Carnivorous plants result from several independent evolutionary processes and are an example of convergent evolution. The American naturalist Thomas Givnish was the first to realize that terrestrial carnivorous plants are mostly restricted to sunny, nutrient-poor and wet environments, where the marginal benefit derived from carnivory exceeds the cost. He introduced a cost/benefit model of carnivory, which soon became a framework for studying functional ecological relationships in carnivorous plants. The cost of carnivory is mainly associated with carbon investment in the production of the lure, mucilage and digestive enzymes in photosynthetically inefficient traps. On the other hand, benefits derived from prey capture associated with increased nutrient uptake include an increased photosynthetic rate, higher seed production and/or direct uptake of carbon from prey.
Since Darwin’s work in 1875, many studies have shown the significant positive effects that prey capture has on the growth and tissue mineral nutrient content, but experimental studies expressing these benefits in terms of increased photosynthetic rate are scarce and ambiguous. A recent paper in Annals of Botany investigates the full hunting cycle of the carnivorous sundew Drosera capensis, including prey attraction, digestion and the benefit from nutrient uptake. This study tests the hypothesis that the red colour of the tentacles may lure insects onto the traps, and measures the activities of different digestive enzymes in response to prey capture and mechanical stimulation of traps. Red coloration of tentacles did not act as a signal to attract fruit flies onto the traps, but the study conforms the hypothesis that there must be a benefit in terms of increased net photosynthesis in response to feeding, prey digestion and nutrient uptake.