The anthropogenic-mediated introduction of exotics has brought into contact taxa that have been otherwise separated by geographical barriers for millennia. This novel encounter, contingent on the degree of reproductive isolation, can result in hybridisation between introduced and resident species (either native or another colonizer). While the stimulating effect of hybridization in driving biological invasions has almost exclusively been attributed to genetic mechanisms enhancing “invasiveness” of hybrid lineages, here we propose a novel role for hybridization in species invasions and range expansion, a purely demographic mechanism without the requirement for any local adaptations. As the founding population in most new species introductions is likely to be small, the colonizer is susceptible to demographic Allee effects driven by pollen or mate limitations. Using a plant population simulation model that incorporates demography, mating systems, quantitative genetics, and pollinators, we show that Allee effects can potentially be overcome by neutral hybridization with a resident species.
Our model showed that hybridization can be completely neutral (i.e. without invocation of adaptive changes) and transient (i.e. hybrids are ephemeral) and yet allow the establishment of an otherwise failed plant invasion. That is, when a cross-compatible resident species co-exists in the habitat, the colonizing species can exploit the individuals of the resident species as its potential mates and overcome the problem of mate limitation. Pure colonizer-type individuals can subsequently be reconstituted through crossing among hybrids or asymmetric introgression with the colonizer parents. The spatial version of our model showed that neutral hybridization can also accelerate the rate of invasions either with or without Allee effects. Conservation programmes should therefore account for this cryptic role that hybridisation could play in biological invasions.