Relative balance of the cost and benefit associated with carnivory in the tropical Utricularia foliosa

Abstract Investment by bladderwort ( Utricularia foliosa ) in carnivory, in terms of total C and N of bladders per leaf, was estimated in places with different nutrient concentrations from the Yahuarcaca Creek in the Colombian Amazon. The aims were to determine whether nutrient limiting conditions stimulate the investment in carnivory, and the relative balance between C and N invested in carnivory versus C and N obtained from prey. There were no significant differences either for phosphate (PO 4 3− ) concentration or for ammonia (NH 4 + ) concentration among five sampling areas, along approximately 5 km long stretch of the creek, with a pooled mean ± S.D. of 0.19 ± 0.06 and 8.6 ± 3.0 μM, respectively. However, there were significant differences in the nitrate (NO 3 − ) concentration ranging from 0.6 to 2.5 μM. Total C and N of bladders per leaf increased with decreasing NO 3 − . This corroborates the hypotheses that the carnivorous plant U. foliosa optimises its investment in carnivory according to nutrient availability in the water, and that N is a limiting factor that stimulates the investment in carnivory. The numbers of prey per bladder were also higher under NO 3 − limitation, thus enhancing the input of nutrients toward the plant through the bladders. The ratio of total C of prey captured/total C invested in bladders was always lower than 1. However, the efficiency of N was higher since when NO 3 − concentration was lower than 1 μM, the ratio of total N of prey captured/total N invested in bladders ranged between 0.97 and 1.67.