The diverse organisms, when facing different environmental conditions, adopt different types of reproductive strategies to maximize the advantages they can have in the environment they find themselves in. The consequence of different strategies result in different patterns of population growth.
The r/K selection theory is the classic and one of the most iconic theories to rationalize the diverse strategies adopted by organisms to address their environment. Though it has come to realize that the theory has its limitations, the r/K theory is still considered an important concept in population ecology.
r-strategy (opportunistic)
Species which adopt the r-strategy tends to reside in unstable environment and habitat. The best reproductive strategy for such a 'turbulent world' is to produce as many offspring as quickly as possible (because most of them will die out anyway – a sad but true fact). Often, they reproduce only once in their lifetime. Because the parents will not take care of their offspring, these organisms tend to have shorter lifespans. One should also infer from these facts that the r-type organisms tend to mature and develop more quickly. They also may have specific strategies to encourage larger amount of offspring production, for example the seeds of weeds may be delivered by air current, so that they can spread to a longer coverage of distance. Many invertebrates are r-strategists. Specific examples are insects, pest species of plants, frogs and fish.
K-strategy (equilibrium)
In contrast, the organisms which adopt K-strategy reside in relatively stable environmental conditions, and they produce further offspring. They often reproduce multiple times. The essence of a K-species is parental investment – the parents take care of their offspring, and the stages of development is much longer. Thus the lifespan is longer when compared to the r-type species. Many vertebrates are K-strategists. Specific examples are large mammals like elephants and deer, plants like palm nut tree (20 kg nut and a few years old!), primates like gorillas, and the most relevant of all, humans.
The comparison between r- and K-type organisms can be neatly summarized in Table 1.
It is worth discussing the theoretical origin of the r/K- selection theory. One may wonder why the two strategies are designated as 'r' and 'K' in the first place. To understand this will require our knowledge of some A-Level Calculus.
In the 19th century, the Belgian mathematician Verhulst has discovered the logistic function. Through further development, the logistic function has found applications in many areas, including population ecology.
Figure 1 shows the basic feature of a logistic function., where:
'a' is a constant related to the system in question;
'r' is a maximum growth rate;
'c' is a carrying capacity, a maximum value the system can achieve (or sustain) before it becomes saturated and cannot 'take any more'.
One can see that when the value of time t tends to infinity, the value of f(t) will reach the carrying capacity c – the maximum capacity the system can sustain.
By solving the logistic differential equation from Figure 2, we can derive the logistic function shown in Figure 1.
If we compare the graphs of an exponential curve (a) and a logistic curve (b) in Figure 3, we may find they appear to have some similarities when the two curve starts evolving. Because an exponential curve will shoot up in the y value when r increases, thus it takes a J-shape. In contrast, a logistic model will soon reach its limit of carrying capacity through time – thus y=K is a horizontal asymptote for the logistic function. The logistic function is thus a S-shaped curve.
The ideal condition for a r-type organism and a K-type organism is different. For the r-strategy, the ideal case is to stick to the trajectory of an exponential curve, meaning to have a growth rate (r) as large as possible. Yet take note that r-type organisms have a short time span, which means that one can imagine the growth curve for r-type organisms is really the left-half of the logistic curve, where the population (N) increases until the organism dies. Thus r-type organisms attempt to maximum growth rate (r) during their existence.
A contrast can be observed for organisms adopting K-strategy. For these organisms the aim is to get as close as possible to the carrying capacity (K) to the system through a longer time span. Because of the limit to infinity consideration for the logistic function in Figure 1, the longer the time, the closer the value is to the carrying capacity. The ideal condition for K-strategy is to extract as much as one can from the environment, before the system reaches a point of saturation.
Critique of the r/K selection theory
It is discovered that the whole r-K spectrum may be a continuous one. Not only certain species may have both attributes at the same time (for example, the 'redundant egg' scenario of birds popularized by evolutionary psychology), some species may even switch between the 2 modes during their lifespans.
More empirical studies have questioned if the selection theory can really successfully predict the correct strategies, given the diverse numbers of influencing parameters.
Though alternative theories have emerged as an attempt to replace the r/K theory, none of them have comparative advantages as they all have their specific limitations, and therefore everyone is returning to this old school theory. Nevertheless, the r/K theory is still a useful heuristic for our understanding of species population, and a good starting point for us to make sense of the complexity of ecological systems.
by Ed Law
