r and k strategies

D Leela and classmates

Haven't we all blown on dandelions and watched the seeds float away? Seeds from plants like dandelions are light and have feathery bristles, can be carried long distances by the wind. With wind dispersal, the seeds are simply blown about and land in all kinds of places. To help their chances that at least some of the seeds land in a place suitable for growth, these plants have to produce lots of seeds. Have you not wondered how many seed they must produce to ensure survival of the species? Similarly insects lay large numbers of eggs to ensure that some of them survive.
At the other end, we have human beings who produce very few offspring. This is true of most large mammals, and they spend a good part of their lives ensuring that their children survive, protecting them and caring for them, even sending them to school and college! We do not see mosquitoes and small fish taking care of their little ones, we do see tigers and lions caring for their cubs.
Scientists say that these are two typical survival strategies found in nature. While you may expect a full range of such strategies, depending on how many offspring you produce and how much care you invest in raising them, nature seems to broadly settle for either end. Most species follow the insect-kind, called r-strategies, or the tiger-kind, called K-strategies.
Those organisms described as r-strategists typically live in unstable, unpredictable environments. Here the ability to reproduce rapidly (exponentially!) is important. They are typically weak and subject to great vulnerability to predators and to changes in their environment. The "strategic intent" is to flood the habitat with offspring so that, regardless of predators or high mortality, at least some of the progeny will survive to reproduce. Organisms that are r-selected have short life spans, are generally small, quick to mature and waste a lot of energy. Typical examples of r-strategists are salmon, corals, and bacteria.
K-strategists, on the other hand, occupy more stable environments. They are larger in size and have longer life expectancies. They are stronger i or are better protected and generally are more energy efficient. They produce, during their life spans, fewer progeny, but place a greater investment in each. Their reproductive strategy is to grow slowly, live close to the capacity of their habitat and produce a few progeny each with a high probability of survival. Typical K-selected organisms are elephants, and humans. 

TABLE

Characteristics of r- and K-selected organisms

r-strategists / K-strategists

short-lived / long-lived
small / large
weak / strong or well-protected
waste a lot of enrgey / energy efficient
less intelligent / more intelligent
have large litters / have small litters
reproduce at an early age / reproduce at a late age
fast maturation / slow maturation
little care for offspring / much care for offspring
small size at birth / large size at birth

We can look at the chance of survival at a young age as one way of characterizing these strategists. If you look at the percentage of survivors among the humans, it is high at a young age, remains more or less constant until old age, and then drops off rapidly. For insects it is very high at birth, and then drops very fast, very quickly. Then there are species like fish, or hydra, for which survival chances are more or less the same, uniform throughout their lives. (See Figure 1 for the survivorship curve that depicts this.)
K-strategists are called that because K is used to denote the "carrying capacity" of a habitat: the population size the habitat can support in terms of available resources. K-strategists produce ``just enough'' offspring so that they are close the carrying capacity of their habitat. r-strategists get their name from r, which is usually used to denote the slope of the exponential growth curve. (See Figure 2 showing the growth curve of bacterial cells.) Isn't it surprising that scientists can be so intelligent
and yet so bad at coming up with good names?
Are ALL species r or K strategists? Of course not. Nature is never so simple, is it? Not all organisms fit such a simple picture: sea turtles are a famous example. They lay a large number of eggs, but walk away without caring for their young, and lead long lives.
In fact, one can even ask wouldn't an organism be better off trying to use both r and K strategies, trying both alternately somehow? Nature is always interesting, and there are such organisms too!
Biofilms are a large kind of planktonic cells that grow slowly over days or weeks, rather than minutes. Their metabolic rate resembles stationary cells more than exponential phase ones. Biofilms are, not only tough; they can alter their local environment in a way that increases their resistance to predation and chemical attack. Their typical mode of reproduction is by fragmentation, in which large fragments separate from the biofilm mass and may form secondary colonies downstream with a relatively large chance of survival (once again in comparison with planktonic cells). Biofilms are also energy efficient in that the metabolic waste product of one organism can supply critical nutrients for another. Thus at nearly every point, bacterial cells incorporated into biofilms appear to behave as K-strategists. 
The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson in 1970 based on their work on bio-geography of islands. Apparently these strategies are even studied by economists. A company trying to determine what products to produce, may decide to follow r-strategies: like soap, in large numbers, small usage time, and low profit; or K-strategies: like refrigerators, in small numbers, long life, and high profit. Within products of the same kind, you have a range in earphones, from cheap ones available in large numbers to expensive custom-made headphones!

Sources: 
https://www.cs.montana.edu/webworks/projects/
https://en.wikipedia.org/wiki/R/K_selection_theory