Nutrient availability and the carnivorous habit in Utricularia vulgaris.

1. Carnivory in plants is thought to enhance growth through an increased supply of nutrients, although there are considerable costs involved. It has been assumed that the relative investment of biomass in traps is inversely proportional to the availability of nutrients from non-carnivorous sources. Our aim was to test the effect of increasing nutrient concentration on investment in carnivory by Utricularia vulgaris. 2. Plants were grown under controlled conditions and nitrogen and phosphorus added at three loadings in a crossed design. Investment in carnivory was assessed as the proportion of (i) leaf biomass and (ii) leaf area comprising traps. 3. There was no effect of nutrient additions on plant growth or periphyton abundance. Investment in carnivory declined with increasing phosphorus loading. There was no effect of nitrogen, despite this being the nutrient commonly thought to be sought by carnivorous plants. Analysis of previously published data also indicated a decline in investment with increasing P availability. 4. Investment in carnivory in U. vulgaris is inversely proportional to the availability of phosphorus from non-carnivorous sources.