The concept of r-strategists and K-strategists links population dynamics to life history: strongly fluctuating environments lead to a strongly fluctuating population density that is low on average. According to the concept, these circumstances select for a high intrinsic growth rate r which is achieved by a distinctive life-history strategy consisting of rapid development, a small body size, early reproduction, semelparity, and a short life span. Species with these characteristics are called r-strategists. On the other hand, relatively constant environmental conditions allow a population to reach its carrying capacity K and thus a high average population density. The concept says that these circumstances select for a high K which is achieved by slow development (associated with great competitive ability), a large body size, delayed reproduction, iteroparity, and a long life span. Species with these characteristics are called K-strategists. The concept proposes to classify natural species on a continuum from r-strategists to K-strategists. It was developed from the 1960s to the 1970s and was very popular at that time. However, its popularity has vanished due to the recognition of serious problems, for example, the lack of a theoretical or empirical justification for the proposed life-history traits of K-strategists.

The r/K concept nonetheless is not only of historical importance, for two of its aspects are still in use today: the first one is the observation that life histories show patterns within and among species, and the second is that selection regimes vary with population density. The best known life-history patterns are continua from fast to slow life histories: species with a fast (slow) life history have characteristics similar to those that were proposed for r-strategists (K-strategists). Explanations for fast and slow life histories offered by the literature include allometric scaling and demographic models. The latter often include density-dependent selection in a way that is much more precise than in the r/K concept.

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