Density Underpins Shifts From Competition to Facilitation in Co-Invading Blowflies

The success of an invasive species is driven by its interactions with both native species and co-occurring invaders. These interactions can range from competitive exclusion to mutual facilitation. However, the factors that govern the direction of species interaction outcomes remain poorly understood in invasive systems, including their environmental dependency. We performed species interactions experiments between two co-occurring invasive blow flies: Calliphora stygia and Lucilia sericata. We varied both total larval density (n = 10, 20, 30, 50) and relative species ratios (100%, 70%, 50%, 30%) and measured outcomes in the form of adult emergence success and adult body size. We found that interaction outcomes varied by species. For intraspecific experiments, C. stygia experienced more pronounced effects of competition, with a more than 2.4-fold greater reduction in body size between low (n = 10) and high (n = 50) densities than experienced by L. sericata between the same densities. Calliphora stygia also experienced reduced adult emergence at n = 30 and n = 50, while L. sericata emergence was not affected. However, interspecific competition drove contrasting density-dependent species-specific responses. At low density interspecific ratios of 70% and 50%, female C. stygia performance reflected facilitation (i.e., increased body size relative to intraspecific competition), with no effect for L. sericata, while increasing density shifted interaction dynamics towards competition (i.e., interspecific and intraspecific effects became comparable) for both body size and adult emergence under all species ratios except 50C:50 L for males, where C. stygia body size decreased. In contrast, L. sericata performed worse under all high density interspecific conditions relative to intraspecific competition when measuring body size, suggesting stronger interspecific effects, regardless of species ratios. The asymmetric response of these two co-occurring blow flies highlights the nuance of co-invader interactions: competition outcomes between invaders are not fixed but rather strongly shaped by both density and species balance.