Priority efects and density promote coexistence between the facultative predator Chrysomya ruffacies and its competitor Calliphora stygia

Highly competitive ephemeral resources like carrion tend to support much greater diversity relative to longer-lived resources. The coexistence of diverse communities on short-lived carrion is a delicate balance, maintained by several processes including competition. Despite this balance, few studies have investigated the efect of competition on carrion, limiting our understanding of how competition drives coexistence. We investigated how priority efects and larval density infuence coexistence between two blowfy species, the facultative predator Chrysomya ruffacies and its competitor Calliphora stygia, which occupy broadly similar niches but difer in their ecological strategies for exploiting carrion. We examined how adult oviposition, larval survival, developmental duration, and adult ftness were afected by the presence of diferently aged heterospecifc larval masses, and how these measures varied under three larval densities. We found C. rufifacies larval survival was lowest in conspecifc masses with low larval densities. In heterospecifc masses, survival increased, particularly at high larval density, with priority efects having minimal efect, suggesting a dependency on collective exodigestion. For C. stygia, we found survival to be constant across larval densities in a conspecifc mass. In heterospecific masses, survival decreased drastically when C. ruffacies arrived frst, regardless of larval density, suggesting C. stygia is temporally constrained to avoid competition with C. ruffacies. Neither species appeared to completely outcompete the other, as they were either constrained by density requirements (C. ruffacies) or priority effects (C. stygia). Our results provide new mechanistic insights into the ecological processes allowing for coexistence on a competitively intense, ephemeral resource such as carrion.