Ref ID : 935
W.T. Edmondson; Ecological studies of sessile rotatoria. Part I. Factors affecting distribution. Ecological Monographs 14(1):33-64, 1944
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1. Many pairs of species of sessile Rotatoria are associated together more frequently than is expected on the hypothesis of random, independent distribution. A few cases of possible mutual exclusion, or replacement are noted. The much greater number of examples of association than of exclusion suggests that while external factors favorable of unfavorable to species are of major importance, competition does not appreciably affect the composition of the fauna. 2. Many species show sensitivity to the chemical nature of the habitat, and occur more frequently in lakes on one side of the median of concentration of certain dissolved materials than on the other side. A discussion of the possible effective factors, out of all correlated variables, leads to the conclusion that some species are very likely excluded from lakes by high bicarbonate concentration, but not necessarily high pH. Examples are Ptygura melicerta and Collotheca algicola which seem to tolerate high pH but not bicarbonate. Beauchampic crucigera and perhaps Ptygura barbata are peculiar in that they tolerate neither high nor low pH nor high bicarbonate, and are thus limited to a narrow range of both. 3. The quantitative importance of surface as such in aquatic habitats is demonstrated by showing that larger populations of rotifers are supported by habitats which have more surface area in unit volume. 4. Many substrate plants show sensitivity to the same chemical factors which affect rotifers. This is very evident in the Winsconsin collections, less so in those from New England. 5. Different plants bear different sets of sessile rotifer species. Utricularia vulgaris americana has by far the most extensive and varied fauna. In general, plants with finely divided leaves have more rotifer species than those with broad leaves. 6. Some sessile species occur more frequently on certain plants than the commonness of the plants alone would warrant. Collotheca gracilipes is almost limited to Utricularia vulgaris americana. Ptygura melicerta and Collotheca algicola occur almost exclusively in colonies of the alga Gloeotrichia, or infrequently, Coleochaete. Several species tend to attach often to epiphytic green algae; for example, Collotheca campanulata and Floscularia janus. Cupelopagis vorax tends to attach most often to broad, flat leaves. Ptygura brevis, in the present series of collections, was found almost invariably in forks of finely divided leaves, usually Ceratophyllum. Ptygura velata also showed strong reaction to shape of substrate in that it favored to concave, curled tips of Sphagnum erythrocalyx. Ptygura beachampi occurred on many substrates, but in any given locality was apt to be highly limited. 7. Experimental analysis of the attachment of Collotheca gracilipes shows that under the experimental conditions, it can attach to a number of plants on which it has not been found in nature, but attaches most frequently by far to Utricularia vulgaris americana. The observed preponderance of C. gracilipes on the young leaves of the plants is shown to be a result of qualitative differences between leaves of different age which seem to be due to the presence in older leaves of a substance which the larvae avoid. The substance apparently is removed by a weak solution of ammonium oxalate and therefore may be a pectin. Floscularia conifera commonly attaches to many substrates. In the experiments it also attaches to many but most often to Utricularia. There is good agreement between field observations and experiment. No reason for the great variety of the Utricularia fauna is evident. 8. The presumed effect of plant succession on the rotifer fauna is discussed. Available evidence suggests that with progressive development of a lake and its flora, the fauna becomes larger in number of species. The presence of Utricularia vulgaris americana in any lake has a greater effect on the potential fauna than any other plant. The rootless independence of Utricularia permits it to occur in many types of localities, but by the same token it is limited to quiet habitats; hence, ponds, bogs, shallow bays of lakes. 9. Species of Flosculariidae are about as common in the littoral of large lakes as in small lakes and ponds, while species of Collothecidae are markedly more abundant in small, protected bodies of water. It is suggested that this difference is correlated with a difference in the feeding mechanisms developed in the two families. 10. The rotifer fauna of different regions is shown to be different in the relative commonness of species. This is largely, but not entirely, correlated with chemical differences. Local differences is distribution are largely substrate effects, while direct or indirect effects of chemical differences produce the major regional differences in distribution, but the effect is obscured by random irregularities in original distribution.