To Eat or Not to Eat: Novel Stable Isotope Models Reveal a Shift in Carnivory with Nutrient Availability for Aquatic Utricularia spp.

Background and aims: Freshwater nitrogen inputs are increasing globally, altering the structure and function of wetland ecosystems adapted to low nutrient conditions. Carnivorous wetland plants, Utricularia spp., are hypothesised to reduce their reliance on carnivory and increase their assimilation of environmental nutrients when the supply of ambient nutrients increases. Despite success in using stable isotope approaches to quantify carnivory of terrestrial carnivorous plants, quantifying carnivory of aquatic Utricularia requires improvement.

Methods: We developed stable isotope mixing models to quantify aquatic plant carnivory and used these models to measure dietary changes of three Utricularia species: Utricularia australis, U. gibba, and U. uliginosa in 11 wetlands across a 794 km gradient in eastern Australia. Diet was assessed using multiple models that compared variations in the natural abundance nitrogen isotope composition (δ15N) of Utricularia spp. with that of non-carnivorous plants, and environmental and carnivorous nitrogen sources.

Key results: Carnivory supplied 40 - 100 % of plant nitrogen. The lowest carnivory rates coincided with the highest availability of ammonium and dissolved organic carbon.

Conclusions: Our findings suggest that Utricularia populations may adapt to high nutrient environments by shifting away from energetically costly carnivory. This has implications for species conservation as anthropogenic impacts continue to affect global wetland ecosystems.