r-and K-selection in soil ciliates: a field and experimental approach

Summary Previously published field studies have suggested that the concept of r/K selection may be applied to terrestrial ciliates. A census was taken of the dominant groups, the Colpodea and Polyhymenophora, to determine species composition and absolute abundances in climatically unpredictable alpine and predictable lowland sites. In addition, two typical representatives of each of the two taxa (Colpoda aspera and Grossglockneria acuta, and Blepharisma undulans and Gonostomum affine, respectively) were selected for examination under laboratory conditions. Variables investigated were: the C/P index (ratio of Colpodea to Polyhymenophora), fertility, tolerance of altered environmental conditions, incidence in predictable or unpredictable biotopes, opportunism, competitive ability, and body size. Our own observations were supplemented by reference to the literature.The field census produced a higher C/P index in the unpredictable sites than in the predictable ones. The experiments show that C. aspera and G. acuta are less sensitive to changes in temperature and exhibit a more rapid rise in individual density as well as a higher number of individuals than do B. undulans and G. affine, although the former species have longer generation times at low temperatures. The explosive increase in number of individuals can be ascribed to the special reproductive strategy of the Colpodea (quadripartition in cysts). These results imply that the Colpodea are more r-selected as compared with the Polyhymenophora. That might be an explanation for their wide distribution and high density in unfavourable terrestrial habitats.