Mutualism in disguise? Isotopic evidence for nutrient transfer from a carnivorous pitcher plant to its insect prey

Carnivorous pitcher plants are regarded as exploiters of arthropod prey, attracting them with nectaries which are densely clustered near the slippery peristome, putting visiting arthropods at risk of becoming trapped. However, most pitcher visitors are not captured and may benefit from access to concentrated, nutrient-rich rewards. This raises the possibility that pitcher plants and their arthropod visitors engage in an indirect mutualism in which both insects and plants incur some cost but reap net nutritive benefits, yet evidence that potential prey species derive measurable nutritional benefits from pitcher plants is currently lacking. To address this knowledge gap, we measured levels of nitrogen isotopic enrichment of vespulid wasps residing in dense stands of the naturally ¹⁵ N enriched pitcher plant Darlingtonia californica relative to those from adjacent forest patches. Wasps occupying pitcher fens had elevated δ ¹⁵ N values, suggesting they consume ¹⁵ N-enriched nitrogen originating from Darlingtonia — either directly through nectar or indirectly via ¹⁵ N-enriched arthropod proteins. These findings contribute support for the hypothesized nutritional mutualism between pitcher plants and local insect populations.