In the ecological research areas of population dynamics and demecology , a distinction is made between two basic reproductive strategies (or reproductive strategies ) when colonizing a biotope , which are referred to as r-strategy and K-strategy . The work of the ecologists Robert H. MacArthur and Edward O. Wilson on the (re-) settlement of islands (see island biogeography ) formed an important basis for this classification .
r-strategists are species that have a high reproductive rate (r) during reproduction (e.g. the common frog with hundreds of eggs left to itself) and use available resources beyond their capacity, while K-strategists use the number of their own Individuals stay at their capacity limit (K; carrying capacity) and thus ensure a smaller number of offspring with a higher chance of survival (e.g. people with often only one offspring who have been looked after over many years). It is about the fundamental question of how limited resources of energy and substances of one generation are made available for the next generation.
However, there is no absolute measure for the development of the specific strategy of a species; different species must always be considered in relation to one another, as the transitions are fluid.
In r / K reproduction theory it is believed that selection pressures drive evolution in one of two general directions: r-selection and K-selection. The notations r and K for the two directions refer to simple mathematical models of population dynamics, such as
where r is the maximum growth rate of the population ( N ) and K is the capacity limit of the ecosystem. The notation dN / dt stands for the change in the population over time.
The so-called r-strategy is based on the growth rate r of a population, which in the discrete case is used to calculate the following generations. The growth rate results from the difference between the birth rate (natality) and the death rate (mortality). With an r-strategy, the number of individuals grows exponentially at first until the capacity limit (K) of the habitat ( habitat or ecosystem ) is reached. The capacity limit is set by intra-specific competition ; Interspecific competition often results in a sharp decline or collapse of the population.
r-strategists usually produce a large number of offspring, but invest little in rearing, to which only small resources are passed on. This has the consequence that often only a small part of the offspring survives. An example of this is the common frog. Each female lays 3,000 to 4,000 eggs with little food in the yolk sac; brood care does not take place. Most tadpoles do not reach adulthood. Such species are able to quickly colonize new habitats. However, this strategy has disadvantages when there is high intraspecific competition and great pressure on predators.
Living beings with a typical r-strategy are most microorganisms (e.g. bacteria ) and small forms of more highly developed organisms (small crabs, aphids, blue tits, sparrows, mice), but also “social insects ” such as bees and ants . Plant-based r-strategists include in particular pioneer plants that can colonize ruderal sites quickly.
Typical characteristics of r-strategists
- Rapid individual development and small body size
- Short life span with a high rate of reproduction
- Early onset of reproduction, short birth intervals, large litter size
- Low parental care
Conditions that favor r-selection
- Environmental conditions (e.g. climate) highly variable
- Variable mortality ratios, often catastrophic population size drops, often extreme offspring mortality
- Mortality factors largely independent of population density
- The size of the population fluctuates extremely, rarely reaching the habitat's capacity limit
- Possibility of repopulating or re-colonizing habitats through spatial expansion ("opportunistic habitat use")
If a population has already reached the capacity limit (K) of the habitat, mass reproduction is hardly worthwhile. Rather, the number of individuals will now remain almost constant over a long period of time and the focus will be placed on “better quality” in the offspring. As a result, the reproduction rate in K-strategists is relatively low, but these species have a high life expectancy , among other things due to the high investment in the offspring (e.g. long breeding or rearing phase). There is also often a safeguarding of the area. This behavior is also known as the “placeholder strategy”.
In addition, K-strategists have the ability to make better use of given resources under strong competitive conditions than is the case with other types of strategy. In this way, K-strategists receive the number of individuals close to the environmental capacity or the ecological capacity (K).
Typical characteristics of K strategists
- Slow individual development and tall body size
- Long life span with a low rate of reproduction
- Later onset of reproduction, long birth intervals, low number of offspring
- Pronounced parental brood care
Conditions that favor K-selection
- Environmental conditions (e.g. climate) relatively constant
- Mortality depends on population density
- Relatively stable death rates, relatively low child mortality
- Population size relatively constant, at the limit of the habitat's capacity
- Saturated habitats, no development of new habitats possible ("consistent habitat use")
- Hardly any spatial expansion
- Rather older habitats
- Nicholas F. Britton: Essential Mathematical Biology. Springer, Berlin / New York 2003, ISBN 1-85233-536-X
- Robert H. MacArthur & Edward O. Wilson: The Theory of Island Biogeography. Princeton University Press, Princeton (NJ) 1967; Reprint: 2001, ISBN 0-691-08836-5
- Gerhard Hornung, Wolfgang Miram & Andreas Paul: Behavioral Biology. In: Biology, Green Series. Materials for secondary level II. Schroedel, Hannover 1998, ISBN 3-507-10530-6 .