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

The evolution of mutualism between host and symbiont communities plays an essential role in maintaining ecosystem function and should therefore have a profound effect on their dynamics of range expansion. In particular, the presence of mutualistic symbionts at the leading edge should enhance the propagation of host-symbiont communities. Here we develop a theoretical framework that captures the eco-evolutionary dynamics of resource exchange between hosts and symbionts, and their dispersal dynamics in space. We provide quantitative insights explaining how the evolution of resource exchange may shape community structure during range expansion. Parasitic symbionts receive the same amount of resource from the host as mutualistic symbionts, but at lower cost. This selective advantage is strengthened with resource availability (i.e. with host density), promoting mutualism at the range edges, where host density is low, and parasitism in the population core, where host density is higher. The host growth rate depends on the average benefit provided by the symbiotic community, and is maximal at the range edges where symbionts are more mutualistic. The expansion of host-symbiont communities is pulled by the hosts, but pushed by the symbionts. This spatial selection also influences the speed of spread. In particular, host-symbiont communities with high symbiont density at their core (e.g. resulting from more mutualistic hosts), or at their leading edge (e.g. resulting from symbiont inoculation) spread faster into new territories, and so do hosts with low dependence on their symbionts