Then autumn arrives, or the ladybirds find them. What happens next?
The population crashes.
That is boom and bust. Now think about survival. If a frog lays over a thousand eggs and only a few become adults, when do most of them die?
Right at the start, as eggs and tadpoles. That is the Type III survivorship curve.
Well done. Huge losses early in life are normal for r-selected species.
Farmers spray a field and kill ninety-nine percent of the aphids. Why might the problem return within weeks?
Because the survivors breed so fast that the population builds up again.
And if some survivors were naturally resistant to the spray?
Their offspring would inherit it, and with such short generations, resistance would spread quickly.
Exactly. Cane toads in Australia are another example. A female can lay thousands of eggs at a time, which helped them spread after they were introduced in 1935.
So r-selected species are great at taking over new places.
r-selected species are adapted to make their population grow as fast as possible.
many small offspring, each with little investment and no parental care.
early maturity, short lifespan and short generation time.
most offspring die early in life.
populations rise steeply in good conditions and crash when conditions change.
the strategy suits disturbed or unpredictable habitats, which is why many pests, weeds and pioneers are r-selected.
A female housefly can lay up to about 500 eggs in her lifetime, and in warm conditions a new generation can be produced in seven to ten days.
A house mouse can have 5 to 10 litters a year, averaging six to eight young each, and females can breed at about six weeks old.
Under ideal conditions, a bacterium that divides every 20 minutes becomes 2^9 = 512 cells after 3 hours.
Dandelions colonise lawns and roadsides quickly because the wind carries their parachute-like seeds over long distances.
r is for rapid, many and small; little care given, few survive at all.
Two families move to a new island. The Flies have hundreds of children in a week and leave them to find their own food; most do not make it, but the island is soon buzzing. The Elephants raise one careful child every few years. When a storm wipes out the island's food, the Flies vanish, then return in a week from a handful of survivors.
Rapid, Rabbits, Rain: r-species breed rapidly, like rabbits, and boom when the rain brings good conditions.
F.A.S.T. - Fecundity high, Adults small, Short life, Tiny parental care.
Term
What does r stand for in r-selected?
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Term
List four traits of r-selected species.
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Term
Which survivorship curve do r-selected species usually show?
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In what kind of environment does r-selection work best?
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Why do r-selected pests recover quickly after spraying?
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Give two examples of r-selected species.
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Species whose life history maximises the population growth rate, with many small offspring, early maturity and little parental care.
The maximum per-individual growth rate of a population when resources are unlimited.
The largest population of a species that an environment can support over time.
A survivorship pattern in which most individuals die very early in life.
A pattern of rapid population growth followed by a sudden crash.
A factor, such as weather or fire, that affects a population regardless of how crowded it is.
One of the first species to colonise bare or disturbed ground.
A species introduced to a new area where it spreads and harms native species or habitats.
r-selected species reproduce early and often, producing many small offspring with little or no parental care, a strategy that lets their populations grow rapidly when conditions are good.
The "r" in r-selected comes from the population growth equation, where it stands for the maximum rate at which a population can increase. r-selected species maximise that rate: they mature quickly, breed early, have many small offspring and give them little care, so most offspring die young. This strategy suits unstable or newly disturbed environments, where populations boom when conditions are good and crash when they turn bad. The lesson explores their traits, examples such as houseflies, aphids, mice and dandelions, their survivorship and growth curves, and why many pests and invasive species follow this strategy.
Ecologists describe a population's growth using two quantities. r is the intrinsic rate of natural increase, the fastest a population can grow per individual when resources are unlimited. K is the carrying capacity, the largest population the environment can support. r-selected species are those whose way of life maximises r.
Typical traits
Examples
Survivorship and population growth
Where the strategy works best r-selection suits unstable, unpredictable or newly opened environments: a ploughed field, a forest after fire, a puddle after rain or a fresh supply of food. Producing huge numbers of cheap offspring is a bet that at least some will land in a good place at a good time. Many pioneer species in ecological succession are r-selected.
Why it matters to people
A continuum, not two boxes r and K describe the ends of a spectrum. Many species mix the traits: sea turtles lay many eggs and give no care, yet they live long and mature late. Modern ecology studies these patterns as life-history strategies, but the r and K model remains a useful way to compare species.