The ecoimmunology of invasive species

Summary Invasive populations frequently harbour a reduced parasite community compared with their native counterparts. The loss of regulating enemies may result in re‐allocation of resources away from costly defences, effects that may be particularly pronounced at the wave‐front of the invasion during the range expansion stage. Bottlenecking and increased genetic drift is also expected to result in a loss of immunogenetic diversity. As the invasive species expands its range, selection will strongly favour increased growth and reproduction, traits that may trade‐off against immune function. Relaxed parasite‐mediated selection is expected to alter the trade‐offs between immune response and immunopathology. Hence, we might expect that a combination of necessity (i.e. loss of specific alleles at immune loci) and natural selection away from immunity towards other fitness traits should result in invasive populations having reduced resistance to parasites but greater potential for demographic growth in the invaded range. Following this argument to its logical conclusion would suggest that invasive species could also be more vulnerable to novel parasites, or to parasites to which the invasive population has lost immunity arriving from the native range. Despite this attractive narrative, there is currently little empirical evidence on the ecoimmunology and immunogenetics of invasions. In this article, we review the evidence for changes in the immune response of invasive species. We also examine the likely effects of range expansions and population genetic processes on the immune systems of invasive species. We conclude that much more empirical research is necessary in this field before general patterns and predictions can emerge.