Niche Breadth and Life History Variation in a Hybrid Daphnia Complex
Laboratory life table experiments using multiple clones of Daphnia galeata, D. hyalina, and D. galeata x hyalina hybrids quantified differences in niche breadth among these taxa along a gradient of food carbon levels (0.2-2.0 mg/L). Two-way analysis of variance revealed significant main effects (i.e., species, food level) for all life history features examined, as well as significant species x food level interactions for several life history characters (e.g., age at first reproduction and intrinsic rate of increase, r). Nested analysis of variance revealed significant species level effects for all life history characters examined, as well as significant clone level effects for all characters except r. Overall, species level effects explained approximately twice the proportion of the total variance for individual traits when compared with clone level effects (means of 32.8 and 15.8%, respectively). Significant interspecific differences in breadth of response (i.e., niche breadth) were observed for estimates of r. D. galeata exhibited a significantly greater niche breadth across food treatments than D. hyalina; D. galeata x hyalina showed an intermediate response breadth, which was not significantly different from either parental species. The existence of considerable intraspecific (i.e., clonal) variation in several life history traits, implies that any interpretation of life history variation among members of hybrid species complexes may depend critically on the genetic (clonal) make-up of the populations examined. These results are discussed in reference to the coexistence of these taxa in nature.
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