The ecology and distribution of the genusDaphnia(Crustacea: Cladocera) in restricted areas: the pattern in Yorkshire

Abstract

The old county of Yorkshire provides a diversity of habitats for freshwater crustaceans. Eight species of Daphnia (Cladocera: Anomopoda) are recorded. Taxonomic problems relating to some of these are discussed where appropriate. The ecological requirements and limitations of the various species are described and the patterns of geographical distribution to which these give rise within the area are reported. Most species are restricted to low altitudes. This is not related to altitude as such, all elevations being relatively modest, but reflects the rather sharp distinction in chemistry of upland and lowland waters in Yorkshire. Upland waters are generally acidic, markedly so on moorlands, and of low ionic content, often even in limestone areas which are frequently plastered with glacial drift: lowland waters are generally alkaline and relatively rich in inorganic ions, though small areas of lowland health are exceptions. Most species are restricted to waters of the latter type. Experiments using D. magna indicate that this limitation is related to the problem of maintaining internal ionic homeostasis in dilute acidic waters. D. obtusa is much the most successful species at colonizing acidic waters and this is reflected by its occurrence in upland regions not frequented by any other member of the genus, and in ponds on lowland heaths that are also evidently beyond the physiological range of other species. Distribution patterns are mapped and the measured ranges of certain chemical variables are recorded. Habitat preferences of individual species, within the tolerated range of water chemistry are described, and the importance of habits and behaviour in habitat selection is stressed. Coexistence of two or more species, even in small water bodies, need not always imply competition, though some indication of similarity of preferences can be obtained from such cases. Of the common species D. obtusa stands out as the most ecologically isolated, but this does not prevent it from coexisting with other species at times. Brief comparisons with other areas are made and certain problems raised by such comparisons are noted.