You don’t have to go home, but you can’t stay here: Disease mediated invasions and the collapse of invasive population

Background/Question/Methods

Invasive species are often able to establish and spread with the help of diseases they bring that can infect native competitors. But sometimes, seemingly successful invasions in which the invasive species reach high densities suddenly collapse, with abrupt decline and extinction of the invader. Diseases have been implicated in invasion collapses, though the underlying mechanism accounting for the entire invasion arc is unresolved: whether the disease is part of the invasion process or comes later as a second instance of invasion. In this study, we simulated a disease-mediated invasion (DMI) by constructing a susceptible-infected-susceptible partial differential equation model for infection and population dynamics of phylogenetically similar native and invasive species. Our model introduces an evolutionary dimension into DMIs and addresses their implications over time and space. The aim of our study was to determine the conditions (if any) under which a disease brought by an invading species could be responsible both for the success of the invasion and for its decline.

Results/Conclusions

We found that when a competitively dominant native species became an inferior competitor due to infection with the invader’s disease, the native population may be able to withstand replacement and halt or reverse the invasion by evolving an effective disease defense—a resistance-driven evolutionary rebound sometimes leading to invasion collapse. The collapse pattern depended on a lag between the disease advance and the invader advance, creating a region of time and space in which the native populations contended only with the disease, evolving disease resistance before confronting the competing invader. We determined the biological conditions and spatial scale in which a DMI may end in evolutionary rebound and collapse, and we present testable predictions for DMI community dynamics. Implications of this model can be applied to the evolution of resistance in Hawaiian honeycreeper species to avian malaria (Plasmodium relictum) carried by invasive birds. Native birds were widely distributed, but by the late 1800's, they were excluded from lower elevations. Recently, low-elevation populations re-emerged by colonization of resistant individuals from higher elevations, suggesting a mechanism of resistance to the disease. This indicates that disease resistance may evolve rapidly enough in such systems to produce local reversal of the invasion.