The r/K selection theory of life-history patterns has been in use for a long time, and serves as a useful tool to introduce the concept of survivorship curves (I recommend that you see this post if you need a refresher on the theory). However, there have been several different definitions used for r/K selection in scientific literature, leading to disagreement regarding it’s application. The r/selection theory has even been used to justify racial differences in humans! In this post, we will look at the various definitions of this theory and the arguments that explore the issues with r/k selection theory, eventually leading to it’s replacement by other theories of life cycles.

The many definitions for r/k survival strategies…

Gregory D. Perry in his 1981 paper identified 4 separate definitions being used for r/K survival strategies, as follows:

• Meaning I
  — r-selection is the selection for maximum population growth in uncrowded populations.
  — K-selection is the selection for competitive ability in crowded populations.
• Meaning II
  — r-selection is the density-independent component of natural selection. This means that crowding or overpopulation is prevented by density-independent mortality along the r-part of the growth curve.
  — K-selection is the density-dependent component of natural selection, leading to mortality because of resource shortage.
• Meaning III
  — r-species occur in habitats which last for very short times.
  — K-species occur in habitats with a long duration of stability.
• Meaning IV
  — r-selection is the allocation of a large proportion of resources to reproduction.
  — K-selection is the allocation of a small proportion of resources to reproduction.

The gaps in the r/k selection theory…

Perry reviews literature that identifies many gaps in this r/K selection theory. Notably, it is important to understand that the growth of populations is not solely dependent on resource availability and stability of the habitat. Many other variations occur in the life-history of a population, which are not adequately captured by this theory.

First, the “stability of the habitat” itself can be questioned when you consider different scales; is the habitat of a dung beetle the soil or a forest patch or the forest? At which scale do you consider a unit population? Further, does instability in the soil (through trampling, for instance) create instability in the entire forest?

Second, the adaptive capacity (genetic or behavioral) of the species to environmental fluxes is a big factor in the survivorship of a population. Human beings are categorized as K-strategists because they live near the carrying capacity and having fewer babies. However, this does not mean that their population cannot grow beyond a certain point (quite the contrary!). Humans have developed abilities that allow them to adapt to the changes in the environment, either by shifting to a new place or finding a new niche or ingeniously devising tools (making and wearing clothes is a way to adapt to the cold), thereby continuing to reproduce and grow their population. Every other organism can adapt and change its response to environmental fluxes. Otherwise, it would likely be extinct.

Third, predator-prey relationships and age-specific mortality also play an important role in determining the survivorship of populations. Changes in prey/predator populations can greatly affect the populations of other organisms in the food web. (Age-specific mortality seems to have found its way into popular usage of the r/K selection criteria, as I was taught in class. The r-strategists tend to have higher mortality in the early years of their life, as opposed to k-strategists who die faster later in life. This is, however, not an original part of the theory, which is Meaning I). Similarly, external factors like climate change/natural disasters may not be consistently present in the life-history of a population, but can have a very strong effect in survivorship when they do occur.

Lastly, migration (natural or adaptive), dispersal patterns and colonization of new ecosystems/niches are an important aspect in the life cycles of many organisms. Plants, for example, disperse their pollen grains and seeds widely, allowing them to enter new ecosystems and capture new niches. Such capabilities have been developed to specifically counter lack of resource availability and instability of the habitat. This questions the supposedly strong influence that resource availability and stability have a on survivorship.

Various other research and analysis has delved into the validity of this theory (see here, here and here) and arrived at a similar conclusion.

Conclusion

So, is the r/k selection theory wrong? I would call it an oversimplification. Currently, this theory has limited validity; it is continued in animal ecology studies for its simplicity and serves as a good introduction to the topic of life-history patterns.