Propagule pressure is often considered a key mechanism of invasion, and studies on the subject have mostly focused on two components: the number of individuals initially introduced (propagule size) and the number of introduction events (propagule number). However, propagule pressure has a genetic component that is often neglected or underappreciated. Propagule diversity, here defined as the number of source populations comprising a propagule, can increase genetic variation, reduce bottleneck and founder effects, and increase the likelihood of introducing preadapted genotypes. In an Araucaria forest in Rio Negro, southern Brazil, we explored the influence of number of source populations and number of individuals introduced in naturalization and invasion success or failure of 18 Pinus species. The species were planted during forestry experiments implemented in 1966 and 1967, and since then very few interventions have occurred in Rio Negro. Most places were left undisturbed since 1970, which allowed the natural regeneration of native and non-native plants. We hypothesized that species with higher numbers of source populations and high number of individuals would be invasive at the site.
Results/Conclusions
Both high number of individuals and high number of source populations were positively related to naturalization success, but high number of source populations explained invasion success better than number of individuals did. That is, for our study system, the number of source populations introduced provides the best explanation for the current observed status (casual, naturalized, or invasive) of Pinus species in Rio Negro. While propagule size is an important driver of naturalization, propagule diversity may be a key component allowing species to invade. This role could be related to the presence of greater genetic diversity, preadapted genotypes, or formation of novel genotypes by hybridization between previously isolated populations. In the studied system, all species but one with more than one source population are invasive (3 species). This clear separation between invasive and non-invasive status is likely the cause of the high fit of the propagule diversity models. On the other hand, propagule size probably resulted in a lower fit to the data because more individuals present did not necessarily enhance reproductive success.