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

Consumer-resource interactions can shift from exploitative to mutualistic when reciprocal benefits such as nutrient exchange offset the costs of damage or mortality, though these benefits are often difficult to detect. Carnivorous pitcher plants are typically viewed as exploiters of arthropod prey, yet most visitors escape capture and may gain 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 costs but reap net nutritive benefits. However, evidence that potential prey species derive measurable nutritional benefits from pitcher plants is lacking. To address this knowledge gap, we measured nitrogen isotopic enrichment in vespulid wasps residing in dense stands of the naturally ¹⁵N-enriched pitcher plant Darlingtonia californica relative to wasps from adjacent forest patches. Wasps occupying pitcher fens exhibited elevated ${\delta }^{15}N$ values, indicating assimilation of ¹⁵N-enriched nitrogen originating from Darlingtonia-either directly through nectar consumption or indirectly via pitcher-enriched arthropod prey. These findings are consistent with reciprocal nutrient exchange between carnivorous plants and local insect populations.