Reassessing protocarnivory-how hungry are triggerplants?

Stylidium species (triggerplants) are claimed to be protocarnivorous based on the presence of glandular hairs, observations of trapped small organisms, and induction of proteinase activity. However, these traits might serve alternative functions. We aimed to re-assess and quantify the degree of carnivory for Stylidium species in an ecological context, by comparing the natural abundance (δ 15 N) of Stylidium species with co-occurring carnivorous (Drosera species) and non-carnivorous plants in their natural habitats. We hypothesised that the δ 15 N signature of Stylidium species would more closely match co-occurring carnivorous plant species than their non-carnivorous counterparts if they rely on captured organisms as a nutrient source, since there is an increase in fractionation by 3-5 ‰ per trophic level. Our results show that the Stylidium species sampled had δ 15 N signatures that matched more closely with co-occurring non-carnivorous plants than with carnivorous Drosera species. This does not support the claim that they rely on captured organisms as a nitrogen source, or the source is negligible. Other studies have shown that protocarnivorous species have a δ 15 N signature that is more similar to that of co-occurring carnivorous than that of non-carnivorous species. Therefore, our findings question the protocarnivory status of Stylidium species.