Jennings J B, Calow P
Department of Pure and Applied Zoology, University of Leeds, Leeds.
Department of Zoology, University of Glasgow, Glasgow.
Oecologia. 1975 Jun;21(2):109-115. doi: 10.1007/BF00345553.
Life-history strategies, ranging from completely free-living types to obligate entoparasitism, through ectocommensalism, ectoparasitism, and entocommensalism, are discussed within the framework provided by the concepts of r- and K-selection. In r-selection density and competition effects are minimal and progeny output (fecundity) can be maximised, while in K-selection competition is keen and the premium is on adult survival with production of a smaller number of extremely fit offspring. Thus r-strategists, with high fecundity, can be expected to have low calorific values because their resources are channelled into production of the maximum number of progeny, while K-strategists will have high calorific values based on lipid reserves which buffer the adults against possible reductions in food supply. This negative relationship between fecundity and calorific value is found in the phylum Platyhelminthes, with entosymbiotes at one end of the spectrum (high fecundity, low calorific value) and ectocommensals and free-living species at the other (low fecundity, high calorific value). This puts the entosymbiotes as more r-selected than the ectocommensal and free-living species, with the ectoparasites occupying an intermediate position. However, it is argued that parasites, and particularly entoparasites, follow both an r- and K-strategy at the same time and that this is only possible because of the stable, nutrient-rich environment provided by the host. Evolutionary theory dictates that all species would follow an r- and K-strategy simultaneously but environmental conditions force them into one alternative or another. Consequently the high fecundity of entoparasites, which has hitherto been viewed as a specific adaptation to entoparasitism, is now viewed as an automatic consequence of the conditions provided by the parasitic environment.
在r选择和K选择概念所提供的框架内，讨论了从完全自由生活类型到专性体内寄生的生活史策略，包括体外共生、体外寄生和体内共生。在r选择中，密度和竞争效应最小，后代产量（繁殖力）可以最大化，而在K选择中，竞争激烈，重点在于成年个体的存活以及产生数量较少但极度适应环境的后代。因此，具有高繁殖力的r策略者预计具有较低的热值，因为它们的资源被用于产生最多数量的后代，而K策略者基于脂质储备具有高热值，这些储备可缓冲成年个体应对食物供应可能的减少。在扁形动物门中发现了繁殖力和热值之间的这种负相关关系，共生生物位于谱的一端（高繁殖力、低热值），体外共生生物和自由生活物种位于另一端（低繁殖力、高热值）。这使得共生生物比体外共生生物和自由生活物种更倾向于r选择，体外寄生虫占据中间位置。然而，有人认为寄生虫，特别是体内寄生虫，同时遵循r策略和K策略，而这之所以可能，只是因为宿主提供了稳定、营养丰富的环境。进化理论表明，所有物种都会同时遵循r策略和K策略，但环境条件迫使它们选择其中一种。因此，体内寄生虫的高繁殖力，迄今为止一直被视为对体内寄生的一种特定适应，现在被视为寄生环境所提供条件的必然结果。
