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Beneficial or not? Decoding carnivore roles in plant protection

Shawn A. Steffan, Yoshito Chikaraishi, David R. Horton, Eugene Miliczky, Juan E. Zalapa, Vincent P. Jones, Naohiko Ohkouchi
Biological control 2015 v.91 pp. 34-41
biological control, carnivores, ecosystem services, herbivores, plant protection, predation, predatory arthropods
Carnivores indirectly protect plants by reducing herbivory. This important ecosystem service can be undermined, however, as carnivores feed upon fellow carnivores. Such intraguild predation is exceedingly common, yet measurement of the degree to which this occurs has remained elusive due to difficulties in measuring the trophic tendencies of free-roaming animals. Conventional molecular methods, such as bulk-isotopic analyses, do not produce reliably accurate trophic position estimates, and often the inaccuracy is substantial. With the advent of compound-specific isotopic analysis (CSIA), it is now possible to accurately quantify the lifetime trophic tendencies of wild carnivore populations. Unfortunately, CSIA is extraordinarily expensive and time-consuming, limiting the number of samples that can be analyzed. The need for high-quality trophic information has to be balanced with the inaccessibility of CSIA. Here, we propose coupling CSIA-derived trophic position estimates with conventional bulk-15N analysis, effectively calibrating site-specific bulk-15N data and thereby allowing for trophic position estimation using bulk data alone. We also create a framework that uses trophic position as a basis to characterize carnivores as beneficial for crop protection. Within an agricultural field, we demonstrated the utility of this new approach by measuring the trophic positions of six common arthropod species. We then compare these trophic position estimates to those deriving from conventional bulk-15N analysis. Our hybrid approach produced more accurate trophic position estimates than the stand-alone bulk-15N method. Ultimately, we were able to examine enough specimens to determine which carnivore populations were likely beneficial for plant protection, and which were not.