Clay larvae do not accurately measure biogeographic patterns in predation

Abstract

Aim: Spatial variation in predation can shape geographic patterns in ecology and evolution, but testing how predation varies across ecosystems is challenging as differing species compositions and defensive adaptations can mask underlying patterns. Recently, biogeography has borrowed a tool from ecology: clay prey models. But clay models have not been adequately tested for geographic comparisons, and a well-known problem –that clay prey only appeal to a subset of potential predators– could bias detected geographic patterns whenever the relative importance of predator guilds varies among sites. Here, we test whether clay larvae accurately capture geographic differences in predation on real larvae. Location: 90° of latitude and >2000 m elevation across the Americas. Taxon: Vertebrate and invertebrate predation on ‘superworms’ (Zophobas larvae). Methods: Across six sites that vary dramatically in latitude, elevation, and biome, we quantified predation on live, dead, and clay larvae. We physically excluded vertebrate predators from some larvae to distinguish total predation and invertebrate-only predation. Results: Predation on live superworms almost doubled from our high-elevation high-latitude site to our low-elevation tropical site. Geographic patterns were consistent among live and dead larvae, but clay larvae missed extremely high predation at some sites and therefore mis-measured true geographic patterns. Clay larvae did a particularly bad job at capturing geographic patterns in predation by invertebrates, although sample sizes for invertebrate predation were small. Main Conclusions: Clay larvae are inappropriate for comparing predation rates across sites. They should be abandoned for biogeographic studies and reserved for comparisons within, rather than across, predator communities.