Invading with biological weapons: The importance of disease-mediated invasions

Invasive organisms and emerging wildlife disease pose two of the greatest threats to global biodiversity and ecosystem functioning. Typically, when parasites are considered in invasion biology, it is in the context of the enemy release hypothesis, wherein a non-indigenous species has greater probability of invasion success by virtue of leaving its natural enemies, including parasites, behind. It is also possible that native parasites may prevent invasions, but it is clear that invasive organisms may bring infectious diseases with them that can infect native competitors (via spillover), or act as competent hosts for native diseases, increasing disease prevalence among native species (via spillback). If the shared disease (either via spillover or spillback) has higher virulence in the native host (which is particularly likely with introduced parasites), there is the potential that the disease can act as a 'biological weapon' leading to a disease-mediated invasion (DMI). Here, we review cases where disease may have been an important factor mediating a wide range of invasions in vertebrates, invertebrates and plants. We then focus on the invasion of the grey squirrel into the UK as a case study of a DMI, and we discuss how mathematical models have helped us to understand the importance of this shared disease and its implications for the management of invasive species. We conclude that (i) DMIs are a widespread phenomenon, that (ii) spillover is more common in animal invasions and spillback more common among plant invasions and that (iii) spillover DMIs are particularly important in explaining the replacement of native animals with phylogenetically similar non-indigenous species.