Beneficial or not? Carnivore trophic position under the lens of amino acid isotopic analysis

The trophic tendency of a carnivore shapes the nature of its contribution to herbivore suppression, as well as its indirect role in crop protection. Unfortunately, measuring the lifetime trophic tendency of a carnivore has remained prohibitively difficult, and as a result, animal trophic function has often been classified within a scheme of overly general categories, such as “omnivore” or “carnivore.” Because carnivory often involves intraguild predation, there is the potential for enemy species to reduce their fellow carnivore populations significantly, releasing pest species from population control. To provide greater resolution to estimates of carnivore trophic tendency, we analyzed the ¹⁵N:¹⁴N ratios of specific amino acids (phenylalanine and glutamic acid) within target organisms, and incorporated these data into a system-specific trophic position formula. Trophic position estimates were then regressed against bulk-¹⁵N data to correlate amino acid isotopic analysis with bulk-¹⁵N data, which allowed for trophic position estimation using bulk-¹⁵N data. Using the frequency distributions of the trophic position estimates, we present the trophic spectrum of each species arrayed across “trophoclines.” Species traditionally thought to be herbivorous were shown to be quite omnivorous, while certain carnivorous species were shown to be both intraguild predators and functional pests of the crop. Our findings reveal the degree to which free-roaming carnivore populations contribute to herbivore suppression, and thus, which species are likely beneficial for crop protection.