BLADDER CONTROL IN UTRICULARIA-MACRORHIZA - LAKE-SPECIFIC VARIATION IN PLANT INVESTMENT IN CARNIVORY

Differences among habitats in the relative availability of prey and inorganic nutrients suggest that carnivorous plants should vary their investment in prey capture. We tested this hypothesis using the carnivorous plant Utricularia macrorhiza, which is common in lakes exhibiting a broad range of nutrient conditions. We assessed U. macrorhiza investment in carnivory as the density of prey-capture bladders per leaf. Field surveys revealed that the number of prey-capture bladders per leaf varied significantly among lakes but was highly consistent within a habitat. Differences in the number of bladders reflected an increasing proportion of plant biomass in bladders. Contrary to our initial hypothesis, the number of bladders was positively, not negatively, correlated with specific conductance of a plant's habitat water. Reciprocal transplant experiments of plants from lakes with many or few bladders per leaf revealed that the number of bladders per leaf is plastic, capable of changing on new leaves over a period of a few weeks. Experimental manipulations of animal prey indicated that a plant's bladder number per leaf is a direct response to water chemistry rather than to prey availability.