How to outrun your parasites (or mutualists): symbiont transmission mode is key

Interspecific interactions shape how and when species, and population, ranges change. Natural enemies, like parasites, can slow population spread, or, conversely, a population can ‘outrun' its enemies and spread uninhibited. Yet, less is known about how mutualistic interactions shape population spread, and what role outrunning mutualistic partners plays. Here, I examine host–symbiont interactions specifically; common across animals and plants, and spanning the spectrum from parasitism to mutualism. I develop a model to determine when a symbiont shapes its host's population spread versus when the host outruns its symbiont. I find that symbiont transmission mode is key. For density-dependent transmission, symbionts cannot be sustained at the low-density population edge and the host outruns its symbiont, whereas frequency-dependent transmission leads to symbionts affecting host spread. However, this pattern breaks down in the presence of a host Allee effect: spread dynamics switch from ‘pulled' to ‘pushed', enabling a symbiont to influence population spread from behind the range edge. Overall, mutualistic symbionts speed up (and parasitic symbionts slow down) host population spread. These findings indicate that contact structures within a population, which shape symbiont transmission, are critical for determining whether host–symbiont interactions influence population spread.