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

Abstract

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.

Mutualism at the leading edge: insights into the eco-evolutionary dynamics of host-symbiont communities during range expansion

“…A similar effect can occur at the edge of an expanding population, if enemies disperse more slowly than their hosts (Fagan et al, 2002;Nomikou et al, 2003). In contrast, if the linked species is beneficial, an expanding host population may also escape its mutualistic symbionts (Dickie et al, 2017;Shaw, 2022), to the detriment of its speed of spread. A similar behaviour occurs in cooperative systems, where the phenotype with the slowest speed of spread limits the spreading speed of the community Li et al (2005).…”

“…In this context, much greater attention has been given to the influence of abiotic factors (Chen et al, 2011;Spence and Tingley, 2020), or to the negative impacts of biotic factors such as predation, competition, or pathogen transmission (Phillips et al, 2010;Brown and Vellend, 2014;Wan et al, 2016). Theoretical approaches have also been scarce (Case et al, 2005;Brooker et al, 2007;Mack, 2012;Kubisch et al, 2014;Shaw, 2022), with very few mathematical models allowing treatment of hosts and their multiple symbionts as separate entities capable of influencing each others' population dynamics and evolutionary trajectories.…”

Mutualism at the leading edge: Insights into the eco-evolutionary dynamics of host-symbiont communities during range expansion

“…In this context, much greater attention has been given to the influence of abiotic factors (Chen et al, 2011;Spence and Tingley, 2020), or to the negative impacts of biotic factors such as predation, competition, or pathogen transmission (Phillips et al, 2010;Brown and Vellend, 2014;Wan et al, 2016). Theoretical approaches have also been scarce (Case et al, 2005;Brooker et al, 2007;Mack, 2012;Kubisch et al, 2014;Shaw, 2022), with very few mathematical models allowing treatment of the host and its multiple symbionts as separate entities capable of influencing each others' population dynamics and evolutionary trajectories.…”