COSTS OF CARNIVORY IN THE COMMON BLADDERWORT, UTRICULARIA-MACRORHIZA

Carnivorous plants are usually restricted to nutrient-poor environments, suggesting that there is a cost to capturing animals that is offset by the benefits of carnivory only under unusual circumstances. One such cost could involve a reduced photosynthetic capacity associated with the growth and maintenance of prey-capture organs. This hypothesis is tested using the common bladderwort, Utricularia macrorhiza, which bears numerous distinct prey-capture bladders. Measurements of the photosynthetic and respiration rates of leaves and bladders were incorporated into growth models to estimate the growth rates of plants with and without bladders. Comparisons were made in three lakes which differed in nutrient status and in which plants exhibited marked differences in their densities of prey-capture bladders. Overall, photosynthetic rates for leaves were approximately twice those for bladders while respiration rates did not differ significantly between tissues. Calculations incorporating these values indicate that plants producing both bladders and leaves would grow to as little as 21% or as much as 83% of plants that produced leaves alone. Comparisons among lakes led to the rejection of the hypothesis that plants from some lakes are able to produce more bladders per leaf because bladders differ in their photosynthetic productivity.