American Society of Naturalists

Philip Erm and Ben L. Phillips (March 2020)

Invasion waves can turn from pushed to pulled due to selection for low-density adapted individuals on invasion fronts

Read the Article (Just Accepted)

Can the evolution of an invasive species fundamentally reshape an entire biological invasion? Like a ripple expanding across the surface of a pond, plant and animal invasions have long been treated by ecologists as travelling waves of catastrophe that flow over entire landscapes. These waves are not all alike, however; they can either be “pushed” waves, so called because migrants from the well-populated core of the invasion spill over onto the invasion front and push it forward, or “pulled” waves, likewise named because isolated pioneers at the very edge of the invasion pull it forward in the absence of competitors. In this way the shape of an invasion wave will depend almost entirely on how successfully an invader can reproduce at low population densities – rely on others for cooperation or reproduction and your invasion will be pushed; thrive when freed from the pressures of competition and your invasion well be pulled. But what if the selective pressures inherent in invasions meant that an invader’s ability to cope with low densities could itself change over time? Could a pushed wave transform into a pulled wave as invaders evolve?

Philip Erm from the University of Cambridge and Ben L. Phillips from the University of Melbourne explored this very question by developing simulation models in which an invading species’ ability to reproduce at low densities was free to evolve as their population spread. They found that intense selection on the sparsely populated invasion front meant that invaders rapidly acquired the ability to cope with low densities. This caused invasions to transform from pushed waves to pulled waves, a result that not only changed the speed of the invasions, but also altered the genetic diversity of the populations left in their wake.

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Abstract

Understanding the dynamics of biological invasions is crucial for managing numerous phenomena, from invasive species to tumors. While the Allee effect (where individuals in low-density populations suffer lowered fitness) is known to influence both the ecological and evolutionary dynamics of an invasion, the possibility that an invader's susceptibility to the Allee effect might itself evolve has received little attention. Since invasion fronts are regions of perpetually low population density, selection should be expected to favour vanguard invaders that are resistant to Allee effects. This may not only cause invasions to accelerate over time, but, by mitigating the Allee effects experienced by the vanguard, also make the invasion transition from a pushed wave, propelled by dispersal from behind the invasion front, to a pulled wave, driven instead by the invasion vanguard. To examine this possibility, we construct an individual-based model in which a trait that governs resistance to the Allee effect is allowed to evolve during an invasion. We find that vanguard invaders evolve resistance to the Allee effect, causing invasions to accelerate. This results in invasions transforming from pushed waves to pulled waves, an outcome with consequences for invasion speed, population genetic structure, and other emergent behaviors. These findings underscore the importance of accounting for evolution in invasion forecasts, and suggest that evolution has the capacity to fundamentally alter invasion dynamics.