Life Histories

Life histories are basic strategies that species employ to maintain their populations. These are not decisions made by individual organisms, they are traits that have been selected for by nature over long periods of time. Many species have similar strategies involving...

How many offspring they have at one time
How often they can have offspring
How mature those offspring will be at birth
How much parental care they will receive

Keep in mind that for a population size to remain constant, the average parents need to average two offspring that will survive and replace them in the next generation. Because nature is so deadly, we can also look at survivorship curves that show the percentage of offspring from each generation that live long enough to reproduce.

r-type

The basic strategy for r-type life histories is to have as many offspring as possible and hope for the best. Lower case "r" in this case is the abbreviation for rate of increase or birth rate. Species with an "r-type" life history tend to...

have many offspring
have offspring that tend to be more developed at birth
have little to no parental care
be smaller in physical size
have a relatively short life span
be prey species instead of predators
have a Type III survivorship curve

Frogs are a good example of an r-type life history.

Females can lay 100s of eggs at a time and then leave forever
Some eggs might be eaten, some will hatch into tadpoles that might be eaten, some tadpoles will turn into frogs that might be eaten.
Most of the babies die in the first few days, weeks or year, but if they survive that, they can reproduce and have babies of their own.

K-type

The strategy for K-type species is quality over quantity. These species tend to live at stable carrying capacities (K) which do not fluctuate very much. K-type species tend to...

Have fewer offspring at a time
Have immature offspring that need more parental care
Live longer lives than most r-type species
Often predators or larger herbivores
Have a Type I survivorship curve

Polar bears are a K-type species.

They have one or two cubs at a time
The mother nurses the cubs in a den for months
When they are old enough to go out of the den, the mother protects them and teaches them how to hunt.
Cubs stay with the mother for 2-3 years, then can live 20 years on their own.
Most cubs survive till adulthood (actually, global warming is making that harder and harder, but it used to be true)

...and then there are others

Not every species fits neatly into the two categories of r-type and K-type. Sometimes a blending of characteristics is best.

Squirrels don't fit neatly into either category:

They can have 5 babies per year, way more offspring than it takes to replace the parents (r-type)
Babies are born helpless in nests and are cared for by the mother for months. (K-type)
Squirrels are likely to die at any age from predators, disease, cars, etc.
They tend to have a Type II survivorship curve.