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Unformatted text preview: Ecology , 89(4), 2008, pp. 10151022 2008 by the Ecological Society of America THE ECOLOGICAL NICHE OF DAPHNIA MAGNA CHARACTERIZED USING POPULATION GROWTH RATE H ELEN L. H OOPER , 1 R ICHARD C ONNON , 1 A MANDA C ALLAGHAN , 1 G EOFFREY F RYER , 2 S ARAH Y ARWOOD-B UCHANAN , 3 J EREMY B IGGS , 4 S TEVE J. M AUND , 5 T HOMAS H. H UTCHINSON , 6,7 AND R ICHARD M. S IBLY 1,8 1 School of Biological Sciences, University of Reading, P.O. Box 68, Reading RG66PS United Kingdom 2 Institute of Environmental and Natural Sciences, University of Lancaster, Lancaster LA14YQ United Kingdom 3 Capita Symonds, Capita Symonds House, Wood Street, East Grinstead RH191UU United Kingdom 4 Pond Conservation: The Water Habitats Trust, Oxford Brookes University, Gipsy Lane, Headington, Oxford OX30BP United Kingdom 5 Syngenta Crop Protection AG, 4002 Basel, Switzerland 6 AstraZeneca R&D, Brixham Environmental Laboratory, Freshwater Quarry, Brixham, Devon TQ58BA United Kingdom Abstract. The concept of an organisms niche is central to ecological theory, but an operational definition is needed that allows both its experimental delineation and interpretation of field distributions of the species. Here we use population growth rate (hereafter, pgr) to define the niche as the set of points in niche space where pgr . 0. If there are just two axes to the niche space, their relationship to pgr can be pictured as a contour map in which pgr varies along the axes in the same way that the height of land above sea level varies with latitude and longitude. In laboratory experiments we measured the pgr of Daphnia magna over a grid of values of pH and Ca 2 , and so defined its laboratory niche in pHCa 2 space. The position of the laboratory niche boundary suggests that population persistence is only possible above 0.5 mg Ca 2 /L and between pH 5.75 and pH 9, though more Ca 2 is needed at lower pH values. To see how well the measured niche predicts the field distribution of D. magna , we examined relevant field data from 422 sites in England and Wales. Of the 58 colonized water bodies, 56 lay within the laboratory niche. Very few of the sites near the niche boundary were colonized, probably because pgr there is so low that populations are vulnerable to extinction by other factors. Our study shows how the niche can be quantified and used to predict field distributions successfully. Key words: calcium; Daphnia magna; ecological niche; fundamental niche; pH; population growth rate. I NTRODUCTION The concept of an organisms niche is fundamental to ecological theory and brings together two core ideas: (1) that environmental factors affect an organisms perfor- mance and thereby limit its geographical distribution, and (2) that organisms are adapted genetically to a limited range of environmental conditions. Although textbooks often treat the concept of an organisms ecological niche historically (emphasizing, among oth- ers, Hutchinsons  rectilinear definition of the...
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