Benefits of increased colonist quantity and genetic diversity for colonization depend on colonist identity

Larger numbers of colonists can be more likely to establish and spread due to the benefits provided by either more individuals (quantity) or a greater diversity of genotypes or phenotypes (genetic diversity). However, the value of higher colonist quantity or genetic diversity varies widely across studies, leaving a great deal of uncertainty in how these respective mechanisms affect colonization success. This variability is potentially driven by differences in which traits are present in respective colonist pools (‘colonist identity’). Studies with high-performing colonizers (e.g. genotypes pre-adapted to the colonizing environment) may find increasing quantity or diversity to be beneficial because it increases the chance high-performers are sampled, while studies with no high-performers may find no effects of quantity or diversity. Alternatively, quantity and genetic diversity may play little to no role if the smallest populations already contain high-performing colonists because there ...