This project will explore a new model of element linkage in organisms. The model is highly general and makes a single assumption Œ that organisms maintain balanced growth or equal specific rates of change of all elements within organism biomass. Using just that assumption, the Element Linkage model ties together three parameters: growth rate, organism stoichiometry, and a new parameter referred to as the catalysis power. Growth rate has obvious ecological relevance. Organism stoichiometry relates directly both to nutrient use efficiency and competitive ability. Catalysis power can be defined as the way the abundance of one element determines the rate of change of another element. The linkage model predicts that there will be a tradeoff between growth rate and competitive ability when the concentration of one element controls the flux of another. Considerable previous work on phosphorus in freshwater zooplankton supports the application of the Element Linkage model to these organisms. This project will focus on the growth-competition tradeoff the model predicts. Seventeen species of cladoceran zooplankton, plus six clones of one species, will enable a nested hierarchical taxonomic approach. Within the order, genera will be compared to genera, within a genus, species will be compared to species, and within a species, clones will be compared to clones. Phylogenetically Independent Contrasts will be used where possible, but at least in the study of species within a genus where the necessary phylogenetic tree is available.  Competitive ability will be measured indirectly by nutrient content of the animals, more directly by numerous measurements of food thresholds, and most directly via competition trials. An animal™s food threshold is the amount of food necessary for zero mass gain. Similar to the R * measure, a lower food threshold indicates superiority in exploitative competition. The threshold measurements and competition trials will be in laboratory flow-through culture systems. This project will look for interspecific growth-competition tradeoffs, which the linkage model predicts will occur on low-P but not high-P foods. These experiments explore thoroughly this tradeoff using an otherwise well- known experimental system and advance the application of stoichiometric models to studies of competitive interactions.

The project will advance a potentially important new model in ecology, and in so doing it will greatly expand the empirical work on competition in freshwater Cladocera. Career and educational opportunities will be generated for the PI, a graduate student, a technician as well as to undergraduates and high school teachers, who will participate in the project via ongoing University of Minnesota initiatives.

This project was funded by the NSF Ecology Program for the period of January, 2004 to December, 2006.