The importance of propagule source region as a component of propagule pressure for marine invasions

A growing body of evidence suggests that propagule supply can strongly influence the likelihood that introduced species will become successfully established. Recent studies that have looked at invasion success as a function of propagule characteristics such as the source region have come to very different conclusions regarding the importance of propagule source vs. propagule supply. In this study, we use the match between the latitude of the native range and the introduced range as an indication of the quality of the source population. Using data for 18 marine and estuarine invertebrates, we compare differences in the latitude of the introduced population and the latitude occupied in the native range. We also examine multiple introduced populations of one species, the European green crab. Our results show that there is no overall trend in the difference between the latitude of the native and introduced ranges. Roughly equal numbers of species invade relatively higher vs. lower latitudes compared to the latitude of their native range. Interestingly, this difference in latitude between native and introduced ranges is greater for organisms from lower latitudes than from higher latitudes. This is consistent with the idea that marine organisms from lower latitudes may be better able to colonize a broader range of latitudes outside their native region than organisms from higher latitudes. These results support the idea that in addition to propagule supply, we need to consider propagule quality including the source region as an important predictor of invasions success.