Aphid‐tending ants on introduced fennel: can resources derived from non‐native plants alter the trophic position of higher‐order consumers?

1. Although plant invasions often reduce insect abundance and diversity, non‐native plants that support phytophagous insects can subsidise higher trophic levels via elevated herbivore abundance. 2. Here ant–aphid interactions on non‐native fennel on Santa Cruz Island, California are examined. Fennel hosts abundant, honeydew‐producing fennel aphids. The patchiness of fennel and the relative lack of honeydew‐producing insects on other plants at our study sites suggest that assimilation of fennel‐derived honeydew would increase the abundance and decrease the trophic position of the omnivorous, aphid‐tending Argentine ant. 3. To assess the strength of the ant–aphid interaction, a comparison of ant abundance on and adjacent to fennel prior to and 3 weeks after experimental aphid removal was performed. Compared with control plants with aphids, ants declined in abundance on and around fennel plants following aphid removal. At the habitat scale, pitfall traps in fennel‐dominated habitats captured more ants than in fennel‐free scrub habitats. 4. To determine if assimilation of aphid‐produced honeydew reduces the ant's trophic position, variation in δ¹⁵N values among ants, plants and other arthropods was analysed. Unexpectedly, δ¹⁵N values for ants in fennel‐dominated habitats were higher than those of arthropod predators from the same sites and also higher than those of ants from fennel‐free habitats. 5. Our results illustrate how introduced plants that support phytophagous insects appear to transfer energy to higher trophic levels via elevated herbivore abundance. Although assimilation of fennel‐derived honeydew did not appear to reduce consumer trophic position, spatial variation in alternative food resources might obscure contributions from honeydew.