Several mechanisms influencing coexistence among clones of D. magna Straus (Crustacea: Cladocera) were studied by investigating competitive interactions, clonal and population dynamics and life-history variation. Exploitative competition for resources was shown to have a greater impact on coexistence and diversity (Chapter 5) than interference competition. Fish predation strongly influenced clonal diversity (Chapter 4). Increased diversity and clonal richness were due both to weak mortality, which stabilised population demography and reduced resource competition, and to genotype-dependent prey-responses to the presence of fish. Variation in prey-response to predation had greatest impact on clonal composition. Additionally, increased temperature hastened the loss of diversity through resource competition (Chapter 4), due in part to faster life-cycles. Sexual reproduction had a considerable influence on clonal coexistence and dynamics (Chapters 4 and 6). Variation in genotype-dependent sex induction altered the clonal composition of the extant population (Chapter 4), while the genotypic composition of ex-ephippial hatchlings determined the clonal and genetic structure of two intermittent, natural populations (Chapter 6). Little change in genotypic or clonal composition occurred except during winter, when diversity declined considerably and single-locus genotypes changed in frequency but no pattern in electrophoretic-clonal composition was detected. Ex-ephippial hatchlings exhibited genotypic variation in response to un-natural compared to natural cues for the termination of diapause (Chapter 6).

A relatively novel approach has been the use in laboratory manipulations of clones that were isolated from the same population and naturally co-exist. Life-history experiments (Chapter 3) demonstrated a population level influence on clone-dependent reaction norms in several traits varying with food and temperature levels, that were consistent with levels of invertebrate predation in the respective native populations. Coexistence was more likely among clones originating from the same population. Hence it is critical to experiment with clones from single populations in order to investigate mechanisms that maintain diversity within natural populations.